CHAPTER 1 INTRODUCTION TO SEMICONDUCTORS, PROPERTIES OF...

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1 CHAPTER 1 INTRODUCTION TO SEMICONDUCTORS, PROPERTIES OF SnS AND ZnO SEMICONDUCTING MATERIALS 1.1 INTRODUCTION Nanoparticles are often defined as particles of size less than 100 nm in diameter. A nanometre is extremely small, equal to one billionth of a metre. For comparison, a human hair is approximately 80,000 nm wide, a red blood cell is approximately 7,000 nm wide, a DNA double-helix is around 2 nm wide and a typical carbon-carbon bond length in the range of 0.12 nm - 0.15 nm. Nanoparticles can be considered as single crystals with a typical size of a few nanometres. They still contain hundred to thousand of atoms. Due to the size quantization effect, these nanoparticles preserve some bulk material properties; in addition, they exhibit more interesting properties. From the technological point of view, the main reason for studying nanostructured materials is the ease of tuning properties by gradually varying the particle size and shape. In recent years, there is a great interest in the synthesis and application of semiconductor nanomaterials, since their properties are size and shape dependent. In particular, research in IV-VI group narrow band gap semiconducting nanomaterials have gained attraction because of their potential applications in field effect transistors, thermoelectric materials, solar cells and near infrared detectors [1-4]. IV-VI group semiconductors are SnSe, SnS, SnTe, PbS, PbSe and PbTe. Among these, much effort has been made on the synthesis of PbS and PbSe due to their large exciton Bohr radii (20 nm for PbS to 46 nm for PbSe) [5]. Since lead is (Pb) a toxic material, synthesis of non-toxic materials with properties similar to lead

Transcript of CHAPTER 1 INTRODUCTION TO SEMICONDUCTORS, PROPERTIES OF...

CHAPTER 1

INTRODUCTION TO SEMICONDUCTORS, PROPERTIES OF

SnS AND ZnO SEMICONDUCTING MATERIALS

1.1 INTRODUCTION Nanoparticles are often defined as particles of size less than 100 nm in

diameter. A nanometre is extremely small, equal to one billionth of a metre. For

comparison, a human hair is approximately 80,000 nm wide, a red blood cell is

approximately 7,000 nm wide, a DNA double-helix is around 2 nm wide and a

typical carbon-carbon bond length in the range of 0.12 nm - 0.15 nm. Nanoparticles

can be considered as single crystals with a typical size of a few nanometres. They

still contain hundred to thousand of atoms. Due to the size quantization effect, these

nanoparticles preserve some bulk material properties; in addition, they exhibit more

interesting properties. From the technological point of view, the main reason for

studying nanostructured materials is the ease of tuning properties by gradually

varying the particle size and shape.

In recent years, there is a great interest in the synthesis and application of

semiconductor nanomaterials, since their properties are size and shape dependent. In

particular, research in IV-VI group narrow band gap semiconducting nanomaterials

have gained attraction because of their potential applications in field effect

transistors, thermoelectric materials, solar cells and near infrared detectors [1-4].

IV-VI group semiconductors are SnSe, SnS, SnTe, PbS, PbSe and PbTe. Among

these, much effort has been made on the synthesis of PbS and PbSe due to their

large exciton Bohr radii (20 nm for PbS to 46 nm for PbSe) [5]. Since lead is (Pb) a

toxic material, synthesis of non-toxic materials with properties similar to lead

chalcogenide is of interest in the new materials research. Semiconductor materials

should fulfill two requirements to be used as solar cells. (i) It should be less toxic

and its constituent elements should be abundant in nature. (ii) It should have good

electrical and optical properties [6]. SnS is a layered structure narrow band gap

semiconductor with less toxicity and its constituent elements are abundant in nature.

SnS has both direct (1.3 eV) and indirect (1.0 eV) band gaps, lying between

Si (1.12 eV) and GaAs (1.43 eV) [7]. It has high absorption co-efficient of 104 cm-1

and it usually exhibits p-type conductivity [8].

For solar cell applications, SnS requires wide band gap n-type

semiconductor as a window material. CdS is a wide band gap (2.4 eV)

semiconductor and it has been extensively used as window material for solar cell

applications. Reddy et al., have reported the highest conversion efficiency of 1.3%

for CdS/SnS heterostructure [9]. However, due to its toxicity Cd free window

material is required for the solar cell applications. ZnO is another wide band gap

(3.37 eV) semiconductor with n-type conductivity and it has potential applications

in the field of optoelectronic and electronic device fabrication and also it is

inexpensive and environmentally friendly material. Ichimura et al., have reported the

fabrication of bulk ZnO/SnS heterostructures for solar cell applications with SnS as

the light absorption layer and have achieved a photo conversion efficiency of 0.01%

[10].

1.2 SEMICONDUCTORS

Semiconductors are defined as materials with electrical resistivity lying

in the range of 10ˉ2 to 109 ohm cm1. Semiconductors occur in many different

chemical compositions in a variety of crystal structures. They are classified as

elemental semiconductors (Si, Ge, Se) and compound semiconductors (GaAs, ZnO,

CdS), based on their constituent elements. An optical property of semiconductor

depends on its band gap. Band gap is the characteristic property of the

semiconductors. Band gap (Eg) of the semiconductor is defined as the energy

difference between the top of the valence band (Ev) to bottom of the conduction

band (Ec) and it is given by

g c vE E E= − (1.1)

There are two types of band gap in semiconductors: (a) direct band gap

and (b) indirect band gap. Few examples for direct band gap semiconductors are:

ZnO, SnO2, SnS2, CdS and ZnS. Si, Ge and GeS are examples for indirect band gap

semiconductors. In the case of direct band gap semiconductors, the minimum of the

conduction band and maximum of the valence band occurs at the same k-point in the

Brillouin zone, whereas, for the indirect band gap semiconductors, the minimum of

conduction band and maximum of valence band occurs at different k-values.

Figure 1.1 Energy level diagram of (a) direct and (b) indirect band gap

semiconductors.

1.3 NANOSCALE SEMICONDUCTORS

Semiconductor nanoparticles demonstrate quantized optical and

electronic properties due to their large surface to volume ratio. Such unique

properties have made these particles useful in the field of nonlinear optics,

luminescence electronics, optoelectronic devices and solar energy conversion [11].

The electronic and optical properties of semiconducting nanoparticles are discussed:

1.3.1 Electronic and Optical Properties of Semiconductor Nanoparticles

In the case of semiconductor nanocrystals, the effect of particle size on

the optical properties is interesting. In order to understand the size dependent

optical and electronic properties of semiconductor nanoparticles, it is important to

know the physics behind what is happening at the nano level. The important

parameters of the semiconducting nanoparticles are discussed in the following

sections.

1.3.1.1 Exciton

When an electron is excited from the valence band to conduction band,

an electron-hole pair is created. This bound state electron-hole pair is called exciton

and it requires a minimum energy to excite it. Changes in the band gap due to

particles size, leads to the corresponding changes in the properties of the material.

The increase in exciton energy with respect to bulk semiconductor band

gap is given by,

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2E

μ⎡ ⎤Δ = ⎢ ⎥⎣ ⎦

h (1.2)

where, R is the radius of spherical quantum dots, µ is the reduced mass of electron-

hole pair and it is given as,

1 1 1

e hm mμ= + (1.3)

where, me and mh are effective mass of electron and hole respectively. The reduced

mass is generally smaller than the electron rest mass m0.

The exciton Rydberg energy is given as,

2

22RB

Eaμ

=h (1.4)

and exciton Bohr radius ( ) is given as, Ba

2

2Bae

εμ

=h (1.5)

where, e-electronic charge, ε-dielectric constant.

Therefore, the energy shift is written as

2

BR

aE ER

π⎡ ⎤Δ = ⎢ ⎥⎣ ⎦ (1.6)

This equation shows that for small quantum dots (R<<aB), the

confinement induced energy shift is large compared to the bulk semiconductor

energy gap [12]. Band gap energies and exciton Bohr radius of some common

semiconductors are given in Table 1.1.

Table1.1 Band gap and exciton Bohr radius of some common semiconductors

 Semiconductors

 Band gap [Eg (eV)] 

 Exciton Bohr radius [aB nm)]

 Si 1.12 4.3

PbS  0.41 20

PbSe  0.26 46

CdS  2.58 2.5

CdSe  1.84 5.5

ZnO 3.37 2.34

SnS 1.3 7

GaAs 1.43 14

1.3.1.2 Quantum Confinement Effect

The tuning of fundamental properties such as optical and vibrational

properties of nanostructured semiconductor material is possible when the size of the

nanostructured semiconductor material approaches the exciton Bohr radius. Though

significant variation in the fundamental properties is observed when the size is less

than the exciton Bohr radius. This is due to the confinement of charge carriers and

phonons within the nanoparticles. This is called quantum confinement effect. Efros

and Efros (1982) introduced three regimes of quantum confinement, depending on

the ratio of the nanocrystallite radius R to the Bohr radius of the electrons, holes and

electron-hole pair. They are:

(i) Strong confinement regime:

For quantum dots with a small radius, the large confinement induced

energy shift is given as,

BR a<<                                                            (1.7) 

here the individual motions of electron and hole are strongly quantized in

all spatial directions.

(ii) Intermediate confinement regime:

When the effective mass of the holes is higher than that of the electrons

( e

h

mm

<<1), the electron and hole Bohr radius are:

2

2ee

am eε

=h and

2

2hh

am eε

=h (1.8)

Therefore, h ea R a< <

(iii) Weak confinement regime:

For large quantum dots ,e hR a a>> , the confinement effects in this size

regime are relatively small.

1.3.1.3 Density of States

In the nanostructured systems such as quantum well, wire and dots the

motion of electrons, holes and excitons are restricted in one, two and three directions

respectively. The dramatic modifications in their density of states due to the

dimensional confinement of electrons in nanomaterials, gives rise to shape

dependent properties. Based on dimensional confinement, the nanomaterials are

classified as two dimensional (2D), one dimensional (1D) and zero dimensional

(0D) nanostructures [13]. Quantum confinement effect arises as a result of changes

in the density of electronic states. The relation between position and momentum of

free and the confined particles explains the quantum confinement effect. For the free

particle, the energy and momentum are defined but there is uncertainty in position.

For localized particle or confined particle the energy is well defined, the uncertainty

of the position decreases, so there is an uncertainty in momentum. Therefore, the

discrete energy of the particle is viewed as superposition of bulk momentum states.

Quantum confinement effect compresses the series of nearby transition occurring at

slightly different energies in the bulk into a single, intense transition in a quantum

dot [14].

The density of states is given by the formula,

( 1

2( )D

E Eρ α)− (1.9)

Where, D is the dimensionality (D =1, 2, 3) of the system.

Figure 1.2 Variation of density of states as a function of dimensionality of the

system.

In the bulk materials, large number of atoms forms a set of molecular

orbitals having negligible difference in their energy levels, resulting in continuum

bands. In the case of nanocrystalline materials, the bands split into discrete

electronic states in the valance and conduction bands. Therefore, photoluminescence

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(PL) and optical absorption (OA) spectra of nanoparticles show a blue shift in the

transition energy [15].

 

Figure 1.3 Energy level diagram of semiconductor quantum dot.

Effective mass approximation and tight binding approximation are used

to calculate the electronic band structure of semiconductor nanoparticles. For

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spatially confined structures, the effective mass approximation is used to calculate

the particle size and size dependent properties of the nanoparticles.

In the nanoparticles, the energy of the lowest excited state is given as,

22 2 2 2

12 2

1.8( )1 12

n

g nn

e h

e e SE R ER R R

Rm m

π αε

=

⎛ ⎞= + − + ⎜ ⎟⎡ ⎤ ⎝ ⎠+⎢ ⎥⎣ ⎦

∑h (1.10)

(1.11)

22 2 2 2

212

1.8( )2

n

gn

e e SE R ER R R Rπ αμ ε

=

⎛ ⎞= + − + ⎜ ⎟⎝ ⎠

∑hn

where, R is the radius of nanoparticles, Eg and E(R) are the bulk band gap and

modified interband transition values respectively, ε2 -dielectric constant of sphere,

αn -coefficient which depends on the ε, S -coordinates of the charge and the bar in

the third term represents the average over the wave function. The first term from

equation (1.10) represents the energy of quantum localization and it also depends on

the size. The second term corresponds to the Coulomb attraction and the third term

represents the solvation energy loss [16].

A number of reports are available on the confinement effect of charge

carriers in semiconductor nanostructures. Silicon has potential applications in the

field of optoelectronics. Silicon normally emits infrared luminescence due to its

small and indirect band gap. The observation of visible luminescence in the highly

porous electrochemically etched silicon is due to the quantum size effect [17].

Highly porous silicon contains the quantum size crystalline materials which are

responsible for the visible luminescence [18]. The room temperature PL spectra of

CdSe quantum rods with 3.7 nm width and with different lengths (9.2 nm, 11.5 nm,

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28.0 nm and 37.2 nm) show length dependent emission. This shows that the

dimension (shape) dependent emission [19]. InAs rods with different lengths along

with InAs dots with diameter similar to that of rods shows red shift in the optical

absorption and PL spectra from dots to short rods to longer rods and also shows

reduction in PL intensity. This is due to the effective mass of electron and hole

( = 0.024 m0 and = 0.4 m0, where, m0 is the free electron mass) are different.

Therefore, the electron wavefunction is delocalized over the entire rod whereas the

hole is in the medium confinement region and its wavefunction is limited. When the

length of the rod is increased, the overlap between the electron and hole

wavefunctions becomes smaller leading to the reduction in radiative rate, resulting

in the decrease of luminescence efficiency [20]. The room temperature optical

absorption spectra of CdSe quantum dots with diameters 2.8 nm, 4.1 nm and 5.6 nm

shows size dependent shift [21].

*em *

hm

The OA spectra of pure and polyvinyl pyrrolidone (PVP) capped ZnO

nanoparticles shows blue shift compared to bulk (~373 nm) value. The excitonic

absorption peak of the PVP-capped and non-capped ZnO nanoparticles are ~ 303 nm

and ~ 312 nm respectively. This may be attributed to the smaller size of the PVP

capped ZnO nanoparticles [22]. Optical absorption spectrum of SnS nanocrystals

with particle size of sub 10 nm shows indirect transition at 1.6 eV and with the

particle size of sub 200 nm shows 1.06 eV [23]. CdS normally exists as yellow

material and it becomes colorless when the particle size is less than 2.2 nm.

Cadmium phosphide is a black material but depending on the particle size, it shows

various colour [15]. The room temperature PL spectrum of thiophenol capped CdS

nanoparticles with diameter 3.6 nm exhibits band-edge emission and defect emission

at 3.0 eV and 2.56 eV respectively. These emissions were observed at higher

energies side compared to bulk CdS (2.43 eV) [24]. PbS quantum dots synthesized

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and stabilized in Nefion polymer showed a blue shift in the optical absorption

spectra from 0.41 eV to 2.3 eV due to strong quantum confinement as the particle

size decreased from 13 nm to 3 nm [25].

1.3.1.4 Phonon Confinement Effect

Phonons are quantized vibrations in crystalline solids. In single crystals,

phonons propagate as a wave. The vibrational properties of nanomaterials get

modified due to confinement of phonons within the nanoparticles. Considerable

changes in the vibrational spectra can be observed only when the particle size is

about 20 lattice parameters. Peak shift and broadening of the Raman spectra are

observed as a result of optical phonon confinement due to break down of lattice

periodicity. In nanomaterials, the periodicity of the crystal lattice is interrupted and

the selection rule q = 0 (zone center optical phonon) is relaxed. Therefore, the

phonons away from the Brillouin zone center also contribute to the Raman spectra.

This leads to changes in the peak shift and asymmetry broadening of the Raman

spectra [26]. Asymmetrical broadening in the optic-phonon spectra of ZnO

nanoparticles with size 8.4 nm and 4.5 nm have been observed due to the

confinement of optical phonons and the effect of phonon confinement depends on

the symmetry of the phonons [27].

1.4 STRUCTURAL AND OPTICAL PROPERTIES OF SnS

1.4.1 Crystalline Structure

Physical and chemical properties of a given material are derived from its

crystalline structure. SnS is a narrow band gap IV-VI group layered semiconductor.

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Figure 1.4 Crystal structure of SnS. The Sn atoms are green and the S atoms

are blue.

SnS was first reported by the German mineralogist Herzenberg. Single crystal of

SnS has been prepared by reacting stoichiometric mixture of Sn and S elements over

the temperature range of 600 - 750 °C [28]. SnS has orthorhombic structure. SnS is

stoichiometric under Sn rich conditions and it will form Sn1-xSx in sulfur rich

conditions.

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1.4.2 Optical Properties

SnS has both direct (1.3 eV) and indirect (1.09 eV) band gaps with a high

absorption coefficient of 104 cm-1 [8]. It has both p-type and n-type conductivity

depending on the departure of Sn stoichiometry from ideal. Optical properties of

SnS can be studied by a variety of experimental techniques such as optical

absorption, transmission, reflection, photoluminescence spectroscopy etc. In the

present work, optical properties of SnS nanostructures are investigated.

1.4.3 Applications

SnS has potential applications in the field of optoelectronic devices [29],

absorber layer in thin film solar cells [30], near infrared detectors [4], holographic

recording systems [31], anode material in lithium ion batteries [32-33] etc.

1.5 STRUCTURAL AND OPTICAL PROPERTIES OF ZnO

1.5.1 Crystalline Structure

Most of II-VI binary semiconductors crystallize in either cubic zinc

blende or hexagonal wurtzite structure where each anion is surrounded by four

cations at the corners of a tetrahedron. This tetrahedral coordination is typical of sp3

covalent bonding nature but these materials also have a substantial ionic character

that tends to increase the bandgap beyond the one expected from the covalent

bonding [34]. ZnO is a II-IV group semiconductor and its crystal structures are

shared by wurtzite (Figure 1.5 a) and zinc blende (Figure 1.5 b) structures. Under

ambient conditions, the thermodynamically stable phase is wurtzite. Wurtzite ZnO

has a hexagonal unit cell with lattice parameters a = 0.325 nm and c = 0.521 nm in

the ratio of c/a =1.6 and belongs to the space group of P63mc [34].

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(a) (b)

Figure 1.5 (a) Wurtzite and (b) Zinc blende crystal structures of ZnO.

1.5.2 Optical Properties

ZnO is a direct wide band gap (3.37 eV) and a transparent conductive

material with large exciton energy of 60 meV. ZnO films are transparent in the

wavelength range between 0.3 - 2.5 μm. Optical properties of ZnO can be studied

using experimental techniques such as optical absorption, reflection, PL and

cathodoluminescence spectroscopy etc. In the present work, room temperature PL

study is discussed in detail. Room temperature PL spectrum of ZnO consists of a

ultraviolet (UV) emission band (375 nm) and a broad emission band (420-700 nm).

At room temperature, the near UV-band is related to the excitonic nature of the

material and may be superimposed with the free exciton emission, its phonon

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replica, bound exciton emission, as well as biexciton emission [36]. The broad

emission band is called deep level emission (DLE) and is observed by several

defects in the crystal structure such as O-vacancy (Vo) [37-39], Zn-vacancy (VZn)

[40-42], O-interstitial (Oi) [43], Zn-interstitial (Zni) [44], and extrinsic impurities

such as substitutional Cu [45].

1.5.3 Applications

ZnO has potential applications in the field of optoelectronics, UV light

emitters, spintronics, solar cells, gas sensors, UV laser from ZnO nanowires, optical

photonic crystals and piezoelectric devices and as antibacterial agent [34, 46-50].

1.6 REVIEW OF LITERATURE

1.6.1 Literature Review of SnS Nanostructures

In recent years, considerable efforts have been made in the synthesis of

SnS nanostructures. Xu et al., have reported the synthesis of SnS quantum dots.

Where, SnBr2 is reacted with sodium sulfide in ethylene glycol (EG) at room

temperature in the presence of various stabilizing ethanolamines as ligands. The

ethanolamines were: triethanolamine (TEA), N-methlydiethanolamine (MDEA) and

N, N-dimethlyethanolamine (DMEA). Among these ethanolamines, small size and

monodispersed SnS nanoparticles with average particles size of 3.2 nm were formed

in the presence of TEA. This could be attributed to two reasons: (i) During

nucleation, the strong binding of multiple hydroxyl groups on the surface of SnS

nanoparticles and (ii) reaction of TEA with Sn2+ forms [Sn(TEA)n]2+ complex [51].

Biswas et al., have reported the synthesis of SnS nanorods and nanosheets through

thioglycolic acid (TGA) assisted hydrothermal process. The diameter of the SnS

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nanorods varied within 30-100 nm and the crystal size and shape depends on the

amount of TGA and sulfur source [52].

Chen et al., have reported the synthesis of SnS nanoflakes through

microwave-assisted polyol process. The final product was amorphous or impurity

when water and ethanol were using as solvent and for benzene no product was

obtained. SnS nanoflakes were formed in EG. There was no effect on the

morphologies and crystallinity of the SnS nanoflakes with increasing the reaction

time [53]. Peng et al., have synthesized SnS nanostructures such as nanobelts,

nanorightangles, nanorods and nanosheets via solvothermal route in EG medium.

SnS nanorods, nanobelts and nanorightangles were formed at reaction temperatures

of 140 °C, 160 °C and 180 °C for 12 hours respectively. The thickness and width of

SnS nanobelts were less than 30 nm and in the range of 50 nm -300 nm respectively.

The thickness of the nanosheets was between 10 nm and 20 nm. A nanorightangles

was composed of two nanobelts. Sheet-like structure was formed when thiourea was

used as sulfur source [54]. SnS nanoparticles were synthesized using tin metal and

elemental sulfur in diethyleneglycoldimethylether (diglyme) at 160 °C. In this

method, SnCl2 was dissolved in diglyme and Li[Et3BH] solution and tetrahydrofuran

was added. SnS nanoparticles with size of 20 nm - 40 nm were obtained [55].

Liu et al., have synthesized monodispersed, size tunable SnS

nanoparticles by using SnCl2 and TMS in oleylamine solution. The size of SnS

particles can be varied as 6 nm, 12 nm and 20 nm by adjusting the hot injection and

reaction temperatures between 120 °C, 150 °C and 210 °C respectively [56].

Hu et al., have reported the synthesis of SnS elegant 3D SnS urchin-like

architectures by solvothermal method. The morphological transformation from 3D

urchin-like architectures into 1D nanofibers with diameters of 20 nm - 60 nm and

lengths of 0.4 μm -3 μm has been obtained only by increasing the reaction time to

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96 hours. Uniform size SnS nanorods with diameter of 100 nm - 400 nm and length

of 0.6 μm - 4 μm were formed for the reaction time of 8-10 days [57].

Nanocrystalline SnS with different morphologies like flakes, sheets, rods and

granules has been synthesized over Sn metal foils via solvothermal method by using

ethylenediamine, water and their mixture as solvents [58]. Rao et al., have

synthesized SnS nanorods and nanoparticles through hydrothermal route at 180 °C

for 2 and 8 hours respectively. The length and diameter of the rods were between

55 nm -250 nm and 10 nm to 50 nm respectively. The formation of SnS nanorods in

the absence of any stabilizing agents was attributed to templating characteristic of

ammonium ion produced during hydrolysis of thiourea [59].

SnS nanoparticles have been synthesized by the reaction of powdered tin

with sulfur in paraffin oil. Room temperature PL spectrum of SnS nanoparticles

shows two strong emission peaks at 480 nm (blue emission) and at 415 nm

(UV emission) respectively. The strong UV emission was due to defects [60]. Single

crystalline SnS nanowires have been prepared in aqueous solution using

cetyltrimethylammoniumbromide (CTAB) as a surfactant together with oxalic acid

at room temperature. For SnS nanowires, three Raman modes were observed at

190.4 cm-1, 223 cm-1 and 273.7 cm-1. The mode observed at 190.4 cm-1 was assigned

to B2g and the modes observed at 223 cm-1, 273.7 cm-1 were assigned to A1g modes

of SnS [61]. SnS nanocrystals with size less than 10 nm have been synthesized

through solvothermal method by decomposition of bis (diethyldithiocarbamato) tin

(II) in oleylamine at elevated temperature. The shape and size tunability of SnS

nanocrystals can be achieved by controlling the reaction temperature, time and

nature of stabilizing ligands. Optical absorption spectrum of SnS nanocrystals with

particle size of sub 10 nm shows indirect band gap transition at 1.6 eV and for the

particle size of sub 200 nm shows 1.06 eV which is close to the bulk SnS [23]. The

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increased band gap value in SnS nanocrystals compared to its bulk value is due to

the quantum confinement effect of the charge carriers in semiconductors [62].

Zhu et al., have reported the synthesis of SnS nanoflowers by TGA

assisted hydrothermal method. The nanoflowers were assembled from more than ten

needle - like SnS nanorods which are 70 nm in width and ~1μm in length. SnS

nanoflower shows both direct and indirect band gap transition at 1.53 eV and

1.43 eV respectively [63]. Salavati-Niasari et al., have reported the synthesis of

different morphologies of nanostructured SnS including nanoparticles, nanosheets

and nanoflowers via a simple hydrothermal process in the presence of TGA. The

UV-Vis optical absorption of SnS nanoflower shows large blue shift compared to

other SnS nanostrutcures. Room temperature PL spectrum of SnS nanoflower shows

a strong peak at 553.05 nm with excitation wavelength of 200 nm [64].

SnS nanowire arrays have been synthesized by pulsed electrochemical

deposition in the porous anodized aluminium oxide template with uniform diameter

of 50 nm and a length up to several tens of micrometers. It exhibits strong

absorption in the visible and near-infrared spectral region and the direct energy gap

of SnS nanowire was 1.59 eV and it has high absorption coefficient (> 105 cm-1) in

the wavelength range from 400 to 800 nm [65]. Uniform ultralarge single crystal

SnS rectangular nanosheets have been synthesized via the pyrolysis of a single

source precursor and it exhibits good electrochemical properties. Therefore, it has

applications in lithium (Li) ion batteries [66]. SnS nanocrystals have been

synthesized using Sn6O4(OH)4 as Sn source. Sn6O4(OH)4 precursor was dissolved in

oleic acid and oleylamine and then thioacetamide. The SnS nanocrystals with

different shape and size can be produced by changing the reaction conditions such as

reaction temperature and Sn/S molar ratio. SnS nanoparticles with size 5 nm with

uniform size distribution were obtained at 150 °C with Sn/S ratio of 1:1. SnS

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nanoflowers and nanosheets were formed at 120 °C with Sn/S ratio of 2:1. The size

of nanoflowers was 13 nm and size of the nanosheet was 40 nm to 100 nm and

shape of the nanosheet was square. With increasing reaction time, the SnS

nanoflowers transforms into amorphous nanosheets. The main reason for the shape

evolution in the SnS nanocrystals is its layered crystal structure. The direct and

indirect band gaps were observed at 3.6 eV and 1.6 eV respectively for SnS

nanoparticles [67].

1.6.2 Literature Review of ZnO Nanostructures

Past few decades optical and vibrational properties of ZnO nanoparticles

have been extensively studied. Highly monodispersed ZnO nanoparticles with size

3.5 nm were prepared using PVP as capping agent. Optical absorption spectra of

PVP capped and uncapped ZnO nanoparticles were observed at 303 nm and 312 nm

respectively. This was due to small size of PVP capped ZnO nanoparticles. PL

spectrum of ZnO nanoparticles shows enhanced near band edge UV emission

(365 nm) and quenched defect related emission (530 nm). The quenching of defect

related emission was due to surface passivation of ZnO nanoparticles by PVP

molecules [22]. ZnO nanoparticles were synthesized using [ZnAc2.2H2O] as zinc

source and with capping agents as 1- octademide (OD), mixture of trioctylamine

(TOA) and OD with of 1:10 ratio and a mixture of trioctylphosphine oxide (TOPO)

and OD with ratio of 1:12. The ZnO particle sizes were 5 ±0.4 nm, 4±0.5 nm and

5±0.8 nm for pure OD, TOA/OD and TOPO/OD respectively. The strong UV

emission and a broad green emission were observed for all the samples. The

quenching of green emission was observed for the samples prepared in the mixture

of TOA/OD and TOPO/OD compared to that of pure OD. The green emission was

due to the oxygen vacancies on the surface. The oxygen vacancies near the surface

were reduced due to introduction of TOA and TOPO [68].

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Yang et al., have observed strong UV emission in ZnO nanoparticles due

to the passivation of surface states of ZnO nanoparticles by adsorption of acetate

molecules. Colloidal ZnO nanoparticles were synthesized in methanol solution using

KOH as precipitating agent [69]. ZnO nanoparticles were synthesized by

thermolysis of [EtZnOiPr] with TOPO at 160 °C for 5 hours without adding any

precipitating agents. The ZnO nanoparticles were monodispersed with average

particle size of 3 nm. The absorption peak was observed at 325 nm, blue shifted

compared to that of bulk ZnO (375 nm). A broad band at 530 nm with a minimum

shoulder was observed from the PL spectrum of ZnO nanoparticles. The broad green

emission was attributed to the transition of photogenerated electron from conduction

band to a deeply trapped hole in the valance band [70]. Zhang et al., have

synthesized ZnO nanorods by microemulsion method. The average diameter of ZnO

nanorods were 15 nm - 20 nm and lengths were in the range of 80 nm - 100 nm. UV

emission and deep-level emission was observed in the room temperature PL spectra

of ZnO nanorods. At 15 K, four peaks were observed from 3.0 eV to 3.5 eV. The

emission observed at 3.351 eV was assigned to donor-bound exciton (DBE) and the

peaks observed at 3.311 eV, 3.237 eV and 3.162 eV correspond to free to bound

transition (FB) and its 2LO phonon replicas (FB1LO and FB2LO) [71].

Phonon confinement of ZnO nanoparticles with different diameters has

been investigated using Raman scattering measurements. The Raman peaks were

found to broaden asymmetrically and also shifted compared to bulk ZnO phonons

[27]. Alim et al., have reported the origin of peak shift in optical phonon of ZnO

with diameter 20 nm using Raman spectroscopy under non-resonant and resonant

conditions. Three factors which responsible for the peaks shift: (i) optical phonon

confinement by the dot boundaries, (ii) the phonon localization by defects or

impurities and (iii) the laser induced heating in nanostructures [72]. Cheng et al.,

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have synthesized ZnO quantum dots by sol-gel method without using any ligands.

The intense UV emission was observed from room temperature PL spectra of ZnO

quantum dots and it shifted from 3.3 eV to 3.43 eV as the particle size reduces from

12 nm to 3.5 nm. From the resonant Raman scattering of ZnO quantum dots, the

intense polar A1 (LO) and E1 (LO) were observed and the non polar E2 phonon was

not observed [73].

1.6.3 Literature Review of SnS/ZnO Heterostructures

Few reports are available on the synthesis of SnS/ZnO herterojunction.

Ichimura et al., have fabricated ZnO/SnS heterostructure using electrodeposition

technique with low conversion efficiency [10]. SnS/ZnO heterojunction has been

fabricated by electrodeposition technique. The heterojunction shows a high

absorption in the visible wavelength range of 400 nm - 700 nm [74].

1.7 OBJECTIVE OF THE THESIS

In the past decade, many new methods have been developed for the

synthesis of SnS nanostructures. However, synthesis of SnS at nanoscale by

chemical route is still a challenge due to its layered structure. The main aim of this

work is to

• Synthesize SnS and ZnO nanostructures by chemical route like

room temperature wet chemical and solvothermal methods

• Study the influence of reaction temperature and time on the SnS

morphology

• Preparation of SnS/ZnO nanocomposites by chemical method

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• Explore the optical properties of SnS, ZnO nanostructures and its

nanocomposite

1.8 LAYOUT OF THE THESIS

In this thesis, the research work carried out is organized in 8 chapters.

Chapter 1 is devoted to the introduction to semiconductors and review of literature.

Chapter 2 is focused on the synthesis and the description of the

characterization techniques used in the research work such as Powder X-Ray

Diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron

Microscopy (SEM), Transmission Electron Microscopy (TEM), High-Resolution

Transmission Electron Microscopy (HRTEM), Photoluminescence, Raman

scattering and optical absorption spectroscopy.

Chapter 3, deals with the synthesis of SnS nanoparticles at room

temperature in aqueous solution and discussion of its structural and optical

properties.

In chapter 4, synthesis, structural and optical properties of SnS

nanosheets prepared at 80 °C in EG medium are discussed.

Chapter 5, gives the synthesis of SnS structures by solvothermal method

and the effect of reaction time on the morphologies of SnS nanostructures.

In chapter 6, synthesis of ZnO nanoparticles in non-aqueous medium

through chemical method and its structural and optical properties of ZnO

nanoparticles annealed at different temperatures are investigated.

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Chapter 7, explains the synthesis of SnS nanorods and SnS/ZnO

nanocomposite by chemical method and its structural and optical properties.

The summary of the work done as well as the scope for future studies are

presented in Chapter 8.