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CHAPTER 1

INTRODUCTION TO NANOTECHNOLOGY AND EXISTING INFORMATION OF TIN SELENIDE

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1.1 INTRODUCTION

Globalization came about as the industrialized countries began to

exploit global fossil fuel and mineral reserves in pursuit of greater

economic efficiency. There is an increasing demand for a cost effective

renewable source of energy. Fossil fuels are assumed to offer low prices

and greater potential. They provide around 66 % of the world's electrical

power, and 95% of the world's total energy demands. But the basic

disadvantage of fossil fuels is pollution. Burning any fossil fuel produces

carbon dioxide, which contributes to the "green house effect" adding to

global warming. The fundamental economic reality of fossil fuels is that

they are found in relatively small number of locations across the globe,

yet consumed everywhere. Now days, the energy question has become

one of most major and worrying problem of the modern society. The

worldwide demand for energy has grown dramatically over the last

century as consequence of the increase in the industrialization of the

world. The need for energy is likely to grow even more in the 21st century

with the improvements in living standards across the planet; moreover,

the economic develop of Asiatic nation, like China and India, has

emphasized the problem of energy demand.

This high demand of energy, the pollution generated by energy

sources currently used and the depletion of natural resources bring into

question. Energy available from the sun outside the Earth‟s atmosphere

is approximately 1367 W/m2; obviously, some of the solar energy is

absorbed as it passes through the Earth‟s atmosphere. As a result, on a

clear day the amount of solar energy available at the Earth‟s surface in

the direction of the sun is typically 1000 W/m2. The high power reaching

every day all regions of our planet makes this primary source the most

interesting between the renewable energy sources. The R & D focused or

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concentrated its attention to improve as well as to investigate the

semiconducting/ photosensitive materials which give sufficient

efficiencies of such Photovoltaic devices (fabricated from such new

investigated materials) making this technology more convenient (also in

term of cost) and more applicable in respect of those based on fossil fuel

[1].

A solar cell is formed by a light sensitive p- n junction

semiconductor. Each photon has an energy associated with it. The

charge carriers in a semiconductor are holes and electrons. If the energy

of the photon incident on the semiconductor is greater than or equal to

the energy required to release the electron it will contribute to the output

of the solar cell.

The theoretical band gap for a maximum efficiency of solar cell lies

in the range of 1.2- 1.8 eV. It has been shown from the solar spectrum

and semiconductor characteristics that the conversion efficiencies of

homojunction solar cells are maximum at a bandgap of about 1.5 eV.

CdTe, a direct band gap semiconductor with a bandgap of 1.45 eV is

ideally suited for solar energy conversion. Direct band gap

semiconductors have high absorption coefficients and hence can be used

in thin films form. Materials like CdTe, CIS, and CdSe are direct band

gap semiconductors. Semiconductors exist both in elemental and

compound form and can be found in single crystalline, polycrystalline or

amorphous structures. Atoms in single crystalline materials are arranged

in a well defined order. Polycrystalline materials consist of randomly

oriented small single crystal grains whereas amorphous materials do not

have any defined ordered structure. The specific properties of a

semiconductor depend on the impurities, or dopants added to it [2]. A

semiconductor doped in such a way that the hole carrier concentration is

greater than the electron carrier concentration is said to p- type. An

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acceptor is an impurity introduced into the semiconductor to generate a

free hole by accepting an electron from the semiconductor. An n- type

semiconductor has a higher electron carrier concentration than the hole

carrier concentration. A donor is an impurity that is added to a

semiconductor to generate free electrons by donating an electron. The

band gap (Eg) is the separation between the energy of the lowest

conduction band and that of the highest valence band. The Fermi level

(Ef) is defined as the energy level at which the probability of occupation

by an electron is half [2]. For a p- type semiconductor, the Fermi level is

close to the valence band whereas for an n- type semiconductor the

Fermi level is close to the conduction band.

Significant optical to electrical/ chemical energy conversion

efficiencies have been obtained in solid state photovoltaic and

photoelectrochemical cells. The potential of this class of materials has

not been fully explored yet but appears to be limited mainly by the

availability of suitable materials. So nowadays, attempts have therefore

been made to produce good quality crystals and thin films of the layered

semiconductors for photoelectronic device applications. Several

approaches including a novel extension of molecular beam epitaxy for the

preparation of layered materials are being actively pursued to produce

high quality single crystals and thin films. The metal chalcogenides

exhibit promising properties for quantum solar energy conversion

because:

- The band gap is typically in the range of 1.0 to 2.0 eV and

therefore ideally fit for the solar spectrum,

- The valence and conduction band width is of reasonable

magnitude due to rather strong metal

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- Chalcogenide hybridization; as a consequence the charge carrier

mobilities are sufficiently large,

- The absorption constants are extraordinarily high, typically in the

range 105 cm-1. Therefore, energy conversion devices fabricated

from these materials may be considered promising alternatives to

more established solar cells.

Among these metal chalcogenides, the binary IV – VI layered

compounds formed with Sn as cations and S, Se and Te as anions form a

very interesting class of semiconductors. SnSe is expected to exhibit

extreme anisotropy in their lattice vibrational, optical and electronic

properties [3] and perhaps show some characteristic features of the two

dimensional or layer type semiconductors [4- 8]. It would appear;

therefore, that SnSe provide an excellent opportunity for investigating the

relations among structure, bonding and the electronic properties of

solids with comparisons possible between:

- two and three dimensional structures

- members of the isomorphic series GeS, GeSe, SnS and SnSe

- Structurally different compounds GeS, GeTe and GaS.

Today, semiconductor materials are the pillars of modern

technology. Without them there would be no electronics industry, no

photonic industry, no fiber optics communications, very little modern

optical equipment and some very important gaps in conventional

production engineering. Progress in synthesis of semiconducting

materials and epitaxial technology is highly demanded in view of its

essential role for the development of several important areas as

production of high efficiency photovoltaic cells and detectors for

alternative energy and medicine and fabrication of light emitting diodes.

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Many of the metal chalcogenides, compounds of metals and

nonmetals with S, Se or Te are important in the new technologies [9].

These chalcogenides considered as “high– tech” materials have many

applications, including the following:

- Tribology (high temperature lubricants )

- Semiconductors, thin films, glasses, photoresists,

photomicrography (electronics)

- Intercalation of alkali and other metals (battery technology)

- Catalysis (dehydrosulphurisation)

- Solar energy conversion

- Extractive metallurgy (de- sulphurisation as practiced in steel

industry)

- Corrosion in sulfur containing atmospheres (e.g. H2 S).

- The properties of IV- VI semiconducting compounds are useful for

infrared detection as sensors of thermal radiation and as wide

band gap detectors in the areas of lasers, radar and laser

communication.

In addition to infrared detectors and emitters, narrow band gap

compound crystals have potential applications in magneto resistive Hall

effect and thermoelectric devices [10, 11]. The layered compounds SnSe

have attracted considerable attention on account of their

semiconducting, anisotropic and optical properties. As stated that these

layered semiconductor SnSe because of their various properties are used

in the field of optoelectronics [12], holographic recording systems [13-

15], electronic switching [16, 17] and infra red production and detection

system [18]. Moreover SnSe is a semiconductor with energy gap of 1 eV

having a potential as an efficient solar cell material [19- 22].

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The interest in the properties of these materials either in the form

of a single crystal, polycrystalline, nanoparticle form or a thin film with

large dimensions for an optimum photoconversion has been showed

several times. Metal selenides have attracted considerable attention

because of their interesting properties and potential applications.

The elemental information about the Sn, Se and Cu used in the

present work for the synthesis of SnSe nanoparticles and copper doped

SnSe nanoparticles are as shown in Table 1.1.

They have been widely used as thermoelectric cooling materials,

optical filters, optical recording materials, solar cell materials, superionic

materials and sensor and laser materials [23]. A polarity dependant

memory switching in devices with crystalline SnSe is also reported [24].

Long time annealing of SnSe films at temperature above 673 K transfers

the SnSe to SnO and SnO2. Such films are very resistive to acids and

could serve as protective coatings [25]. A single phase SnSe films can be

used as an absorber layer in the fabrication of heterojunction solar cells

[26]. The bulk material corresponding to SnSe are low band gap

semiconductors and have been the subject of numerous applications due

to technological importance as infrared and visible radiation detectors

[27]. The anisotropic behavior of tin Chalcogenide makes them attractive

layered compounds, used as cathodic materials in lithium intercalation

batteries [28].

The compound SnSe in the form of high quality single crystal or

epitaxial layered is used in optoelectronic devices and nuclear radiation

detectors [29]. From technological applications point of view of tin

chalcogenides include the constructions of LASER‟s and detectors in the

infrared region [30, 31]. Various researchers in the study of these SnSe

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or tin chalcogenides therefore have a great amount of interest all over the

world and till today.

Tin (Sn):

Tin is a malleable silvery- white metal which takes a high polish. It

possesses a highly crystalline structure and is moderately ductile. Two or

three allotropic forms of tin exist. Gray or a tin has a cubic structure.

Figure 1.1 shows solid state structure of the Tin. Upon warming, at

13.2°C gray tin changes to white or b tin, which show a tetragonal

structure. This transition from a to the b form is termed the tin pest. A g

form may exist between 161°C and the melting point. When tin is cooled

below 13.2°C, it slowly changes from the white form to the gray form,

although the transition is affected by impurities such as zinc or

aluminum and can be prevented if small amounts of bismuth or

antimony are present.

Tin is resistant to attack by sea, distilled, or soft tap water, but it

will corrode in strong acids, alkalis, and acid salts. The presence of

oxygen in a solution accelerates the rate of corrosion.

Selenium (Se):

It is a member of group VI elements. It is a covalent

semiconductor. It has been metallized by application of pressure. At

room temperature Se is rhombohedral. The solid state structure of

Selenium is shown in figure 1.2. They are also metastable monoclinic

forms of Se based on Se8 rings, similar to S8. Selenium exhibits both

photovoltaic action, where light is converted directly into electricity and

photoconductive action where the electrical resistance decreases with

increased illumination. These properties make selenium useful in the

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production of photocells and exposure meters for photographic use, as

well as solar cells. Selenium is also able to convert a. c. electricity to d. c.

and is extensively used in rectifiers. This solid is a p- type semiconductor

and is useful in electronic and solid-state applications.

It has been used in photocopying for reproducing and copying

documents, letters, etc. Se used by the glass industry to decolorize glass

and to make ruby colored glasses and enamels and as photographic

toner and additive for stainless steel. The solid-state structure of

selenium is shown in figure 1.3. Author has chosen to work on the

synthesis and characterization of lesser studied pure SnSe nanoparticles

and copper doped SnSe nanoparticles. Most of research work is available

or published on SnSe nanoparticles but according to authors knowledge

no work has been reported but on copper doped SnSe nanoparticles yet

or few/ rare publications are found. So in this chapter the general

introduction on the existing information about Tin Selenide has been

provided as a guideline for the work described in this thesis.

Copper (Cu):

Copper, a transition metal, is the lightest element in Group IB and

is in the 4th period. The heavier members of the group are silver (Ag) and

gold (Au). The crystal structure of copper is FCC. Each copper atom has

12 nearest neighbors. Silver and gold also have FCC crystal structures

and are "isostructural" with copper. Solid state structure of copper is

shown in figure 1.3 Copper and copper alloys are some of the most

versatile engineering materials available. The combination of physical

properties such as strength, conductivity, corrosion resistance,

machinability and ductility make copper suitable for a wide range of

applications. These properties can be further enhanced with variations in

composition and manufacturing methods. The electrical conductivity of

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Copper is second only to silver. The conductivity of Copper is 97% that of

silver. Although additions of other elements will improve properties like

strength, there will be some loss in electrical conductivity.

As an example a 1% addition of cadmium can increase strength by

50%. However, this will result in a corresponding decrease in electrical

conductivity of 15%. The corrosion resistance of copper alloys comes

from the formation of adherent films on the material surface. These films

are relatively impervious to corrosion therefore protecting the base metal

from further attack.

Tin Selenide (SnSe)

SnSe compound is prominent among the layered semiconductors

and it crystallize in orthorhombic Bravaise lattice with structure [32, 33]

having lattice constant a= 4.46, b= 4.19 and c= 11.57 [3]. The unit cell of

SnSe contains eight atoms with the symmetry of the space group Pnma

(D162h).

The Sn and Se atoms are arranged in two adjacent double layers

orthogonal to largest cell dimensions [34]. The SnSe crystal is made up of

tightly bound double layers of Sn and Se atoms stacked along the c axis.

Each atoms has three strongly bonded neighbors within its own layer

and three more distant weakly bonded neighbors, two of which lies

within the same double layer and remaining one in the adjacent layer as

shown in figure 1.4. Due to such types of layered structure of IV- VI

compound, all the physical properties of the compound or crystals

depend on the direction (anisotropic behavior). But in the present

investigation author has synthesize pure SnSe nanoparticles instead of

SnSe single crystals. In presented work, instead of anisotropic behavior

(direction dependant properties of all the characterization), it is possible

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to study the crystallite size or particle size dependant characterization of

pure SnSe nanoparticles.

In both these compounds the cation (Sn) and anion (Se) atoms

from zigzag cation- anion chains perpendicular to c- axis. Each atom has

the coordination environment of a heavily distorted octahedron [35].

Each atom forms six dominant heteropolar bonds, the stronger of which

are in three bonds to nearest neighbors in the same double layer. Three

weaker bonds are to further neighbours, two of which are in the same

double layer, one to an atom in the next adjacent layer [32]. The bonding

between the layers is weak, being of van der walls type.

In this chapter, discussion on introduction to the history and early

observations of the size related physical properties of nanosized

semiconductor materials is presented. Nanosized materials show

fascinating and unique differences in optical and electronic properties

with respect to bulk materials. The distinct physical and chemical

features of these nanomaterials make them an exciting and attractive

class of novel materials with an enormous potential for various

applications. The size related properties can be rationalized quantum

mechanically using the concept of size quantization. In the nanometer

regime, opto-electronic properties also depend on shape and can be

controlled by the number of dimensions in which size is confined. The

synthetic strategies towards nanocrystalline semiconductors with

controlled size and shape are outlined.

Opto- electronic properties also depend on the composition and in

this respect nanocrystalline heterostructures are of interest.

Heterostructures of different epitaxially grown crystalline materials allow

selective carrier confinement and further control over both the emissive

and electronic properties. The incorporation of atomic impurities is an

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alternative way to modify the physical properties of a semiconductor.

Such doping can also be applied in nanosized semiconductors. This

enables the formation of materials where the properties are determined

by both size effects and atomic band transitions of the dopants. The

applications discussed in this work relate to the luminescent properties

as active component in light- emitting diodes and nanophosphors.

1.2 INTRODUCTION TO NANOTECHNOLOGY

The greek word „nano‟ is referred to the length scale of one billionth

of a meter. Thus nanoscience deals with the science of materials and

technologies in the scale range of ~ 1- 100 nm. This means, the

nanoscience deals with a few hundred to a few thousand atoms or atomic

clusters, whereas the microscopic word is made out of trillion of atoms or

molecules. Nanoparticles are larger than individual atom and molecules,

but are smaller than bulk solid; hence they obey neither absolute

quantum chemistry nor laws of classical physics and have properties

that differ markedly from those expected. Presently, the nanoscience and

technology represents the most active discipline all around the world and

is considered as the fastest growing technology revolution in the human

history had ever seen. This intense interest in the science of the

materials confined within the atomic scales stems the fact that these

nanomaterials exhibit fundamentally unique properties with great

potential of bringing plethora of next generation technologies in

electronics, computing, optics, biotechnology, medical imaging, medicine,

drug delivery, structural materials, aerospace, energy etc.

Miniaturization is a concept nurtured by nature since the process

of evolution and with time, the control of biological processes at small

length scales has become immaculate. The origin of the field of

nanoscience and nanotechnology has primarily been a motivation to

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mimic the programmed synthesis and manipulation of materials at

similar length scale, an art perfected by nature.

Richard Feynaman was the visionary who first drew attention

towards this possibility in his speech titled “There‟s plenty of room at the

bottom”. Since then, several significant achievements have been made

towards the process of miniaturization, even though control at

complexity levels manifested by biological systems is still a dream. Figure

1.5 sums the success of the man in competition to nature on fabrication

of material at small scale routinely. Two different approaches have been

undertaken to achieve this goal, which are “top- down” and “bottom up”.

The top- down approach thrives on the principle that large sized

objects can be chiseled to obtain smaller objects. Humans have been

following this approach since the beginning of civilization and with time,

this art has been mastered to achieve size limits of submicron levels.

However, physical constraints limit the application of this approach to

achieve nano domain precision. Thus, the bottom up approach has taken

over where small scale objects can be assembled to build up larger sized

materials for various applications. This includes the synthesis of

nanostructures of desired characteristics, their self assembly and

eventual formation of larger sized particles.

1.2.1 HISTORY AND EARLY OBSERVATIONS

Exposure of mankind to the nano world is not new and the history

of noble metal colloids has been reported in ancient literature. History

claims that Moses, the great physician, gave the first recipe for colloidal

gold. The verses 19 and 20 in the book “Exodus” mention the preparation

of the first colloidal gold. According to verse 20,

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“And he took the [golden] calf which they had made, and burnt it in

the fire, and ground it to powder, and strewed it upon the water, and

made the children of Israel drink of it.”

The mention above indicates that the potential of noble metal

colloidal suspension in therapeutics has been realized since ancient time.

The great German physician and alchemist, Paracelsus (1493- 1541) had

once quoted, “Of all Elixirs, Gold is supreme and the most important for

us, for it can keep the body indestructible. Drinkable gold will cure all

illnesses, it renews and restores”. Aurum potable (drinking gold) and luna

potable (drinking silver) have been considered as elixirs by the alchemist

since 1570. The optical properties of colloidal gold and silver have

specially drawn attention due to their absorption maxima in the visible

region of the electromagnetic spectrum, which leads to colored colloidal

suspension.

In 1818, Jeremias Benjamin Richters gave an explanation for the

different colors seen in drinkable gold indicating that the pink or purple

color was due to finest degree of subdivision while yellow color arises due

to the aggregation of very fine particles. In earlier days, colloidal gold and

silver has been used as colorant. The colorant in glasses, “Purple of

Cassius”, is a colloid with heterocoagulation of gold nanoparticles and tin

oxide. Similarly, the Lycurgus cup of fourth century AD [37] (Figure 1 (b).

Which looks green in reflected and red in transmitted light, has been

reported to contain colloidal gold and silver. In 1857, Faraday reported

the preparation of deep red colored solution of gold nanoparticles by the

reduction of aqueous chloroaurate ions using phosphorus in CS2. This

probably was the first rationalized report on the purposeful synthesis of

colloidal gold nanoparticles. Soon thereafter, the term colloid was coined

by Thomas Graham (1861) for suspended particles in liquid medium and

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was categorized to be in the size range 1 nm to few μm. However, Norio

Taniguchi gave the term „nanotechnology‟ for the colloidal particles,

which have at least one dimension of the length scale of 1-100 nm. Ever

since the report of Faraday, several different approaches have been

developed towards the synthesis of colloidal noble metal nanoparticles by

physical, chemical and biological routes.

Nanostructures with their dimensions between 1 to 100 nm attract

an enormous attention in the last two decades. The development of the

modern micro (electronic) integrated circuits stimulated an extensive

research effort to create smaller structures in order to reach higher

performances, less power consumption, and lower costs. The scaling

down of bulk metals and semiconductors to the nanometer regime

revealed several exciting phenomena, such as size-dependent excitation,

quantisized conductance, and metal to semiconductor to insulator

transitions [36]. Modern physics and mathematics make it possible to

study, simulate, and explain these size and shape dependent properties.

The application of nanosized materials is however much older than

today‟s science, and dates back to ancient Egypt and Roman times. In

the ancient times metal nanoparticles were formed in molten glass, and

used to made stained glass objects. Such a magnificent example of

ancient glass is the Lycurgus Cup [37]. (AD fourth century), illustrating

myth of King Lycurgus (Figure 1.6). The dispersed gold nanoparticles in

the glass make the glass appears green, when viewed in reflecting

daylight. When the cup is illuminated from the inside it appears red by

the transmitted light.

The first study on the size dependence of the physical properties of

metals was reported by Faraday 1856 [38]. Faraday observed that the

electronic structure of a metal can become size dependent below a

certain size. The size dependent phenomena were also observed in 1960‟s

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for semiconductors [38, 40] where for colloidal dispersions of AgBr and

AgI a shorter absorbance wavelength was observed compared to

macroscopic material [40]. The study of layered MoS2 and quantum wells

revealed spatial dependent optical properties for quantum wells with

different layer thickness [41]. Evans and Young were among the first who

related these findings to size quantization of the electronic structure of a

semiconductor [41]. It took however until the 1980‟s when the first

theoretical explanation was proposed for colloidal spherical nanocrystals

(NCs) by Brus [42]. Together with advances in the synthetic procedures

[43, 44] this lead to a rapid increase in research in the field of nanosized

materials. A breakthrough in the synthesis of high quality monodisperse

semiconductor NCs or quantum dots (QDs) was made by the work of

Murray, Norris, and Bawendi in 1993 [45]. They separated the initial

nucleation from the particle growth, by rapid injection of reagents into a

hot coordinating solvent.

The sudden increase in precursor concentration above the

nucleation threshold at sufficient high temperature triggers a short burst

of nucleation, leading to a rapid decrease in precursor concentration

below the nucleation point. At this point, no new particles are formed

and the growth proceeds by the consumption of monomers from solution

by the QD nuclei. This “hot injection method” enabled the creation

various types (CdSe, CdTe, CdS, PbSe, and ZnSe) of monodisperse (<10%

size distribution) and high quality NCs. In past few years, various

alternative methods have been developed resulting in high quality

monodisperse NCs in both hydrophobic [41, 43] and aqueous

environments [46, 47, 48]. The work presented in this thesis is mainly

based on the hot injection method [41, 49] and on the formation of

colloidal QDs from temperature initiated growth using preformed atomic

clusters [50]. Colloidal semiconductor NCs with various shapes and

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properties, are prepared, studied, and used as a novel class of

luminescent materials with distinct optical properties.

1.2.2 NANOSTRUCTURE MATERIALS

Nanostructured materials are materials with the characteristic

length scale of the order of a few (typically 1 to 100) nanometers. The

structure refers to the chemical composition, the arrangement of the

atoms, and the size of a solid in one, two or three dimensions; effects

controlling the properties of nanostructure materials include size effects

(where critical length scales of physical phenomenon become comparable

with the characteristic size of the building blocks of the micro structure),

changes of the dimensionality of the system, changes of the atomic

structure and alloying of components, e.g. elements that are not miscible

in the solid and/or the molten state. The synthesis, characterization and

processing of nanostructure materials are part of an emerging and

rapidly growing field.

Research and development in this field emphasizes scientific

discoveries in the generation of materials with controlled micro structural

characteristics. Nanostructured materials may be grouped under

nanoparticles, nano intermediates, and nano composites. They may be in

or far away from thermodynamic equilibrium. For example

nanostructured materials consisting of nanometer sized crystallites of Au

or NaCl with different crystallographic orientation and chemical

compositions vary greatly from their thermodynamic equilibrium.

Research in present day nanomaterials is based largely on the

investigating fundamental properties of matter in nanoscale regime,

however, the timely attention and efforts are essential for the

transformation of these new findings into technology product so that the

well assumed technology revolution will become day light reality. By now,

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a variety of chemical, biological as well as physical processes are

established for the preparation of different kinds of nanoparticle systems

and the ground breaking inventions such as scanning tunneling

microscopy (STM), atomic force microscopy (AFM), etc, made the

characterization and atomic scale manipulation of nanoscale materials a

practical reality.

1.2.3 SEMICONDUCTOR NANOPARTICLES

Almost all materials system including metal, insulators and

semiconductors show size dependent electronics or optical properties in

the quantum size regime. Among these, the modification in the energy

band gap of semiconductors is the most attractive one because of the

fundamental as well as technological importance. 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. When dimension of a

material is continuously reduced from macroscopic size to nanometers,

the physical and chemical properties drastically change. If one dimension

is reduced to nanometer range, so that the size is comparable to the de-

Broglie wavelength of the exciton; while other two dimensions remain

large, one obtains a structure known as quantum well. If two dimensions

are reduced and one remains large, the resulting structure is referred as

quantum wire. And if all three dimensions are reduced, the material is

called a quantum dot.

The word quantum is associated with these three types of the

nanostructures because the change in properties arises from quantum

mechanical nature of physics in ultra small domain. In, quantum dots,

the surface to volume ratio is large and surface effects dominate.

Quantum dots, also known as nanocrystals, are a special class of

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materials, composed of periodic groups of II-VI, III-V, or IV-VI

semiconductors. Semiconductors derive their great importance from the

fact that their electrical conductivity can be greatly altered via an

external stimulus (voltage, photon flux, etc), making semiconductors

critical parts of many different kinds of electrical circuits and optical

applications.

Quantum dots are unique class of semiconductor because they are

so small, ranging from 2- 10 nanometers (10- 50 atoms) in diameter. At

these small sizes materials behave differently, giving quantum dots

unprecedented tunability and enabling never before seen applications to

science and technology. Optical properties of the quantum dots can be

easily tuned with the particle size. The band gap can be controlled with

the change in size of the nanomaterial, so the different colored emission

can be observed from the same material. These quantum dots of same

material can be used for fabrication of LEDs having emission over the

whole visible spectrum.

The usefulness of quantum dots comes from their peak emission

frequency's extreme sensitivity to both the dot's size and composition,

which can be controlled using Evident Technologies' proprietary

engineering techniques. This remarkable sensitivity is quantum

mechanical in nature, and is explained as follows. The electrons in

quantum dots have a range of energies. The concepts of energy levels,

band gap, conduction band and valence band still apply. However, there

is a major difference. Excitons have an average physical separation

between the electron and hole, referred to as the Exciton Bohr Radius,

this physical distance is different for each material. In bulk, the

dimensions of the semiconductor crystal are much larger than the

Exciton Bohr radius, allowing the exciton to extend to its natural limit.

However, if the size of a semiconductor crystal becomes small enough

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that it approaches the size of the material's Exciton Bohr radius, then

the electron energy levels can no longer be treated as continuous - they

must be treated as discrete, meaning that there is a small and finite

separation between energy levels. This situation of discrete energy levels

is called quantum confinement, and under these conditions, the

semiconductor material ceases to resemble bulk, and instead can be

called a quantum dot. This has large repercussions on the absorptive

and emissive behavior of the semiconductor material.

1.2.4 DOPED NANOPARTICLES SEMICONDUCTOR CHALCOGENIDE

The incorporation of atomic impurities in semiconductors is a

common method to modify and tailor the electrical and optical properties

of semiconductors. This technique is also known as doping. Selective

doping of semiconductors, such as silicon and germanium, enables the

creation of p-n diodes and transistors which eventually lead to integrated

circuits and microelectronics that we use in every day live. The doping of

semiconductor NCs can also result in drastic changes of both optical and

electronic properties. The ability to use atomic impurities in NCs

therefore extends the already exciting size- depend properties as shown

previously for doped NCs like CdSe, CdS, ZnSe, and ZnS doped with Cu,

Mn and more exotic dopants [51, 52]. These dopants often function as

emissive traps, with trap energy levels between the valence and

conduction band of the host crystal. The creation of highly luminescent

and stable doped NCs turns out to be a difficult task. In previous work

the luminescence efficiency often does not exceed a QY of 20% [53] which

is rather poor compared to the 70% routinely achieved for common core-

shell QDs. Peng et al. showed the creation of highly efficient ZnSe:Mn

QDs with QYs up to 70% [54].

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The commonly used transition metal Cu and Mn dopants, are

substitutional as they are located in the host lattice replacing the metal

ion of the host. Norris et al [53] shows that the match between the

crystal structure and lattice constants of the dopant and the host

material is crucial for effective incorporation of the dopants. The system

discussed in this thesis is based on ZnSe doped with Mn. The Mn2+ ion

shows visible luminescence due to a strong sensitivity to the crystal field

of the host ZnSe lattice. The emission arises from the transition between

the d-orbitals of the manganese atom (4T1→6A1), and can be shifted by

influencing the crystal field splitting. Excitation of the Mn2+ ion is

expected to be the result of carrier or exciton trapping. The band

alignment of the valence and conduction band of ZnSe are such that the

atomic Mn trap levels are in a type-I arrangement, and therefore function

as a (emissive) trap state in the ZnSe: Mn particle. These doped

luminescent ZnSe: Mn nanowires were formed by oriented attachment.

The possibility to align these anisotropic structures, creates the

opportunity to study the optical properties of substitutional Mn dopants

and of zinc blende ZnSe exciton emission from oriented crystals.

Light emission from nanocrystalline phosphors is possible through

a radiative recombination process of charge carriers generated by higher

energy photon absorption. The colour of the emission can be tailored by

changing the crystalline size and appropriate doping. Recently, doped

semiconducting nanocrystals (NC) have attracted considerable interest

due to their interesting properties such as high luminescence quantum

efficiency, short radiative lifetime, size dependent emission color

tunability, low voltage cathodoluminescence, multicolor

electroluminescence etc. These materials are considered to be the

luminophors for next generation displays, bio- labels, lasers, etc. ZnS

and CaS phosphors doped with various activators are very attractive

luminescent materials used in variety of applications such as screens of

22

cathode ray oscilloscopes, fluorescent lamps, television picture tubes,

cameras and radars. Besides these direct applications of the

luminescence of these phosphors certain indirect applications of these

phosphors have also been reported. Karvets [56] has reported CaS doped

with Sr and Ce as an optical memory storage material having erasable

and rewriteable memory. Khare et al [57] have reported CaS:Cu as a

temperature sensor. Due to extensive applications of these phosphors,

the luminescent properties of these phosphors had been investigated by

large number of workers [58- 59] with different concentration of

activators and co-activators. Different concentration of a dopant gives

different result in the same host lattice. The decay of luminescence in

these phosphors is primarily determined either by the electron transition

within the activator ion or by carriers temporarily captured in various

trapping states depending upon which is slower. Since the traps are

always present in these phosphors, the phosphorescence decay even with

relatively fast activator ions is slower than might be desired for some

applications of particular interest. In the present case singly doped

phosphors as well as multiple doped phosphors are studied. Also the

effect of killer impurities (Fe, Co, Ni) on the luminescence of these

phosphors is studied, as a parameter of concentration of the activators.

The room temperature photoluminescence (PL) of copper doped

zinc sulfide (ZnS:Cu) nanoparticles were investigated [60]. These ZnS:Cu

nanoparticles were synthesized by a facile wet chemical method, with the

copper concentration varying from 0 to 2 mol%. The PL spectrum of the

undoped ZnS nanoparticles was deconvoluted into two blue

luminescence peaks (centered at 411 nm and 455 nm, respectively), With

the increase of the Cu2+ concentration, the green emission peak is

systematically shifted to longer wavelength. In addition, it was found that

the overall photoluminescence intensity is decreased at the Cu2+

23

concentration of 2%. The concentration quenching of the luminescence

may be caused by the formation of CuS compound.

Colloidal semiconductor nanocrystals (NCs) with dimensions below

the bulk exciton radius exhibit size related optical and electronic

properties [61]. Their stable, tunable, bright, and narrow photoemission

make them favorable for application as emitters in biomedical labeling,

LEDs, and lasers [62]. By introducing transition metal dopants into the

semiconductor NCs [63, 64] their functionality can be further enhanced

by combining quantum size effects and atomic band transitions,

resulting in a novel class of materials. ZnSe has shown to be a very

suitable host material for creating Mn doped NCs with various doping

levels [63, 65]. Up to now, mainly spherical doped ZnSe colloidal NCs

have been reported [66, 67, 68] while anisotropic Mn doped ZnSe

materials have been produced by templated growth and thermal

deposition techniques [69, 70, 71]. In fact, only limited attention has

been given to shape control of colloidal ZnSe nanocrystals. One

dimensional (1D) ZnSe NCs such as nanowires [72, 73] and nanorods

[74] have been described. Panda et al.,[75] demonstrated the formation of

narrow nanorods and nanowires at moderate temperatures.

From one report, stable solution based synthesis used to create the

first colloidal wurtzite ZnSe nanowires, with Mn doping. These colloidal

ZnSe Mn doped nanowires have optical properties that depend on size,

shape, and doping level in one single crystal. The ZnSe:Mn nanowires are

highly crystalline and can be synthesized with ultra-narrow diameters

from 1 to 3 nm, well below the bulk exciton Bohr radius (3.8 nm) of

ZnSe. The luminescent properties of these nanowires were further

improved by the formation of CdSe on the crystal surface increasing the

photoluminescence quantum yield up to 40%. These doped nanowires

combine quantum size and shape effects with atomic band transitions,

24

making them particular interesting for anisotropic optical studies and for

anisotropic spin related studies.

In addition, advances in nanoparticle fabrication have allowed

researchers to produce well formed nanostructured particles doped with

transition metal ions [76]. These studies have led researchers to believe

that the next magnetic storage devices can be fabricated using doped

nanoparticles. A large obstacle, however, toward these goals is to

understand the chemistry and physics of the impurity ion in a

nanoparticle lattice and its impact on useful and desirable properties. To

understand the effect an impurity or dopant ion has on the structural

and electronic properties in a quantum dot, one must first examine how

these types of effects are understood in its corresponding bulk lattice. In

reported study of Cu ion doping in a CdSe nanoparticle lattice, they

begin by examining Cu doped into bulk CdSe. Experimentally, CdSe:Cu

bulk materials have been studied previously and have been targeted for

uses in photovoltaics; many studies have been aimed at understanding

the defect and transport properties of such materials [77, 78].

Several of these studies have observed that Cu in CdSe can act as

both an acceptor and donor, with reports of Cu acting as a deep electron

acceptor about 0.5 eV below the conduction band minimum (CBM). In

addition, many computational studies have been targeted on transition

metal doped semiconductors, with doping described in terms of the

“doping pinning rule”. Zhang et al. have established that doping in

semiconductors can be explained by considering the valence band

maximum (VBM) offset with respect to a p(n)-like pinning energy. This

has established a simple yet effective way to determine the relative

carrier concentration if a system can be doped [79]. As predicted by

Zhang et al., CdSe can easily be n doped, and lightly p doped [80]. The

prediction of n type doping is consistent with the observation of deep

25

levels below the CBM as mentioned above. Finally, Wei et al. have

predicted the formation of Cu substitutional defects in CdX (where X = S,

Se, Te) to be energetically favorable [81]. In addition, they predict a lattice

compression with Cu single ion substitution, as expected from the

smaller Cu ion. With this understanding of CdSe: Cu bulk, they will

exploit it in our efforts in studying CdSe:Cu nanoparticles. And this lead

to the synthesis of copper doped SnSe nanoparticles at different copper

concentration level. The effect of doping in SnSe nanoparticles can be

understand if we have all the data (i. e. structural, electrical, dielectrical,

optical and thermal data) of pure SnSe nanoparticles. Hence author also

presents synthesis of pure SnSe nanoparticles by using aqueous solution

technique.

A study of structural, optical and electrical properties of CdS and

CdS:Cu nanoparticles has been studied in [82]. From XRD and TEM they

reveal that the prepared samples are nanocrysatlline in nature and their

particle size shows a decreasing trend with the increase in doping

concentration of Cu in CdS nanoparticles. The band gap of all the

samples increases with the increase in doping concentration of Cu in

CdS nanoparticles. This increase is the outcome of quantum confinement

effect originated from smaller size of nanoparticles produced at higher

concentration of Cu doping. The electrical conductivity of the samples at

room temperature as well as at elevated temperature increases with the

increase in doping concentration of Cu in CdS nanoparticles. This is due

to the increased carrier density of free electrons.

The physical properties of nanostructures are substantially

influenced by the synthesis route and have a strong bearing on the

structure and microstructure of the specimens. Preparation of

nanomaterial samples, which are uniform in composition, size, shape,

internal structure and surface chemistry, is essential to successfully

26

mapping their size- dependent materials properties. These materials have

been prepared by different techniques like low- pressure chemical vapour

deposition (LPCVD) [83], dc/ rf sputtering [84], molecular beam epitaxy

(MBE) [85]. But these techniques are not very cost effective. Thermal

evaporation of a material in the presence of a carrier gas is another

technique, which is simple and less expensive. This technique has been

utilized for the deposition of the metals and nanoclusters [86].

Nanocrystalline thin films of semiconductors have been the subject of

intensive studies from fundamental, experimental and applied interests.

Most of the studies have been done on II-VI and III-VI nanocrystalline

materials. However, not much work has been reported in the IV- VI

nanomaterials. Here author reports synthesis of pure and copper doped

SnSe nanoparticles by using aqueous solution technique in double

distilled water.

Among the IV-VI compounds, tin selenide (SnSe) has potential

applications in memory switching devices, as efficient solar material and

in holographic recording systems. So, the authors have decided to

prepare thin films of SnSe in nanocrystalline form and to study its

electrical and optical properties. And according to our knowledge no work

is reported on Cu doped SnSe nanocrystalline and it give me motivation

to work which is reported in thesis.

1.3 EFFECT OF SIZE ON PROPERTIES OF NANOMATERIALS

While most microstructured materials have similar properties to

the corresponding bulk materials, the properties of materials with

nanometer dimensions are significantly different from those of atoms and

bulk materials. This is mainly due to the nanometer size of the materials

which renders them: (i) large fraction of surface atoms, (ii) high surface

energy, (iii) spatial confinement and (iv) reduced imperfections, which do

27

not exist in the corresponding bulk materials [87]. Nanomaterials have a

relatively larger surface area when compared to the same mass of

material produced in a bulk form. As the particle size decreases, a

greater proportion of atoms are found at the surface compared to those

inside. For example, variation of percentage of atoms at surface with

particle size is shown in Table 1.2. in the present investigation, in order

to obtain the crystallite size dependant characteristic (i. e. electrical,

dielectrical, optical and thermal properties) of pure SnSe nanoparticles,

author kept pure SnSe nanoparticle at different three temperature (i. e.

200, 300 and 400 °C). By this way author synthesized pure SnSe

nanoparticles having different range of crystallite size (obtained from the

XRD technique and range of particle size obtained from UV- VIS- NIR

optical absorption spectra as well as Transmission Electron Microscopy

images.

As growth and catalytic chemical reactions occur at surfaces, this

can make materials more chemically reactive (in some cases materials

that are inert in their bulk form are reactive when produced in their

nanoscale form) and affect their strength. Thus, the entire material will

be affected by the surface properties of nanomaterials [88, 89]. For other

materials such as crystalline solids, as the size of their structural

components decreases, there is much greater interface area within the

material; this can greatly affect both mechanical and electrical

properties. For example, most metals are made up of small crystalline

grains; the boundaries between the grain slow down or arrest the

propagation of defects when the material is stressed, thus giving it

strength. If these grains can be made very small, or even nanoscale in

size, the interface area within the material greatly increases, which

enhances its strength. For example, nanocrystalline nickel is as strong

as hardened steel.

28

1.3.1 MECHANICAL PROPERTIES

Due to the nanometer size, many of the mechanical properties of

the nanomaterials are different from the bulk materials including the

hardness, elastic modulus, fracture toughness, scratch resistance and

fatigue strength etc. An enhancement of mechanical properties of

nanomaterials can result due to this modification, which are generally

resulting from structural perfection of the materials [90, 91]. The small

size either renders them free of internal structural imperfections such as

dislocations, micro twins, and impurity precipitates or the few defects or

impurities present cannot multiply sufficiently to cause mechanical

failure. The imperfections within the nano dimension are highly energetic

and will migrate to the surface to relax themselves under annealing,

purifying the material and leaving perfect material structures inside the

nanomaterials. Moreover, the external surfaces of nanomaterials also

have less or free of defects compared to bulk materials, serving to

enhance the mechanical properties of nanomaterials [90].

The enhanced mechanical properties of the nanomaterials could

have many potential applications both in nanoscale such as mechanical

nano resonators, sensors, microscope probe tips and nanotweezers for

nanoscale object manipulation, light weight high strength materials,

flexible conductive coatings, wear resistance coatings, tougher and

harder cutting tools etc. Among many of the novel mechanical properties

of nanomaterials, high hardness has been observed for many

nanomaterials. A variety of superhard nanocomposites can be made of

nitrides, borides and carbides by plasma- induced chemical and physical

vapor deposition [92]. The excellent mechanical properties of

nanomaterials could lead to many potential applications in all the nano,

micro and macro scales. High frequency electromechanical resonators

have been made from carbon nanotubes and nanowires. Nanostructured

29

materials can also be used as nanoprobes or nanotwizzers to probe and

manipulate nanomatierals in a nanometer range [93]. Due to their high

aspect ratio and small dimensions, one dimensional nano structures

such as carbon nanotubes can also be used as nano probe tips.

The nanotweezers can be made from two individual carbon

nanotubes. This nanotweezers operated under electrical stimulus was

used to probe the electrical characteristics of nanostructures. It could be

useful both in the nanostructure characterization and the manipulation.

The nanotweezers could be used as a novel electromechanical sensor

that can detect pressure or viscosity of media by measuring the change

in resonance frequency. They can also be explored into manipulation and

modification of biological systems such as structures within a cell.

1.3.2 THERMAL PROPERTIES

By controlling the structures of materials at nanoscale dimensions,

the properties of the nanostructures can be controlled and tailored in a

very predictable manner to meet the needs for a variety of applications.

The engineered nanostructures include metallic and non metallic

nanoparticles, nanotubes, quantum dots and superlattices, thin films,

nano composites and nanoelectronic and optoelectronic devices which

utilize the superior properties of the nanomaterials to fulfill the

applications. Many properties of the nanoscale materials have been well

studied, including the optical electrical, magnetic and mechanical

properties. However, the studies on thermal properties of nanomaterials

have only seen slower progresses. This is partially due to the difficulties

of experimentally measuring and controlling the thermal transport in

nanoscale dimensions. Atomic Force Microscope (AFM) has been

introduced to measure the thermal transport of nanostructures with

30

nanometerscale high spatial resolution, providing a promising way to

probe the thermal properties with nanostructures [94].

Moreover, the theoretical simulations and analysis of thermal

transport in nanostructures are still in infancy. As the dimensions go

down into nanoscale, the availability of the definition of temperature is in

question. In non metallic material system, the thermal energy is mainly

carried by phonos, which have a wide variation in frequency and the

mean free paths. The heat carrying phonons often have large wave

vectors and mean free path in the order of nanometer range at room

temperature, so that the dimensions of the nanostructures are

comparable to the mean free path and wavelength of phonons. However

the general definition of temperature is based on the average energy of a

material system in equilibrium. For macroscopic systems, the dimension

is large enough to define a local temperature in each region within the

materials and this local temperature will vary from region to region, so

that one can study the thermal transport properties of the materials

based on certain temperature distributions of the materials. But for

nanomaterial systems, the dimensions may be too small to define a local

temperature. Moreover, it is also problematic to use the concept of

temperature which is defined in equilibrium conditions, for the

nonequilibrium processes of thermal transport in nanomaterials, posing

difficulties for theoretical analysis of thermal transport in nanoscales

[94].

In nanomaterials systems, several factors such as the small size,

the special shape, the large interfaces modify the thermal properties of

the nanomaterials, rendering them the quite different behavior as

compared to the macroscopic materials. As the dimension goes down to

nanoscales, the size of the nanomaterials is comparable to the

wavelength and the mean free path of the phonons, so that the phonon

31

transport within the materials will be changed significantly due the

phonon confinement and quantization of phonon transport, resulting in

modified thermal properties. The special structure of nanomaterials also

affects the thermal properties. For example, because of the tubular

structures of carbon nanotubes, they have extremely high thermal

conductivity in axial directions, leaving high anisotropy in the heat

transport in the materials [95]. The interfaces are also a very important

factor for determining the thermal properties of nanomaterials.

The use of nanofluid to enhance the thermal transport is another

promising application of the thermal properties of nanomaterials.

Nanofluids are generally referred to the solid- liquid composite materials,

which consist of nanomaterials of size in the range 1- 100nm suspended

in a liquid. Nanofluids hold increasing attentions in both research and

practical applications due to their greatly enhanced thermal properties

compared to their base fluids. Many type of nanomaterials can be used

in nanofluids including nanoparticles of oxides, nitrides, metals, metal

carbides, and nanofibers such as single walled and multi walled carbon

nanotubes, which can be dispersed in to a variety of base liquid

depending on the possible applications, such as water, ethylene glycol,

and oils. The most important features of nanofluids are the significant

increase of thermal conductivity compared with liquids without

nanomaterials.

1.3.3 STRUCTURAL PROPERTIES

The increase in surface area and surface energy with decreasing

particle size leads to changes in interatomic spacings. For copper

metallic clusters the interatomic spacing is observed to be decreasing

with decreasing cluster size. This is due to the compressive strain

induced by the internal pressure arising from the small radius of

32

curvature in the nanoparticle. Conversely, for semiconductors and metal

oxides there is evidence that interatomic spacings increase with

decreasing particle size.

1.3.4 CHEMICAL PROPERTIES

The change in structure as a function of particle size is

intrinsically linked to the changes in electronic properties. The ionization

potential (the en- ergy required to remove an electron) is generally higher

for small atomic clusters than for the corresponding bulk material.

Furthermore, the ion-ization potential exhibits marked fluctuations as a

function of cluster size. Such effects appear to be linked to chemical

reactivity of the materials. Nanoscale structures such as nanoparticles

and nanolayers have very high surface area to volume ratios and

potentially different crystallographic structures which may lead to a

radical alteration in chemical reactivity. Catalysis using finely divided

nanoscale systems can increase the rate, selectivity and efficiency of

chemical reactions such as combustion or synthesis whilst

simultaneously, significantly reducing waste and pollution. Nanoparticles

often exhibit new chemistry as distinct from their larger particulate

counterparts; for example, many new medicines are insoluble in water

when in the form of micron-sized particles but will dissolve easily when

in a nanostructured form.

1.3.5 MAGNETIC PROPERTIES

The large surface area to volume ratio results in a substantial

proportion of atoms (those at the surface which have a different local

environment) having a different magnetic coupling with neighboring

atoms, leading to differing magnetic properties. Ferromagnetic particles

become unstable when the particle size reduces below a certain size,

33

since the surface energy provides a sufficient energy for domains to

spontaneously switch polarization directions. As a result, ferromagnetics

become paramagnetics. However, nanometer sized ferromagnetic turned

to paramagnetic behaves differently from the conventional paramagnetic

and is referred to as superparamagnetics. An operational definition of

superparamagnetism would include at least two requirements. Firstly,

the magnetization curve should not show hysteresis, since that is not a

thermal equilibrium property. Secondly, the magnetization curve for an

isotropic sample must be temperature dependent to the extent that

curves taken at different temperatures must approximately superimpose

when plotted against H/T after correction for the temperature

dependence of the spontaneous magnetization.

At the same time bulk ferromagnetic materials usually form

multiple magnetic domains, small magnetic nanoparticles often consist

of only one domain and exhibit a phenomenon known as

superparamagnetism. In this case the overall magnetic coercivity is then

lowered: the magnetizations of the various particles are randomly

distributed due to thermal fluctuations and only become aligned in the

presence of an applied magnetic field. Giant magnetoresistance (GMR) is

a phenomenon observed in nanoscale multilayers [96] consisting of a

strong ferromagnet (e.g., Fe, Co) and a weaker magnetic or non- magnetic

buffer (e.g., Cr, Cu); it is usually employed in data storage and sensing.

In the absence of a magnetic field the spins in alternating layers are

oppositely aligned through antiferromagnetic coupling, which gives

maximum scattering from the interlayer interface and hence a high

resistance parallel to the layers. In an oriented external magnetic field

the spins align with each other and this decreases scattering at the

interface and hence resistance of the device.

34

1.3.6 ELECTRONIC PROPERTIES

The changes which occur in electronic properties as the system

length scale is reduced are related mainly to the increasing influence of

the wave-like property of the electrons (quantum mechanical effects) and

the scarcity of scattering centres. As the size of the system becomes

comparable with the de Broglie wavelength of the electrons, the discrete

nature of the energy states becomes apparent once again, although a

fully discrete energy spectrum is only observed in systems that are

confined in all three dimensions. In certain cases, conducting materials

become insulators below a critical length scale, as the energy bands

cease to overlap. Owing to their intrinsic wave-like nature, electrons can

tunnel quantum mechanically between two closely adjacent

nanostructures, and if a voltage is applied between two nanostructures

which aligns the discrete energy levels in the DOS, resonant tunneling

occurs, which abruptly increases the tunneling current.

In macroscopic systems, electronic transport is determined

primarily by scattering with phonons, impurities or other carriers or by

scattering at rough interfaces. The path of each electron resembles a

random walk, and transport is said to be diffusive. When the system

dimensions are smaller than the electron mean free path for inelastic

scattering, electrons can travel through the system without

randomization of the phase of their wave functions. This gives rise to

additional localization phenomena which are specifically related to phase

interference. If the system is sufficiently small so that all scattering

centres can be eliminated completely, and if the sample boundaries are

smooth so that boundary reflections are purely specular, then electron

transport becomes purely ballistic, with the sample acting as a

waveguide for the electron wave function. Conduction in highly confined

35

structures, such as quantum dots, is very sensitive to the presence of

other charge carriers and hence the charge state of the dot.

These Coulomb blockade effects result in conduction processes

involving single electrons and as a result they require only a small

amount of energy to operate a switch, transistor or memory element. All

these phenomena can be utilized to produce radically different types of

components for electronic, optoelectronic and information processing

applications, such as resonant tunneling transistors and single electron

transistors.

1.3.7 OPTICAL PROPERTIES

The reduction of materials dimension has pronounced effects on

the optical properties. The size dependence can be generally classified

into two groups. One is due to the increased energy level spacing as the

system becomes more confined and other is related to surface plasmon

res- onance. The quantum size effect is most pronounced for

semiconductor nanoparticles, where the band gap increases with a

decreasing size, resulting in the interband transition shifting to higher

frequencies [97, 98]. In a semiconductor, the energy separation, i.e. the

energy difference between the completely filled valence band and the

empty conduction band is of the order of a few electron volts and

increases rapidly with a decreasing size [99].

Quantum confinement produces a blue shift in the band gap as

well as appearance of discrete subbands corresponding to quantization

along the direction of confinement. The optical properties of

nanostructured semi- conductors are highly size dependent and thus

can be modified by varying the size alone, keeping the chemical

composition intact. The luminescent emission from the semiconductor

36

nanostructures can be tuned by varying the size of the nanoparticles. In

the case of nanostructured semiconductor lasers, the carrier

confinement and nature of electronic density of states of the

nanostructures make it more efficient for devices operating at lower

threshold currents than lasers with bulk materials. The size dependent

emission spectra of quantum wells, quantum wires and quantum dots

make them attractive lasing media. The performance of quantum dot

lasers is less temperature dependent than conventional semiconductor

lasers [100].

1.4 OPTICAL PROPERTIES OF NANOCRYSTAL SUSPENSIONS

A nanocrystal (NC) or “quantum dot” (QD) is a small particle of a

given material having the crystal structure of the corresponding bulk

material but exhibiting vastly different electronic properties due to its

small size. Quantum mechanically confining the electrons inside the QD

leads to a change in the electronic states: the effective energy gap

increases relative to the bulk material and discrete exciton states emerge.

The effects of quantum confinement on the electronic states were

investigated theoretically by Efros and Efros. They classified the size

effects into three different confinement strength regimes- weak,

intermediate, and strong by comparing the radius (R) of the QD to the

Bohr radius of the electron (ae) and hole (ah) in the parent bulk medium.

The Bohr radii are defined as given in following equation 1.1.

𝑎𝐵 =4𝜋𝜖 ∞ 𝑕2

𝑚𝑒 ,𝑕 𝑒2 (1.1)

where ε(∞) is the optical frequency dielectric constant, and me,h

are the electron and hole effective masses. In the case of weak

confinement (R > ae, ah), the electron and hole retain their bulk-like

37

character as an electron- hole pair bound by the Coulomb interaction (an

exciton) and are virtually unaffected by the quantum confinement. In the

intermediate regime (ae > R > ah), the electron is quantized and, as a

result, experiences an increase in its energy (confinement energy).

The hole is still unaffected by confinement and remains influenced

by the Coulomb attraction to the confined electron. However, when the

confinement is strong (ae, ah > R), both the energies are significantly

increased due to quantum confinement. In fact, the energy of the

strongly- confined charge carriers is often much greater than the energy

of the Coulomb interaction. The optical properties undergo the greatest

enhancement when the QD radius is smaller than the exciton Bohr

radius, aB as given by equation 1.2.

𝑎𝐵 =4𝜋𝜖 ∞ 𝑕2

𝑒2 ( 1

𝑚𝑒+

1

𝑚𝑕𝑕 𝑕 𝑕 𝑕𝑕 𝑕 𝑕 𝑕 𝑕 𝑕 𝑕 𝑕𝑕

) (1.2)

of the bulk semiconductor (aB > R). This is based on the observation that

as the dimensionality of the bulk semiconductor is reduced, the density

of states becomes concentrated in narrowing energy bands and

eventually discrete energy states. The effect of this narrowing has on the

strength of the transitions is profound; the oscillator strength of a

strongly-confined electron-hole pair3 inside a QD is predicted to be a

factor of (aB /R)3 times larger than that of the bulk exciton. This implies

that quantum dots in the strong confinement limit have the potential for

greatly enhanced optical properties and motivates the study of

semiconductor QDs in this limit. For other phenomena, such as the

excited state relaxation of charge carriers inside a QD, it is the

confinement of the electrons and holes separately that matters. Little

work has been done on QDs that strongly confine both the electrons and

38

the holes (ae, ah > R). This regime requires either the synthesis of very

small QDs or materials with large electron and hole Bohr radii.

Typically, QDs with a radius between 1.5 nm and 8 nm can be

fabricated and attempts to make them smaller results in molecular

clusters with a different crystal structure than the bulk semiconductor.

As shown in Table 1-1, the problem encountered with II-VI and III-V

semiconductors is that the hole Bohr radii are too small; in other words,

the holes are too massive. Therefore, these materials do not allow access

to the regime of strong confinement.

Due to confinement, nanocrystals possess discrete electron and

hole energy levels [100, 101, 102], and the energy of the exciton

increases. This is schematically shown in figure 1.4 (b). The separation

between the filled and unfilled levels (∆Enc) increases with respect to the

value for a bulk crystal of the same material (∆Ebulk) and is determined by

the nanocrystals' size: in smaller nanocrystals the separation is larger.

Furthermore, decreasing the size of the crystal induces a gradual change

from continuous bands in the bulk crystal to discrete electron and hole

levels in the nanocrystal. The gradual change starts at the band edges,

as schematically shown in figure 1.7.

In the bulk crystal the conduction band (CB) and the valence band

(VB) are separated by the bandgap ∆Ebulk. In the nanocrystal there are

discrete energy levels at the band edges, and the filled and unfilled levels

are separated by ∆Enc which is larger than ∆Ebulk.

In spite of the discrete electron and hole levels in the nanocrystals,

the absorption and emission spectra are broad. This is related to the size

dispersion of the nanocrystal suspension. A suspension of nanocrystals

contains a collection of differently sized particles, distributed around a

39

certain average size. Each individual nanocrystal absorbs and emits light

in a narrow frequency window, leading to broad spectra for the

suspension [103, 104]. Despite the size dispersion of the suspension,

several transitions in the absorption spectrum can be distinguished.

The position and the strength of the transitions have been studied

as a function of particle size and assigned using a system of quantum

numbers which take into account the complex valence band structure

[105]. Since its publication in 1993 the organometallic synthesis route

[106] has been adapted for many types of semiconductor nanocrystals

[107]. After years of modification and improvement CdSe and CdTe

suspensions can be prepared with high photoluminescence efficiency a

high degree of monodispersity and with a good control of the size and

shape of the nanocrystals. An important step forward in improvement of

the optical properties was made by capping the nanocrystals, besides the

organic coating, with an inorganic semiconductor with a larger band gap

[108]. Capping with an inorganic semiconductor prevents interaction of

the photo-excited charge carriers with the surface of the nanocrystals

and thereby improves the photoluminescence quantum yield and the

stability towards photo-oxidation. Because of these reasons,

inorganically capped nanocrystals are used as light sources in various

fields such as biological labeling [109] and photonics [110, 111], and as

buildings blocks for solar cells, light- harvesting materials [112] and light

emitting diodes [113].

1.4.1 PARTICLE PLASMONS

The same quantum size effect is also known for metal

nanoparticles. However, in order to observe the localization of the energy

levels, the size must be very small, as the level spacing has to exceed the

thermal energy (26 meV). Surface plasmon resonance is the coherent

40

collective excitation of all the free electrons within the conduction band,

leading to an inphase oscillation [114, 115]. When the size of a metal

nanocrystal is smaller than the wavelength of incident radiation, a

surface plasmon resonance is generated [116]. The energy of the surface

plasmon resonance depends on both the free electron density and the

dielectric medium surrounding the nanoparticle. The width of the

resonance varies with the characteristic time before electron scattering.

For larger nanoparticle, the resonance sharpens as the scattering length

increases. Noble metals have the resonance frequency in the visible

range of electromagnetic spectrum.

In general, we have seen that plasmons arise from interplay of

electron density oscillations and the exciting electromagnetic fields. In

this sense, we should talk about surface plasmon polaritons and also

distinguish the propagating (evanescent) modes at the interface of a

metal and a dielectric from their localized counterpart at the surface of

metallic particles (so called particle plasmon polaritons). If an

electromagnetic wave impinges on a metallic nanoparticle (whose spatial

dimension is assumed to be much smaller than the wavelength of light),

the electron gas gets polarized (polarization charges at the surface) and

the arising restoring force again forms a plasmonic oscillation, as

presented in Figure 1.8.

At the resonance frequency the plasmons are oscillating with a 90°

phase difference (180° above resonance). Also a magnetic polarization

occurs, but most of the time it can be neglected. The metallic particle

thus acts like an oscillator and the corresponding resonance behavior

determines the optical properties [117]. When a metallic nanoparticle is

illuminated by white light, the plasmonic resonance determines the color

we observe, see Fig. 2.8. This behavior is nothing new: Microscopic gold

and silver particles incorporated in the stained glasses of old church

41

windows are responsible for their beautiful lustrous colors14. Another

very famous example dates back to antiquity – a Roman cup made of

dichroic glass illustrating the myth of King Lycurgus15 [118, 119].

We can tune the resonance of the surface plasmon polariton by

changing the size or shape of a metallic nanoparticle, as plotted in figure

1.10. This figure also shows that the resonance intensity has a maximum

for the aspect ratio somewhere between 0.3 and 0.4. The plasmonic

resonance is not only sensitive to the shape and size of a nanostructure.

The upper panels show the scattering cross section, in the lower

panels they report the corresponding density plots also the dielectric

medium surrounding the particle plays a key role [120]. Even the

slightest change in the dielectric surrounding leads to a detectable shift

of the resonance energy. That is the reason why metallic nanoparticles

are very suitable for sensing applications: Placing a molecule in the

vicinity of a nanoparticle effects the dielectric environment and therefore

shifts the plasmon peak.

42

Figure 1.1 The solid state structure of Tin.

Figure 1.2 The solid state structure of Selenium.

Figure 1.3 The solid state structure of Copper.

43

Figure 1.4 The layered structure of Tin Selenide.

Figure 1.5 Picture showing the relative size scale of various miniaturized

natural and manmade material.

44

Figure 1.6 The Lycurgus Cup (British Museum).

Figure 1.7 Schematic band structure of a bulk semiconductor crystal

(left) and nanocrystals (right).

Figure 1.8 Excitation of particles plasmons through the polarization of

metallic nanoparticles.

45

Figure 1.9 Nanoparticles of various shape and size in solution – the

plasmonic resonance determines the color.

Figure 1.10 Tuning the resonance of a surface plasmon polariton by

changing the diameter of a gold nanosphere (a) or by

squeezing its aspect ratio (b).

46

Table 1.1 Physical properties of Sn, Se and Cu elements.

Parameters Sn Se Cu

Group, period, block 14, 5, p 16, 4, p 11, 4, d

Crystal structure Tetragonal

(white), diamond

cubic (gray)

hexagonal face-

centered

cubic

Electronic configuration

[Kr] 4d10 5s2 5p2 [Ar] 4s2 3d10 4p4

[Ar] 3d10 4s1

Atomic Number 50 34 29

Atomic Weight (g) 118.710 78.96 63.546

Atomic radius (pm) 140 120 128

Covalent radius (pm) 139±4 120±4 132±4

Density (g cm-3) 7.365 4.81 8.94

Melting point (0C) 231.93 221 1084.62

Boiling Point (0C) 2602 685 2562

Electrical resistivity (nΩm)

(0 °C) 115 10-6 (20 °C) 16.78

Thermal conductivity (W·m−1·K−1)

66.8 0.519 401

Heat of vaporization

(kJ·mol−1) 296.1 95.48 300.4

Heat of fusion

(kJ·mol−1) 7.03 6.69 13.26

Thermal expansion (µm·m−1·K−1)

(25 °C) 22.0 37 16.5

Molar heat capacity

(J·mol−1·K−1) 27.112 25.363 24.440

47

Table 1.2 Variation of percentage of atoms at surface with grain size.

Particle size (nm) Number of atoms Atoms at surface

(%)

10 30000 20

5 4000 40

2 250 80

1 25 99

Table 1.3 Exciton, electron, and hole Bohr radii of typical

semiconductors used to make colloidal QDs.

aB (nm) ae (nm) ah (nm)

II- VI: CdSe 3 3 <1

II- VI: CdS 2 2 <1

III- V: InP 9 7 2

III- V: GaAs 10 8 1

IV- VI: PbS 20 10 10

IV- VI: PbSe 47 23 24

48

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