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CHAPTER 1

SEMICONDUCTOR NANOMATERIALS

1.1 INTRODUCTION

Nanocrystalline materials are single or multi-phased polycrystalline

solids with a grain size of a few nanometers, typically less than 100 nm. Since

the grain sizes are so small, a significant volume of the microstructure in

nanocrystalline materials is composed of interfaces, mainly grain boundaries,

i.e., a large volume fraction of the atoms resides in grain boundaries.

Consequently, nanocrystalline materials exhibit properties that are

significantly different from and often improved over their conventional

coarse-grained polycrystalline counterparts (Suryanarayana and Froes 1992).

Nanostructured materials can be made by attrition of parent coarse grained

materials using the Top-down approach from the macroscale to the nanoscale,

or conversely, by assembly of atoms or particles using the Bottom-up

approach. The control of arrangement in atoms from the macroscale to the

nanoscale is indeed the strength of materials chemistry. Therefore, it is not

surprising that increasing attention has been paid to the chemical synthesis

and processing of nanostructured materials. Nanostructures formed

chemically under ambient conditions can also be found in natural biological

systems from seashells to bone and teeth in the human body. These materials

are notable in that they are simultaneously hard, strong and tough. Therefore,

a number of investigations have been conducted to mimic nature

(biomimetics), artificially synthesize nanostructured materials and study their

properties and behaviour. These investigations have clearly shown that one

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could engineer (tailor) the properties of nanocrystalline materials through

control of microstructural features, more specifically the grain size (Burda et

al 2005). In this chapter, classifications and important aspects of

nanomaterials are presented in the following sections.

1.2 CLASSIFICATION OF NANOMATERIALS

Recently, nanostructures have attracted increased interest because

of their enhanced physical and chemical properties in the nanoscale regime.

“Nanomaterial” is a material of interest which represents a system or object

with at least one of its dimensions approximately one hundred nanometer or

less. For example, nanorods have two dimensions in the nanoscale, i.e., the

diameter of the nanorods is between 1 and 100 nm and its length can be very

large. In the case of spherical particle, it has all three dimensions between

1 and 100 nm. The nanostructures can be classified into three categories,

i) Zero-dimensional (0-D) (e.g. quantum dots and nanoparticles)

ii) One-dimensional (1-D) (e.g. nanorod, nanowire, nanobelt and

nanoneedle)

iii) Two-dimensional (2-D) (e.g. ultra-thin films)

Synthesis and understanding the growth of these nanostructures are

important to effectively make nanoscale devices. 0-D nanostructures

electrostatically or structurally isolated from the outside in which electrons

are confined. Typically quantum dots consist of a few hundreds to a few

millions of atoms, but only a small number of electrons ( 100) are free.

Depending on electron confinement, it is possible to distinguish between

planar quantum dots, vertical quantum dots and self-assembled quantum dots.

The 1-D nanostructures have a dimension that is outside the nanometric size

range. These 1-D nanostructures have a shape like a rod and consist of

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nanotubes, nanorods, nanoneedles and nanowires. The 2-D nanostructures

have two dimensions outside the nanometric size range. Hence these 2-D

nanostructures display plane-like structures and consist of thin films,

nanocoatings and nanolayers. Table 1.1 shows the general features and

different classes of nanostructures with their dimensionality.

The nanostructured materials may contain crystalline,

quasicrystalline, or amorphous phases and can be metals, ceramics, polymers,

or composites. If the grains are made up of crystals, the material is called

nanocrystalline. On the other hand, if they are made up of quasicrystalline or

amorphous (glassy) phases, they are termed nanoquasicrystals and

nanoglasses respectively. Gleiter (1995) has further classified the

nanostructured materials according to the composition, morphology and

distribution of the nanocrystalline component.

Table 1.1 Classification of nanocrystalline materials

General feature

Dimensionality Separated

Nanomaterials

Surface

NanomaterialsBulk Nanomaterials

0-DWell-dispersed

nanopowders

Nanocrystalline

thin layers

Nanocrystalline

materials

1-DNanorods and

nanotubesNano connections

Nanotube reinforced

nanocomposites

2-D Thin nanofilms Nano layersNanostructured

multilayer

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1.3 STRUCTURE OF THE GRAINS AND THE GRAIN

BOUNDARIES OF NANOMATERIALS

The structure of the grains (crystallites) in nanocrystalline materials

has been normally accepted to be the same as in a coarse-grained material.

High-resolution transmission electron microscope (HRTEM) experiments

have indicated that nanocrystalline materials consist of small crystallites of

different crystallographic orientations separated by grain boundaries. Even

though not frequently reported, the grains contain a variety of crystalline

defects such as dislocations, twin boundaries, multiple twins and stacking

faults. The structure of the grain boundaries has received a lot of attention and

has been discussed extensively in the literature, especially to decide whether it

is different in nanocrystalline and coarse-grained materials of the same

composition. The grain boundary structure determines the diffusivity and

consequently the rate of deformation by grain boundary diffusion (Coble

creep) and the rates of sintering and grain growth.

Figure 1.1 Schematic depiction of nanostructured materials showing

atoms in grain interiors (gray) and atoms in grain

boundaries (white)

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The conclusions differ and some believe that the structure is

fundamentally different while others believe that it is the same. The present

status of the structure of grain boundaries in nanocrystalline materials can be

found in some recent publications (Baier et al 2011; Nowak and Carter 2009).

Figure 1.1 shows a schematic representation of hard-sphere model of an

equiaxed nanocrystalline metal. Two types of atoms can be distinguished:

crystal atoms with nearest neighbour configuration corresponding to the

lattice and boundary atoms with a variety of interatomic spacing, differing

from boundary to boundary. Gleiter (1995), his co-workers and others

(Suryanarayana and Froes 1992) studied the structure of nanocrystalline

materials using a number of techniques and showed that the grain boundaries

in nanocrystalline materials may be random, rather than possessing either the

short-range or long-range order normally found in conventional coarse-

grained materials. This randomness has been associated with either the local

structure of individual boundaries or the structure co-ordination among

boundaries. EXAFS studies also indicated a much larger reduction in the

atomic co-ordination numbers than that detected by diffraction studies,

supporting the concept of widely disordered grain boundaries in

nanocrystaline materials (Baker et al 2009; Frenkel et al 2011).

1.3.1 Grain Size Determination

In nanocrystalline single-phase alloys and pure metals, the most

important structural parameter is grain size. The properties of materials are

mostly dependent on the grain size and therefore, an accurate determination of

the grain size is important. Both direct (imaging) and indirect (scattering)

techniques have been employed to determine grain sizes. Transmission

electron microscopy (TEM) techniques (especially, the HRTEM studies) are

ideal to determine the grain sizes of nanocrystalline materials directly, using

the dark-field technique. The width of the Bragg reflection in an X-ray

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(large-angle) diffraction pattern can provide grain (or crystal, i.e., the size of

the coherently diffracting domain) size information after incorporating the

appropriate corrections (for instrumental and strain effects).

The TEM techniques can clearly indicate whether there is

distribution of grain sizes and it is also possible to obtain a grain size

histogram by measuring the grain sizes and by counting the number of grains.

On the other hand, the X-ray diffraction technique gives only the average

crystal size and this value depends strongly on which function is used when

averaging over the size distribution. A number of recent studies discuss the

techniques for an accurate measurement of grain sizes and application of

X-ray peak shape analysis to nanocrystalline materials (Dieckmann et al

2009; Pyrz and Buttrey 2008; Rehani et al 2006). As the properties are size-

dependent it is essential to know the size distribution in a synthesized material

and to obtain narrow size distribution. Grain size determination will be dealt

with in detail in chapters 3, 4 and 5.

1.3.2 Morphology of Nanocrystals

The nanoparticles in various applications require consistent

production of particles with uniform size and shape. The particle size and

shape not only affects their surface area, but also give rise to new properties.

Quantum confinement effect in semiconductor nanostructures with reduced

dimensions and different shapes (rods, wires, tubes, ribbons) have attracted

considerable interest over the past decade and they show good optical and

electronic properties. The properties of nanocrystals are drastically altered in

the shape and size change, making nanocrystals as ideal candidates for many

applications (Hao et al 2010; Jun et al 2006; Kinge et al 2008; Na et al 2009).

Very small particles can have morphologies which differ from that of bulk

material if this provides for lower-energy surfaces. Small particles have been

observed to take on specific geometric shapes. Simple morphologies can

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easily be deduced through traditional imaging and diffraction techniques in

the TEM and scanning electron microscope (SEM). For more complex

geometries tilting experiments provide useful information about particle

morphology, though there is considerable difficulty in tilting the nanoparticle

while keeping it in the field of view. The different morphological

transmission electron micrographs of CdSe quantum dots, CdSe nanorods,

CdSe tetrapods, hollow Co8O9 spheres, Ag2S-CdS striped binary nanorods,

CdSe tetrapods with CdTe dendrimer branches, Pt@CoO yolk-shell particles,

Au-tipped CdS nanorods and Bi2Se3 nanoflowers are shown in Figure 1.2

(Choi and Alivisatos 2010).

Figure 1.2 Different morphologies of semiconductor nanocrystals.

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1.4 PROPERTIES OF NANOMATERIALS

The physical properties of nanostructured materials differ

fundamentally from that of the bulk materials as the system size approaches

quantum mechanical scale. Optimization of geometry, structure, morphology,

electronic, mechanical and the optical properties of nanometer-sized systems

are of fundamental importance for the design of nanostructures with

favourable properties. Essentially, the reduction in the particle size from bulk

to nanoscale results in an increase in the proportion of surface energy and

alters the inter particle spacing. Because of the very fine grain sizes and

consequently high density of interfaces, nanocrystalline materials exhibit a

variety of properties that are different and often considerably improved in

comparison with those of conventional coarse-grained materials. These

include increased strength/hardness, enhanced diffusivity, improved

ductility/toughness, reduced density, reduced elastic modulus, higher

electrical resistivity, increased specific heat, higher coefficient of thermal

expansion, lower thermal conductivity and superior soft magnetic properties.

But, it is becoming increasingly clear that the early results on the properties of

nanocrystalline materials are not very reliable, mainly due to the significant

amount of porosity present in those samples. Thus, for example, the room

temperature ductility in ceramic samples has not been reproduced. The

properties of nanocrystalline materials are summarized and compared with

those of coarse-grained materials in the following sub headings.

1.4.1 Mechanical Properties

The strength and hardness of the nanocrystalline materials are 4-5

times greater, when compared to the coarse grained material and the elastic

constant of these materials have been found to be reduced by 30% or less. The

various results on the variation of hardness with grain size reveal a fact that at

very small grain sizes, the hardness also decreases with a decrease in grain

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size. The critical grain size at which this reversal takes place is dependent on

the material. The mechanical properties of solids depend strongly on the

density of dislocations, grain size and hence interface-to-volume ratio. An

enhancement in damping capacity of a nanostructured solid may be associated

with grain-boundary sliding or with energy dissipation mechanism localized

at interfaces. A decrease in grain size significantly affects the yield strength

and hardness. The grain boundary structure, boundary angle, boundary sliding

and movement of dislocations are important factors that determine the

mechanical properties of nanostructured materials. Grain boundary diffusion

and sliding are the key requirements for super plasticity.

1.4.2 Diffusion in Nanomaterials

In general, atomic transport in nanocrystalline materials differs

substantially from that in coarse-grained or single-crystalline materials. This

is due to the fact that, in nanocrystalline solids, the crystallite interfaces

provide paths of high diffusivity, whereas in more coarse-grained crystals,

volume self-diffusion or substitutional atom diffusion is substantial generally

only at temperatures greater than approximately half the melting temperature.

Since nanocrystalline materials contain a very large fraction of atoms at the

grain boundaries, the numerous interfaces provide a high density of short-

circuit diffusion paths. Consequently, they are expected to exhibit an

enhanced diffusivity in comparison to single crystals or conventional coarse-

grained polycrystalline materials with the same chemical composition. This

enhanced diffusivity can have a significant effect on mechanical properties

such as creep and super plasticity.

Interface diffusion, in combination with a high fraction of

interfaces, gives rise to modified physical properties of nanocrystalline solids.

For instance, enhanced ductility of nanocrystalline ceramics and intermetallic

compounds have recently been analyzed in the framework of models of

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mesoscopic sliding or grain switching according to Ashby-Verrall, both of

which are controlled by diffusion in interfaces (Kao 2007; Song et al 2010).

With respect to nanocrystalline magnetic materials, atomic diffusion enables,

for example, a controlled stress-induced adjustment of magnetic anisotropies

in soft-magnetic alloys or a texturing of hard-magnetic Nd2Fe14B

nanocomposites (Hofmann and Kronmüller 1996). Furthermore, diffusion

processes may control the formation of nanocrystalline materials, by means of

crystallization of amorphous precursors, as well as the stability of

nanocrystalline materials (relaxation, crystallite growth), their reactivity,

corrosion behavior, or interaction with gases.

1.4.3 Thermal Properties

Since nanocrystalline materials contain a large amount of

interfacial volume, the coefficient of thermal expansion (CTE) is expected to

be higher than in a coarse-grained material. Accordingly, measured values of

CTE of nanocrystalline Cu, Pd, Fe-B-Si and Ni-P alloys are almost twice the

value for single crystals (Suryanarayana 1995). CTE for nanocrystalline

(8 nm) Cu obtained by the inert gas condensation technique has been reported

to be 31 × 10-6

K-1

in comparison with 16 × 10-6

K-1

for copper single crystals.

A comparison of the specific heats of different nanocrystalline,

coarse-grained polycrystalline and amorphous materials suggests that, at room

temperature, the specific heat in the nanocrystalline state is much higher than

that in the coarse-grained material and even that of the amorphous material.

While most of the investigators reported a nonlinear (parabolic) variation of

specific heat with temperature, some researchers have reported a linear

variation. It has also been noted that the specific heat increase at a constant

temperature was linear with the reciprocal crystal size.

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1.4.4 Optical Properties

Nanocrystalline systems have attracted interests for their novel

optical properties, which differ remarkably from bulk crystals. With the

growing technology of these materials, it is essential to understand the

detailed basis for photonic properties of nanoparticles. The linear and

nonlinear optical properties of such materials can be finitely tailored by

controlling the crystal dimensions, the chemistry of their surfaces and method

of synthesis. The size dependence of optical properties has been explained in

the following subtitles.

1.4.4.1 Optical absorption

The simplest experiment to determine the size dependence in

semiconductor nanoparticles is to study absorption spectrum of the material as

a function of wavelength of incident photons. Optical spectroscopic methods

probe the energy difference between electronic states as well as lifetimes of

excited states and their respective energy relaxation channels using time-

resolved techniques. The decrease in particle size shifts the absorption edge

from the infrared to the visible region of the electromagnetic spectrum as the

band gap energy of the semiconductor increases. In a molecular type of

description is equivalent to an energy decrease of the highest occupied

molecular orbital (HOMO) and an energy increase of the lowest unoccupied

molecular orbital (LUMO) due to the spatial confinement of the charge carrier

wave functions. By changing the size of semiconductor nanoparticles, the

colour of their colloidal solutions as well as their oxidation reduction

properties can be tuned.

Metallic nanoparticles have fascinated scientists because of their

colorful colloidal solutions long before semiconductors and their applications

became an integral part of modern technology. Gold nanoparticles were used

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as a pigment of ruby-colored stained glass dating back to the 17th century.

The physical origin of the light absorption by metallic nanoparticles in a

certain size range is the coherent oscillation of the valence band electrons

induced by an interaction with the electromagnetic field. These resonances

are known as surface plasmons and are indeed a small particle effect as they

are absent in the individual atoms as well as in the bulk. However, the size

dependence of the surface plasmon absorption is not as easily explained as in

the case of semiconductor nanoparticles, where a shift in the HOMO and

LUMO results in a larger band gap and a blue shift of the absorption onset.

Studies of the electron phonon relaxation time following the different

plasmon excitations have been carried out for gold nanorods and nanodots. It

has been demonstrated that how the surface plasmon absorption in colloidal

gold nanostructures can be used as a sensitive monitoring tool to probe the

stability of capping miscelles. Figure 1.3 illustrates a ‘blue shift’ with

absorption in smaller and smaller particles, which is indicative of increasing

energy gap.

Figure 1.3 Size dependence of the optical absorption spectra of

colloidal CdSe nanoparticles (Bawendi groups MIT)

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1.4.4.2 Luminescence properties

Luminescence may be defined as the emission of light from certain

substance, when excited with radiations like X-ray/UV/electrons and

mechanical stress/chemical reaction/electric discharge/thermal heating etc.

The emitted radiation from a luminescent material is free from heating effect

and hence is also called as ‘cold emission’. Optical excitation of

semiconductor nanoparticles often leads to both band-edge and deep trap

luminescence. The size dependence of the excitonic or band-edge

fluorescence has been studied extensively and can be reasonably explained by

the effective-mass approximation. The fluorescence process in semiconductor

nanoparticles is very complex and most nanoparticles exhibit broad and

Stokes-shifted luminescence arising from the deep traps of surface states.

Only clusters with good surface passivation show high band-edge emission.

The absence of band-edge emission has been previously attributed to the large

non-radiative decay rate of the free electrons trapped in these deep trapped

states. As the particles become smaller, the surface/volume ratio and hence

the number of surface states increases rapidly, reducing the excitonic

emission. Thus, surface states often determine the physical properties,

especially the optical properties of nanoparticles.

Photoluminescence is classified into two types, depending upon the

nature of the ground and the excited states. In a singlet excited state, the

electron in the higher energy orbital has the opposite spin orientation as the

second electron in the lower orbital. These two electrons are said to be paired.

In a triplet state these electrons are unpaired, that is, their spins have the same

orientation. Returning to the ground state from an excited singlet state does

not require an electron to change its spin orientation. A change of spin

orientation is needed for a triplet state to return to the singlet ground state

(Figure 1.4).

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Fluorescence is the emission which results from the return to the

lower orbital of the paired electron. Such transitions are quantum

mechanically, “allowed” and the emissive rates are typically near 108 s

1.

These high emissive rates result in fluorescence lifetimes near 108 s or 10 ns.

The lifetime is the average period of time a fluorophore remains in the excited

state. Phosphorescence is the emission which results from transition between

states of different multiplicity, generally a triplet excited state returning to a

singlet ground state. Such transitions are not allowed and the emissive rates

are slow. Typical phosphorescent lifetimes range from milliseconds to

seconds, depending primarily upon the importance of deactivation processes

other than emission.

Figure 1.4 Fluorescence and phosphorescence mechanism

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1.4.5 Electrical and Electronic Properties

The electrical conductivity of the solids is determined by its

electronic structure or band structure. A crystalline solid is called a metal if

the uppermost energy band is partly filled or the uppermost filled band and

the next unoccupied band overlap in energy. In the case of semiconductors,

the completely filled valance band and the empty conduction band are

separated by an energy gap (Eg) which is small ( 3 eV). The electrons can be

excited from the valence band to conduction band using light or heat, which

results in partial conductivity. In insulator, the Eg is high and the electrical

conductivity is restricted. The conducting nature of the solids can be affected

by various factors like, temperature and particle size. When the particle size

is reduced to nanometer range, the Eg increases and hence the conductivity is

reduced. In the case of metal nanoparticles, the density of states in the

conduction and valence bands are reduced and electronic properties changed

drastically i.e., the quasi-continuous density of states is replaced by quantized

levels with a size dependent spacing, in this situation, the metal does not

exhibit bulk metallic or semiconducting behaviour.

The size quantization effect may be regarded as the onset of the

metal to non-metal transition. The size at which transition occurs depends on

the nature of the metal. Because of the increased volume fraction of atoms

lying at the grain boundaries, the electrical resistivity of nanocrystalline

materials, as affected by grain boundary scattering, is found to be higher than

that in the coarse-grained material of the chemical composition. It has also

been shown that the electrical resistivity of nanocrystalline materials is

sensitive not only to the grain boundaries but also to other types of

imperfections and/or stresses introduced by the synthesis process. At a

constant temperature, the electrical resistivity increases with a decrease in

grain size and for a constant grain size, the electrical resistivity increases with

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temperature and both these observations are consistent with the theoretical

analysis of scattering of electrons by grain boundaries. The magnitude of the

electrical resistivity (and hence conductivity) in nanocomposites can be

changed by altering the grain size. For example, by changing the volume

fraction of iron particles in a nanocrystalline iron–silica system, the electrical

conductivity could be changed by 14 orders of magnitude.

1.4.6 Magnetic Properties

Many properties of magnetic systems are fundamentally determined

by the underlying electronic structure. Magnetic behaviour, electronic

transport and even structural stability are intimately related to electronic

properties. Reducing the size or dimension of magnetic systems changes the

electronic properties by reducing the symmetry of the system and by

introducing a quantum confinement, the strength of a magnet in terms of

coercivity and saturation magnetization value. These values increase with

decrease in the grain size and an increase in the specific area (surface per unit

volume) of the grains.

Nanoparticles exhibit magnetic properties that are different from

bulk materials. These are due to the following reasons: (i) As the grain size of

these systems reaches the typical lengths of few nanometers, the response of

the system depends on the boundary conditions (which are no longer periodic,

but determined by the particle size) and therefore, to be different from bulk

material. (ii) Because of the large ratio of surface to volume atoms in

nanoparticles, the surface energy becomes important when compared with

volume energy and therefore, the equilibrium situation can be different that

for bulk materials. The energy barrier to overcome for magnetization

inversion is KV, where K is the anisotropy constant and V the particle

volume. In the case of nanoparticles, the volume is so small, therefore the

thermal energy (KBT) is enough to invert the magnetization with relaxation

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time as low as few seconds. Thus, the material loses coercivity and

remanence, giving rise to the so-called super paramagnetic behaviour. It is

found that the changes in the interatomic distances have a strong influence on

the magnetic properties. The saturation magnetization M and curie transition

temperature of nanocrystalline materials are considerably varied with respect

to the bulk materials.

The coercivity of the fine particles increases as the particle size is

reduced. The increase in coercivity attains a maximum and then tends to zero.

In certain solid solutions, there may be small clusters containing more than

the average number of magnetic ions, surrounded by non-magnetic ions.

These magnetic clusters within the solid solution act as super paramagnetic in

nature.

1.5 APPLICATIONS OF NANOMATERIALS

Nanomaterials offer an extremely broad range of potential

applications from electronics, optical communications and biological systems

to new materials. Many possible applications have been explored and many

devices and systems have been studied. More potential applications and new

devices are being proposed in literature. It is obviously impossible to

summarize all the devices and applications that have been studied and it is

impossible to predict new applications and devices. It is interesting to note

that the applications of nanotechnology in different fields have distinctly

different demands and thus face very different challenges, which require

different approaches.

Applications of nanostructures and nanomaterials are based on (i)

the peculiar physical properties of nanosized materials, e.g. gold nanoparticles

used as inorganic dye to introduce colors into glass and as low temperature

catalyst, (ii) the huge surface area, such as mesoporous titania for photo

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electrochemical cells and nanoparticles for various sensors and (iii) the small

size that offers extra possibilities for manipulation and room for

accommodating multiple functionalities. For many applications, new

materials and new properties are introduced. For example, various organic

molecules are incorporated into electronic devices, such as sensors (Briseno

and Yang 2009; Star et al 2003). One important branch of nanotechnology is

nanobiotechnology. Nanobiotechnology includes; (i) the use of nanostructures

as highly sophisticated scopes, machines or materials in biology and/or

medicine and (ii) the use of biological molecules to assemble nanoscale

structure (Parak et al 2003).

The II-VI semiconductor nanostructures represent ideal systems for

dimension-dependent properties and are expected to play an important role as

building blocks in devices and processes, such as light emitting diodes, solar

cells, single electron transistors, lasers and biological labels (Deka et al 2009;

Friedman et al 2005; Grimsdale et al 2009; Kamat 2008). Semiconductor

nanoparticles are successfully utilized in photocatalytic applications.

Colloidal semiconductors were first introduced in 1976. Since then, the

promising application of nanoparticles in photocatalysis has achieved great

attention and been extensively investigated (Palmisano et al 2007). The

semiconductor catalysts form a heterogeneous photocatalysis system with

pollutant molecules and exhibit high degradation effectiveness. The design of

highly efficient and selective photocatalytic systems that can work in the

reduction of global air and water pollution is of vital interest. Especially, the

modified WO3 and CdS semiconductors are well known as photocatalysts that

can operate under visible light, even though they are not stable and have small

reactivity (Kim et al 2011). On the other hand, sensitization using dye

molecules loaded on semiconductor photocatalysts has been successfully and

widely applied to utilize solar energy. Although this dye-sensitized system

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has been developed to be utilized as a Graetzel cell, which can convert solar

energy to electronic energy efficiently and directly, the dye molecules are not

stable when this system is applied as a photocatalysts under environmental

conditions (Grätzel 2003; Senthilnathan and Ganesan 2010).

1.6 II-VI SEMICONDUCTOR NANOPARTICLES

Over the past two decades, there have been extensive experimental

and theoretical studies of optical responses of semiconductor nanoparticles,

because they have a wealth of quantum phenomena and show unique size-

dependent materials properties. In particular, there are two active research

fields in semiconductor nanoparticles optics: indirect-gap group IV

semiconductor nanoparticles and II–VI compound semiconductor

nanoparticles. The electronic and optical properties of II-VI compound

semiconductor nanoparticles have been extensively investigated in view of a

wide variety of applications. With change in the particle size, dramatic

modifications of their electronic and optical properties take place due to the

three-dimensional quantum confinement of electrons and holes when the size

of the particle approaches the Bohr radius of an exciton (Kukushkin et al

2011; Yadav et al 2010). In addition to the change in electronic and optical

properties, the structural behavior also exhibits changes with change in

particle size. Semiconductors with widely tunable energy band gap are

considered to be the materials for next generation flat panel displays,

photovoltaic, optoelectronic devices, laser, sensors, photonic band gap

devices, etc.

Artificially obtained semiconductor structures with reduced

dimensions present a large variety of new interesting properties in comparison

to the bulk material and open new ways in the engineering of semiconductors.

By simple combination of two semiconductor materials of different band gap

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energies a spatial confinement of carrier motion in the direction of the growth

axis is obtained. Further, the decrease of dimensions to one-dimensional and

quasi-zero dimensional structures leads to large enhancement of their optical

properties. Confinement of carriers in all 3 spatial directions consequently

involves a redistribution of the energy in well-like energy levels. Moreover,

quantum dots are known as prototypes in the physics of quantum

confinement. A brief review of the basic properties of II-VI semiconductor

nanocrystals are dealt with in this chapter.

In industry, the most frequently used II-VI semiconductor

nanostructures are cadmium and zinc chalchogenides. Cadmium sulphide is

yellow in color and is a chemical compound with the chemical formula CdS.

It is a direct band gap semiconductor (2.42 eV) and has many applications.

Cadmium sulphide has, like zinc sulphide, two crystal forms: the more stable

hexagonal wurtzite structure (found in the mineral Greenockite) and the cubic

zinc blende structure (found in the mineral Hawleyite). The wurtzite

(hexagonal) structure is stable from room temperature to melting point.

However, metastable cubic phase has been found in thin films and

nanocrystalline powders (Murray et al 1993; Wang et al 2002). The two types

of structures are illustrated schematically in Figure 1.5. Both wurtzite and zinc

blend structures show tetrahedral coordination, i.e. each cation is surrounded

by 4 anions and vice versa. The bulk material usually crystallizes in wurtzite

structure, whereas in the case of nanocrystals these two structures coexist.

Their ratio can be controlled through appropriate selection (control of

temperature) of growth conditions. Another important parameter for the

characterization of the crystallographic structure of II-VI quantum dots is the

lattice constant. Table 1.2 contains information about the lattice constants of

the two structures for the most commonly used bulk semiconductors.

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Hexagonal Cubic

Figure 1.5 The crystallographic structures of II-VI semiconductors.

Black spheres are cations (and can be Zn, Cd or Hg atoms),

whereas white spheres are anions S, Se or Te atoms.

Table 1.2 The lattice constant a in Å of II-VI semiconductors and the

numerical values in brackets indicate the lattice constant of

the metastable structure

CompoundsHexagonal

(Wurtzite)

Cubic

(Zinc Blend)

ZnSe (4.01) 5.669

CdS 4.137 (5.838)

CdSe 4.298 (6.084)

CdTe (4.57) 6.481

The crystallographic structure of mixed semiconductor nanocrystals

i.e., CdSxSe1-x is also determined by the preparation conditions, depending on

the (stable) crystallographic structure of the compounds CdS and CdSe.

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1.7 DOPED II-VI SEMICONDUCTOR NANOPARTICLES

Doping is very important not only to control the transport

properties of the semiconductors but also to modify their optical properties.

Hence, they find applications in display screens of computer monitors,

screens of cathode ray tubes (CRT), fluorescent lamps, X-ray detectors, light

emitting diodes (LED), laser materials, etc. Semiconductors used in such

applications are doped with metal ions, especially transition, or rare earth

metal ions. Sometimes, additional metal ions are used as co-activators to

enhance the emission. Doped semiconductors have been widely investigated

in the past. Depending upon the size and charge on the ion, substitutional

doping of impurity ions can produce either excess electrons or holes in a

semiconductor or just lattice distortion can take place. This gives rise to

localized levels in the band gap of a semiconductor. Such states are

responsible for transport properties, photo luminescence, electro

luminescence and other optical properties in these semiconductors.

In the doped nanoparticles, the number of doped atoms therefore

should be still smaller, that is, just a few atoms per particle. The alteration of

electronic structure, the optical and other properties, of doped semiconductor

nanoparticles, is an interesting area of nanoparticles research. There are

attempts not only to investigate the static equilibrium properties of the doped

nanoparticles but also the charge dynamics in nanoparticles. The first report

on the doped nanoparticles appeared in 1983 (Becker and Bard 1983).

However, it was an unintentional synthesis of nanoparticles. Wang et al

(1991) reported the results of their work on clusters. Bhargava et al (1994)

reported a large enhancement of photoluminescence in Mn-doped ZnS

nanoparticles. They further stressed that doped nanoparticles form a new class

of materials in which photoluminescence efficiency increases and decay time

reduces dramatically by some orders of magnitude. Following this report,

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many groups made investigations on doped semiconductor particles of

various sizes as well as materials. Synthesis of doped nanoparticles by various

commonly used methods is discussed in chapter 2.

1.7.1 Doped CdS Nanoparticles

Metal sulfides CdS, ZnS, CdxZn1-xS and CdxMn1-xS are

technologically important materials which have been used in the form of thin

films for a variety of applications including optical coatings, solid-state solar

cell windows, electro optic modulators, photoconductors, field effect

transistors, sensors and transducers. The impurity ions were doped into the

inorganic core of the CdS nanostructures, various factors such as mismatch in

the ionic radius, charge imbalance, different coordination, as well as

differences in chemical properties between the Cd2+

, Zn2+

and Mn2+

metal

ions are observed. A great deal of attention has been paid to doped

semiconductor nanostructures with magnetic ions such as Mn2+

, Cr2+

, Fe2+

and Co2+

to impart the unusual giant Zeeman, Faraday rotation and magnetic

polaron effects that characterize this class of materials, known as diluted

magnetic semiconductors (DMS).

1.7.1.1 CdS: Zn Nanoparticles

CdS is structurally very similar to ZnS and belongs to the same

II–VI group of semiconductors. Bulk CdS has a large and direct band gap of

2.42 and 2.56 eV at 300 and 0 K, respectively. Cd xZnxS is a promising

material for optoelectronic applications in the blue and UV spectral region

due to its wide direct bandgap and a lattice constant which can be matched to

common substrates such as GaAs, GaP or Si (Bailey and Nie 2003; Li et al

2005). Cd1-xZnxS in bulk form has a band gap which is tunable from 2.4 to

3.7 eV and, hence, can emit at different wavelengths (different band-to-band

transitions) by varying the x values from 0 to 1. In solar cell systems, where

24

CdS films have been demonstrated to be effective, the replacement of CdS

with the higher band gap Cd1-xZnxS alloys has led to a decrease in window

absorption losses and an increase in the short circuit current ( Gaewdang and

Gaewdang 2005).

A Cd1-xZnxS alloy has higher efficiency than those of pure CdS and

ZnS as photocatalysts in hydrogen production. CdS alone, however, shows

negligible photocatalytic activity because of its instability and rapid electron-

hole pair recombination rates. Studies have proven that with the appropriate

particle interaction, CdxZn1-xS nanoparticles can efficiently decompose

organics such as phenol and methylene blue under visible light irradiation.

Moreover, the optical properties of Cd1-xZnxS can likewise be

effectively tuned through control of its phase, morphology and size; for

example, more efficient luminescence has already been achieved in ternary

Cd1-xZnxS, as well as in binary ZnS and CdS nanoparticles, in comparison

with their bulk counterparts (Chen and Gao 2005; Liu et al 2005; Villoria et al

2010; Kim et al 2007; Zhang et al 2007). The tunability of the optical

properties of Cd1-xZnxS relative to its composition, in addition to its phase,

morphology and size, should make it a more interesting material to study,

considering the excitement of understanding new science and the potential

hope for novel applications and even economic impacts. Unfortunately, to our

knowledge, the literature reporting the synthesis and characterization of

Cd1-xZnxS (0 < x < 1) nanostructures is limited so far.

1.7.1.2 CdS: Mn Nanoparticles

Magnetic semiconductors are key materials for device ideas in

quantum computing and spin electronics. The 2+ magnetic ions are easily

incorporated into the host II-VI crystals by replacing group II cations. In such

II-VI based DMS such as (CdMn) Se, magneto-optical properties were

25

extensively studied and optical isolators were fabricated using their large

faraday effect (Tanaka 1998). Although this phenomenon makes these DMS

relatively easy to prepare in bulk form as well as thin epitaxial layers, II-VI

based DMS are difficult to dope to create p- and n-type, which makes the

material less attractive for electro optic applications.

In II-VI DMS, crystallizing in zinc blende/wurtzite structure the

‘d-d’ and ‘sp-d’ exchange interactions play an important role. The ‘sp-d’

exchange interaction influences the transport properties, while the ‘d-d’

exchange interaction controls the static and dynamic magnetic properties of

DMS. The exchange integrals have been reported from different experiments

like high field magnetization, neutron scattering, Raman Scattering and

susceptibility measurements. Larson et al (1988) calculated Jdd using

perturbation technique and showed that it has three contributions arising from

super exchange, the Bloebergen-Rowland process and Ruderman-Kittel-

Kasuya-Yosida (RKKY) like process between conduction electrons. The

dominant contribution comes from the hybridization induced super exchange

interaction involving only anion derived upper valence band states and the

Mn ‘d’-states. ‘sp-d’ exchange interaction is described by the super exchange

constants and arising respectively from the electron or hole interactions

with the ‘d’ electrons at the center of the Brilliouin zone. These super

exchange contributions aid ferro and antiferromagnetic orderings respectively.

Due to the higher value of , II-VI based DMS are antiferromagnetic in

nature, whereas in Cr based II-VI DMS, with larger values are

ferromagnetic.

1.8 II-VI SEMICONDUCTOR PHOTOCATALYSTS

In recent years, interest has been focused on the use of

semiconductor materials as photocatalysts for the removal of organic and

26

inorganic species from aqueous or gas phase. This method has been suggested

in environmental protection due to its ability to oxidise the organic and

inorganic substrates. In heterogeneous photocatalysis two or more phases are

used in the photocatalytic reaction. The semiconductor photocatalysts have

large band gap (Eg > 2.2 eV). Size dependent band gap expansion is helpful in

development of photocatalytic properties in nanocrystals of semiconductors.

A combined effect of size dependent band edge potential growth and the

photo induced polarization of semiconductor nanocrystals determine their

photocatalytic properties. A light source with semiconductor material is used

to initiate the photoreaction. The catalysts can carry out substrate oxidations

and reductions simultaneously. UV light of long wavelengths can be used,

possibly even sunlight.

Semiconductor photocatalysis with a primary focus on TiO2 as a

durable photocatalyst has been applied to a variety of problems of

environmental interest in addition to water and air purification. Several

semiconductors such as ZnO, ZnS, CdS, TiO2 and Fe2O3 have been used for

heterogeneous photocatalytic destruction of organic pollutants in waste water

(Chen et al 2010, Xiong et al 2010). Dyes are the important class of organic

water pollutants and therefore, many studies have been conducted on the

photodegradation. Among these photocatalysts, nanosized CdS is an

interesting photocatalyst material, since it has a narrow band gap (2.4 eV) and

a suitable conduction band potential for photocatalysis. Incorporation of

elements in the structure of CdS making solid solutions is a powerful strategy

for improving the photocatalytic properties of CdS. The solid solution

photocatalysts have advantages compared to doped photocatalysts since solid

solution allows one to control the potentials of the conduction and valence

bands, and the photogenerated electrons and holes are able to move in the

continuous valence and conduction bands instead of the discrete donor levels

seen in the doped photocatalysts (Villoria et al 2010). The photocatalytic

27

properties of synthesized CdS nanoparticles for the degradation of Rhodamine

B under UV irradiation have been studied and discussed in chapter 4.

1.9 AIM OF THE PRESENT WORK

In view of the above importances, the present work is focused on

the synthesis and property studies of pure and doped CdS nanostructures. The

structural and optical properties of the nanoparticles were studied and the

results are presented in the thesis. Specifically, the present work concerns the

following objectives:

To synthesize the molecular precursors Metal-pyrrolidine

dithiocarbamate complexes by chemical precipitation

technique.

To synthesize pure, Zn and Mn doped CdS nanoparticles by

thermal decomposition of Metal complexes by conventional

and microwave heating.

To study the structural and optical properties of synthesized

nanoparticles using different characterization techniques.

To compare the properties of the nanoparticles obtained from

both conventional and microwave heating processes.

To study the photocatalytic activities of these nanoparticles by

photo degradation of Rhodamine B dye under UV irradiation.