Chapter 1 Existing information about transition metal...

21
1 Chapter 1 Existing information about transition metal dichalcogenides (TMDCs)

Transcript of Chapter 1 Existing information about transition metal...

1

Chapter 1

Existing information about transition metal

dichalcogenides (TMDCs)

2

11 INTRODUCTION

Transition metal dichalcogenides characteristically contain layered crystal

structures Number of researchers have fascinated by the motivating properties of the

compounds of this family For the last few decade scientists are beginning to turn to

transition metal dichalcogenides shortly known as TMDCs As their name suggests these

are made up of transition metals such as molybdenum tungsten or niobium linked with

chalcogens such as sulfur selenium etc They comprise a layer of transition metal atoms

sandwiched between two layers of chalcogen atoms However the atoms in these layers

are strongly held together by covalent bonds whereas each layer sheet is only associated

to its neighbouring layer by weak van der Waals bonds allowing individual sheets to be

separated from each other [1]

Transition metal dichalcogenides (TMDCs) are layered materials with strong in plane

bonding and weak out of plane interactions permits two dimensional layers of single unit

cell thickness Although TMDCs have been studied for decades recent advances in these

materials characterization and device fabrication have opened up new opportunities for

two dimensional layers of thin TMDCs in electronics and optoelectronics TMDCs such

as MoS2 MoSe2 WS2 and WSe2 and some mix compounds of these materials have

valuable band gaps that change from indirect to direct in single layers allowing

applications such as transistors photo detectors and electroluminescent devices [1] The

historical developments of TMDCs methods for preparing atomically thin layers their

electronic and optical properties have been reviewed

The compounds of Transition metal dichalcogenides group can be represented by

the formula of the type MX2 (where M is the transition metal group VIB and X2 is the

chalcogen element such as Se S Te etc) TMDCs with various characters of metal

semiconductor and magnetic substances have been studied widely These materials are

considered structurally as strongly bonded two dimensional X-M-X layers loosely

coupled to one another by relatively weak Van der Waals type forces [1]

3

In X-M-X sandwich layer the metal atom (M) is bonded to six nearest neighbour

atoms (X) Figure 11 shows the schematic diagram of two dimensional model of

transition metal dichalcogenides

Fig11 Schematic model of atomic layers of TMDCs

The optical and electrical properties of TMDCs have been investigated by several

researchers The structural and bonding properties of transition metal have become an

important field in recent solid state research It is well known that the electronic structure

of transition metal dichalcogenides is characterized by two types of states First there is a

strong interaction between the outer sp orbitalrsquos of the metal and outer p and s chalcogen

orbital The electronic states resulting from this interaction form a broad bonding and a

broad anti bonding bond commonly referred to as the valence and the conduction band

Secondly there is a much weaker interaction between the outer d orbitalrsquos of the

transition metal and the outer p chalcogen orbital [2]

4

Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are

fundamentally and technologically intriguing They are found to be chemically versatile

Multi layered TMDCs are direct band gap semiconductors whose band gap energy as

well as carrier type (n- or p-type) varies between compounds depending on their

composition structure and dimensionality They have been investigated as chemically

active electro catalysts for hydrogen evolution and hydrosulfurization as well as

electrically active materials in opto-electronics devices Their morphologies and

properties are also useful for energy storage applications such as electrodes for Li-ion

batteries and super capacitors

Structure of these transition metal dichalcogenides can be described as solid

containing molecules which are in two dimensions extends to infinity and which are

loosely staked on top of each other to form three-dimensional crystals Several layered

materials have promising semiconducting properties and have attracted attention as a new

class of solar cell materials Very important optical energy electrical energy and chemical

energy conversion efficiencies have been obtained in photovoltaic and

photoelectrochemical solar cells The potential of this group of materials has not been

fully discovered yet It appears to be limited mainly by the availability of appropriate

materials Attempts have been made to produce good quality crystals and thin films of the

layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar

cells devices applications Several approaches actively pursued to produce high quality

single crystals and thin films of layered transition metal dichalcogenides The layered

transition metal dichalcogenides exhibit promising properties for quantum solar energy

conversion Few of these properties are listed below

The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is

ideal for the solar energy absorption

Due to strong metal dichalcogenides hybridization the width of valance and

conduction band is of considerably high magnitude and because of this the charge

carrier mobility are sufficiently high

5

The absorption coefficients are found to be high for TMDCs materials It largely falls

in the range of 105 cm-1

Therefore solar energy conversion devices produced form TMDCs may be

considered as a bright option to more known solar cells Out of the entire TMDCs family

single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport

technique are chosen for the present investigation The advantages of crystal growth by

direct vapour transport technique are discussed in detail in chapter 2 The elemental

information about the material molybdenum (Mo) tungsten (W) and chalcogen element

selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2

single crystals are shown in Table 11

Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)

Parameters Mo W Se

Atomic Number 42 74 34

Atomic Weight (amu) 9595 18384 7897

Group 6 6 16

Density(kg m3) 10280 19250 4810

Melting point (K) 2896 3695 494

Boiling point (K) 4912 6203 958

Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High

Thermal conductivity(WmK) 138 173 20

Molar specific heat (JmolK) 2406 2427 2536

Heat of fusion (kJmol) 3748 353 669

Heat of vaporization(kJmol) 598 774 9548

Covalence radius (pm) 130 139 120

6

12 MOLYBDENUM (Mo)

Molybdenum is a Group VI chemical element with the symbol Mo and atomic

number 42 Mo display body centered cubic structure at room temperature There is no

confirmation for face changes up to 280 Gpa in experiments It is likely that the

solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard

steady carbides in alloys and due to this reason most of world production of the element

is in making many kinds of steel alloys including high strength alloys and super alloys

[3]

Fig 12 The solid state structure of molybdenum

The majority of molybdenum compounds have low solubility in water Molybdenum

enclosing enzymes are by far the most general catalysts used by some bacteria to break

the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen

fixation Owing to the diverse functions of the various auxiliary types of molybdenum

enzymes molybdenum is a required element for life in all higher organisms in the

majority of the bacteria [3]

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

2

11 INTRODUCTION

Transition metal dichalcogenides characteristically contain layered crystal

structures Number of researchers have fascinated by the motivating properties of the

compounds of this family For the last few decade scientists are beginning to turn to

transition metal dichalcogenides shortly known as TMDCs As their name suggests these

are made up of transition metals such as molybdenum tungsten or niobium linked with

chalcogens such as sulfur selenium etc They comprise a layer of transition metal atoms

sandwiched between two layers of chalcogen atoms However the atoms in these layers

are strongly held together by covalent bonds whereas each layer sheet is only associated

to its neighbouring layer by weak van der Waals bonds allowing individual sheets to be

separated from each other [1]

Transition metal dichalcogenides (TMDCs) are layered materials with strong in plane

bonding and weak out of plane interactions permits two dimensional layers of single unit

cell thickness Although TMDCs have been studied for decades recent advances in these

materials characterization and device fabrication have opened up new opportunities for

two dimensional layers of thin TMDCs in electronics and optoelectronics TMDCs such

as MoS2 MoSe2 WS2 and WSe2 and some mix compounds of these materials have

valuable band gaps that change from indirect to direct in single layers allowing

applications such as transistors photo detectors and electroluminescent devices [1] The

historical developments of TMDCs methods for preparing atomically thin layers their

electronic and optical properties have been reviewed

The compounds of Transition metal dichalcogenides group can be represented by

the formula of the type MX2 (where M is the transition metal group VIB and X2 is the

chalcogen element such as Se S Te etc) TMDCs with various characters of metal

semiconductor and magnetic substances have been studied widely These materials are

considered structurally as strongly bonded two dimensional X-M-X layers loosely

coupled to one another by relatively weak Van der Waals type forces [1]

3

In X-M-X sandwich layer the metal atom (M) is bonded to six nearest neighbour

atoms (X) Figure 11 shows the schematic diagram of two dimensional model of

transition metal dichalcogenides

Fig11 Schematic model of atomic layers of TMDCs

The optical and electrical properties of TMDCs have been investigated by several

researchers The structural and bonding properties of transition metal have become an

important field in recent solid state research It is well known that the electronic structure

of transition metal dichalcogenides is characterized by two types of states First there is a

strong interaction between the outer sp orbitalrsquos of the metal and outer p and s chalcogen

orbital The electronic states resulting from this interaction form a broad bonding and a

broad anti bonding bond commonly referred to as the valence and the conduction band

Secondly there is a much weaker interaction between the outer d orbitalrsquos of the

transition metal and the outer p chalcogen orbital [2]

4

Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are

fundamentally and technologically intriguing They are found to be chemically versatile

Multi layered TMDCs are direct band gap semiconductors whose band gap energy as

well as carrier type (n- or p-type) varies between compounds depending on their

composition structure and dimensionality They have been investigated as chemically

active electro catalysts for hydrogen evolution and hydrosulfurization as well as

electrically active materials in opto-electronics devices Their morphologies and

properties are also useful for energy storage applications such as electrodes for Li-ion

batteries and super capacitors

Structure of these transition metal dichalcogenides can be described as solid

containing molecules which are in two dimensions extends to infinity and which are

loosely staked on top of each other to form three-dimensional crystals Several layered

materials have promising semiconducting properties and have attracted attention as a new

class of solar cell materials Very important optical energy electrical energy and chemical

energy conversion efficiencies have been obtained in photovoltaic and

photoelectrochemical solar cells The potential of this group of materials has not been

fully discovered yet It appears to be limited mainly by the availability of appropriate

materials Attempts have been made to produce good quality crystals and thin films of the

layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar

cells devices applications Several approaches actively pursued to produce high quality

single crystals and thin films of layered transition metal dichalcogenides The layered

transition metal dichalcogenides exhibit promising properties for quantum solar energy

conversion Few of these properties are listed below

The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is

ideal for the solar energy absorption

Due to strong metal dichalcogenides hybridization the width of valance and

conduction band is of considerably high magnitude and because of this the charge

carrier mobility are sufficiently high

5

The absorption coefficients are found to be high for TMDCs materials It largely falls

in the range of 105 cm-1

Therefore solar energy conversion devices produced form TMDCs may be

considered as a bright option to more known solar cells Out of the entire TMDCs family

single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport

technique are chosen for the present investigation The advantages of crystal growth by

direct vapour transport technique are discussed in detail in chapter 2 The elemental

information about the material molybdenum (Mo) tungsten (W) and chalcogen element

selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2

single crystals are shown in Table 11

Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)

Parameters Mo W Se

Atomic Number 42 74 34

Atomic Weight (amu) 9595 18384 7897

Group 6 6 16

Density(kg m3) 10280 19250 4810

Melting point (K) 2896 3695 494

Boiling point (K) 4912 6203 958

Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High

Thermal conductivity(WmK) 138 173 20

Molar specific heat (JmolK) 2406 2427 2536

Heat of fusion (kJmol) 3748 353 669

Heat of vaporization(kJmol) 598 774 9548

Covalence radius (pm) 130 139 120

6

12 MOLYBDENUM (Mo)

Molybdenum is a Group VI chemical element with the symbol Mo and atomic

number 42 Mo display body centered cubic structure at room temperature There is no

confirmation for face changes up to 280 Gpa in experiments It is likely that the

solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard

steady carbides in alloys and due to this reason most of world production of the element

is in making many kinds of steel alloys including high strength alloys and super alloys

[3]

Fig 12 The solid state structure of molybdenum

The majority of molybdenum compounds have low solubility in water Molybdenum

enclosing enzymes are by far the most general catalysts used by some bacteria to break

the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen

fixation Owing to the diverse functions of the various auxiliary types of molybdenum

enzymes molybdenum is a required element for life in all higher organisms in the

majority of the bacteria [3]

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

3

In X-M-X sandwich layer the metal atom (M) is bonded to six nearest neighbour

atoms (X) Figure 11 shows the schematic diagram of two dimensional model of

transition metal dichalcogenides

Fig11 Schematic model of atomic layers of TMDCs

The optical and electrical properties of TMDCs have been investigated by several

researchers The structural and bonding properties of transition metal have become an

important field in recent solid state research It is well known that the electronic structure

of transition metal dichalcogenides is characterized by two types of states First there is a

strong interaction between the outer sp orbitalrsquos of the metal and outer p and s chalcogen

orbital The electronic states resulting from this interaction form a broad bonding and a

broad anti bonding bond commonly referred to as the valence and the conduction band

Secondly there is a much weaker interaction between the outer d orbitalrsquos of the

transition metal and the outer p chalcogen orbital [2]

4

Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are

fundamentally and technologically intriguing They are found to be chemically versatile

Multi layered TMDCs are direct band gap semiconductors whose band gap energy as

well as carrier type (n- or p-type) varies between compounds depending on their

composition structure and dimensionality They have been investigated as chemically

active electro catalysts for hydrogen evolution and hydrosulfurization as well as

electrically active materials in opto-electronics devices Their morphologies and

properties are also useful for energy storage applications such as electrodes for Li-ion

batteries and super capacitors

Structure of these transition metal dichalcogenides can be described as solid

containing molecules which are in two dimensions extends to infinity and which are

loosely staked on top of each other to form three-dimensional crystals Several layered

materials have promising semiconducting properties and have attracted attention as a new

class of solar cell materials Very important optical energy electrical energy and chemical

energy conversion efficiencies have been obtained in photovoltaic and

photoelectrochemical solar cells The potential of this group of materials has not been

fully discovered yet It appears to be limited mainly by the availability of appropriate

materials Attempts have been made to produce good quality crystals and thin films of the

layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar

cells devices applications Several approaches actively pursued to produce high quality

single crystals and thin films of layered transition metal dichalcogenides The layered

transition metal dichalcogenides exhibit promising properties for quantum solar energy

conversion Few of these properties are listed below

The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is

ideal for the solar energy absorption

Due to strong metal dichalcogenides hybridization the width of valance and

conduction band is of considerably high magnitude and because of this the charge

carrier mobility are sufficiently high

5

The absorption coefficients are found to be high for TMDCs materials It largely falls

in the range of 105 cm-1

Therefore solar energy conversion devices produced form TMDCs may be

considered as a bright option to more known solar cells Out of the entire TMDCs family

single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport

technique are chosen for the present investigation The advantages of crystal growth by

direct vapour transport technique are discussed in detail in chapter 2 The elemental

information about the material molybdenum (Mo) tungsten (W) and chalcogen element

selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2

single crystals are shown in Table 11

Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)

Parameters Mo W Se

Atomic Number 42 74 34

Atomic Weight (amu) 9595 18384 7897

Group 6 6 16

Density(kg m3) 10280 19250 4810

Melting point (K) 2896 3695 494

Boiling point (K) 4912 6203 958

Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High

Thermal conductivity(WmK) 138 173 20

Molar specific heat (JmolK) 2406 2427 2536

Heat of fusion (kJmol) 3748 353 669

Heat of vaporization(kJmol) 598 774 9548

Covalence radius (pm) 130 139 120

6

12 MOLYBDENUM (Mo)

Molybdenum is a Group VI chemical element with the symbol Mo and atomic

number 42 Mo display body centered cubic structure at room temperature There is no

confirmation for face changes up to 280 Gpa in experiments It is likely that the

solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard

steady carbides in alloys and due to this reason most of world production of the element

is in making many kinds of steel alloys including high strength alloys and super alloys

[3]

Fig 12 The solid state structure of molybdenum

The majority of molybdenum compounds have low solubility in water Molybdenum

enclosing enzymes are by far the most general catalysts used by some bacteria to break

the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen

fixation Owing to the diverse functions of the various auxiliary types of molybdenum

enzymes molybdenum is a required element for life in all higher organisms in the

majority of the bacteria [3]

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

4

Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are

fundamentally and technologically intriguing They are found to be chemically versatile

Multi layered TMDCs are direct band gap semiconductors whose band gap energy as

well as carrier type (n- or p-type) varies between compounds depending on their

composition structure and dimensionality They have been investigated as chemically

active electro catalysts for hydrogen evolution and hydrosulfurization as well as

electrically active materials in opto-electronics devices Their morphologies and

properties are also useful for energy storage applications such as electrodes for Li-ion

batteries and super capacitors

Structure of these transition metal dichalcogenides can be described as solid

containing molecules which are in two dimensions extends to infinity and which are

loosely staked on top of each other to form three-dimensional crystals Several layered

materials have promising semiconducting properties and have attracted attention as a new

class of solar cell materials Very important optical energy electrical energy and chemical

energy conversion efficiencies have been obtained in photovoltaic and

photoelectrochemical solar cells The potential of this group of materials has not been

fully discovered yet It appears to be limited mainly by the availability of appropriate

materials Attempts have been made to produce good quality crystals and thin films of the

layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar

cells devices applications Several approaches actively pursued to produce high quality

single crystals and thin films of layered transition metal dichalcogenides The layered

transition metal dichalcogenides exhibit promising properties for quantum solar energy

conversion Few of these properties are listed below

The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is

ideal for the solar energy absorption

Due to strong metal dichalcogenides hybridization the width of valance and

conduction band is of considerably high magnitude and because of this the charge

carrier mobility are sufficiently high

5

The absorption coefficients are found to be high for TMDCs materials It largely falls

in the range of 105 cm-1

Therefore solar energy conversion devices produced form TMDCs may be

considered as a bright option to more known solar cells Out of the entire TMDCs family

single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport

technique are chosen for the present investigation The advantages of crystal growth by

direct vapour transport technique are discussed in detail in chapter 2 The elemental

information about the material molybdenum (Mo) tungsten (W) and chalcogen element

selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2

single crystals are shown in Table 11

Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)

Parameters Mo W Se

Atomic Number 42 74 34

Atomic Weight (amu) 9595 18384 7897

Group 6 6 16

Density(kg m3) 10280 19250 4810

Melting point (K) 2896 3695 494

Boiling point (K) 4912 6203 958

Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High

Thermal conductivity(WmK) 138 173 20

Molar specific heat (JmolK) 2406 2427 2536

Heat of fusion (kJmol) 3748 353 669

Heat of vaporization(kJmol) 598 774 9548

Covalence radius (pm) 130 139 120

6

12 MOLYBDENUM (Mo)

Molybdenum is a Group VI chemical element with the symbol Mo and atomic

number 42 Mo display body centered cubic structure at room temperature There is no

confirmation for face changes up to 280 Gpa in experiments It is likely that the

solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard

steady carbides in alloys and due to this reason most of world production of the element

is in making many kinds of steel alloys including high strength alloys and super alloys

[3]

Fig 12 The solid state structure of molybdenum

The majority of molybdenum compounds have low solubility in water Molybdenum

enclosing enzymes are by far the most general catalysts used by some bacteria to break

the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen

fixation Owing to the diverse functions of the various auxiliary types of molybdenum

enzymes molybdenum is a required element for life in all higher organisms in the

majority of the bacteria [3]

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

5

The absorption coefficients are found to be high for TMDCs materials It largely falls

in the range of 105 cm-1

Therefore solar energy conversion devices produced form TMDCs may be

considered as a bright option to more known solar cells Out of the entire TMDCs family

single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport

technique are chosen for the present investigation The advantages of crystal growth by

direct vapour transport technique are discussed in detail in chapter 2 The elemental

information about the material molybdenum (Mo) tungsten (W) and chalcogen element

selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2

single crystals are shown in Table 11

Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)

Parameters Mo W Se

Atomic Number 42 74 34

Atomic Weight (amu) 9595 18384 7897

Group 6 6 16

Density(kg m3) 10280 19250 4810

Melting point (K) 2896 3695 494

Boiling point (K) 4912 6203 958

Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High

Thermal conductivity(WmK) 138 173 20

Molar specific heat (JmolK) 2406 2427 2536

Heat of fusion (kJmol) 3748 353 669

Heat of vaporization(kJmol) 598 774 9548

Covalence radius (pm) 130 139 120

6

12 MOLYBDENUM (Mo)

Molybdenum is a Group VI chemical element with the symbol Mo and atomic

number 42 Mo display body centered cubic structure at room temperature There is no

confirmation for face changes up to 280 Gpa in experiments It is likely that the

solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard

steady carbides in alloys and due to this reason most of world production of the element

is in making many kinds of steel alloys including high strength alloys and super alloys

[3]

Fig 12 The solid state structure of molybdenum

The majority of molybdenum compounds have low solubility in water Molybdenum

enclosing enzymes are by far the most general catalysts used by some bacteria to break

the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen

fixation Owing to the diverse functions of the various auxiliary types of molybdenum

enzymes molybdenum is a required element for life in all higher organisms in the

majority of the bacteria [3]

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

6

12 MOLYBDENUM (Mo)

Molybdenum is a Group VI chemical element with the symbol Mo and atomic

number 42 Mo display body centered cubic structure at room temperature There is no

confirmation for face changes up to 280 Gpa in experiments It is likely that the

solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard

steady carbides in alloys and due to this reason most of world production of the element

is in making many kinds of steel alloys including high strength alloys and super alloys

[3]

Fig 12 The solid state structure of molybdenum

The majority of molybdenum compounds have low solubility in water Molybdenum

enclosing enzymes are by far the most general catalysts used by some bacteria to break

the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen

fixation Owing to the diverse functions of the various auxiliary types of molybdenum

enzymes molybdenum is a required element for life in all higher organisms in the

majority of the bacteria [3]

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

7

121 Application of Molybdenum

Molybdenum is used in steel alloys for its high corrosion resistance and weld ability

The ability of molybdenum to withstand extreme temperatures without significantly

expanding or softening makes it useful in applications that involve intense heat including

the manufacture of armor aircraft parts electrical contacts industrial motors and

filaments Approximately all the high strength steel enclose Mo in amounts from 025

to 8 Molybdenum improves the strength of steel at high temperatures It is used as

electrodes in electrically heated glass furnaces It is also employed in nuclear energy

applications as well as for missiles and air craft applications It is a valuable catalyst in

petroleum refining [4]

122 Electronic Configuration of Molybdenum

The following symbolize the electronic arrangement for the ground state neutral

gaseous atom of molybdenum The pattern associated with molybdenum in its compounds

is not necessarily identical

Ground state electron configuration [Kr] 5s1 4d5

Shell structure 2818131

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

8

13 TUNGSTEN

Tungsten is a chemical element with the chemical symbol W and atomic number 74

Tungsten is a hard and rare metal under standard conditions and found naturally on Earth

only in chemical compounds Tungsten exists in two major crystalline forms which are α

and β The former has a body centered cubic structure and is the most stable form The

structure of the β phase is called A15 cubic Tungsten has the highest melting point

(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile

strength among all the elements The density of tungsten is 193 times than that of water

and about 17 times than that of lead Tungsten has the lowest coefficient of thermal

expansion The low thermal expansion and high melting point and tensile strength of

tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d

electrons [5]

Fig 13 The solid state structure of tungsten

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

9

131 Application of Tungsten

Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens

alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both

the filament and target) electrodes in TIG welding super alloys and radiation shielding

Due to hardness and high density tungsten is widely used in military field Tungsten

compounds are also often used as industrial catalysts Tungsten alloys are sometimes used

in low temperature superconducting circuits Tungsten with some percentage of

chalcogen elements found to have semiconducting nature Semiconductor crystal of

tungsten dichalcogenides are used in PEC solar cell [5]

132 Electronic Configuration of Tungsten

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of tungsten The pattern associated with tungsten in its compounds is not

necessarily identical

Ground state electron configuration [Xe] 4f14 5d4 6s2

Shell structure 2 8 18 32 12 2

Magnetic ordering Paramagnetic

Crystal structural Body Centered Cubic (BCC)

Element category Transition metal

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

10

14 SELENIUM (Se)

Selenium is a chemical element with symbol Se and atomic number 34 It is a

non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium

converts in to gray selenium at the temperature of 180degC The gray selenium is the most

stable and dense form of selenium The gray selenium has hexagonal crystal lattice

consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes

by slow cooling of molten Se or by condensing Se vapours just below the melting point

Red and black coloured Se are insulators while gray Se is a semiconductor showing

appreciable photoconductivity It can resists process of oxidation by air and it is not

affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual

changes at high temperature [6]

Fig14 The solid-state structure of selenium

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

11

141 Application of Selenium

The selenium exhibits photovoltaic and photoconductive properties Due to that

selenium is used in photocopying photocells light meters and solar cells Zinc selenide

was the first material used for blue LEDs Cadmium selenide has recently played an

important part in the fabrication of quantum dots Sheets of amorphous selenium convert

x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray

cameras Selenium is a catalyst in some chemical reactions but it is not widely used

because of issues with toxicity In X-ray crystallography incorporation of one or more

selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and

single wavelength anomalous dispersion phasing Selenium is used in the toning of

photographic prints and it is sold as a toner by numerous photographic manufacturers Its

use intensifies and extends the tonal range of black-and-white photographic images and

improves the permanence of prints [6]

142 Electronic Configuration of Selenium

The following correspond to the electronic arrangement of the ground state neutral

gaseous atom of selenium The configuration associated with selenium in its compound

form is not necessarily identical

Ground state electron configuration [Ar] 3d10 4s2 4p4

Shell structure 2 8 18 6

Magnetic ordering Diamagnetic

Element category Polyatomic nonmetal

Crystal structure Hexagonal

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

12

15 APPLICATION OF TMDCs

Significant growth has been made in the application areas of semiconducting

TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an

equivalent electrical performance to a 10 nm thick organic or amorphous oxide

semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible

electronics The charge transport and scattering mechanisms have been recognized and it

is found to be useful in many electronic devices This fundamental knowledge of TMDCs

has begun to convert into essential functional of an electronic circuit is well The need for

developing an inexpensive and efficient method of converting solar energy into electrical

or chemical energy inspired fast development of semiconductor electrochemistry in the

past decades The method of converting solar energy with the support of semiconductor

photo electrochemical cells has been advanced as an option to the well known energy

conversion method involving the use of solid state semiconductor solar cells TMDCs

materials are used in optoelectronics holographic recording systems switching infrared

generation and detection system [6]

Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may

be considered as an ideal model system for the investigation of fundamental aspects of

semiconductor metal interface The TMDCs appears to be an appropriate electrode for

solar energy conversion due to its ability to obtain relatively large photocurrents in

aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell

N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device

These properties suggest its stable and efficient application in the photo electrochemical

solar cell [6]

Similarly access to high-quality large area substrates is rising with the entrance of

chemical vapor deposition grown materials although improvements are needed for high

performance circuit applications In the field of optoelectronics methods for improving

light absorption fluorescence and electroluminescence quantum yields in ultrathin

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

13

materials will be desirable if they are to be feasible with conventional bulk

semiconductors Finally in the field of sensing applications chemical fictionalization

methods are preferred that impart high chemical selectivity and robustness without

unsettling the excellent electronic properties of the TMDCs semiconductor However the

chemical vapour deposition technique is the single identified method to obtain electronic

grade TMDCs over big areas Similarly solution based methods for arranging and

depositing TMDCs materials require large improvement particularly for high

performance electronic and optoelectronic use While many applications of

semiconducting TMDCs are almost the same to other electronic materials the atomically

slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on

such exclusive opportunities the technological strength of semiconducting TMDCs can

be maximized

16 OCCURRENCE AND SYNTHESIS

Since TMDCs single crystals are not identified to occur naturally and so they have to

be produce in the laboratory For the growth of this crystal a range of growth techniques

are accessible at present These contain both growth from the melt as well as growth

from the vapour The well acknowledged methods over the years for the growth of binary

IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical

vapour transport (CVT) techniques The VIB metal compounds have been originally

produced as single crystals by Kjekshus [7] and coworkers Since then a number of

research groups have deal with the growth of single crystals constantly from the vapour

phase The compounds are always produced from the pure elements The single crystals

are developed from the vapour phase either by conscious cooling creating sublimation or

by a mineralization Chemical transport responses are frequently used to develop single

crystals of TMDCs The developed crystals are stable in standard laboratory

circumstances [7]

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

14

17 EXISTING INFORMATION REGARDING CHARACTERISTICS

OF TMDCs

171 Structural Investigation

Small dimensional crystals are of immense importance because of their exacting

properties associated to the crystalline anisotropy Transition metal dichalcogenides that

crystallize in the form of parallel fiber represent a diversified family ranging from super

conductors to widespread band gap semiconductors Diverse physical properties initiate

from minute difference of the X-X and M-M legend lengths consequential in structures

possessing three different varieties of trigonal prismatic chains Their essential structural

units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains

parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16

Every chain shifted with regard to two adjacent ones by half the lattice factor all along

the c direction [7]

The chains are connected by metal-chalcogen bonds and shape layers are associated

by a lot weaker Van der Waals forces Their structure formulates them beneficial for

battery cathode intercalation and photochemical cell purposes These layered compounds

crystallize hexagonally in strong strands or filamentary strip created platelets As shown

in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)

and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism

The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms

which distribute trigonal faces consequently forming isolated columns The columns

scuttle parallel to the crystallographic axis (c-axis) and are relocating from the

neighboring columns through one-half the unit cell along the c axis [7]

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

15

Fig 15 Crystal structure of TMDCs

Fig 16 Primitive cell of MoSe2 WSe2 single crystal

The distance among metal atoms by the side of the b axis is a lot shorter than the

inter prism distance This structure has the similarity with a bundle of metallic chains

each with an insulating covering The Van der Waals selenium-selenium bonds are in

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

16

plane perpendicular to the columns and the Van der Waals gap is almost vertical to the

c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16

The transition metal dichalcogenides (TMDCS) where M representing a transition

metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise

in layered structures consisting of sandwiches of three hexagonally formed sheets of

atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B

dichalcogenides are metallic and turn out to be superconductors at low temperatures The

crystal formation of these materials can be separated into two central groups Depending

on the crystal field symmetry about the metal atom it can be octahedral or trigonal

prismatically coordinated by the six nearest neighbour chalcogens The group 4B

dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have

prismatic while several group 5B compounds shows evidence of either or both structures

[7]

172 Electrical Properties of TMDCs

From the literature survey it is found that TMDCs are diamagnetic Every

characteristic of layer materials are found to be anisotropic The magnitude of the

conductivity at 300K fluctuates widely from sample to sample possibly because of

varying contamination concentrations However there is now noticeable proof that the

minimum indirect band gap in these semiconductors is higher than 03 eV and the

electrical conductivity in these materials is minimum at 300K It has been found in the

current research work that electrical conductivity of TMDCs increases with the

temperature (chapter 4)

It is found from literature review that early studies were made by hicksetal in 1967

[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2

specimen were of the order of 1016cm-3 An intensive study has been made on mobility of

charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated

the temperature dependence of electrical conductivity and Hall coefficient of

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

17

semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has

explain various scattering mechanisms involved in these materials He also stated that in

these materials the free charge carriers have a tendency to become localized within each

layer Therefore they behave as if they are moving through a part of independent layers

Moreover it was investigated that this tendency is related by a strong interaction between

the free carriers and the optical phonons polarized vertically to the layers [8] Table 12

and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by

various researchers

Table 12 Electrical data of Tungsten diselenide

SR

NO

Research

scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal P 0570 8 x1016 99 78 [5]

2 Fivazetal N 123 1 x1017 100 --- [8]

3 Deshpandeetal P 341 601x1015 304 103924 [9]

4 Lux-Steineretal P 40 6 x1015 250 --- [10]

5 Spah etal N 080 74x1016 236 --- [11]

6 Mahalawyetal N 0166 388 x1017 1215 --- [12]

7 Present work P 08635 6429 x 1014 1561 9709

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

18

Table 13 Electrical data of Molybdenum diselenide

SR

NO Research scholar

Carrier

type

Resistivity

ρ(Ωcm)

Carrier

concentrations

N(cm-3)

Hall

mobility

microH (cm2VS)

Hall

coefficient

RH(cm3C)

Ref

1 Hicksetal N 06 56 x1016 15 -110 [5]

2 Grantetal N 1 16 x1017 40 -078 [6]

3 Pathaketal N 01769 11 x1017 213 ---- [13]

4 Agarwaletal N 115 178 x1016 303 -650 [14]

5 Huetal N 25 35 x1016 314 ----- [15]

6 Sumeshetal N 4751 174 x1017 7558 ----- [16]

7 Present work P 09635 813 x 1014 1452 7526

173 Optical Properties of TMDCs

The investigation of optical properties of TMDCs gives important information of the

electronic properties and band structures of the crystal The wilsonetal [1] have

investigated the optical spectra of transition metal dichalcogenides having trigonal

prismatic coordination The transmission spectra and optical absorption of group VI A

materials have been investigated at liquid nitrogen temperature (77K) and room

temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in

1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

19

been investigated at liquid helium temperature by bealsetal [20] In the present

investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out

Optical band gap of all the samples under investigation are found to be around 14eV

The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the

application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the

working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed

that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)

which absorb visible solar energy in the neighborhood of infrared light are mainly

attractive materials for photoelectrochemical solar energy translation Among TMDCs

family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of

these materials contain polymer based TMDCs solar cells

It is seen from literature review that optical band gap of TMDCs material falls near to

maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix

compound of both can absorb maximum of the solar radiation incident on it This

increases the possibility of generation of photo electron hole pairs Therefore TMDCs

material must be useful in photo sensitive application Thus it is worth investigating the

use of TMDCs in PEC solar cell

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

20

REFERENCES

[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441

[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728

21

[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575

[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728