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CHAPTER 1

INTRODUCTION TO NANOPARTICLE

This chapter emphasize the general introduction, properties and the

applications of the nanoparticle. A brief review of literature pertaining to the

present work is also presented.

1.1 INTRODUCTION

Nanotechnology is an advanced technology, which deals with the

synthesis of Nanoparticles, processing of the Nanomaterials and their

applications. Normally, if the particle sizes are in the 1-100 nm ranges, they are

generally called Nanoparticles or Nanomaterials. For oxide materials, the

diameter of one oxygen ion is about 1.4 Å. So, seven oxygen ions will make

about 10 Å or 1 nm, i.e., the ‘lower’ side of the Nano range. On the higher side,

about 700 oxygen ions in a spatial dimension will make the so-called ‘limit’ of

the Nano range of materials. Nanoparticles constitute a major class of

nanomaterials. Nanoparticles are zero-dimensional, possessing Nano metric

dimensions in all the three dimensions. The diameters of nanoparticles can vary

anywhere between one and a few hundreds of nanometers. Accordingly, the

electronic and atomic structures of such small nanoparticles have unusual

features, markedly different from those of the bulk materials. Large

nanoparticles (>20–50 nm), on the other hand, would have properties similar

to those of the bulk (Jortner and Rao 2002). At small sizes, the properties vary

irregularly and are specific to each size. The size-dependent properties of

nanoparticles include electronic, optical, magnetic, and chemical

characteristics. Nanoparticles can be amorphous or crystalline. Being small in

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size, crystalline nanoparticles can be of single domain. Nanoparticles of metals,

chalcogenides, nitrides and oxides are often single crystalline.

Nanoscience and nanotechnology primarily deal with the synthesis,

characterization, exploration and exploitation of nanostructured materials.

These materials are characterized by at least one dimension in the nanometer

range. Nanostructures constitute a bridge between molecules and infinite bulk

systems. Individual nanostructures include clusters, quantum dots,

nanocrystals, nanowires, and nanotubes, while collections of nanostructures

involve arrays, assemblies and super lattices of the individual nanostructures

(Rao et al 2004). The physical and chemical properties of nanomaterials can

differ significantly from those of the atomic-molecular or the bulk materials of

the same composition. The uniqueness of the structural characteristics,

energetics, response, dynamics, and chemistry of nanostructures constitutes the

basis of nanoscience.

Some of the important concerns in the nanoscience area are:

i. Nanoparticles or nanocrystals of metals and

semiconductors,nanotubes, nanowires and Nano biological

systems.

ii. Assemblies of nanostructures (e.g., nanocrystals and nanowires)

and the use of biological systems, such as DNA as molecular

nanowires and templates for metallic or semiconducting

nanostructures.

iii. Theoretical and computational investigations provide the

conceptual framework for structure, dynamics, response and

transport in nanostructures.

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iv. Application of nanomaterials in biology, medicine, electronics,

chemical processes, high-strength materials etc.

One potential applications of nanotechnology are the production of

novel materials and devices in Nano electronics, computer technology,

medicine, and health care. Generally, nanotechnology therefore describes any

activities at a magnitude of less than 100 nm. Nanotechnology refers to the

creation, investigation and application of structures, molecular materials,

internal interfaces or surfaces with at least one critical dimension or with

manufacturing tolerances of (typically) less than 100 nanometers. Research and

technology development at the atomic, molecular or macromolecular levels, in

the length scale of approximately 1-100 nanometer range, to provide a

fundamental understanding of phenomena and materials at the nanoscale and to

create and use structures, devices and systems that have novel properties and

functions because of their small and /or intermediate size. The novel and

differentiating properties and functions are developed at a critical length scale

of matter typically under 100 nm.

Nanoparticles measure only a few nanometers and can consist of just

a few or several thousand atoms. The material out of which nanoparticles are

made is nothing out of the ordinary. The basic material of nanoparticles can be

organic or inorganic, for example silver or ceramic. They can be elements such

as carbon, or compounds such as oxides, or they can be a combination of

different compounds and elements. The key characteristic is not the material

itself but the size of the particles. In comparison to their size nanoparticles have

a vast surface area. At this size, a relatively inert material can become highly

reactive and therefore potentially interesting for many different uses, for

example as a catalyst. In addition, nanoparticles have a tendency to form

agglomerations. Nanoparticles with less than 1000 atoms, i.e. very small

nanoparticles, are called clusters. Nanoparticles are invisible due to the fact that

they are smaller than the wavelength of visible light and therefore unable to

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scatter light. Aside from synthetic production, nanoparticles are also present in

natural materials, for example in clay, a constituent of loam, which contains a

high proportion of natural nanoparticles. These are responsible for properties

such as frost-resistance, durability and strength.

1.2 Classification of Nanomaterials

Nanomaterials have extremely small size which having at least one

dimension 100 nm or less. Nanomaterials can be nanoscale in one dimension

(e.g. surface films), two dimensions (e.g. strands or fibers), or three dimensions

(e.g. particles). They can exist in single, fused, aggregated or agglomerated

forms with spherical, tubular, and irregular shapes. Common types of

nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes.

Nanomaterials have applications in the field of nanotechnology, and displays

different physical and chemical characteristics from normal chemicals (i.e.,

silver Nano, carbon nanotube, fullerene, photo catalyst, carbon Nano, silica).

According to Siegel, Nanostructured materials are classified as Zero

dimensional, one dimensional, two dimensional, three dimensional

nanostructures. Classification of Nanomaterials (a) 0D spheres and clusters, (b)

1D nanofibers, wires and rods, (c) 2D films, plates and networks, (d) 3D

nanomaterials. Nanomaterials are materials which are characterized by an ultra-

fine grain size (< 50 nm) or by a dimensionality limited to 50 nm. Nanomaterials

can be created with various modulation dimensionalities as defined by Richard

W. Siegel: zero (atomic clusters, filament and cluster assemblies), one

(multilayers), two (ultrafine-grained over layers or buried layers), and three

(Nano phase materials consisting of equalized nanometer sized grains) as

shown in Figure 1.1.

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Figure 1.1 Classification of Nanomaterials (a) 0D spheres and clusters,

(b) 1D nanofibers, wires and rods, (c) 2D films, plates and

networks, (d) 3D nanomaterials

The self-assembly of these Nano sized building blocks are 2D and

3D. “zero-dimensional” structure is the simplest building block that may be

used for nanomaterials design. These materials have diameters <100 nm, and

are denoted by nanoparticles, nanoclusters or nanocrystals. The term

nanoparticle is generally used to encompass all 0D Nano sized building blocks

(regardless of size and morphology). For amorphous / semi crystalline

nanostructures smaller in size (i.e., 1–10 nm), with a narrow size distribution,

the term nanocluster is more appropriate. This distinction is a simple extension

of the term “cluster,” which is typically used in inorganic / organometallic

chemistry to indicate small molecular cages of fixed sizes. Analogous to bulk

materials, any nanomaterial that is crystalline should be referred to as a

nanocrystal. A special case of nanocrystal that is comprised of a semiconductor

is known as a quantum dot. Typically, the dimensions of these nanostructures

lie in the range 1–30 nm, based on its composition. Quantum dots currently find

applications as sensors, lasers, and LEDs.

1.3 SYNTHESIS OF NANOPARTICLES

Nanomaterial fabrication methods can be classified according to

whether their assembly followed either the so called bottom-up approach,

where smaller components of atomic or molecular dimensions self-assemble

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together, according to a natural physical principle or an externally applied

driving force, to give rise to larger and more organized systems or the top-down

approach, a process that starts from a large piece and subsequently uses finer

and finer tools for creating correspondingly smaller structures.

1.3.1 Bottom-up and top-down methods of synthesis

There are two approaches to the synthesis of nanomaterials: bottom-

up and top-down. In the bottom-up approach, molecular components arrange

themselves into more complex assembly of atom-by-atom, molecule-by-

molecule, cluster-by cluster from the bottom (e.g., growth of a crystal). In the

top-down approach, nanoscale devices are created by using larger, externally-

controlled devices to direct their assembly. The top-down approach often uses

the traditional workshop or microfabrication methods in which externally-

controlled tools are used to cut, mill and shape materials into the desired shape

and order. Attrition and milling for making nanoparticles are typical top-down

processes. Bottom-up approaches, in contrast, arrange molecular components

themselves into some useful conformation using the concept of molecular self-

assembly. Synthesis of nanoparticles by colloid dispersions is an example of

the bottom-up approach. The bottom-up approach plays a very important role

in preparing nanomaterials having very small size where the top-down process

cannot deal with the very tiny objects. The bottom-up approach generally

produces nanostructures with fewer defects as compared to the nanostructures

produced by the top-down approach. The top-down and the bottom-up approach

are illustrated in Figure 1.2. The main driving force behind the bottom-up

approach is the reduction in Gibbs free energy. Therefore, the materials

produced are close to their equilibrium state.

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Figure 1.2 Schematic illustration of the preparative methods of

nanoparticles

In top-down techniques such as lithography, significant

crystallographic defects can be introduced to the processed patterns. For

example, nanowires made by lithography are not smooth and can contain a lot

of impurities and structural defects on its surface. Since the surface area per unit

volume is very large for the nanomaterials, these defects can affect the surface

properties, e.g., surface imperfections may cause reduced conductivity and

excessive generation of heat would result. The top-down approach plays an

important role in the synthesis and fabrication of nanomaterials. Figure 1.3

compares the bottom-up and the top-down approach of nanomaterials.

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Figure 1.3 Comparison of the “top-down” and “bottom-up” approach to

nanomaterial synthesis

1.3.2 Mechanical grinding

Mechanical attrition is a typical example of ‘top down’ method of

synthesis of nanomaterials, where the material is prepared not by cluster

assembly but by the structural decomposition of coarser-grained structures as

the result of severe plastic deformation. This has become a popular method to

make nanocrystalline materials because of its simplicity, the relatively

inexpensive equipment needed, and the applicability to essentially the synthesis

of all classes of materials. The major advantage often quoted is the possibility

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for easily scaling up to tonnage quantities of material for various applications.

Similarly, the serious problems that are usually cited are;

i. contamination from milling media and / or atmosphere, and

ii. to consolidate the powder product without coarsening the

nanocrystalline microstructure.

Mechanical milling is typically achieved using high energy shaker,

planetary ball, or tumbler mills. Figure 1.4 illustrates the mechanical milling

method of nanomaterials. The energy transferred to the powder from refractory

or steel balls depends on the rotational (vibrational) speed, size and number of

the balls, ratio of the ball to powder mass, the time of milling and the milling

atmosphere. Nanoparticles are produced by the shear action during grinding.

Milling in cryogenic liquids can greatly increase the brittleness of the powders

influencing the fracture process. As with any process that produces fine

particles, an adequate step to prevent oxidation is necessary. Hence this process

is very restrictive for the production of non-oxide materials since then it

requires that the milling take place in an inert atmosphere and that the powder

particles be handled in an appropriate vacuum system or glove box. This

method of synthesis is suitable for producing amorphous or nanocrystalline

alloy particles, elemental or compound powders. If the mechanical milling

imparts sufficient energy to the constituent powders a homogeneous alloy can

be formed. Based on the energy of the milling process and thermodynamic

properties of the constituents the alloy can be rendered amorphous by this

processing.

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Figure 1.4 Schematic representation of the principle of

mechanical milling

1.3.3 Sol-gel process

The sol-gel process, involves the evolution of inorganic networks

through the formation of a colloidal suspension (sol) and gelation of the sol to

form a network in a continuous liquid phase (gel). The precursors for

synthesizing these colloids consist usually of a metal or metalloid element

surrounded by various reactive ligands. The starting material is processed to

form a dispersible oxide and forms a sol in contact with water or dilute acid.

Removal of the liquid from the sol yields the gel, and the sol / gel transition

controls the particle size and shape. Calcination of the gel produces the oxide.

Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based

precursors such as Si (OEt) 4 (tetraethyl orthosilicate or TEOS). The reactions

involved in the sol-gel chemistry based on the hydrolysis and condensation of

metal alkoxides M (OR) z can be described as follows:

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MOR + H2O → MOH + ROH (hydrolysis)

MOH + ROM → M-O-M + ROH (condensation)

Sol-gel method of synthesizing nanomaterials is very popular

amongst chemists and is widely employed to prepare oxide materials. The sol-

gel process can be characterized by a series of distinct steps.

i. Formation of different stable solutions of the alkoxide or

solvated metal precursor.

ii. Gelation resulting from the formation of an oxide- or alcohol-

bridged network (the gel) by a polycondensation reaction that

results in a dramatic increase in the viscosity of the solution.

iii. Aging of the gel, during which the polycondensation reaction

continue until the gel transforms into a solid mass, accompanied

by contraction of the gel network and expulsion of solvent from

gel pores. Ostwald ripening (also referred to as coarsening), is

the phenomenon by which smaller particles are consumed by

larger particles during the growth process and phase

transformation.

iv. Drying of the gel, when water and other volatile liquids are

removed from the gel network. This process is complicated due

to fundamental changes in the structure of the gel. The drying

process has itself been broken into four distinct steps: the

constant rate period, the critical point, the falling rate period and

the second falling rate period. If isolated by thermal

evaporation, the resulting monolith is termed a xerogel. If the

solvent (such as water) is extracted under super critical or near

super critical conditions, the product is an aerogel.

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v. Dehydration, during which surface- bound M-OH groups are

removed, there by stabilizing the gel against rehydration. This

is normally achieved by calcination of the monolith at

temperatures up to 8000 °C.

vi. Densification and decomposition of the gels at high

temperatures (T > 8000 °C). The pores of the gel network are

collapsed, and remaining organic species are volatilized. The

typical steps that are involved in sol-gel processing are shown

in the schematic diagram below (Figure 1.5).

Figure 1.5 Schematic representation of sol-gel process of synthesis of

nanomaterials.

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1.3.4 Chemical Vapour Deposition

Chemical vapour deposition (CVD) is the method of depositing a

solid material on a hot surface. This method is a suitable versatile process for

coatings, powders, fibers and monolithic components. And it is also used to

produce metals, metal oxides and non-metallic elements such as carbon and

silicon. CVD method is shown in Figure 1.6. The high deposition rate is the

main advantage of CVD method. Thick coatings or nanoparticles can be

obtained by this method. This method is more economical than the physical

vapour deposition method. In the last two decades, CVD have been applied in

the areas of semiconductor industry and in metallurgical coating industry.

Recently more importance has been given to the CVD process because of the

mass production of monodisperse nanoscale powders (Cheng et al 1994, Kear

and Skandan 1997, Kim et al 1999).

Figure 1.6 Schematic diagram of chemical vapour deposition method

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1.3.5 Low Temperature Wet–Chemical Synthesis: Precipitation

Method

Precipitation method plays an important role in the preparation of

metal oxide nanoparticles. In this process, salt precursor such as AlCl3 is

dissolved to prepare Al2O3, Y (NO3) to make Y2O3 and ZrCl2 to make ZrO2.

The metal hydroxides form a precipitate in water by adding a base solution such

as sodium hydroxide or ammonium hydroxide solution (Gao et al 1999, Rao et

al 1996). The resulted chloride salts such as NaCl or NH4Cl are washed and

then the hydroxide is calcined followed by filtration and finally it is washed

thoroughly to get an end product (Oxide powder). This method is also used to

prepare composites of different oxides by co-precipitation. The main drawback

of this method is the difficulty of controlling the particle size.

1.3.6 Hydrothermal Synthesis

This method is the popular technique to synthesize mixed metal

oxides. Metal oxides can be prepared either directly from homogeneous or

heterogeneous solution. In order to speed up the reactions between the solids,

hydrothermal method utilizes water under temperature and at pressure above its

normal boiling point. An excellent solvent is water because it has high dielectric

constant. Due to high temperature, it decreases and it increases with a rise in

pressure.

This is the best property for increasing the solubility of many

sparingly soluble compounds under hydrothermal conditions. The

hydrothermal condition leads to large number of useful chemical reactions such

as co-precipitation, precipitation, crystal growth and the hydrolysis. The

hydrothermal reaction is performed by means of closed vessels. The reactants

used in this method are suspended or dissolved by means of small quantity of

water. They are generally transferred to acid digestion autoclaves or reactors.

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The autoclave model is shown in Figure 1.7. The reactants which are difficult

to dissolve under hydrothermal conditions turn to solution and precipitate.

Figure 1.7 Schematic diagram of an autoclave

The single step process for preparing several oxides and phosphates

are the hydrothermal process (Clearfield 1991, Haushalter and Mundi 1992).

The narrow size distribution of spherical submicron titanium hydrous oxide was

obtained by Oguri et al (1988), which could be transformed into polycrystalline

anhydrous anatase with a spherical morphology. Ferroelectric lead titanate with

high Curie temperature can also be prepared by this method (Cheng et al 1996).

The same technique was also used for the nanocrystalline metal oxide

fabrication. Nano sized α-alumina with a particle size of 10 nm was synthesized

via hydrothermal method by Sharma et al (1998). Further this method was

employed for fabricating several other metal oxides.

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1.3.7 Microwave Synthesis

Figure 1.8 Schematic diagram of microwave used for the powder

Microwave processing plays a vital role in the areas of food

processing, Medical applications and chemical applications. The processing of

ceramics by means of microwave includes interaction of materials,

measurement of electric, designing of microwave equipment, development of

the new materials, sintering, connecting and modeling. Therefore, the

successful alternative to conventional processing emerges out from the

microwave processing of Ceramics (Krage 1981, Roy et al 1985). The main

advantage of this method is the uniform heating of the materials at low

temperature and time than the conventional method. At low temperature and

time, the micro wave energy can be utilized successfully for the fabrication of

ceramics as well as carbon based fibers. Various electroceramices such as lead

zirconate titanate through this method are synthesized by Varandan et al (1990)

and Sharma et al (2001). The schematic diagram of the microwave unit is shown

in Figure 1.8. In order to observe the materials, mechanical, electrical and

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electronic properties have to be improved. Micro coiled Carbon fibers with

large surface area have been fabricated recently by this method.

1.4 PROPERTIES OF NANOPARTICLES

There are numerous material properties that are affected by

decreasing the grain size within the material. Due to their nanometer size,

nanomaterials are already known to have many novel properties. Many novel

applications of the nanomaterials arise from these novel properties have also

been proposed. In this chapter, the properties of nanomaterials including the

mechanical, thermal, optical and chemical properties of nanomaterials will be

addressed together with the possible applications of nanomaterials (Guozhong

Cao 2004).

1.4.1 Melting Point and Vapour Pressure

Melting point and vapour pressure are the essential thermodynamic

properties of a material. When matter is reduced in size, there will be an

increased number of atoms or molecular units that lie on the surface. The

physical implications for this are a significant reduction in melting point. The

reduction in melting point can be explained by considering the surface energy

contribution to the Gibbs free energy of the nanoparticle. The reduction in the

melting point is inversely proportional to the particle radius (Buffat and Borel

1976, Coombes 1972). For 5 nm particle of gold, a quite large depression of

melting point has been observed on an inert unreactive support (Buffat and

Borel 1976). Alivisatos and his colleagues noticed a larger depression of

melting point for CdS nanoparticle (Goldstein et al 1992).

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1.4.2 Mechanical Properties

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

nanomaterials are modified to be different from the bulk materials including the

hardness, elastic modulus, fracture toughness, scratch resistance, fatigue

strength etc. An enhancement of mechanical properties of nanomaterials can

result due to this modification, which are generally the resultant from structural

perfection of the materials (Guozhong Cao 2004, Herring and Galt 1952). The

elastic constants of nanocrystalline materials have found to be reduced by 305

or less. These results were interpreted as a result of the large free volume of the

interfacial component resulting from the increased average interatomic spacing

in the boundary regions. Generally, the hardness increases with a decrease in

grain size. At very small grain sizes, the hardness decreases with a decrease to

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

on one material (Nohara 1982).

1.4.3 Thermal Properties

Many properties of the nanoscale materials have been well studied,

including the optical, electrical, magnetic and mechanical properties. However,

the 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 used to measure the thermal transport of nanostructures with

nanometer-scale high spatial resolution, providing a promising way to probe

the thermal properties with nanostructures (David et al 2003). Moreover, the

theoretical simulations and analysis of thermal transport in nanostructures are

still in infancy. Available approaches including numerical solutions of Fourier's

Law, computational calculation based on Boltzmann transport equation and

Molecular-dynamics (MD) simulation, all have their limitations (David et al

2003). More importantly, as the dimensions go down into nanoscale, the

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availability of the definition of temperature is in question. In non-metallic

material system, the thermal energy is mainly carried by photons, which have a

wide variation in frequency and the mean free paths. 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 (David et al 2003).

1.4.4 Optical Properties

The optical properties of small particles have received considerable

attention because of potential applications in optical sensors (Elghanian et al

1997) and lasing devices (Klimov et al 2000). Nanocrystalline systems have

attracted interest for their novel optical properties, which differ remarkably

from bulk crystals. The factors include quantum confinement of electrical

carriers within nanoparticles, efficient energy and charge transfer over

nanoscale distances in many systems and a highly enhanced role of interfaces.

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

detailed basis for photonic properties of nanoparticles. The linear and non-

linear optical properties of such materials can be finely tailored by controlling

the crystal dimensions and the chemistry of their surfaces, fabrication

technology becomes a key factor for the applications. Size-dependent optical

absorption and photoluminescence as a result of the creation and recombination

of excitons have been studied extensively (Empedocles et al 1999, Nirmal et al

1999). In nanocrystal arrays, it has been found that interactions between

nanocrystals can lead to long-range resonance transfer (Kagan et al 1996).

Optical absorption exhibited by these crystallites arises due to transitions

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involving the molecular orbital which have nodes on the grain surface. The

semiconductor devices like CdS, CdSe, ZnS, ZnSe have been investigated for

their optical absorption of a function of particle size, exhibited blue shifts as

particle size decreases. Size effect on optical absorption becomes significant

when the cluster diameter becomes equal to or similar than electron hole exciton

diameter in a bulk semiconductor. The surface conditions do not show much

effect on the observed luminescence spectra (Fitzgerald 1995).

1.4.5 Electrical and Electronic Properties

According to the theory of electron scattering in solids, the electrical

resistivity of nanocrystalline materials is expected to be higher than that in the

corresponding coarse-grained polycrystalline ones due to the increased volume

fraction of atoms lying on the grain boundaries. The electrical resistivity of

nanocrystalline material is also found to be higher than that of the amorphous

solids. As the volume fraction of the interface in the nanocrystalline materials

is inversely proportional to the grain size, then the dependence of residual

resistivity on grain size can be correlated with that of the interfacial volume

fraction (Guozhong Cao 2004). It is well known that the electrical conductivity

of the solids is determined by its electronic structure. Generally, in solids, the

valence band is completely filled by electrons and separated from the empty

conduction band with the energy gap of Eg (bandgap). For metals, Eg = 0, which

results in the mixing of the valence and conduction bands. In the case of

semiconductors, the value of Eg is small. The electrons can be excited from the

valence band to the conduction band using heat, light etc., which results in

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

is restricted. The conducting nature of the solids is affected by various factors

like temperature and particle size (Charles Kittel 1953). When the particle size

is reduced to nanometer range, the bandgap (Eg) value increases and hence the

conductivity is reduced. In the case of metal nanoparticles, the density of states

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in the conduction and valence bands are reduced and electronic properties

changed drastically, i.e., the quasi-continues 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. This size quantization

effect may be regarded as the onset of the metal to non-metal transition. The

size at which the transition occurs depends on the nature of the metal (Charles

Kittel 1953).

1.4.6 Magnetic Properties

The extrinsic magnetic properties of particles depend strongly upon

their size and shape. Among the magnetic properties, Hc shows a remarkable

size effect and saturation magnetization is independent of the particle size

(Bhargava and Gallagher 1994). When the particle size is reduced in

ferromagnetic and ferroelectric materials to sizes of the order of microns, the

particles become single domains. As the particle size reduced further, the

materials become super paramagnetic or super ferroelectric respectively, at

temperature below Curie point. At these conditions they do not exhibit any

hysteresis effects and they retain very high permeability and lose their

magnetism or polarization when the external field is removed. The super

paramagnetic nanoparticles can be used for separation processes in bio-

chemistry. The potential applications of nanoscale magnetic particles are in

colour imaging, Ferro fluids and magnetic refrigeration. Co, Fe, Ni metals are

used for this purpose since they are easy to synthesis and cost effective (Chen

and Zhang 1998).

1.4.7 Surface Atom / Volume Atom Ratio

Nanoparticles have interesting properties due to their small size. For

most materials, if the surface is formed with particles size of approximately 3

nm diameter, a 2/3 of the atoms lie on the surface. When the matter is

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subdivided, the surface area is large and it becomes more reactive. Therefore,

the nanoparticles will be an attractive method for providing a matrix for any

chemical reaction, such as pollution cleanup. This is being seriously pursed to

destroy chlorinated hydrocarbons (Koper and Klabunde 1997).

1.4.8 Transport Properties

There are two important ways in which materials can conduct

electrical current. Both electrons and ions can carry electric charge. Diffusion

usually takes place by the movement of ions to neighboring vacancies. In the

stoichiometric compounds, the vacancy concentration and ionic conductivity

are very small. The smaller particle size increases the non-stoichiometry of a

material. The defect thermodynamics is dominated by interfaces when the

particle size is in nanometer regime. The unusual defect thermodynamics of the

nanocrystals are attributed to interfacial reduction (Somorjai 1994).

1.5 APPLICATIONS OF NANOPARTICLES

Nanoparticles offer radial breakthrough in areas such as materials

and manufacturing, electronics, medicine and health care, environment and

energy, chemical and pharmaceutical, biotechnology and agriculture,

computation and information technology and national security. Nano carbon is

used to make rubber tires wear resistant. Nano phosphorous are used for Laser

coupled devices (LCD'S) and Cathode Ray Tubes (CRT'S) to display colours.

Nano alumina and silica are used for super fine polishing compounds, Neon

iron oxide is used to create the magnetic material used in disk drives and audio

/ video tapes. Nano zinc oxide or Nano titanium are used in many sunscreens

to block harmful UV rays.

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1.5.1 Nanoparticle Applications in Medicine

The use of polymeric micelle nanoparticles to deliver drugs to

tumors. The use of polymer coated iron oxide nanoparticles to break up clusters

of bacteria, possibly allowing more effective treatment of chronic bacterial

infections.

The surface change of protein filled nanoparticles has been shown to

affect the ability of the nanoparticle to stimulate immune responses.

Cerium oxide nanoparticles act as an antioxidant to remove oxygen

free radicals that are present in a patient's bloodstream following a traumatic

injury. The nanoparticles absorb the oxygen free radicals and then release the

oxygen in a less dangerous state, freeing up the nanoparticle to absorb more

free radicals.

Carbon nanoparticles called Nano diamonds in the field of medicine.

For example Nano diamonds with protein molecules attached can be used to

increase bone growth around dental or joint implants.

Chemotherapy drugs attached to Nano diamonds are used to treat

brain tumors.

1.5.2 Applications in Manufacturing and Materials

Ceramic silicon carbide nanoparticles dispersed in magnesium

produce a strong, lightweight material. A synthetic skin that may be used in

prosthetics has been demonstrated with both self-healing capability and the

ability to sense pressure. The material is a composite of nickel nanoparticles

and a polymer. If the material is held together after a cut it seals together in

about 30 minutes giving it a self-healing ability. Also the electrical resistance

of the material changes with pressure, giving it sense ability like touch.

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Silicate nanoparticles can be used to provide a barrier to gases (for

example oxygen), or moisture in a plastic film used for packaging. This could

slow down the process of spoiling or drying out in food.

Zinc oxide nanoparticles can be dispersed in industrial coatings to

protect wood, plastic and textiles from exposure to UV rays.

Silicon dioxide crystalline nanoparticles can be used to fill gaps

between carbon fibers, thereby strengthening tennis racquets.

Silver nanoparticles in fabric are used to kill bacteria, making

clothing odor-resistant.

1.5.3 Applications and the Environment

The photo catalytic copper tungsten oxide nanoparticles are used to

break down oil into biodegradable compounds. The nanoparticles are in a grid

that provides high surface area for the reaction is activated by sunlight and can

work in water, making them useful for cleaning up oil spills.

Gold nanoparticles are embedded in a porous manganese oxide as a

room temperature catalyst to breakdown volatile organic pollutants in air.

Iron nanoparticles are being used to clean up carbon tetrachloride

pollution in ground water.Iron oxide nanoparticles are being used to clean

arsenic from water wells.

1.5.4 Applications in Energy and Electronics

Nanoparticles called nanotetrapods studded with nanoparticles of

carbon are used to develop low cost electrodes for fuel cells. This electrode may

be able to replace the expensive platinum needed for fuel cell catalysts.

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To print prototype circuit boards using standard inkjet printers, silver

nanoparticle ink was used to form the conductive lines needed in circuit boards.

Combining gold nanoparticles with organic molecules creates a

transistor known as a NOMFET (Nanoparticle Organic Memory Field-Effect

Transistor). This transistor is unusual in that it can function in a way similar to

synapses in the nervous system.

A catalyst using platinum-cobalt nanoparticles is being developed

for fuel cells that produce twelve times more catalytic activity than pure

platinum.

When sunlight is concentrated on nanoparticles, it produces steam

with high energy efficiency. The solar steam device is intended to be used in

areas of developing countries without electricity for applications such as

purifying water or disinfecting dental instruments.

A lead free solders reliable enough for space missions and other high

stress environments using copper nanoparticles.

Silicon nanoparticles coating anodes of lithium-ion batteries can

increase battery power and reduce recharge time.

Semiconductor nanoparticles are being applied in a low temperature

printing process that enables the manufacture of low cost solar cells.

A layer of closely spaced palladium nanoparticles is being used in a

hydrogen sensor. When hydrogen is absorbed, the palladium nanoparticles

swell, causing shorts between nanoparticles. These shorts lower the resistance

of the palladium layer.

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1.5.5 Nanomaterials in Electronics

Nantero is developing a high density nonvolatile random access

memory chip called NRAM (Nanotube-based / Non-volatile random access

memory) chip.

Carbon nanotubes are used as active memory elements and

integrated with traditional semiconductor technology. NRAM is slated to

replace DRAM (dynamic RAM), SRAM (static RAM), flash memory and

ultimately hard disk storage. NRAM is a universal memory chip suitable for

countless existing and new applications in the field of electronics.

Solid state lighting (SSL) encompasses technology to make lighting

technologies more energy efficient, longer lasting and cheaper. Instead of using

inert gases or vacuum tubes, it relies on light being emitted from a

semiconductor.

Quantum dots have been investigated as building blocks for tunable

optical devices such as light emitting devices and lasers.

Carbon nanotubes can be either ‘metallic’ or semi-conducting

depending on the actual way in which the carbon atoms are assembled in the

tube. The metallic forms possess electrical conductivities 1000 times greater

than copper and are now being mixed with polymers to make conducting

composite materials for applications such as electromagnetic shielding in

mobile phones and static electricity reduction in cars.

1.6 LITERATURE REVIEWS ON NANOPARTICLES UNDER

HIGH PRESSURE AND HIGH TEMPERATURE

When the size of the materials is reduced, the kinetics of the phase

transition is simplified. The phase diagram and kinetic stability of a crystalline

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phase depend on size. Thus size serves as a synthetic tool. Pressure combined

with size can be used to alter the structural stability of the material.

Semiconductor nanocrystals remain stable well above the pressure at which the

extend semiconductor changes phase.

Bulk CdSe transforms from a wurtize structure to a rock salt

structure at 3.0 GPa with hydrostatic pressure (Edwards and Drickamer 1961,

Yu and Giellisse 1971). CdS undergoes an analogous transition between 2.7

and 3.1 GPa (Samara and Drickamer 1962, Corll 1964). Bulk silicon transforms

from the diamond structure to the β-Sn phase at approximately 11 GPa and then

further transforms to a primitive hexagonal structure at above 16 GPa

(McMohan et al 1994). In all significantly elevated in examined, the phase

transition pressure is significantly elevated in nanocrystals compared to the bulk

materials (Variano et al 1998). Further, the elevation is a function of crystallite

size with smaller diameter crystallites undergoing transition at higher pressure.

Some reviews of the proceeding work on nanoparticles under high pressure are

consolidated in this section.

Peppiatt and Sambles (1975) observed that melting point decrease

as the particle size is reduced. In recent years, this is observed in semiconductor

like Cadmium Selenide (CdSe) and Cadmium Sulphide (CdS) by Goldstein et

al (1992). Tolbert and Alivisatos in the following years contributed much to

know the state of the phase transition in semiconductor nanoparticles under

high pressure. According to Tolbert and Alivisatos (1991, 1994, 1995), the

phase transition is enhanced in CdSe, Si and Indium Phosphide (InP). The cause

for enhancement in solid-solid first order transition is discussed with the help

of effects such as single nucleation, surface effects and shape changes.

Qadri et al (1996) reported that the effect grain size in PbS

nanocrystals is to elevate the transition pressure. He observed that the

compressibility increase with decreasing grain size. Herhold et al (1996) have

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studied CdSe, CdS, InP and Si in Nano regime. All the above nanocrystals

transform via single nucleation with a kinetic barrier that increasing cluster size.

The structural transition path causes a shape change in the nanocrystals, which

alters the surface energy and thus the kinetic and thermodynamic stability of

the transformed nanocrystals provide enhanced metastability which allows

structural and optical measurements in this regime. This make possible to

recover the dense high pressure phase pressure which is inaccessible in the bulk

solids.

Also an enhancement of transition pressure in nanocrystals such as

ZnS (Jiang et al 1999), ZnO and PbS (Jiang et al 2000) is observed when

compared with their corresponding bulk materials. However, Jiang et al (1998)

reported that that for nanometer sized γ-Fe2O3 particles, the phase transition

pressure (from γ-Fe2O3 to α-Fe2O3) is much lower than that for bulk material.

They suggested that the larger volume change upon for electrical property of

CoFe2O4 nanocrystals investigated under pressure up to 20 GPa using DAC at

ambient temperature. The experimental results indicate that the phase transition

from the spinel to a tetragonal structure takes place at 7.5 GPa and 12.5 GPa for

6 nm and 8 nm respectively.

A reduction of transition pressure is also reported in TiO2

nanocrystals for the rutile to α-PbO2 transition (Olsen et al 1999). Wang et al

(2001) also found that fluorite-type CeO2, undergoes a phase transition to an

orthorhombic PbCl2 - type structure at pressure of GaAs from I → II transition

as 17 GPa and 20 GPa respectively, for both bulk and Nano phase material.

Jorgensen et al (2003) reported high pressure energy dispersive X-ray

diffraction of nanocrystalline GaN. Pressure-induced structural phase transition

from the wurtzite to the NaCl phase is obtained at 60 Gpa for nanocrystalline

GaN which is greater than the bulk.

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Jiang (2004) reported the recent development of pressure-induced

phase transformation in crystals. The thermodynamic theory is presented and

three components viz., the ratio of volume collapse, the surface energy

difference and the internal energy differences, governing the change of

transition pressure in nanocrystals are uncovered. These parameters can be used

to explain the results reported in the literature and to identify the main factor to

change the transition pressure in nanocrystals.

Anna N. Treflova et al (2005) reported the high pressure and high

temperature electrical resistivity studies of ZrO2. The resistivity of

nanocrystalline praseodymium-doped zirconia powders has been measured in

the pressure and temperature ranges between 15 and 50 GPa and 77 and 400 K

respectively. Around 30-37 GPa the resistivity of all samples decrease by 3-4

orders of magnitude. Also it is found that the activation energy of the samples

depends on the crystallite size.

Ana Akrap et al (2007) reported the high pressure and the high

temperature studies of β–SrxV6O15. By applying pressure, one can change the

order of the transition. The temperature dependence of the transport coefficients

shed light on the possible mechanism of electrical conductivity. The room

temperature of the electrical resistivity indicates that the system is either a

semiconductor or a bad metal. However, even up to 650 K, there is not a trace

of a metallic temperature dependence of resistivity. From the activation energy

studies of the samples it is clear that the gap between the temperature range 165

K and 300 K are very low.

Yang Jie et al (2013) reported the high pressure, high temperature

and the activation energy studies of the solid C60. The investigation from the

high pressure electrical resistivity of the materials helps us to understand the

transport properties of the carriers under high pressure. The C60 sample is a

molecular crystal in which C60 molecules are arranged in an fcc structure and

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are bound together by van der Waals forces. However, with increasing applied

pressure, the individual molecules are bound together into pairs or larger

clusters by covalent intermolecular bonds because of polymerization which the

electron transport easier. At high temperatures, the carrier concentration is

determined by the intrinsic properties of the pure semiconductors. While at low

temperatures, the carriers are affected by the impurity content. the temperature

dependence of resistivity for C60 material were studied in the temperature range

of 300-423 K at different pressures.