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CHAPTER 1

NANOPARTICLE: A GENERAL INTRODUCTION

1.1 INTRODUCTION

Nanotechnology is a rapidly growing area of importance and

interest, incorporating a wide range of research fields. It deals with materials

or structures in nanometer scale, typically ranging from sub nanometers to

several hundred nanometers. One nanometer is 10-3

micrometer or 10-9

meter.

It deals with single nano-objects, materials and devices based on them and

with processes that take place in the nanometer range. Nanomaterials are

those materials whose key physical characteristics are dictated by the nano-

objects they contain. Nanomaterials are classified into compact materials and

nanodispersions. The first type includes nanostructured materials (Moriarty

2001), i.e., materials isotropic in the macroscopic composition and consisting

of contacting nanometer-sized units as repeating structural elements (Gusev

and Rampel 2004). The particles with small size in the range from a few to

several tens of nanometers are called quasi zero-dimensional mesoscopic

systems, quantum dots, quantized or Q-particles, etc.,(Khairutdinov et al

1996). The reason that nanoscale materials and structures are so interesting is

that size constraints often produce qualitatively new behavior.

Nanoparticles can basically differ in their properties from larger

particles, for example, from long- and well-known ultra dispersed powders

with a grain size above 0.5 µm. As a rule, nanoparticles are shaped like

spheroids. Nanoparticles with a clearly ordered arrangement of atoms (or

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ions) are called nanocrystallites. Nanoparticles with a clear-cut discrete

electronic energy levels are often referred to as ‘‘quantum dots’’ or ‘‘artificial

atoms’’; most often, they have compositions of typical semiconductor

materials, but not always. Many magnetic nanoparticles have the same set of

electronic levels. Nanoparticles are of great scientific interest because they

represent a bridge between bulk materials and molecules and structures at an

atomic level. The term ‘‘cluster,’’ which has been widely used in the chemical

literature in previous years, is currently used to designate small nanoparticles

with sizes less than 1 nm. Magnetic polynuclear coordination compounds

(magnetic molecular clusters) belong to the special type of magnetic materials

often with unique magnetic characteristics. Unlike nanoparticles, which

always have the distributions in sizes, molecular magnetic clusters are the

fully identical small magnetic nanoparticles. Their magnetism is usually

described in terms of exchange-modified paramagnetism (Alivisatos 1996,

Gubin 2009).

Nanorods and nanowires, as shown in Figure 1.1, are quasi-one-

dimensional nanoobjects. In these systems, one dimension exceeds by an

order of magnitude than the other two dimensions, which are in the

nanorange. The group of two-dimensional objects includes planar structures–

nanodisks, thin-film magnetic structures, magnetic nanoparticle layers, etc., in

which two dimensions are an order of magnitude greater than the third one,

which is in the nanometer range. The nanoparticles are considered as giant

pseudomolecules having a core and a shell and often also external functional

groups. The unique magnetic properties are usually inherent in the particles

with a core size of 2–30 nm. For magnetic nanoparticles, this value coincides

(or less) with the size of a magnetic domain in most bulk magnetic materials

(Knauth and Schoonman 2004).

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Figure 1.1 The classification of metal containing nanoparticles by the

shape

1.2 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 rose 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).

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1.2.1 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. According to the

reduction in the melting point is inversely proportional to the particle radius

of the material (Buffat and Borel 1976, Coombes 1972). For 5 nm particle of

gold a quite large depression of melting point have 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).

1.2.2 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

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 30% 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).

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1.2.3 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 nano scale 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 and 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

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

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

a wide variation in frequency and the mean free paths (mfp). 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).

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1.2.4 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

(LRRT) (Kagan et al 1996).

Optical absorption exhibited by these crystallites arises due to

transitions 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).

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1.2.5 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 (band gap). 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

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 value (Eg) increases and hence the conductivity

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

conduction and valence bands are reduced and electronic properties changed

drastically, i.e., the quasi-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 behavior. This size quantization

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

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

Kittel 1953).

1.2.6 Magnetic Properties

The extrinsic magnetic properties of particles depend strongly upon

their shape and size. 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 superparamagnetic or super ferroelectric respectively, at

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

hysterisis effects and they retain very high permeability and loose their

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

paramagnetic nanoparticles can be used for separation processes in

biochemistry. 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).

1.2.7 Surface Atoms/Volume Atom Ratio

Nanoparticles have interesting properties due to their small size,

hence a high surface to volume ratio. 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 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

clean up. This is being seriously pursed to destroy chlorinated hydrocarbons

(Koper and Klabunde 1997).

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1.2.8 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 neighbouring 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).

1.3 SYNTHESIS OF NANOPARTICLES

The preparation of nanoscale structures and devices can be

accomplished through “bottom-up” or “top-down” methods. In the bottom-up

approach, small building blocks are assembled into larger structures; chemical

synthesis is a good example of bottom-up approach in the synthesis of

nanoparticles. In the top-down approach, large objects are modified to give

smaller features, attrition or milling is a good example of top-down approach.

Both approaches play very important roles in modern industry and most likely

in nanotechnology. There are advantages and disadvantages in both

approaches.

Methods to produce nanoparticles from atoms are chemical

processes based on transformations in solution e.g. sol-gel processing,

chemical vapour deposition (CVD), plasma or flame spraying synthesis, laser

pyrolysis, atomic or molecular condensation. These chemical processes rely

on the availability of appropriate “metal-organic” molecules as precursors.

Figure 1.2 shows the preparation methods of nanoparticles (Guozhong Cao

2004).

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1.3.1 Bottom-Up and Top-Down Approaches

Obviously there are two approaches to the synthesis of

nanomaterials and the fabrication of nanostructures: top-down and bottom-up.

Attrition or milling is a typical top-down method in making nanoparticles,

whereas the colloidal dispersion is a good example of bottom-up approach in

the synthesis of nanoparticles. Lithography may be considered as a hybrid

approach, since the growth of thin films is bottom-up whereas etching is top-

down, while nanolithography and nanomanipulation are commonly a bottom-

up approach. Both approaches play very important roles in modern industry

and most likely in nanotechnology as well (Guozhong Cao 2004).

There are advantages and disadvantages in both approaches.

Among others, the biggest problem with top-down approach is the

imperfection of the surface structure. It is well known that the conventional

top-down techniques such as lithography can cause significant

crystallographic damage to the processed patterns (Das et al 1993) and

additional defects may be introduced even during the etching steps (Vieu et al

2000). For example, nanowires made by lithography are not smooth and may

contain a lot of impurities and structural defects on surface. Such

imperfections would have a significant impact on physical properties and

surface chemistry of nanostructures and nanomaterials, since the surface over

volume ratio in nanostructures and nanomaterials is very large. The surface

imperfection would result in a reduced conductivity due to inelastic surface

scattering, which in turn would lead to the generation of excessive heat and

thus impose extra challenges to the device design and fabrication. Regardless

of the surface imperfections and other defects that top-down approaches may

introduce, they will continue to play an important role in the synthesis and

fabrication of nanostructures and nanomaterials.

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Bottom-up approach is often emphasized in nanotechnology

literature, though bottom-up is nothing new in materials synthesis. Typical

material synthesis is to build atom by atom on a very large scale, and has been

in industrial use for over a century. Examples include the production of salt

and nitrate in chemical industry, the growth of single crystals and deposition

of films in electronic industry. For most materials, there is no difference in

physical properties of materials regardless of the synthesis routes, provided

that chemical composition, crystallinity, and microstructure of the material in

question are identical. Of course, different synthesis and processing

approaches often result in appreciable differences in chemical composition,

crystallinity, and microstructure of the material due to kinetic reasons.

Consequently, the material exhibits different physical properties (Guozhong

Cao 2004). In organic chemistry and/or polymer science, we know polymers

are synthesized by connecting individual monomers together. In crystal

growth, growth species, such as atoms, ions and molecules, after impinging

onto the growth surface, assemble into crystal structure one after another (Das

et al 1993).

Although the bottom-up approach is nothing new, it plays an

important role in the fabrication and processing of nanostructures and

nanomaterials. There are several reasons for this. When structures fall into a

nanometer scale, there is little choice for a top-down approach. All the tools

we have possessed are too big to deal with such tiny subjects. Bottom-up

approach also promises a better chance to obtain nanostructures with less

defects, more homogeneous chemical composition, and better short and long

range ordering. This is because the bottom-up approach is driven mainly by

the reduction of Gibbs free energy, so that nanostructures and nanomaterials

such produced are in a state closer to a thermodynamic equilibrium state. On

the contrary, top-down approach most likely introduces internal stress, in

addition to surface defects and contaminations (Vieu et al 2000).

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PERPARATION

Break-up Built-up

Solid phaseGas phase1µm-1nm

Liquid phase10µm-1nm

Dry100µm-100nm

Wet100µm-10nm

Media mil

Media mil

Jet mil

Liquid jetmil

Grinding mil

Physical PhysicalChemical Chemical

Plasma

Sputtering

ElectronbeamLASER

Electric

furnace

Combustion

Plasma

Thermaldecomposition

Freeze drying

Precipitation

Spray drying

Sol gel

Polymerization

Figure 1.2 Preparation methods of nanoparticles

1.3.2 Sol-Gel processing

Sol-gel processing is a wet chemical route for the synthesis of

colloidal dispersions of inorganic and organic-inorganic hybrid materials,

particularly oxides and oxide-based hybrids. From such colloidal dispersions,

powders, fibers, thin films and monoliths can be readily prepared. Although

the fabrication of different forms of final products requires some specific

considerations, the fundamentals and general approaches in the synthesis of

colloidal dispersions are the same. Sol-gel processing offers many

advantages, including low processing temperature and molecular level

homogeneity. Sol-gel processing is particularly useful in making complex

metal oxides, temperature sensitive organic-inorganic hybrid materials, and

thermodynamically unfavorable or metastable materials (Brinker 1990).

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Typical sol-gel processing consists of hydrolysis and condensation of

precursors. Precursors can be either metal alkoxides or inorganic and organic

salts (Pierre 1998, Wright and Sommerdijk 2001). Organic or aqueous

solvents may be used to dissolve precursors, and catalysts are often added to

promote hydrolysis and condensation reactions:

Hydrolysis:

M(OEt)4 + xH2O M(OEt)4-x(OH)x + XEtOH

Condensation:

M(OEt)4-x(OH)x + M(OEt)4-x(OH)x (OEt)4-x(OH)x-1MOM(OEt)4-x(OH)x-1

+H2O

Where, M Metals like Si, Ti, Sn, Cu etc.,

Et Ethanol

O Oxygen

H Hydrogen

Hydrolysis and condensation reactions are both multiple-step

processes, occurring sequentially and in parallel. Each sequential reaction

may be reversible. Condensation results in the formation of nanoscale clusters

of metal oxides or hydroxides, often with organic groups embedded or

attached to them. These organic groups may be due to incomplete hydrolysis,

or introduced as non-hydrolysable organic ligands. The size of the nanoscale

clusters, along with the morphology and microstructure of the final product,

can be tailored by controlling the hydrolysis and condensation reactions

(Guozhong Cao 2004).

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1.3.3 Low-Temperature Wet-Chemical Synthesis: Precipitation

Method

One of the conventional methods to prepare nanoparticles of metal

oxide ceramics is the precipitation method. This process involves dissolving a

salt precursor, usually chloride, oxychloride, or nitrate, such as AlCl3 to make

Al2O3, Y(NO3)3 to make Y2O3, and ZrCl2 to make ZrO2. The corresponding

metal hydroxides usually form and precipitate in water by adding a base

solution such as sodium hydroxide or mmonium hydroxide solution. The

resulting chloride salts, i.e., NaCl or NH4Cl, are then washed away and the

hydroxide is calcined after filtration and washing to obtain the final oxide

powder (Gao et al 1999, Qian and Shi 1998, Rao et al 1996). This method is

useful in preparing composites of different oxides by co-precipitation of the

corresponding hydroxides in the same solution. One of the disadvantages of

this method is the difficulty to control the particle size.

1.3.4 Hydrothermal Synthesis

Hydrothermal methods are becoming a popular technique to

precipitate mixed metal oxides directly from either homogeneous or

heterogeneous solution. Hydrothermal method utilizes water under pressure

and at temperatures above its normal boiling point as a means of speeding up

the reactions between solids (Rabenau 1985). Water is an excellent solvent

because of its high dielectric constant. This decreases with rising temperature

and increases with rising pressure, with temperature effect predominating. In

addition, the high dielectric constant of water is confirmed to a region of low

temperature and high densities (pressure). This property is mainly responsible

for increasing the solubility of many sparingly soluble compounds under

hydrothermal conditions leading to many useful chemical reactions such as

hydrolysis, precipitation, co-precipitation, and crystal growth. Hydrothermal

reactions are usually performed in closed vessels. The reactants are either

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dissolved or suspended in a known amount of water and are transferred to

acid digestion reactors or autoclaves as shown in Figure 1.3. Under

hydrothermal conditions, reactants otherwise difficult to dissolve can go into

solution and reprecipitate.

Hydrothermal reaction is a single-step process for preparing several

oxides and phosphates (Rabenau 1985, Clearfield 1991, Haushalter and

Mundi 1992). Oguri et al (1988) obtained narrow size distribution of spherical

submicron titanium hydrous oxide, which could be readily transformed into

polycrystalline anhydrous anatase with spherical morphology. Fine particles

of ferroelectric lead titanate with high Curie temperature were prepared via

hydrothermal technique (Cheng et al 1996). This technique was further used

for the fabrication of nanocrystalline metal oxides. Sharma et al have

synthesized nanosize -alumina using hydrothermal method with particle size

of 10 nm. Quantum size particles (<10 nm) of Y2O3 could also be achieved by

this technique at 170ºC using seeds (Sharma et al 1998). This method was

further employed for the fabrication of several other metal oxides, e.g., ZnO,

TiO2, and ZrO2, with nanosize particles (Sharma et al 1998, Yang et al 2000).

Figure 1.3 Schematic diagram of an autoclave

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1.3.5 Chemical Vapour Deposition

Chemical vapour deposition (CVD) may be defined as the

deposition of a solid on a heated surface from a chemical reaction in the vapor

phase. It is a versatile process suitable for the manufacturing of coatings,

powders, fibers, and monolithic components. It is possible to produce most

metals, metal oxides, and nonmetallic elements such as carbon and silicon as

a large number of compounds including carbides, nitrides, oxides,

intermetallics and many others. Figure 1.4 shows a schematic diagram of

CVD method. The main advantage of CVD is that the deposition rate is high

and thick coatings or nanoparticles can be readily obtained. The process is

generally competitive and, in some cases, more economical than the physical

vapor deposition (PVD). Additionally, it is not restricted to a line of sight

deposition, which is a general characteristic of sputtering, evaporation, and

other PVD processes. However, two major areas of applications of CVD have

rapidly developed in the last 20 years, the semiconductor industry and in the

metallurgical coating industry which includes cutting tool fabrication. Very

recently, the CVD process has been given enormous attention owing to the

possibility of mass production of monodisperse nanoscale powders; however,

the mechanism of powder synthesis kinetics is still not clear (Cheng et al

1994, Kear and SKandan 1997, Kim et al 1999)

Figure 1.4 Schematic diagram of chemical vapour deposition method

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1.3.6 Microwave Synthesis

The interest in the use of microwave processing spans a number of

fields from food processing to medical applications to chemical applications.

A major area of research in microwave processing of ceramics includes

microwave material interaction, dielectric measurement, microwave

equipment design, new material development, sintering, joining, and

modeling. Therefore the microwave processing of ceramics has emerged as a

successful alternative to conventional processing (Krage 1981, Roy et al

1985). Nevertheless, microwave method not only offers the advantages of a

uniform heating at lower temperature and time than the conventional method,

but also provides an economic method of processing. The microwave energy

has been already successively utilized in the fabrication of ceramics as well as

carbon fibers at low temperature and time. Varadan et al (1990) and Sharma

et al (2001) have synthesized various electroceramics such as barium

strontium titanate (BST) and lead zirconate titanate (PZT) by microwave.

Figure 1.5 shows the schematic diagram of a typical domestic microwave unit

used by Sharma et al. These materials are observed to have improved

mechanical, electrical, and electronic properties. Until recently, microcoiled

carbon fibers with large surface area have also been fabricated by using

microwave aid (Xie et al 2002).

Figure 1.5 Schematic diagram of microwave used for the powder

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1.3.7 High-Energy Ball Milling Processes

Ball milling has been utilized in various industries to perform size

reduction for a long time. Recently, materials with novel microstructures and

properties have been synthesized successfully via high-energy ball milling

processes (Koch el al 2000, Suryanarayana 2001). Although different terms

have been used to describe the high-energy ball milling processes, three terms

are generally used to distinguish powder particle behavior during milling:

mechanical alloying (MA), mechanical milling (MM), and mechanochemical

synthesis (MS). Mechanical alloying is referred to when mixtures of powders

are milled together. In this case, materials transfer is involved to obtain a

homogeneous alloy. Mechanical milling describes a milling process when no

material transfer is involved; that is, only powder with uniform composition is

milled. Mechanochemical synthesis, on the other hand, is a special MA

process where chemical reactions between the powders take place during

milling. The unique feature of MS process is that grain refinement and

chemical reactions take place at low temperatures under far-from-equilibrium

conditions (Miao et al 1996).

1.4 APPLICATIONS OF NANOPARTICLES

Nanoparticles offer radical breakthroughs 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 tyres 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, nano iron oxide is used to create the magnetic material used in

disk drives and audio/ video tapes. Nano zinc oxide or nano titania are used in

many sunscreens to block harmful UV rays.

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1.4.1 Nanocrystalline Materials in Electronics

Nanostructured materials have an increased impact on electronics;

smaller dimension in electronics provides higher functionality, increased

memory density and higher speed. Quantum effect devices or single electron

devices are great potential utility for future electronic circuits. Better

resolution of television screens could be achieved by reducing the size of the

phosphors. Nanocrystalline zinc sulphide, cadmium sulphide and lead

telluride synthesized by sol-gel technique are noteworthy candidates for

improving the resolution of monitors. Nanocrystalline phosphor plays a major

part in enhancing the resolution of the display devices. Due to enhancement in

electrical and magnetic properties of nanomaterials, high brightness and

contrast is expected from flat panel displays.

1.4.2 Application of Nanostructured Magnetic Materials

Nano materials are interesting due to their micro structural features

and magnetic properties (Kneller and Hawing 1991, Wu et al 2001). With the

aid if nano magnetic materials, fast, more compact and less power consuming

memory systems with greater storage capacity can be designed. Magnet made

of nanocrystalline yttrium-samarium-cobalt grains posses very fascinating

magnetic properties due to their extremely large surface area. Some typical

application includes quieter submarines, automobile alternators, land-based

power generators, and motors for ships, ultra sensitive analytical instruments

and magnetic resonance imaging in medical diagnostics.

1.4.3 Application of Nanoparticles in Biology and Medicine

Understanding of biological processes in the nanoscale level is a

strong driving force behind development of nanotechnology (Whitesides

2003) out of the plethora of size dependent physical properties available in the

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practical side of nanomaterials, optical (Parak et al 2003) magnetic (Pankhurst

et al 2003) effects of the nanoparticles are used for biological application.

Hybrid bio-nanomaterials can also be applied to build novel electronic,

optoelectronics and memory devices (Yan et al 2003 and Keren et al 2003).

Some of the application of nanomaterials to biology or medicine is listed

below.

Drug and gene delivery (Panatarotto et al 2003)

Bio detection of pathogens (Edelstein et al 2000)

Detection of proteins (Nam et al 2003)

Probing of DNA structure (Mahtab et al 1995)

Tissue Engineering (Ma et al 2003, Isla et al 2003)

Tumor destruction through heating (hyperthermia) (Shinkai

et al 1999)

Separation and purification of biological molecules and cells

(Molday and Mackenzie 1982) MRI contrast enhancement

(Weissleder et al 1990)

Phagokinetic studies (parka et al 2002 )

Fluorescent biological labels (Bruchez et al 1998)

1.4.4 Application in Thermal Engineering

There is a great need for more efficient heat transfer fluids in many

industries, from transportation, energy supply to electronics. The coolant,

lubricants, oil and other heat transfer fluids used in today’s conventional

thermal systems have inherently poor heat transfer properties. The

conventional working fluids that contain millimeter or micrometer size

particles cannot be used in the newly emerging “miniaturized technology”

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since they clog in micro channels. these problems can be solved with the help

of nanotechnology to thermal angering called nanofluids. The nanofluids have

two important factors such as extreme stability and ultra thermal conductivity

(Gerold et al 2004).

Nanofluids are innovative class of heat transfer fluids created by

dispersing solid particles smaller than 100 nm in diameter in traditional heat

transfer fluids. Solid Nanoparticles (Eastman et al 2001 and Xie et al 2002,

2002a) are added since they conduct heat better than liquid and they stay

suspended much longer than the larger particles. This provides better mileage,

fuel savings to consumers, fewer emissions and a cleaner environment for

vehicles, Efficiency of the nanofluids can be increased by using CNT (Carbon

Nanotubes) treated with nitric acid (Xie et al 2003).

1.5 REVIEWS ON NANOPARTICLES UNDER HIGH

PRESSURE

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 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 -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

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phase transition pressure are 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.

In 1975, Peppiatt and Sambles observed that melting point decrease

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

semiconductor like Cadmium Selenide (CdSe) amd 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 effect 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

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 nano crystals provide enhanced metastability

which allows structural and optical measurements in this regime. This make

possible to recover the dense high pressure phase at atmospheric 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

23

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

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

pressure (from -Fe2O3 to -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 take place at 7.5 GPa and

12.5 GPa for 6 and 80 nm respectively.

A reduction of transition pressure is also reported in TiO2

nanocrystals for the rutile to -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 around 26.5 GPa for

nanocrystalline CeO2, which is less than 32 GPa for bulk CeO2, Jiang et al

(2002) reported the onset and transition pressure of GaAs from I II

transition as 17 GPa and 20 GPa respectively, for both bulk and nanophase

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 nano

crystalline GaN which is greater than the bulk.

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

phase transformation in nanocrystals 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.

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1.6. SCOPE OF THE PRESENT WORK

Significant introduction to nanomaterials and the review of

nanoparticles under high pressure are elaborated in this chapter. In this

dissertation, preparation and characterization of some transition metals and

cerium doped SnO2 nanoparticles prepared by co-precipitation method are

discussed in detail. Essential characterization studies such as structural,

morphological, optical, electrical and high pressure studies were carried out

for all prepared nanoparticles. In electrical and dielectric studies, all the

prepared samples exhibit excellent properties. All prepared samples shows

decreasing trend of electrical conductivity with the addition of dopants. The

dielectric loss of all the prepared samples decreases with the increase in the

dopant level and frequency. This shows the capability of these materials to be

used in high frequency device applications. In high pressure studies, SnO2

nanoparticle while doping with Mn, Fe, Co, Zn has a significant property. It

corresponds well to the anomalous behavior in elastic constants under

pressure. These results suggest that there remains a short-range order and

local strain distributed throughout the sample, which results in the anomalous

properties. Structural stability was identified in all the samples under high

pressure presented in forthcoming chapters.