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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