CHAPTER 1

GENERAL INTRODUCTION

1.1 Introduction

Nano, derived from the Greek word nanos meaning dwarf, indicates the

billionth Nanostructured materials, nanophase materials or nanoparticles are

defined as materials, which are made up of grains with grain sizes in the range 1-

100 nm. Technology in the current century requires the miniaturization of devices

with no compromise on quality and performance. Nanostructured materials are

attracting a great deal of attention because of their potential applications in areas

such as electronics, optics, catalysis, ceramics and magnetic data storage.'" The

distinct properties and the improved performances of nanomaterials are determined

by their sizes, surface structures and inter particle interactions. In the present

century, all countries are utilizing a major chunk of exchequer on nanotechnology.

Nano business alliance forecasts that the nanotech market will reach 225 billion

dollars by 2005 and the global market for nanotechnology related products and

sewices would reach one trillion dollars by 2015.' Based on these reports,

nanotechnology will decide the economy of the future world. This chapter opens

the portals of various application fields in nanotechnology especially in electronics

and telecommunications.

1.2 Birth of Nanoscience

The fruitfulness of nanotechnology is basically the production of

nanometer-sized materials with unique, physical, mechanical, electrical and optical

properties. Nanometer sized materials can be prepared from the bulk material by

means of milling or etching. This process is referred to as "top-down" process.

Nanometer sized materials can also be prepared by staking atoms with atoms or

molecules with molecules. This is called molecular engineering in which the

required number of molecules can be selected. This process is referred to as

"bottom-up" approach. Depending on the property, "bottom-up'' and "top-down"

methods have importance in present day.

According to authentic studies, meteorites and stable element like C60,

which exist from the formation of universe, were nanostructures'condensed from

solar nebu~a .~ Colloidal materials are supposed to be the first form of nano

materials. Earlier, dispersed systems of particles with grain sizes 1 to 100 nm were

designated as colloid^.^ The epoch making work conducted by Kubo in 1962 is

believed to be the preliminary work in nanoparticles. He predicted that a small

metal particle should exhibit physical properties quite different from the bulk. He

also showed this distinct behaviour in aluminium particles regarding their magnetic

After this, the efforts for the synthesis of nanometer size atom clusters

of metals and ceramics by means of gas condensation were carried out. Then

various physical and chemical methods were invented to prepare atom cluster in the

nanometer size regime, which have the potential to show various quantum size

effects. The possibilities to prepare materials by stacking atom by atom created a

revolution in the capability to engineer materials with controlled optical, electronic,

magnetic, mechanical and chemical properties"-14 for many future technological

applications.

Rarely has a scientific field created so much enthusiasm and expectation as

nanotechnology. At the beginning stage of nanotechnology a lot of attention was

focussed on synthetic nanostructured materials in anticipation of enhanced

properties in comparison with its conventional form.'5 This imagination became

successful and gave birth to zero-dimensional quantum well structures, one-

dimensional modulated, multilayered material and the three-dimensional analogue,

bulk nanophase material.I6 Research on nanomaterials literally exploded in mid

1980's in the United states." Nanotechnology is not a new field but rather a

confluence of many fields such as physics, chemical and electrical engineering,

mechanics, material science, chemistry and biology coming together at the

nanometer scale. Much hope exists for vast improvements in each of these areas

through developments in such fields as nanoelectronics, information technology,

nanomachines, molecular electronics, nanotables, micro electro mechanical system

(MEMS) and micro fluidity. Today research and development in nanostructured

materials is one of the most intensely pursued areas in science. The areas of

research now include medical science, defence, analytical chemistry, drug delivery,

bio encapsulation and electronic, magnetic, optical devices. Without

nanotechnology human life will come to a standstill but with it the scopes are

unimaginable. According to Business Communication Company (BCC) reports,

electronic, magnetic and optoelectronic applications for nanoparticles will account

for 74% of the 2005 market currently and for the foreseeable future, the most

important types of nanoparticles are simple oxides in particular silica, alumina and

titania as well as metallic nanoparticles used in established applications. K. Eric

Drexler is the renowned and responsible personality for the present day

nanotechnology revolution. He has numerous publications and has written lot of

articles about possibilities of nanotechnology in molecular manipulation. He

beautifully explained this idea in his published paper entitled "Molecular

Engineering: An Approach to the Development of General Capabilities for

Molecular ~ a n i ~ u l a t i o n " . ~ ~ Other books written by Eric Drexler that have

revolutionised nanotechnology are "Engines of Creation: The Coming Era of

~ a n o t e c h n o l o ~ ~ " ~ ~ and "Unbounding the Future: The Nanotechnology

evolution".^^ The real excitement for nanotechnology, however, is driven by the

integration of nanodevices and sensors into biological systems for diagnosis, drug

delivery and homeland security. Nanotechnology has thus evolved into a particular

interdisciplinary science.

1.3 Synthesis and Processing Methods

Numerous methods have been reported for the synthesis of nanoparticles

depending on the material of interest, size regime and application requirement. 21-27

Whatever be the method, the attraction of the method depends on its capability for

the reproduction of size and shape and the control over the average size. We can

prepare nano grains either by allowing the atom to fuse together untilthe desired

size is attained (construction) or by breaking a bulk material (destruction) to the

predetermined size. The building up method is chosen for the preparation of

nanomaterials rather than the breaking down process since, the former method

allows better control in the preparation of particles of desired size2'. The various

methods used for the preparation of nanoparticles are listed below.

1. Co-precipitation methods 5. Micro emulsion method

2. Gas evaporation technique 6 . Laser ablation.

3. Sputtering 7. Sol- gel method

4. Mechanochemical method

In recent years, attention is focussed on materials obtained by chemical

precipitation route. The chemical method is characterised by a complete and

homogeneous mixing of the solutions of the starting compounds to the molecular or

atomic level. In comparison with other conventional methods, this method requires

only low thermal treatment. Also, synthesis by this method yield small particle size,

high homogeneity and stoichiomefcy, which could not be achieved at high

temperatures. This method has been adopted to prepare high quality f e m t e ~ ~ ~ and

aluminates without high temperature treatment. Solvent extraction method was

carried out to prepare high purity femtes from chemically beneficiated iron ore

rejects." Chemical methods are used by several persons to exploit the low-grade

iron ores to prepare impurity-free fine particles of high surface area and hence

oxides of desired crystal structure. 31-33

In inert gas evaporation technique a metal is evaporated in high vacuum

chamber in the presence of an inert gas. The vaporised metal atoms collide with

inert gas atom to form clusters and are collected in a cold substrate. The pressure of

the inert gas decides the cluster size.

The sputtering method is used to fabricate multilayered nanocomposite

materials. The target material is bombarded with a beam of inert gas ions and the

sputtered atoms are collected on a temperature-controlled substrate. Sputtering

using magnetically confined plasmas is generally known as magnetron sputtering.

Sputtered nanocomposites consist of magnetic particles embedded in non-magnetic

matrix.

Mechanochemical method is a commercial technology to produce large

quantities of materials. The nanoparticles are obtained by repeated mechanical

deformation and alloying as the powder is subjected to vigorous shaking in a ball

mill. The oxidation of the fine particle during milling may be avoided by setting an

inert atmosphere.

Micro emulsion method is used for fabricating the particles of magnetic

oxides using reverse micelles. One of the key factors of this method is that the

surfactant molecules help in preventing unwanted particle growth and protect

particles from oxidation providing the passivation to the active nanoparticles. This

is an apt method to prepare nanomaterials with highly uniform morphologies.

Laser ablation technique uses high power pulsed laser to vaporise cluster

ions from surfaces. The wavelength of the laser should be adjusted with the

material. This is a general and versatile method virtually applicable to any material.

In the arc method, the metal atom to be evaporated serves as electrodes in the

vacuum chamber and generates a discharge between the electrodes. When arc

current is increased metal droplets in the form of smoke is produced. The small

metal particles are formed from the smoke particles.

A traditional method for the synthesis of nanoparticles faces the problem of

incapability of reproduction in nanometer dimensions. In addition, the nanoparticles

obtained usually have a strong tendency to aggregate, which makes the study of

their behaviour difficult. The sol-gel method is an accepted method to avoid these

difficulties. Nickel fenite / SiOz nanocomposites were synthesised by embedding

nickel ferrite in silica matrix through sol-gel method based on hydrolysis and

condensation of a silicon alkoxide and thermal treatment.34 Nanocomposites can

effectively retain the nature of the nanocrystals by dispersing them in diamagnetic

organic or inorganic matrices.35

1.4 Characterisation Techniques

Preparation of materials of desired quality and property has upmost

importance in the field of research, even though modem sophisticated and

technologically foolproof methods are available for the synthesis of materials.

Various characterization techniques are available to test the quality, chemical

composition, abundance of different materials, internal ordering of atoms, porosity

surface morphology, size of particles etc. The properties of a given material depend

on the size of the particle. Nanophase materials are artificially synthesized with

microstructures modulated in zero to three dimensions on length scales less than

100 nm. These new ultra fine materials have properties significantly different and

considerably improved, relative to those of their coarser grained counterparts. The

property changes result from their small grain sizes, because in small grains, large

percentage of their atoms are in grain boundaq environments and they interact

between neighbouring grains also. The size dependent property changes in nano-

structured materials have great technological importance. The modem world uses

the following methods for the characterization of nano-structured materials.

X-ray diffraction technique is the most common and efficient method for the

determination of structure and crystallainity and material identification. Structural

model of nano crystalline materials identified by XRD can be checked using

another fine structure analysis namely Extended X-ray Absorption Fine Structure

(EXAFS). XRD is an apt method to examine whether a resultant material has

amorphous or crystalline nature. Crystalline phases can be identified by just

comparing the d values obtained from XRD data with the fundamental data in Join!

Committee on Powder Dtffraction Standards (JCPDS).

1.4.2 TEM

Transmission Electron Microscopy (TEM) is an effective direct observation

method to see the atomic and molecular arrangement. TEM is an effective probe to

study the size and shape of nano particles. The crystallanity and size of the particles

can also be determined from TEM analysis. This is actually a supporting evidence

for XRD analysis. The sharp rings observed in the selected area electron diffraction

pattern can confirm the crystallainity of the sample. This method can also be used

to study the porosity of rnateria~s.'~ Characterisation of nano particles can also be

performed with the help of techniques like High Resolution Electron Microscopy

(HREM) and Selected Area Electron Diffraction (SAED)?' In TEM, the sample is

made sufficiently thin such that it transmits a mono energetic beam of electrons.

Rudimentary lattice fringes are formed by interference between the transmitted

beam and diffracted beam. In HREM, this principle is generalised by admitting

many diffracted beams, which leads to the formation of a two-dimensional image of

the atomic ~ t ruc ture .~~~rna l l angle X-ray scattering technique together with TEM

can be effectively employed to study the distribution of crystal sizes in nano

crystalline material^.'^

1.4.3 PAS

Positron Annihilation Spectroscopy (PAS) is an important technique to

measure the porosity of the nano phase materials. PAS is sensitive to small pores

ranging from single vacant lattice sites to larger voids.40 The porosity measurements

using BET (Braunauer - Emmett - Teller) nitrogen adsorption method probe only

pore structures open to the free surface of the sample, but can yield pore size

distributions, which are not available from PAS. This method can be used to

interrogate the micro pores present in the nano materials. Also this method gives us

the specific surface area of nano particles (surface area of 1 gm). BET results show

that as the temperature of calcinations increases the surface area decreases. This is

due to the higher crystallinity of the spinel and increase in particle size at higher

sintering temperature.41 BET is an ideal method to study the structural surface

properties.

1.4.4 FTIR

Fourier Transform Infrared Spectroscopy (FTIR) is an important tool for the

structural study. Vibrational spectroscopy is a characteristic feature for a material.

Vibrational frequencies of atoms or group of atoms are specific. So comparative

study will yield the structure of the material. For example, the frequency at 3450

cm-' can be attributed to the hydroxyl groups, which are extensively hydrogen

bounded. The vibrational band at 1640 cm-I is assigned to the bending vibrational

mode of the interlayer water molecules.42 The crystallinity in nano materials

develops on sintering the precursor. The surge in the crystallinity can be asserted by

the change in the vibrational frequency with the generation of new frequencies in

the FTIR spectrum. The presence of absorbed or adsorbed water in a given material

can be traced out with the help of FTIR.~' Being simpler and more accurate, the

FTIR spectroscopy is now widely used in nanotechnology for material

characterisation.

1.4.5 SEM

Scanning Electron Microscopy (SEM) is used to "see" the surface

morphology of materials. The atomic arrangements are governed by the inter

atomic forces and chemical bonds. The ratio of the number of atoms on the surface

to the total number of atoms in a nanoparticle (surface to volume ratio) is inversely

proportional to particle size. Also the surface energy increases with decreasing

particle size. So clearly a nano particle would prefer a shape, which would

minimize its surface energy.u In the nano world, since the properties of materials

depend on the size and shape of the particles, the morphology study has relevance.

SEM can be effectively used to test the amorphous or crystalline texture of nano

materials. This is also an effective tool to check the porosity and the presence of

physically absorbed and chemically bound water. In the case of nano materials the

amorphous raw powder reveals micrometer sized agglomerated particles with a

highly porous structure while the calcined products shows well faceted grains made

up of several tiny crystallites. This interesting structural change can be easily

visualised using SEM photograph.

1.4.6 AFM & STM

Methods like Atomic Force Microscopy (AFM) and Scanning Tunnelling

Microscopy (STM) are very much efficient for the study of nano materials. The

AFM utilises the presence of repulsive short range contact forces and long range

non contact forces for imaging surface topography. The forces between the atoms

of the tip and those in the sample cause the deflection of a micro fabricated

cantilever. The deflection produced in the cantilever depends strongly on the tip-to-

surface separation, and can be used to map the surface topography with atomic

resolution. The spring constant of the cantilever is much smaller than typical inter

atomic spring constants. Deflections of 0.01 nm can be measured by modem optical

deflection ~ ~ s t e m s . 4 ~

STM can provide atomic images of the surface of a sample. An almost

automatically sharp metallic tip is brought close enough to the sample surface, such

that there is a measurable tunnelling current at a convenient operating voltage. The

tip is scanned over the surface, while the current between it and surface is sensed. A

feedback circuit constantly moves the height of the tip so as to keep the current

constant. Since the current varies exponentially as the separation 'z' between the tip

and the surface, the action of feedback circuit is to keep '2' constant. The STM

image is actually a map, z (x, y); of the tip height 'z' as a h c t i o n of its lateral

position (x, y). The output is normally plotted as a pseudo three-dimensional graph.

This mode of operation is known as constant current mode. This STM technique

can also be operated in the constant height mode, in which the gap is kept constant

and the current monitored as a function of (x, y).46

Thermo Gravimetry (TG), Differential Thermal Analysis (DTA),

Differential Scanning Calorimetry (DSC), can be used to study the thermodynamic

stability of nan~~articles." These methods can effectively be used to identify the

phase transition temperature and also can be employed to assess the temperature at

which physissorbed water, interlayer water, structural water etc escapes from the

precurser.48 The crystallisation in nanomaterials occurs only at some elevated

temperatures during sintering. The crystallisation in a material is indicated by a

broad exothermic feature in the TG 1 DTA curve. This can be used as a supporting

evidence for XRD result regarding crystallinity.

The other relevant techniques used for the characterisation of nanoparticles

are EDX, UVI Vis spectral studies, MAS NMR, CBED, TPD etc.

Energy dispersive X-ray (EDX) analysis is used to study the elemental

composition. So this can be used to study the material stoichiometry at different

sintering temperatures. This method can also be used to detect contamination in

materials.

The optical properties of materials can be studied with the help of UV-Vis

spectra. In the case of nanomaterials the size dependent optical properties usually

manifest as colour changes. This colour change is due to the change in the co-

ordination state of ions. This type of migration can easily be understood by using

UV - VIS spectra?'

MAS Nuclear Magnetic Resonance (NMR) spectroscopy in an extremely

sensitive probe in identifying the co-ordination state of metals in metal containing

materials. Quadrapole moment as well as the magnetic nature of materials can be

examined with NMR spectra.50

Convergent Beam Electron Diffraction (CBED) technique has been used to

determine the degree of cation inversion i.e. the magnitude of the cation ordering

parameter in spinels. By this CBED technique the experimental data can be

obtained from small illuminated areas several tens to 100 nm in diameter. So this is

a very useful technique for intimating the extent of cation inversion in multi-phase

mineralogical specimen containing spinels."

Temperature Programmed Desorption (TPD) or Thermal Desorption

Spectroscopy or Temperature Programmed Reaction Spectroscopy (TPRS) involves

heating a sample while contained in a vacuum and simultaneously detecting the

individual gas in the vacuum by means of a mass analysis. As the temperature rises,

certain absorbed species will have enough energy to escape and will be detected as

a rise in pressure for a certain mass. There are two approaches to this thermal

evaporation technique.s2 One is by raising the temperature of the sample very

quickly and this is known as flash desorption and the second is by rising slowly the

temperature of the sample and is known as TPD. As the temperature rises and a

particular species is able to desort from the surface the pressure will rise. As the

temperature rises further the amount of the species on the surface will reduce

causing the pressure to drop again. This results in a peak in the pressure versus time

plot. The temperature of the peak maximum provides information on the binding

energy of the bound species.

1.5 Properties of Nanoparticles

A thorough knowledge in the properties of nanoparticles with particle size is

essential for the fundamental understanding of condensed matter and to use these

materials for a variety of applications. The manner in which the properties change,

when a bulk material formed from the assembly of clusters, gives an insight to

develop the theory to explain property change.

In nanocrystalline materials the physical properties are predominantly

controlled more by the grain boundary than by the grains?3s" The physical

properties of nanoshvctured materials differ considerably from that of bulk, since

the system size approaches quantum mechanical length scales. The size dependent

magnetic properties of nanoscale particles are a subject of interest in the present

decade.55 With increase in temperature MgFe204 a ferromagnetic material,

transforms to a paramagnetic state. A rise in annealing temperature increases the

Curie temperature and hence the coercivity. However the saturation magnetization

decreases with increase in heating time.s6 Nanometer sized magnetic data storage,

magnetic sensor and electronic devices not only use the charge of the electrons but

also its spin. Many properties of magnetic systems are determined by the

underlying electronic structure. Reducing the sizes of magnetic systems changes the

electronic properties by reducing the symmetry of the system and by introducing a

quantum confinement. Most types of size-induced changes that exhibited by the

physical and chemical properties can be attributed to the following three distinct

origins.

a. Quantum size effect.

b. Surface and interface effects.

c. Change in the lattice symmetry due to strain.

a. Quantum size effect

Ultrafine materials that contain grains or clusters below 100 nm, or layers of

filaments of that dimension are defined as nanostructured materials currently.57

Owing to small size of building blocks (particle, grain or phase) and high surface to

volume ratio these materials show distinct properties related to mechanical,

electrical, optical, electronic and magnetic aspects.'' The properties of

nanostmctured materials depend on features like quantum size effect, chemical

composition of the constituent phases, presence of grain boundaries, heterophase

interface or the free surface and interactions between the constituent domains. Most

of the properties shown by nanomaterials can be explained by the quantum size

effect. What is the optimum size a nanoparticle can have to show the properties of

the bulk material, and how the number of atoms in a nanophase affects in

determining the property of a material is still in discussion. If a metal particle,

initially having bulk properties is reduced to a size which contains a few hundreds

of atoms and then to a state which contains few atoms the density of states in the

valance and conduction bands decreases and electronic properties change

drastically and finally may result in the disappearance of conductivity and

magnetism.

metal 1 colloid cluste~

(a) (b)

molecule

(c)

Fig.l.l Band structure in (a) a bulk metal or colloid, (b) a large metal cluster, (c) a triatomic cluster

The quasi-continuous density of states is replaced by a discrete energy level

structure, with a level spacing larger than the characteristics thermal energy KBT. A

three-atom assembly introduces a larger band gap compared to the bulk structure.

Thus with size reduction a conducting material becomes a non-conductor. This is

referred to as size induced metal-insulator transition ( s I M I T ) . ~ ~ . ~ ~ This refers to the

transition caused by a geometrical limitation of extended states with a

corresponding De Broglie wavelength when the volume of metallic particles is

reduced to 20 nm from lpm initially. This effect was detected by measuring the

microwave absorption of small particles of solid or liquid dispersed in oil.

Size quantization effect may be the onset of the metal semiconductor

transition at the very end of metallic behaviour of such smaller particles. The size at

which the transition occurs depends on the type of the metal and the criterion

chosen for metallicity as well as the method of investigation. The reason for the

new electronic properties exhibited by nanoparticles due to quantum size effect may

be explained on the basis of quantum mechanics with Heisenberg's uncertainty

relation. The position and momentum of an electron cannot be determined

simultaneously with same accuracy. So the average energy is not determined by its

chemical origin but only by the dimension. Electrons localised as "particle in a box"

with zero dimensional quantum dots lose their freedom in all three dimensions,

leading to discrete energy states, as long as their energy is not great enough to

breakout of this confinement. Whenever it is in the magnitude of the electron

wavelength A /2 or even smaller, quantum effects govern the wave propagation of

the system. This effect is called the Quantum Size Effect (QSE).

6. Surface and interface effects

Surfacelinterface plays an important role in the properties of materials when

their characteristic size approaches nanoscale. Metal and semiconductor nano

particles arranged in a space or on a substrates are now used for the development of

a new class of plasmonic devices for integrated nanophotonic circuits. Depending

on the substrates on which the nanoparticles are deposited, the same material will

show different properties. For example nanoparticles when deposited on a dielectric

or semiconductor substrate and irradiated with an external light beam, induced

surface charges at the interface between the substrate and material will decide the

properties. As a result the local electric field acting on the nanparticles is modified

and the resonant extinction of the light is red-shifted with respect to the single

nanoparticle system.

c. Change in lattice symmetry due to strain

In nanoparticles with size reduction a significant increase in the lattice

parameter in usually observed. In the case of Ce02 nanoparticles this change in

explained in terms of an associated reduction in the valency of the ce4+ ions to ce3+

ions caused by an increasing molar fraction of oxygen vacancies. In a nanoparticle

system that naturally possesses an enormous surface area per unit volume, such

vacancies can enhance the ability of a volume of this material to store and release

oxygen. The change in lattice constant with particle size can easily be studied with

Raman spectra. In CeOz it is observed that the position of strong first order Raman

line at 464 cm-' in shifted to progressively lower energies as the particle size gets

smaller, and the peak gets progressively broader and asymmetric on the low energy

side. The change in line width is due to inhomogeneous strain broadening

associated with dispersion in particle size and by phonon confinement. In the case

of nanocrystal a change in the magnetic and electronic degrees of freedom was

observed, when a guest ion is incorporated on a host lattice. Vacancy formation due

to charge imbalance and lattice strain can self-limit inclusion of guest ions into a

host lattice. At nanoscales further complications arise from the propensity for the

ion to migrate to less strained surface sites, rather than incorporate as substitutional

ions in the crystal lattice.

1.5.1 Structural Properties

The physics and chemistry of solids reveal that most properties of solids

depend on the microstructure, i.e. the chemical composition, the atomic structure

and the size of the solid which builds it. In perfect crystalline solids the building

blocks are the unit cells and single crystal solid which have translational periodicity

with point group and space group symmetries. If a big single crystal is made into

powder form and if the powders are consolidated to form a bulk material, the

system consists of an assembly of smaller single crystals. The bulk form obtained in

this process is called a polycrystalline material. The translational periodicity is

retained in each small single crystals, but the same is not retained in adjacent small

single crystals. Thus there is a boundary or interface between these small single

crystals. These srnaIl crystals are called grains and the boundary of grains in called

as grain boundary. An assembly of grain will constitute a particle. Thus nanometer

sized building blocks are microstructurally heterogeneous, consisting of building

blocks and the regions between adjacent building blocks (grain boundaries). It is

this inherently heterogeneous structure on a nanometer scale that is crucial for many

of their properties which distinguish them from bulk materials. The grain boundary

density is directly related to the size of the grain, for a given volume. If the material

is made of nanometer sized grain, the density of grain boundary will be very high

and the number of atoms occupying the boundary will be almost equal to those

within the grain.

Fig.l.2 Arrangement of atoms in a nanometer sized grain.

Fig.l.3 Distribution pattern of grains in a particle.

If we assume the grains as spheres of diameter d and the width of the grain

boundary as w then the volume fraction of the nanostructured materials associated

with the boundaries can be estimated as

In the case of grains within 5 nm range, volume fraction of atoms in the

grain boundaries is about 50% and in the case of 10 nm size it is 30% and for 100

nm size it is 3%. So the properties striki'ngly depend on the size of the grains.

1.5.2 Opticalproperlies

The optical properties of metallic nanoparticles have been extensively

studied and recent trends in nanotechnology have brought this fascinating area to

the forefront of current scientific re~earch.~' This opened a new area of sub-

diffractional optics known as plasmonics. This branch is applicable where light is

supposed to play the role of information carrier rather than the electrons in

traditional nanoelectronics. Nanoparticles of metals and semiconductors arranged

on a substrate may be used to build a new variety of plasmonic devices for

integrated nanophotonic circuits. When these nanoparticles are irradiated with an

external beam, the surface charges induced modifies the local elechic field acting

on the nanoparticle and the resonance excitation of the light is red shifted with

respect to the single nanoparticle ~ ~ e c t r u m . ~ ~ ' ~ ' An interesting non-linear

phenomenon of optical bistability has been reported in semiconductor

nanoparticles. This property can be effectively use4 for the fabrication of

nanophotonic switching devices." Comparable variations have been noted in the

atomic structure when the size is reduced to a few inter atomic spacing. Due to this,

the change in colour is observed when the CdS crystal size is reduced to few

nanometer. In the case of nano gold, the colour is red instead of its parent glittering

yellow colour.

1.5.3 Magnetic properties

Magnetic properties of nanomaterials differ considerably from that of glassy

form. Measurements of the saturation magnetisation Ms of nanocrystalline iron

(crystal size 6 nm) revealed a reduction of Ms from 220 e .m .~ .~ - ' (a - Fe) to about -1 65 130 e.m.u.g . This suggests that atomic and magnetic structures are different in

glassy and nanocrystalline iron. A recent study in magnetic phase transitions in

nanocrystalline erbium66 shows that at 10-70 nm the three normally observed

magnetic transitions vanish and a new low-temperature transition to super

paramagnetic behaviour arises. On the other hand, for nanocrystalline erbium with

larger grain diameters, the normal magnetic transition reappears at different

temperatures, while the low temperature super paramagnetic behaviour is retained.

1.5.4 Thermal properties

Thermal properties such as specific heat, entropy and thermal expansion of

nanoparticles are strikingly different from their bulk form. It has been reported that

enhancement of specific heat (A Cp) occurs in nano crystalline palladium (6 nm) in

comparison with polycrystalline palladium.67 The enhancement is from 29% to 53%

when temperature varies from 150 K to 300 K In the case of nanocrystalline copper,

the reported enhancement is 9% to 11% corresponding to the temperature variation

150 K to 300 K. Since copper and palladium are dia and paramagnetic, the

enhanced Cp may be from vibrational and or configurational entropy effects.

The measured excess specific heat may be used to compute the excess

entropy of palladium nanocrystal relative to palladium single crystal. At 300 K the

excess entropy of the nanocrystalline material is about twice the entropy of melting

of palladium.68 An enhancement of coefficient of thermal expansion occurs in

nanocrystalline copper. A similar enhancement was also observed for

nanocrystalline palladium and Ti02. Total thermal expansion coefficient consists of

two parts. The crystalline component (an,) of a nanocrystalline material and the

boundary component (%$'. Systematically varying the crystal size and the

chemical composition, the thermal expansion of nanocrystalline materials can be

tuned to any predetermined value.

1.5.5 Electrical properties

Nanostructured systems are useful in tailoring electronic properties ,of

materials. It is obvious that nano structured materials will have an increasing impact

on electronics, since the modern world demands electronic devices of smaller

dimensions with higher functionality, increased memory density and higher speed.

This can only be achieved by the manipulation of the properties of materials by

systematic nanoscale variation of composition. Quantum effect devices or single

electron devices are of great potential utility for future electronic circuits. A

significant reduction in size of a device requires a considerable increase in the

operating speed. There are several technological impediments to these

developments, namely lack of the ultrafine precursors to manufacture these

components, poor dissipation of the tremendous amount of heat generated and poor

reliability. Ultra pure nanocrystalline starting materials can help the industry to

overcome these barriers with better thermal conductivity and longer lasting durable

interconnections.

1.6 Applications

1.6.1 Electronics

Nanostructured systems are widely used in electronic industry, for making

various electronic de~ices .~~." Quantum size effect devices or single electron

devices now modified the electron transport phenomena. In the case of computer,

the reduction in the size of a microprocessor developed a considerable increase in

the operating speed even though there are some curable impediments such as lack

of ultra-pure and ultra fine precursors for the manufacture of these components,

poor dissipation of the heat and poor reliability. There is a class of materials called

electrochromic materials in which an optical absorption band can be introduced or

an existing band can be altered by the passage of current or the application of an

electric field. Nanocrystalline oxide gels are used for this purpose. These devices

are used in public billboards and ticker boards to convey information. They can

change colour depending on the voltage applied. The resolution, brightness and

contrast depend on the tungstic acid gel grain size. The resolution of TV screen can

be increased by reducing the size of the phosphors, which make up the pixels.

Nanocrystalline zinc selenide, zinc sulphide, cadmium sulphide and lead telluride

are now used for improving the resolution of the monitors. Most of the portable

computers are now equipped with flat panel displays which use nanocrystalline

phosphors. In addition, much higher brightness and contrast could be expected from

those panel displays constructed out of nanomaterials due to their enhanced

electrical and magnetic properties.

Nanocrystalline spinel ferrites have high electrical resistivities and

consequently low eddy currents and dielectric losses. These properties find

applications in microwave devices, computer memories and magnetic recording.72

By doping, important changes in structural, electrical and magnetic properties can

be introduced7'. Humidity dependent conductivity in soft ferrites can be enhanced

by doping small percentage of alkali salts.74

1.6.2 Magnetism

Many of the modem technological devices such as electrical power

generator, transformers, electric motors and computers rely on magnetism and

magnetic materials75. Novel nanomagnetic materials have widespread appreciation

due to relationship between microstructural features like particle size, size

distribution, chemical inhomogeneities, crystalline defects, crystallographic texture

and magnetic properties.76.77 Soft magnetic nanocrystalline alloys have high

coersivity and low remnant magnetisation. Nanocrystalline grain size and degree of

coherence across interphase boundaries improve the remnant magnetization. Soft

magnetic materials can be used for data storage applications which are dependent

on the microstructure and geometry of the material^.^^'^^ Magnetic films are used in

a variety of applications such as recording media and heads, magneto-optical

storage and sensors. The responses of these types of films are influenced by factors

like grain size and distribution, presence of non-magnetic phases of the grain

boundaries, non-magnetic interlayer and magnetostriction.

The route to progress in memory technology is cost reduction and the

development of ever faster, more compact and less power consuming memory

systems, with greater storage capacity. A small basic storage cell and a system to

read and write are imperative. Now very compact memories based on scanning

tunnelling microscopy employs storage cells approaching the size of a single atom.

The magnetic strength of a material is measured in terms of coercivity and

saturation magnetization values. These values increase with decrease in grain size

because smaller grains have greater specific surface area. Magnets made up of

nanocrystalline yttrium-samarium-cobalt shows some specific magnetic properties,

which finds applications in quieter submarines, motors for ships and magnetic

resonance imaging in medical diagnosis.

Nanocrystalline soft femtes are used to make magnetic recording heads,

inductor and transformer cores, filter cores, magnetostrictive vibrators etc." Super

paramagnetic properties are unique features of magnetic nan~~art ic les .~ ' They are

of broad interest for potential applications like ferrofluid technology,

magnetocaloric refrigeration, as contrast agents in Magnetic Resonance Imaging

(MRI) and as drug carriers for magnetically guided drug delivery.82s83

1.6.3 Biology and medicine

Living organisms are built of cells that are about 10 pm size. The size of the

protein is about 5 nm, which is comparable with the size of the man made

nanoparticles. This allows to spy the cellular machinery without introducing too

much interferen~e.'~ The optical and magnetic properties of nanomaterials are most

used for biological applications.85386 Nanomaterials are suitable for biotagging.

Nanoparticles coated with antibodies can interact only with desired biological

target."." Photodynamic cancer therapy is used for the destruction of the cancer

cells by laser generated atomic oxygen, which is cytotoxic. A high quality special

dye, which is capable of producing atomic oxygen is absorbed by the affected cell.

Hence only the cancer cells are destroyed when exposed to laser radiation. Proteins

are the important part of a human cell. The basics of protein structure and their

functionalities are important for further progress in human well being. Gold

nanoparticles are widely used in immunohistochemistry to identify protein-protein

interaction. Modem pharmaceutical companies are now exploiting the quantum size

effect in nano crystals for tagging bio molecules or using bio conjugated gold

nanoparticles for labelling various cellular parts. Nanoceramic materials are used in

tissue engineering and orthopaedics. An intense research is going on in drug

delivery that are on formulations or dispersion containing components down to

nano sizes. Colloidal silver is widely used in anti microbial formulations and

dressings. The futuristic trend in medical field is to use nanomaterials as

multifunctional and controllable by external signals or by local environment, thus

turning them into nanodevices

1.7 Nano Muminates

Nanoceramic materials that are in the early stages of development already

show many processing and property advantages over conventional coarse grained

a~tematives.'~ In recent years there has been increasing interest in the synthesis of

nanocrystalline metal oxide^.^^.^^ Spinel ceramics have been used as high

temperature materials in various applications because of their chemical stability at

high temperature and good physical, electrical and thermal properties combined

with high melting point and mechanical strength. The most typical material in the

family of chemical compounds with spinel structure is M a 2 0 4 spinel, which has

many applications. In ideal case, MgA1204 has an f.c.c structure with magnesium

ions occupying eight tetrahedral sites and the aluminium ions in 16 octahedral sites,

having a lattice parameter 0.808 nm. At high temperature a disorder from this

occurs and this study is currently Magnesium aluminate

has radiation resistance together with good insulating properties. These properties

are considered in using them for making dielectric windows of fusion reactors or

electrical insulators in radiation environments.9899 Spinel refractory bricks are used

in steel industry as fumace lining because they are very resistant to corrosions by

slag. '"

Spinel type compounds are used in electrochemical devices and batteries as

energy sources for portable telecommunication devices. 101,102 Spinels are very

promising research objects in material sciences owing to their applications in solar

cells and displays. '03*'" MgA1204 is used in a wide range of applications such as

catalysis and sensors. 105,106 For many of its application a large surface area is

required.lo7 Nano MgA1204 having a large surface area fulfils this requirement.

Besides the general uses as a dielectric and optical material, spinel has received an

increasing attention recently for use as a superior substrate for epitaxial silicon -

integrated electronic devices. 108,109 Nowadays MgA1204 is highly applicable in

metallurgical, radiotechnical and chemical industries.'l0

Strontium aluminate is usually used as a host material for EU~ ' because of

their long after glow properties."' Stoichiometric SrA1204 has cubic spinel

structure. 112.113 Usually different host materials will give different colours for the

same dopant."4 But now SrA1204 with slight modification in physico-chemical

parameters is able to produce variety of colours ranging from blue to red for the

same dopant. The emissions from SrA1204 are strong enough to use them in

fluorescent lamps and plasma displays. The Sr0-A1203 system actually consists of

four well-known phosphor hosts namely SrA1204, SrAl12019, Sr2AlsOll and

Sr4A114025. 115,116 Owing to this unique structure, strontium aluminate doped with

E U ~ + have very high quantum efficiency, long persistence and better stability than

the other alkaline earth aluminates. The efficiency of SrA1204 crystal depends on

thermal processing parameters such as heating rates, time of constant heating,

subsequent cooling rates and the role of inert gases. Even though the chemical

composition of the starting materials was optimised, sometimes the phosphor may

lose its initial brightness, decay time, and other decay characteristics during the

process of high temperature treatment."' Good quality SrA1204 has been developed

at lower calcination temperature. Photoluminescent materials should have the

properties of storing light energy and glowing slowly at dark place. When

compared with other materials SrA1204 phosphor is chemically stable and shows

spectacular photo resistance, which has excellent properties such as high brightness,

free from radiation, safety, long duration, which resulted in its wide applications in

many fields. It may be used as luminous paints in different places, such as high

ways, railways, airports e t ~ . " ~ When added to transport glaze luminous glaze

develops and is used to coat the surface of ceramic product^."^ This can also be

used in textile field, in the dial plates of night watch, automobile, ship and other

i n s t ~ m e n t s . ' ~ ~

The grain size of phosphorous powders prepared by solid-state reaction is of

micrometer size. Nanoparticles of phosphorous can be obtained by grinding the

larger phosphor particles. But this process will introduce additional defects and

greatly reduce luminescence efficiency.121 For the preparation of nanophosphorous

materials several methods are available among which ~ o - ~ r e c i ~ i t a t i o n ' ~ ~ and

combustion synthesis are prominent. Phosphor nanoparticles are potential

construction materials in detecting damage in bridges or high buildings, as well as

in display and decorative materials. For many luminescent materials, the

luminescence properties are grain size dependent. When the grain size reaches

nanometer level the luminescent materials show the property such as the blue shift

of excitation and emission spectra.'23

One of the other members in the spinel aluminate family here studied is

Nickel Aluminate. NiA1204 belongs to the normal spinel family and is a transition

metal spinel with aluminium in octahedral sites and nickel in tetrahedral sites.'"

Spinels are used as catalyst supports since they are inert towards acids and alkalies, 2 -1 125 and possess high melting point and surface area in the range 10-100 m g . High

surface area of NiA1204 is of much importance for catalystic purposes. Transition

metal spinels are used in sensor technology also.'26 Since the surface area of

nanoparticles are high compared to that of bulk materials, the synthesis of nano

sized nickel aluminate particles will be worthwhile to study. As a result of high

surface area nanocrystalline NiA1204 display improved properties such as lowered

sintering temperature, increased hardness, stability, diffusion and du~tility.'~'

Preparation condition, calcinations temperature and time have significant impact on

the final structure and texture of nickel aluminate. So the chemical co-precipitation

method was employed to produce fine quality NiA1204 at low calcinations

temperature. Oxides with mixed electronic and oxygen ionic conductivities have

been widely studied to use it as components in high temperature fuel cells. NiA1204

is used as anodes for Internal Reforming Solid Oxide Fuel Cell (IR-SOFC).'~'

As an electrode material, the study of physical properties especially

electrical conductivity has importance. NiA1204 can transform from normal spinel

form ( N ~ ~ + ) [ A I ~ ~ + ] o ~ to an inverse spinel (A13+)[~i2+A13+]04. This movement can be

studied in terms of an inversion parameter I, which is the fraction of tetrahedral

sites occupied by A13+ ions. This efficiency manifests the lattice constant. The

lattice constant decreases as the disorder parameter increases.

1.8 Present work

The present work consists of synthesis and study of different properties of

three technologically important materials MgA1204, SrA1204, and NiA1204. These

spinel ceramics were used as high temperature materials in various applications

because of their chemical stability at high temperature and good physical and

chemical properties combined with high melting point and mechanical strength.

The effects of finite size on the vibrational, structural and electrical properties of

nanoparticles of these materials were investigated in this study. The size and

structure of particles were determined at room temperature using X-ray diffraction.

The vibrational properties were studied using FTIR. The temperature at which

crystallisation starts were identified using TG/DTA study. The surface morphology

of systems were identified by SEM. Also an investigation on the change in

dielectric properties with temperature and frequency of the applied ac signal was

canied out. The electrical properties of the system at microwave frequency were

also systematically investigated.

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