1

Chapter 1

Introduction

2

Chapter 1

Table of Contents

Chapter 1: Introduction 1-36

1.1 Nanomaterials 3

1.2 Nanoscience and nanotechnology 5

1.3 Physics of nanoparticles 5

1.4 Applications of nanomaterials 7

1.5 Silicates 10

1.5.1 Structural unit of CaSiO3 13

1.6 Types of silicates 14

1.7 Silicate chemistry 16

1.7.1 Silicon Vs Carbon 16

1.7.2 Si-O bond 17

1.8 Why the selection of Calcium silicate 17

1.8.1 Physical properties of CaSiO3 18

1.8.2 Research on CaSiO3 18

1.9 Nanophosphors 21

1.10 Luminescence 22

1.10.1 Physical aspect of photoluminescence 23

1.10.2 Excitation of photoluminescence 25

1.10.3 Radiative and non-radiative ecombination 28

1.11 Objectives and scope of present work 29

3

1.1 Nanomaterials

In the recent scenario, nanomaterials are one of the prominent areas

for research in material science and related aspects. A very small

particles, with dimensions of about 1 to 100 nanometres consisting of

about 101 to 104 atoms, which are too large to be considered as

molecules and too small for the bulk material, which belong to a new

class of materials called nanoparticles.

In the early 1850’s Faraday quantitatively studied the nanoparticles

of gold in a solution phase which he referred as pseudo solution and

these are nothing but the modern day colloids. A study on the

characteristics of such colloids was made by Albert Einstein and John

Tyndall in 1900’s. However it was Richard Feynman, a famous

physicist through whom the widespread interest on nanaomaterials

was developed almost 5 decades ago. Richard Feynman in his classic

lecture to the American Physical Society titled “There’s Plenty of Room

at the Bottom – An invitation to Enter a New Field of Physics” [1]

detailed the indication of miniaturisation of devices in other words

manipulation and controlling things on smaller scale)

Research on small particles was initiated in 1970s with experimental

and theoretical calculations of their atomic and electronic structure.

The synthesis of materials by consolidation of small clusters was first

suggested in 1980s by Herbert Gleiter and was applied initially to

metals and then to nanophase ceramics. After the advent of beams of

atom clusters with selected sizes in the 1980s, the physics and

4

chemistry of confined ensembles explored and there had been a rapid

expansion of research on isolated atom clusters. In 1990s a variety of

carbon based clusters called fullerenes and carbon nanotubes have

been synthesized. Recently, however, the interest in the creation of an

entirely new class of materials via nano structuring arose from the

realization that by controlling the sizes of atomic ensembles in the

range of 1-100 nrn, one could alter and prescribe the properties of the

assembled new materials [2]. In the last two decades, there has been a

tremendous increase in industrial and academic interest of

nanostructured materials. Scientists have acquired the ability to

produce different types of nanostructured materials and control their

properties. These materials might be influencing the day-to-day life of

human beings in many ways in the future. These developments seem

to bring Feynman's dream of nanotechnology closer to reality.

when mixed with other materials the independent nanoparticles

demonstrate fine properties and the compounds thus formed when

coupled with nanotechnology are stronger, lighter and more durable

[3].These are designed at the molecular level to take advantage of their

small size and properties which are generally not seen in their bulk

counterparts. The properties of nanomaterials are significantly affected

by their structural or compositional modulations, their spatial

confinement and their interfaces or by a combination thereof. In

general spatial confinement affects any property, as the size of the

atomic ensemble becomes equal to or smaller than critical length

scales for the mechanism that is responsible for that property. The

5

uniqueness of nanoparticles lies in its large surface to volume ratio,

since nearly all the atoms occupy surface positions.

The dependence of the specific properties of clusters on their

geometrical dimension is termed as quantum size effect and this effect

originates from two interrelated causes. They are boundary scattering

effects resulted when effective wavelength of excitations is comparable

with the cluster size and the other is the smaller the sample size, so

that the energy gap for excitations is comparable to the thermal

energy, to the magnetic interactions or to the spectral resolution. in

comparison to the bulk lattice the nanoparticles have an increased

band gap and discrete exited electronic states and the wave functions

of electrons are confined to the particle volume in small particles.

Thus, when the size of the particle becomes equal to or smaller than

de-Broglie wavelength the confinement increases the energy which

gives rise to electron-hole pair and shifts the absorption spectra

towards shorter wavelengths, which is the so-called blue shift in

absorption spectra due to quantum size effect.

The kinetic enhancement of luminescence quantum yield and an

increase of band gap [4] are caused in silicon nanoparticles due to

quantum confinement but luminescence increases by many orders of

magnitude with respect to indirect gap bulk silicon in individual and

porous Si nanoparticles. Another interesting property of a nanoparticle

can be viewed in the melting point of a cluster. The melting point of

gold in the bulk form is about ~1300 K and is reduced to ~700 K in a

cluster of about 2.5nm diameter [5].

6

1.2 Nanoscience and nanotechnology

Being an emerging area in science nanoscience, deals with the

structures and study of fundamental principlesof molecules with one

dimension ranging nearly 1 and 100 nanometers. Another important

aspect of this science is that it unites the science of living-world and

the science of ‘non-living’ world with the unlimited potential to develop

superior devices that are not available in the present time [6].

nanotechnology is based on the fact that material properties are size

dependent at nanoscale and is a result of tangible nanoscience that

describes matter at quantum level. It deals with the designing,

production, and characterization of nanostructures, devices and

entities which are smaller, faster, lighter, efficient, and cost effective

and eco friendly. More precisely, it is a field of science that controls

individual atoms and molecules in creating devices that are thousands

of times smaller than current day products and having more efficiency

and low cost. In this context that nanoscale silicate devices are being

given increased attention as they have novel electrical properties that

can be utilized for many of the practical applications. The emergent

field of nanophotonics deals more particularly with the interaction of

optical fields with matter at nano regime. It creates a technological

impact which perhaps cannot match up to any other technological

developments that have taken place till date as it deals with every

aspect of human life that ranges from building novel materials to

medicine.

7

1.3 Physics of Nanoparticles

The extremely small size of nanomaterials has two important effects:

First is the size effect- the continuous energy levels (density of states) of

bulk matter are replaced by set of discrete energy levels which is shown

in the Fig 1.1 as Quantum size effect. Second is the surface or interface

induced effects this is due to the fact that, as the size decreases, the

surface to volume ratio increases massively due to this the atoms

having different chemical environment will be on the surface than in

bulk. The size effect mainly modifies the physical properties of

nanoparticles whereas the surface effect induces a lot of modification

in chemical, optical and

Fig 1.1. The quantum size effect:

Available energy levels with different state of matter, such as molecular

clusters, nanoparticles and bulk mechanical properties. This is

evidenced in the experimental investigations on properties of

thermodynamics such as thermal conductivity, specific heat, melting

point of metals, semiconductors nanoparticles and vapour pressure [7-

11].

HOMO

LUMO

Ener

gy

Band Gap

Conduction Band

Valence Band

NanoparticlesMolecules Bulk

8

Although the semiconductor nanoparticles or quantum dots (QD) are

said to be having a zero dimension, yet for electrons in the QD this

dimension is still an infinite 3-dimensional well. The potential energy is

zero in the well but it is infinite at the wall (boundary) of dots. An

electron-hole pair can be created in a quantum dot either by photo-

excitation or by charge injection processes (by electric field). Then the

minimum energy required to create a pair of electron-hole in a

quantum dot that has several contributions and these are the bulk

energy gap (Eg), charge carrier confinement energy (Econf) and coulomb

interaction energy (Ecoul). By combining all three terms, one can

estimate the size dependent energy gap of quantum dot semiconductor

which is given by

where is the disparity in band gaps of nanoparticles

and that of bulk , ‘h’ is the Planck’s constant, is the

diameter of the particle and and are effective masses of

electrons and holes respectively [12]. The metal nanoparticles also

show size dependent optical properties called surface plasma

resonance. This effect is observed due to collective oscillations of

conduction electrons and the effect is more pronounced in the case of

noble metal nanoparticles such as silver and gold [13-14]. Upon light

excitation the collective oscillations couples with electromagnetic

radiation which produces large enhancement of electromagnetic field in

9

the metal nanoparticles. Then the effective interaction between this

internal field and incident light leads scattering and absorption of light.

The radius of nanoparticle and the surface to volume ratio are

inversely proportional. For example, in case of a cluster containing 13

atoms the ratio of surface to volume is close to 92.3% [15]. The

variation of theoretically predicted surface to volume ratio as a function

of particle radius is plotted in Fig.1.2. It is evident from this figure that

the surface to volume ratio increases rapidly with decreasing size for

the cluster below 3 nm.

Fig.1.2. Surface/volume ratio as a function of the particle size. [Image:

Chem. Rev.105, 1025 (2005)]

The surface states are unsaturated atoms that are highly reactive

and hence play a significant role in controlling the fundamental

properties of structural phase transition via light emission to solubility.

Electronically, these states create a set of discrete energy levels in the

energy gap and for smaller particle size these can substantially mix up

with intrinsic discrete states of nanoparticles.

10

1.4 Applications of Nanomaterials

The vast range of technological and industrial applications of nano

particles is largely a direct result of their diversity of structure on

microscopic and mesoscopic scales. The industry has long been used

in fine particles in dyes, pigments, adhesives and catalysts. As the

reduction of the grain size to nanometer-dimensions provides increased

strength and hardness, super tough and superstrong ceramics can be

synthesized [16]. These superceramics can be used in various

application of engineering such as in aerospace and automotive

components and high efficiency gas turbines. Being made into wear-

resistant coatings or pressed, the nanocrystalline materials are

sintered to make rigid objects, the application of which includes cutting

tools and fine drill bits. The nanorystalline ceramics are used in optical

filtration technology since their optical transparency can be controlled

by controlling the porosity and grain size.

The requirements for high energy laser mirrors gives rise to demand

for optics having surface with roughness of 0.1 nm in order to

minimise X-ray optics, scattering in laser gyroscopic optics and laser

induced damage. Due to an increase in the rate of diffusion in

nanocrystalline metals and ceramics there is a considerable reduction

in the temperature during the occurrence of sintering [16]. This

enhanced diffusion is useful in making the fuel cells operate at lower

temperature and to make gas sensors. The enhanced diffusivity is used

to make gas sensors and fuel cells capable of operation at much lower

temperatures [17]. The magnetic and electrical properties of

11

nanocrystalline materials lead way to their widespread application in

industries. The occurrence of giant magneto resistance (GMR) is now

present even in equiaxed nanocrystalline materials. Hence for the

reading heads of the next generation of information systems

nanocrystalline materials exhibiting large GMR are promising

candidates. The exceptional combination of soft magnetic property is

handy in the production of saturable reactors high frequency

transformers, magnetic heads and tape-wound cores for common mode

chokes, nanocomposites have another useful property of

magnetocaloric effect, which is used for magnetic refrigeration,

replacing compressed gas containing harmful chlorofluro carbons. Fine

particles of latex, which is an organic polymer, serve as a vehicle for

most of biological and chemical manipulations in the field of

pharmaceutics. Aerosols, which are fine dispersions of a solid material

into a gas, are used in agriculture, forestry, military technologies and

medicine.

Nanoparticulate thin films possessing desirable electronic properties

are of great interest in microelectronics. To meet the increasing

demand for improved performance in storing and processing data,

researchers are working harder than ever on techniques and

equipments to shrink silicon circuitry. Chipmakers are entering in to

the field where transistors will be too tiny to print on silicon. Instead,

they will be grown in the material as clusters of atoms [18, 19].

In nanophase materials, due to a large number of surface atoms with

dangling bonds on the nanoparticle surface, they are far more reactive

12

than any other samples of coarser grained materials and act as very

good catalysts. Compared to the conventionally produced metals the

nanophase Cu and Pd, assembled from clusters showed an increase in

the mechanical properties such as hardness and yield-strength up to

500% greater [17]. The increased strength is due to the difficulty of

moving dislocation in the spatially confined grains of nanophase

materials. But in the case of ceramics, due to significant porosity and

ultrafine grain sizes, the mechanical property behaviour is different

from metallic property [20, 21]. Coarser grained ceramics are brittle

while nanophase ceramics are found to be ductile due to the grain

boundary sliding and diffusion process in nanomaterials. Similar

behaviour can be observed in the microhardness of nanoparticles of

metals and ceramics. In semiconductor nanocrystals or quantum dots,

the spatial distribution of excited electron hole pairs is confined within

a small volume resulting in the enhanced non-linear optical properties

[22-24].

The quantum confinement of carriers converts the density of states

to a set of discrete quantum levels, which is the fundamental

requirement of semiconductor lasers. When the nanocrystal is very

small, the transport of single electron affects the electronic property,

giving the possibility of producing mono electron devices [25].

Nanostructured porous Si has been found to give visible

photoluminiscence which can be correlated with its electronic

properties, possibly leading to a new approach to optoelectronic

devices. There is a significant amount of concentration on nano devices

13

that are based on GaAs due to their high digital speed in processing

with the additional advantages over the silicon that has greater

radiation resistance and high temperature tolerance. A bridge-like

Josephson junction having a bridge length of the order of the

coherence length of superconducting electrons, which is about a few

nanometers, offers an entirely different approach to the development of

high-speed digital circuits.

Semiconductor nanoparticles hold the potential for making more

efficient solar cells. Another dream of scientists in this area concerns

with the use of quantum confined semiconductor systems for spatial

light modulations, and optical transitions, other devices depending on

non-linear optical properties [26, 27]. Semiconductor materials

composed of layers of different phases/compositions are particularly

interesting if the layer thickness is less than the mean free path of the

electrons, providing an ideal system for quantum well structure. For

the future using micro electro-mechanical systems Nanotech organic

films will be used as the data storage medium and the information

thus stored will be in inexpensive films in the form of cluster of

molecules.

Although much of the work being done at present is in the

development of nano- materials, there are indeed some of

nanostructured products and consumer goods currently available in

the market. Some industries are producing a tyre-sealant made of

nanoparticles, which help the tyres to run cooler and safer and allow it

lighter and increase the fuel efficiency of vehicles. Textile companies

14

are producing wrinkle free and stain resistant cotton fabrics and khaki

jeans by adding 'nano whiskers' to ordinary cotton through a chemical

process. Another application of nanoparticles is in pharmaceuticals.

Biotech companies are building fullerenes into molecules help to

deactivate HIV molecules, blow up cancer cells and repair neurological

damage in a nerve, it is called Lon Gehrig's disease. 21st century is

expected to be the century of nanotechnology and more efficient and

long lasting materials made of nanoparticles will come into our daily

use.

1.5 Silicates

There is a dramatic expansion in the range of crystallographically

defined pore size from micropore to the mesoporous regime due to the

invention of mesoporous silica materials [28]. Amongst all the minerals

silicates are the most interesting, complicated and largest class of

minerals [29]. Up to 90% of the Earth's crusts are made up of silicates

and 30% of all minerals are silicates. Anionic group with a negative

four charge (-4), can be linked to each other in different modes and

forms a single unit, double units, chains, sheets, rings and framework

structure as shown in Fig.1.3 basic unit of silicates being tetrahedron-

shaped. The silicates affluent chemistry, physics and materials content

including Geolites [30], mesoporous materials [31], and inorganic-

organic composites [32] are endowed by the richness in crystal

structures. Due to their low cost and richest resources the exploration

of new type of nano structures of silicates with unexpected properties

seems technologically and economically important. The economic

15

implications and environmental concerns have motivated and created

an interest in the synthesis of -Ca2SiO4 to be used as a cementatious

material. (Many positive aspects, such as energy, raw material savings

and the possible higher durability are presented by the partial or total

replacement of conventional Portland cement by Ca2SiO4 [33-35].

Fig.1.3.Structure of CaSiO3 [36]

The development of nano science and nanotechnology requires

precise control over crystal structure, compositions, sized and

dimensionality of materials in nano scale [37-39]. Nano structures have

been recognized as an important physical systems in view of the

remarkable alteration of the bulk properties such as structural,

magnetic optical, dielectric, thermal, etc. due to surface and quantum

confinement effects. The potential applications of these quantum-

confined atoms in the realm of nanotechnology field are expected to

dominate the materials development. The quantum-confined atoms

provide a novel way to modulate and control the luminescent

16

properties of activators, which would generate a new class of

phosphors. Rapidly growing field of contemporary scientific interest is

based on the materials with nanosized structures and these materials

with nanostructure are intermediate between the microsized entities

and classical molecular scale. Because of their unique optical,

electronic, and magnetic features One dimensional nanoscale building

blocks such as nanotubes [40, 41], nano wires [42], nanobelts [43] and

nanorods [44] have striking interest. The manipulation of material

properties of one-dimensional, 2-dimensional, 3-dimensional

confinement will revolutionize the luminescence devices area with huge

advances in digital memory storage and optical accumulated radiation

damage at the scale of 1,00,000 years. It became an effective method in

archaeology [45].

Inorganic luminescent materials (β-CaSiO3) have practical

applications in many devices involving the artificial production of light,

e.g. cathode ray tubes, flat panel screens and field emission displays

[46, 47]. Recently, noteworthy efforts have been devoted to research on

modern materials to be usage for white light- emitting-diodes and

optical storage. White LEDs sustain great potential for application in

flat-panel displays due to fast progress in material design and device

fabrication [48]. The prospective of industrialized world largely depends

on the entity of sufficient energy sources.

Further, the interest in luminescent materials showing reversible

photo induced spectral changes at room temperature has significantly

proved that the combination of such phenomena with near field optical

17

spectroscopy is very promising when applied to high density optical

storage with nanometer resolution [49]. The application of

nanotechnology in recent years has lead to the development of

materials with porous architecture and high surface area, it includes

catalysis and absorption science [50-53], molecular sieves,

bioceramics, bioactivity, hybrid optics and many more are the areas of

interest. Technological progress serves as a motive for the need of such

materials. Nano materials are useful in protecting our environment.

The nanomaterials have significant relevance to our society including

energy conversion, emission monitoring and acting as remedies for

nuclear waste disposal, atmospheric and water pollution [54].

1.5.1 The Main Structural Unit

Fig.1.4.Structural unit of CaSiO3

As shown in Fig.1.4 the silicates contain one silicon atom and four

oxygen atoms with tetrahedral structural unit. There is a relative

stabilisation in size of [SiO4]4- tetrahedral, when Si-O bond length

varies from 0.161 to 0.164 nm at ambient conditions. The chemical

resistance and thermal stability of the majority of silicate compounds

18

are produced due to the relatively high strength of Si-O bonds. The

tetrahedral unit can be polymerized i.e., linked to each other through

the oxygen atoms. They are able to form polymers by means of linkage

with one, two, three or four neighbouring tetrahedra, forming siloxane

Si-O-Si bonds. Other ions can be located in the silicate lattices such as

to Lithium (Li+), Sodium (Na+), Potassium (K+), Calcium (Ca2+), Zinc

(Zn2+), Beryllium (Be+), Magnesium (Mg2+), Boron (B3+), Aluminum

(Al3+), Beryllium (Be2+) etc. as well as ions of Titanium, Manganese, and

Iron in various oxidation states. Some cations such as Boron,

Aluminum, Berryllium are able to isomorphically substitute silicon

atoms in the silica-oxygen tetrahedra. However, most of them are

located out of the anionic framework and play the role of "charge

balanced cations" which are usually six-coordinate. These tetrahedra

can then share one or more vertices to form an enormous variety of

polymeric structures. By contrast, carbon chemistry is hardly

characterized by polymeric oxyanions. Indeed, the only well-

characterized carbon oxyanion is carbonate (CO32-); even

orthocarbonate (CO44-) has not been isolated, though certain of its

esters exist, and polycarbon oxyanions do not exist at all. Presumably

the relative weakness of the C-O bond results both from the small size

of the C atom and the competing stability of the C=O double bond [55].

1.6 Types of Silicates

The compounds containing [SiO4]4- anions can be termed as

“Silicates”. However, the silicon atoms in silicates may exist with higher

coordination numbers than four, for example six as in the case of

19

Table 1.1Different types of silicate structures

Silicate structures

Silicate

type

Unit

Structure

Mineral

Example

Connectivity Schematic

Orthosilicate SiO44- Zircon

Zr[SiO4]

Q0

Pyrosilicate Si2O76- Thortveitite

Sc[SiO7]

Q1

Cyclosilicate, discreet cyclic units

Trimer Si3O96- Bentoite

BaTi[Si3O9]

(Q2)3

Polysilicates, infinite chains

Pyroxenes (SiO32-)n Wollastonite

Ca[SiO3]

(Q2)n

Phyllosilicates, infinite sheets

(Si4O102-)n Talc

Mg[Si4O10](OH)2

(Q3)n

Tectosilicates, unbound frameworks

(SiO2)n Zeolites,

Quartz

(Q4)n

20

of magma. Some silicates like gneisses are formed in metamorphic

rocks. Because the minerals such as Kaolinite (Al2Si2O5(OH)4) or

Montmorillonite were formed due to the weathering of primary rocks

they are exogenous. The existence of Natural silicates is in both

amorphous and crystalline states. Precious opal is one of the typical

examples of silicate minerals, containing amorphous which consists of

silica particles and silicic acid xerogel as a binder. The products such

as pottery were the first silicate products which were obtained down

the civilization. According to archeologists the age of the earliest

fragments of man–made burnt clay is about 15 century BC but in 5000

BC the first industrial pottery has been produced in Egypt. Ceramics

are used in manufacturing durable containers, tools and even a roof.

Glass making was started by about third century BC. The silicate

industry is successfully growing up till date. Since silicates are the

most common raw material in nature, it is not surprising that silicate

products are inherently woven into human life. A huge number of man-

made silicates are existing nowadays.

The precursor used in the sol-gel processing are the inorganic

binders like catalysts made of synthetic zeolites, organosilicate

compounds such as tetraethyl orthosilicate (TEOS) and cement and

water glasses. The flexibility of the Si-O-Si linkage, in combination with

the different degrees of connectivity between the tetrahedral building

blocks, leads to a wide variety of silicate structures that can be formed

are shown in Table 1.1. The connectivity of these building blocks can

be described with “Q” notation where a superscript defines the number

21

of nearest neighbour silicon atoms attached to the tetrahedral corners.

In Table 1.1 Q0 represents discreet tetrahedra where the silicon atoms

are not bound to any other silicon atoms over an oxygen bridge. Q1

represents where a tetrahedron shares only one apical oxygen with

another tetrahedron, such as for either a dimer or the end-member of a

chain. Q2 represents tetrahedra that are arranged in chains or in

discreet cyclic units, as each tetrahedron is bound to two other

tetrahedra over oxygen bridges. Q3 connectivity is found in branched

chains and sheet structures, while Q4 is found in three-dimensional

frameworks such as quartz. Silicate structures can accommodate a

wide variety of cation to balance the charge of the fundamental

structural unit, as well as a possible substitution of silicon atoms in

the structure (for example, by aluminium to form aluminosilicates. The

Pillared clays are silicates that find applications based upon their

porosity due to cavities between the layers that make up the structure.

These clays are able to selectively intercalate species as the interlayer

spacing is uniform. However, the silicates most commonly used due to

their porous structures are zeolites, which are produced in millions of

tons per annum.

1.7 Silicate Chemistry

1.7.1 Silicon Vs. Carbon

The silicon chemistry cannot be understood by using carbon

chemistry although Si and C contrasts markedly, and fundamentally

being adjacent to each other in Group IV of the Periodic table and the

reason for this is the differences between first and second row

22

elements, the atoms in the second row are greater in size.

Electronegative of silicon being significantly less than oxygen, the Si–O

bond is intermediate in character between ionic and covalent [56].

Pauling (1929) put forward that the Si–O bond is approximately 50%

covalent and 50% ionic in character; there has been an ongoing debate,

whether the bond is predominantly ionic or covalent. Nevertheless, as

ionic bonding is non-directional, the partially ionic character of the

bond allows for more variation of bond angles for Si–O–Si siloxane

linkages with a soft bending potential of the oxygen atom. When the

calculated bending potential for Si–O–Si is compared with that of Si–S–

Si has been found that not only is less energy required to distort the

geometry from the minimum energy at the oxygen centre, but also the

potential at the minimum is much softer. Although Si=Si double bonds

have been synthesized, they remain largely a laboratory tour de force

[57], such compounds are highly reactive, they must be kinetically

stabilized by such subterfuges as bulky side chains to inhibit auto

decomposition in addition, despite many attempts, no Si=O double

bonds or delocalized bonds have been synthesized except fleetingly, in

stark contrast to C=O. Thus the multiple and delocalized bonds so

characteristic of carbon chemistry are essentially absent. However, the

Si-O single bond is very stable when compare to C-O.

1.7.2 The Si-O bond

The Si-O bond has ~50% covalent character [58] which has the

anomalously short (161 pm) Si-O distance in the SiO4 tetrahedron had

originally been interpreted as reflecting a degree of 3d-hybridization

23

although numeric resonance (partial occupancy of an antibonding

orbital by a lone pair on an adjacent electronegative atom) has been

suggested. Recent ab initio modeling of H4SiO4 [59] and H6Si2O7 [60]

indicate that only a small amount of 2p-3d hybridization is present.

Cruickshank (1985) came out with a conclusion that even though his

original model overestimated the d-character, the d-orbits participation

cannot be neglected in the detailed models, because the second-row

elements are intermediate between those of the first row, in which d-

orbital participation is not important, and in transition metals it plays

a very important role [61].

1.8 Why Calcium Silicate (CaSiO3 - selection of calcium silicate)

Several branches of national economies over the world have been

using Calcium silicate (CaSiO3).The growing demand for nano CaSiO3

in recent is proved by the steady increase in its production. Good and

high temperature strength, creep resistance, low thermal expansion,

chemical inertness, conductivity and thermal stability are some of the

mixed set of properties of silicates of calcium [62]. These are widely

used in technological applications, such as the production of porcelain

materials, special radio ceramics, flux, glaze, lining bricks and sanitary

components [63]. In the medical industry for artificial bone and dental

root CaSiO3 is used as a biomaterial as it shows good bioactivity and

biocompatibility [64-68]. CaSiO3 is described as a very fine, white or off-

white powder with low bulk density and high physical water

absorption, in the solution of ethanol and water the substance is

practically insoluble.

24

Wollastonite, another term for Calcium silicate (CaSiO3) is a superior

matrix of luminescent material [69]. It is displayed by the theoretical

and experimental studies that a lot of interest in lamp industry and

cathode ray tubes is due to the CaSiO3 doped with appropriate

transition metal ions [70, 71]. In view of the practical importance and

potential applications, we have chosen a nano CaSiO3 for present work.

1.8.1 Physical properties of CaSiO3

Crystal symmetry Monoclinic, Triclinic

Space group P1 (1A polytype or P21)

Unit cella =7.925Å, b= 7.32 Å,c =7.065 Å;

α=90.055°,β= 95.217°, γ =103.42°

Colour White, colourless or gray

polytype Exists

Twinning Common, twin axis [010],

CleavagePerfect in two directions at near

90°

Specific gravity 2.85–3.10

Optical properties Biaxial (-)

Melting point 1550 0C

Refractive indexnα=1.616–1.640, nβ=1.628–1.650,

ny=1.631–1.653

Solubility Soluble in HCl, insoluble in water

Table 1.2. Physical Properties of calcium silicate

25

1.8.2 Research on CaSiO3

As part of our research program we have focused on the preparation

of calcium silicate, since a little work on the synthesis of this material

has been carried out even though it has been proved to be an excellent

luminescent host. In advanced materials Calcium silicate has a

promising future as a highly versatile luminescent host material

because various guest ions can produce a wide range of multi-colours.

It is only in the recent years that the luminescence study of CaSiO3

doped with transition metal ions and rare earth was initiated. In this

host matrix, good mechanical resistance and stability for the phosphor

is provided by the tetrahedral silicate (SiO42-) ion. For the efficient

luminescence yielding phosphors the silicate based host lattices have

been proposed. As a mineral in nature Calcium silicate has mainly two

normal modifications one is the high-temperature phase

pseudowollastonite (α- CaSiO3), which also belongs to the triclinic

system with a pseudo-hexagonal structure and the other is the low-

temperature phase wollastonite (β-CaSiO3), which belongs to the

triclinic system, while α-CaSiO3 is rare in nature β-CaSiO3 is a natural

silicate mineral. Ringwood et.al [72] O’Neill and Jeanloz et.al [73] have

studied that the lower mantle generally consist of (Mg, Fe)SiO3

perovskite with CaSiO3 perovskite and its importance is been

highlighted by Stacey and Isaak et.al [74], however there are several

unanswered questions about the stability, the equation of state, the

26

properties of CaSiO3 under pressure, temperature that complicates any

attempt to model the lower mantle and its structure.

Shieh et. Al [75] reported the equation of state of the cubic

perovskite phase of CaSiO3 have been measured up to core-mantle

boundary pressures by different groups. The average values of the

experimental results by third order Birch-Murnigham EOS, yields a

unit cell volume, V0 = 45.542 Å, The average of the theoretical results is

similar to the one obtained for the experimental data V0 = 45.728 Å. As

observed by Kaili Lin et.al [76] holding time on the mechanical

strength, sintering temperature and microstructure of the CaSiO3

ceramics are the effects of sintering processes. Phase transition from β-

CaSiO3 to α-CaSiO3 occurred at around 1100 0C. However, the phase

transition should not be the main reasons for the change of the

compressive strength, since the density difference of samples are

negligible. In addition, the in vitro biocompatibility of the CaSiO3

ceramics have studied by examining the adhesion and proliferation of

the bone marrow mesenchymal stem cells the study on in vitro

biocompatibility of the CaSiO3 ceramics has been conducted and the

results imply that in the preparation of bioactive coatings on metallic

implants or bioactive bone substitutes the CaSiO3 ceramics may be a

potential candidate.

R. P. Sreekanth Chakradhar et.al [77] reported the luminescence

phenomenon of β-CaSiO3 and studied the phase transition

temperatures of combustion derived wollastonite powder by powder X-

ray diffraction method. They also reported that the morphology and

27

porosity of the prepared powders are dependents an calcination

temperature. The pores size of the samples are in the range 0.25–

1.5μm for 950 0C and 0.5–8μm for 1200 0C. The porous nature of the

powders is supported by the increase in surface areas with calcination.

J.M. Henriques et.al [78] have reported that CaSiO3 triclinic crystal

unit cell parameters have been optimized using the local density

approximation (LDA) within the density functional theory (DFT)

formalism in order to minimize the total energy. A comparison was

made between the theoretical results and X-ray data, and also

calculated the electronic band structure, optical absorption, and

density of states. They obtained the values of Indirect band gap Eg=

5.43- 5.44 eV, and a direct band gap Eg = 5.52 eV of CaSiO3.

Wei Xia et.al [79] have prepared the calcium silicate by precipitation

method, the phase transition temperature of β-CaSiO3 and α-CaSiO3 is

870 0C (from amorphous to β-CaSiO3) and 1125 0C (from β-CaSiO3 to

α-CaSiO3) respectively.

Lei zhou et al [80] reported the luminescence phenomenon of rare

earth ion doped calcium silicate phosphor. The luminescence

properties and possible mechanism of this kind of phosphors and

energy transfer from host toEu3+ were investigated in detail. When rare

earth ion (Eu3+) introduced into CaSiO3 host lattice, the sharp and

intense emission peak located at 611 nm, it is due to typical

hypersensitive transition of Eu3+ ion. F. A. Kroger et al [81] prepared

CaSiO3doped Ce3+ and studied the luminescence properties of

28

phosphor. The results reveal that the incorporation of Ce3+ ion into the

CaSiO3 gives violet emission nearer to UV region.

Yang Liangzhun et.al [82] has studied that the energy transfer

mechanism of luminous CaSiO3: Pb2+, Mn2+. The prepared phosphor

powder was characterized by PXRD, TEM and SEM respectively. The

results indicated that the single phase CaSiO3: Pb2+, Mn2+ powders

with good crystallization could be obtained by sol-gel method. They

observed reddish-orange emission of the phosphor. The emission

intensity of phosphors prepared by the sol-gel method is about two

times stronger than that prepared by the high temperature solid-state

reaction. They also found that the emission intensity increase in the

mol concentration of Pb2+.

Jinghai Yang et.al [83] and Yang Jing-Hai et al [84] studied the

optical properties of rare earth doped luminescence materials co-doped

with transition metal ions in β-CaSiO3 matrix. It was found that When

the Eu3+ and Bi3+ dopant concentration were 0.08 and 0.002 mol % of

Ca2+ ions in CaSiO3 respectively. They have discussed the possible

energy transfer mechanism between Bi3+ and Eu3+ ions in calcium

silicate host. Indeed they conclude that the luminescence material of

CaSiO3:Eu3+Bi3+, Eu3+ acts as the luminescence centre and Bi3+ plays

the role of sensitizer. Being divalent or trivalent europium has different

luminescence characteristics due to the different valences. The electron

configuration of Eu3+ ions is 4F6. The red light emission of Eu3+ ions

has been comprehensively applied in colour television, cathode ray

29

tube, panel display and many fluorescent powders of three primary

colours.

1.9 Nanophosphors

Phorphors are the materials which convert absorbed energy into

visible light without going to high temperatures, i.e. incandescence.

They are also called as luminescent materials. A phosphor basically

consists of a host lattice in which activator ions are incorporated. The

activator absorbs excitation energy and converts it into visible

radiation. The structure of hosts, activator, impurity ions etc,

influences the luminescent properties of lamp phosphors.

Rare earth ions which usually correspond to electronic transitions

within the incomplete 4f shell stimulate luminescence in phosphors

resulting in narrow band spectra. The spectra, however is not

dependent on the nature of host lattice. Materials which possess these

characteristics are used in display systems such as television i.e.

cathode ray tubes, plasma display panels, electroluminescence based

displays and field emission displays. They also find application in light

sources like fluorescent tubes, compact fluorescent lamps and cold

cathode lamps, as diagnostic tool in medicine and biology.

Usually, the phosphors are in the form of crystalline powder with

size ranging between 1 µm and 100 µm. Unidimensional materials of

less than 100 nm sizes are termed as nanophosphors. They are

characterized by marked absorption and emission with higher

efficiency and lifetime. The remarkable luminescence efficiency and

greatly reduced radiative lifetime have rendered nanophosphors,

30

compounds of considerable interest. Numerous avenues of designing

phosphors have been proposed since the particle size varies with the

band gap energy in the nanometer range. Nanophosphors have many

known applications and new possibilities are being explored every day.

1.10 Luminescence

Luminescence is generally defined as light emitted from cold objects,

in contrast to incandescence, which is the light emitted by hot bodies.

There are various types of luminescence, named according to the

nature of the excitation causing the light emission, different type of

luminescence’s are tabulated in Table 1.3. The emission of light by a

substance not under heat is called luminescence. Chemical reactions,

electrical energy, subatomic transitions and crystal stress are some of

the causes for this behavior. These factors differentiate luminescence

from incandescence. In the case of incandescence, light is emitted by a

substance as a consequence of heating. The term ‘luminescence’ was

introduced in 1888 by Eilhard Wiedmann.

An electron in a state of higher energy is unstable. It loses energy

when it returns to its ground state. During luminescence, light is

emitted when an electron at a higher energy level returns to lower

energy level. A system gets excited on absorption of a photon and stays

in that intermediate stage with “non-radiative relaxation.” The return

from intermediate energy state to ground state is accompanied with the

emission of a photon possessing lower energy than that of the absorbed

one. A material gives strongest luminescence from the lower levels of

the ground state and this phenomenon is called as fluorescence. For

31

example, semiconductors give out most of the light corresponding to

the band gap energy – that is to say from the bottom of the conduction

band to the top of the valence band.

There are two conditions for a luminescence material.

(i)The luminescent material should have a structure with nonzero band

gap energy (Eg)

(ii)Energy should be supplied to the nanomaterial before luminescence

Means of Excitation Luminescence Applications

UV / Visible light PhotoluminescenceLamps, Displays,

Diagnosis, Dentistry

Mechanical forces Triboluminescence Crash prevention

Cathode rays (

electron beam)Cathodoluminescence CTRs, FEDs, VFDs

Chemical reactions ChemiluminescenceImmunoassay, ATP,

LLD

Biochemical

reactionsBioluminescence

Chem. Assay, Oxy.

Detection

Heating after prior

storage of energyThermoluminescence

Radiation dosimetry,

Environmental

Protection

Electric field ElectroluminescenceElectron discharge, EL

panels

Ions (particles) IonoluminescenceMaterial analysis,

Defect studies

32

Ultrasound Sonoluminescence Marine biology

Ionizing radiation

(X-rays, α, β, γ)Radioluminescence Radiology

Infrared photonsAnti-stokes

luminescence

Security,

Authentication,

Medical, Displays

Table 1.3. Types of luminescence

1.10.1 Photoluminescence and its Physical aspects

Photoluminescence (PL) is a process in which a substance absorbs

photons and then re-radiates photons. An excitation to a higher

energy state and then a return to a lower energy state accompanied by

the emission of a photon is described by quantum mechanics.

Photoluminescence is quite distinguished by photoexcitation. The time

period between absorption and emission is significantly short, ranging

in the order of 10 to 15 nanoseconds. Under certain circumstances,

this period could extend into minutes or nearly hours.

Resonant radiation is one of the elementary photoluminescent

processes. During resonant radiation, a photon with a particular

wavelength is absorbed and re-emitted. The process is not

accompanied by any notable internal energy transitions. It is extremely

speedier, in the order of ~10 ns. With the sample undergoing internal

energy transitions, more interesting phenomena surface. The most

familiar fluorescence effect occurs accompanied by loss of energy with

33

emitted light photons possessing lower energy compared to the

absorbed ones

The process shown in the Fig 1.7 is called as internal conversion or

vibrational relaxation which usually occurs in a picosecond or less.

Molecules undergo complete vibrational relaxation during their excited

period since a notable number of vibration cycles emerge during that

stage. Neighbouring solvenet molecules absorb excess vibrational

energy converted into heat following collision with excited state

fluorophore. An excited molecule stays in the lowest excited singlet

state S(1) for nanoseconds before returning to ground state. This

return journey, followed by an emission of a photon is called as

fluorescence. Together with the normal thermal motion, the tightly

spaced vibration energy levels at ground state result in a wide range of

photon energies during emission. It is because of this reason that

fluorescence is observed as emission intensity across a band of

wavelengths rather than a sharp line.

Phosphorescence is a more specialized form of photoluminescence.

In this process, the energy of the absorbing photons undergo

intersystem crossing into a state of higher spin multiplicity, usually a

triplet state. The transition to lower singlet energy states is restricted

quantum mechanically, once the energy is trapped in the triple state.

This causes slow radiative transition back to the singlet state, which

may take minutes or hours. This is the origin for "glow in the dark"

substances. The study of PL spectroscopy, a technique to obtain the

relation between the intensity and the wavelength of the luminescence,

34

has become one of the most powerful techniques for the comprehensive

and non-destructive assessment of many materials. It yields a huge

data pertaining to the electronic structure of the materials under

study.

Fig.1.5. Internal conversion process

1.10.2 Excitation of photoluminescence

Several materials are capable of giving out visible luminescence

when subjected to some form of excitation. The excitation could be

achieved with UV light nuclear radiation such as α rays,

β and γ particles, mechanical shock, heat, chemical reactions, and

electric fields. However, the scope of this section is restricted to

photoluminescence. The process of photoluminescence is schematically

35

represented in Fig 1.6 with E-k diagrams for a direct and indirect gap

material, where E and k stand for kinetic energy and wave vector of the

electron or hole respectively.

The relative positions of the conduction band minimum and the

valence band maximum in the Brillouin zone, distinguishes the direct

and indirect gap materials. In the case of a direct gap material,

conduction band minimum and valence band maximum appear at the

zone centre where the value of k is zero. But, as far as indirect gap

materials are concerned, the conduction band minimum does not

occur at the value of k is zero, but rather at some other values

of k which is usually at the zone edge or close to it.

The electrons and holes created due to absorption of photons are

represented by shades states at the bottom (for conduction band) and

empty states at the top (for valence band) in the following fig 1.6.

Thermalization of the excited electrons and holes through emission of

phonon is represented by the cascade of transitions within the

conduction and valence bands. The conduction band minimum and the

valence band maximum occur at the same k values in a direct gap

material. As the momentum of the absorbed or emitted photon is

negligible compared to that of the electron, both the photon absorption

and emission processes can conserve momentum without the

assistance of phonons. Hence, photon absorption and emission are

presented by vertical arrows on E-k diagrams.

Conduction band minimum and valence band maximum occur at

different k values in case of an indirect gap material. In an attempt

36

aimed at conservation of momentum, the photon absorption process

must involve either absorption or emission of a phonon while it

requires emission of a phonon for the PL. For an indirect gap material,

the energy of a phonon is much smaller than that of the PL photon and

hence the peak energy of the PL also reflects its band gap.

Fig 1.6 Schematic representation of photoluminescence processes

(a) Direct band gap material (b) Indirect band gap material.

An electron-hole pair is created following the Absorption of a UV or

visible photon with an energy hυ exceeding the band gap energy (Eg) of

the material and this electron (hole) is excited to states up in the

conduction band (Ref Fig 1.6).

Both energy and momentum must be conserved during a photon

absorption process in materials. The conduction band minimum and

valence band maximum have the same k values in a direct band gap

37

material, the conservation of momentum is guaranteed for the photo

excitation of the electron that involved by a visible photon or UV and

there is no significant change in the electron wave vector during the

absorption process of a photon, it is represented by vertical arrows in

the Fig 1.6. The electron-hole pair recombines radiatively with an

emission of a photon or nonradiatively due to the arrival of the electron

to the bottom of the conduction band, by transferring the electron's

energy to impurities in the material or dangling bonds at the surface.

There is no involvement of any phonons in the electron-hole

recombination in a direct band gap material just like the

photon absorption process discussed in the above section as there is

no need for momentum change for the electron. In contrast, in an

indirect gap material, the excited electron located in the conduction

band needs to undergo a change in momentum state before it

recombines with a hole in the valence band, conservation of

momentum demands that the electron-hole recombination must be

accompanied by the emission of a phonon, since it is not possible to

make this recombination by an emission of a photon alone. In

comparison to the photon absorption process in an indirect gap

material for which conservation of momentum can be fulfilled by either

absorption or emission of a phonon, in electron-hole

recombination process the phonon absorption becomes negligible,

whereas phonon emission becomes the dominant momentum

conservation because the following reasons (i) the number of phonons

available for absorption is small and is rapidly decreasing at lower

38

temperatures, whereas the emission of phonons by electrons which are

already at a high-energy state is very probable (ii) the optical transition

assisted by phonon emission occurs at a lower photon energy Eg -

hυphonon than the band gap energy, whereas phonon absorption results

in a higher photon energy of at least Eg + hυphonon,which can be more

readily re-absorbed by the semiconductor nanoparticle. But we note

that the energy of a phonon (hυphonon) is just in the order of ~ 0.01 eV,

much smaller than the energy of the electron-hole recombination

luminescence photon. Also, prior to the recombination, the electrons

and holes respectively accumulate at the bottom of conduction band

and top of the valence band, the energy separation between the

electrons and the holes approximately equals to the energy of the band

gap. Hence, luminescence emitted by both the types of semiconductors

occurs at energy close to the band gap Eg.

1.10.3 Radiative and non-radiative recombination

Figure 1.7 Band diagrams shows non-radiative recombination

(a) deep level (b) Auger process (c) Radiative recombination.

39

The competition between radiative and nonradiative recombination

determines the PL efficiency. The PL process, requires change in both

momentum and energy of the exited electron for materials with indirect

band gap and therefore involves both phonon and photon, also a

second-order process with a long radiative lifetime (~ 10-5 -10-3 s),and

hence a relatively small efficiency due to the competition with

nonradioactive combination. Therefore the PL efficiency is higher in

direct band gap material in comparison with an indirect band gap

material and the PL process in a direct band gap material is a first-

order process with a much shorter radiative lifetime (~ 10-9 -10-8 s).

Substantial changes in both the efficiency and the peak energy of the

photoluminescence can be expected for particles in nanometer size

domain because of the quantum confinement effect and can be

understood by the principle of Heisenberg uncertainty. As the

confinement energy EQC becomes comparable to or greater than their

thermal energy the quantum confinement effect becomes important.

Therefore, the quantum confinement effect would lead to a progressive

widening of the band gap of a nano-sized silicates as its size is reduced,

along with a broadening of the electron-hole pair state in momentum

space and a decreasing probability for the pair to find a nonradiative

recombination centre, provided that the surface dangling bonds are

passivated which would otherwise act as traps for the carriers and

quench the PL.

Initially, the PL peak would shift to the higher energies; latter two

effects would highly enhance the electron-hole radiative recombination

40

probability resulting in a higher PL efficiency. This is one of the main

reasons why silicon nanocrystals are proposed by Witt, Ledoux and his

coworkers.

1.11 Objectives and Scope of Present Work

Nanoscaled materials can exhibit new or enhanced structural,

electronic, magnetic and optical properties. These size– dependent

properties have stimulated the researchers worldwide to study their

unique properties dependent on their size and shape. The choice of

host material is an important issue for new phosphor materials which

can be made functional in the display of emission as the colours of

emission of the activator (dopants) can be changed by ligand field of the

hosts. Therefore, the studies of luminescence behaviour of activators in

different hosts create new opportunities for researchers to develop

them as a new luminescent material.

Several phosphors have developed based on sulphides, oxides,

aluminates and silicates host. As compared with sulphides, oxides and

aluminates based hosts, silicate based phosphors are generally

preferred due to their stability. Among a silicate based host, very little

work has been published on doped, codoped and undoped CaSiO3

phosphor even though it has been proved to be an excellent

luminescent host. Therefore, we selected CaSiO3 material because of

the unique structure, different physical and chemical properties than

other porous materials and therefore the potential to perform more

favourably in certain relevance very large pore volume and

consequently large surface capacity are one of the vital properties of

41

materials due to the macropores in the framework, as observed with

scanning electron microscopy.

An attempt is made to explain a novel and simple

technique of synthesizing nanocrystalline CaSiO3 with the help of low

temperature solution combustion route using calcium nitrate as

oxidizer and Diformyl-hydrazine (DFH) as fuel.

Due to the overall simplicity of the process it is anticipated

that there should not be undefeatable issues in potentially scaling-

up of the method of production to meet potential commercial

demands.

Europium (EU), Dysprosium (Dy), Lead (Pb), Manganese

(Mn), Chromium (Cr), and Bismuth (Bi) doped, co-doped CaSiO3

nanophosphors were synthesized.

The synthesized phosphors were characterized by Powder

X-ray diffraction (PXRD), Scanning electron microscopy (SEM),

Transmission electron microscopy (TEM), Fourier transform infrared

(FTIR) spectroscopy, UV–Visible spectroscopy and

Photoluminescence.

The PL measurements of the synthesized samples were

studied by spectroscopic technique under UV excitation.

To analyze the impact of calcination and crystallinity of

combustion derived powders.

The bulk physical properties of luminescence capacity and

surface area are able to be specifically designed through control of

42

the preparation process and the use of calcinations upon the

material.

The thesis aims at increasing the level of understanding

and applicability of nano-structured calcium silicate.

With particular reference to rare earth and transition metal

ion dopants the study helps to acquire insight on structural and

photophysical properties.

The results obtained from the above studies in undoped

and doped nanocrystalline CaSiO3 the subject matter of the present

thesis.