chapter 1

Introduction

Glass has fascinated mankind from thousands of years as it possess several

desirable properties which make it an interesting material [1-2]. However, its usage

has always been limited due to its unwanted properties such as low tensile strength,

resistance against fracture, low hardness etc. Recently, formation of composite

materials by incorporation of metal nanoparticles in glass matrices has attracted

increasing attention as they exhibit striking optical, electrical, thermal effects that

make them promising candidate for various nanotechnological applications [3-8].

The remarkable modifications in properties displayed by these nanocomposites

depend on the amount, size and shape of embedded nanoparticles [9-10].

To realize metal doped glass nanocomposites a number of techniques such as

low energy ion beam mixing [11], sol-gel [12], direct metal-ion implantation [13],

ion-exchange [14-15], vacuum deposition [16], field-assisted ion diffusion [17] etc.

have been utilized. In most of these techniques dopants are first introduced into the

glass matrix and then the doped glasses are treated by proper combination of

treatments such as irradiation by either low-mass ion beams or electrons, heat

treatment in reducing atmosphere or pulsed laser irradiation [18]. Such post-

treatments aggregate the nanoparticles into nanometer sized clusters.

In the present research work, vacuum deposition and ion-exchange techniques

followed by thermal annealing in air has been chosen to synthesize silver-soda glass

nanocomposites. These methods are highly favorable as they are very efficient for

introducing very high concentrations of metal ions into the glass matrix and

simultaneously keeping it intact. Variations in optical, structural, mechanical and

electrical properties of fabricated nanocomposites with annealing temperature have

been investigated.

Chapter-1 Introduction

2

A brief outline about concept of nanomaterials with their classification is

described in section 1.1. Section 1.2 describes the concept of nanocomposites, their

classification and history of metal-glass nanocomposites. Selection of host matrix

with its basic structure is discussed in section 1.3. Section 1.4 contains the choice of

filler and its applications. Overview about the methods of synthesis of

nanocomposites is given in section 1.5. Section 1.6 describes the modified

properties of nanocomposites and related literature. Justification and aim of the

present work are described in the section 1.7.

1.1 Concept of Nanomaterials

Miniaturization is a general aim of all the research and technological

developments that are taking place to produce smaller, faster, lighter and cheaper

devices with greater functionality while using fewer raw materials and consuming

less energy. Research in the field of nanomaterials is a step towards miniaturization

of technology that will contribute significantly towards a sustainable usage of raw

material and energy [19]. The history of nanomaterials is quite long; however, major

developments within nanoscience have taken place during the last three decades.

The idea of nanotechnology was first highlighted by noble laureate Richard

Feynman, in his famous lecture at the California Institute of Technology on 29th

December, 1959 and he also discussed the idea of nanomaterials in one of his

articles titled, “There is plenty of room at the bottom”. He underlined that from this

nanoscale should arise new physical and chemical properties [20-21]. The term

nanoparticle, which represents another form of nanomaterials, came into frequent

use in the early 1990s by the materials science community to represent particles that

are composed of up to tens of thousands of atoms but confined to size less than 100

nm, until then, more general terms like submicron and ultra-fine particles were in

use.

One nanometer (abbreviated as 1 nm) is one billionth of a meter. To get a

sense of the nanoscale, a human hair measures 75,000 nm or 8-10 hydrogen atoms

lined up end to end make 1 nm [22]. Research in the field of nanoscience is a

multidisplinary effort that involves interaction between researchers in the field of

physics, chemistry, materials science, mechanics and even biology and medicine

[23].

Chapter-1 Introduction

3

Nanomaterials are those materials that possess at least one characteristic

dimension in nanoscale i.e. 1 to 100 nm. Nanomaterials exhibit many unique

properties such as enhanced chemical reactivity, lowering of melting point,

nonlinear optical behaviour, increased mechanical strength, enhanced diffusivity,

high specific heat, magnetic behaviour and electric resistivity [24-26] etc. for which

they are being extensively studied in various research fields. Researchers have

proposed a huge range of potential scientific applications of nanomaterials in the

fields of biotechnology, sensors, medical diagnostics, catalysis, high performance

engineering materials, magnetic recording media, optics, conducting adhesives [27-

28] etc. Two principal factors that are responsible for the alteration in properties of

materials when they are reduced to nanoscale are increased surface to volume ratio

resulting in a large number of surface atoms relative to the total number of atoms

and quantum confinement effects [29]. For example, thermodynamic processes

change at the nanoscale due to increase of surface energy as the size of the material

decreases [29]. In addition, many nanomaterials act as catalysts as their high energy

surfaces are more reactive than the surfaces of bulk material such as gold which is

chemically inert at normal scale while act as a powerful chemical catalyst at

nanoscale [30-31].

1.1.1 Classification of Nanomaterials

Based on the number of dimensions which lie within nanometer range,

nanomaterials are usually classified into three classes (Fig. 1.1): (1) 1-Nano

Dimensional Materials: The materials having one dimension in nanoscale are

called 1-nano dimensional materials such as nanofilms, coatings, multilayers etc.

(Fig. 1.1 a). (2) 2-Nano Dimensional Materials: The materials having two

dimensions in nanoscale are called 2-nano dimensional materials such as carbon

nanotubes, nanofibres, nanowires etc. (Fig. 1.1 b). (3) 3-Nano Dimensional

Materials: The materials which have all the dimensions in nanoscale such as

nanopowders, nanoparticles, nanocrystalline materials, quantum dots etc. are called

3-nano dimensional materials (Fig. 1.1 c) [32-33].

1.1.2 Properties of Nanomaterials

Nanomaterials exhibit many unique properties, for which they are intensely

being studied in diverse research fields. Unlike bulk materials that have constant

Chapter-1 Introduction

4

physical properties regardless of mass, nanomaterials have unique size dependent

properties. Indeed the possibility to control the properties, by tuning the size of the

nanoparticle, has been the cause and subject of many investigations. One such

example is that the reduction of material’s dimension has pronounced effects on its

optical properties such as absorbance, luminescence, reflectivity, refractive index

etc. For example, colloidal solutions of gold nanoparticles have a deep red colour

which becomes progressively more yellow as the particle size increases. Mechanical

properties (especially hardness and strength) are highly dependent on the presence

of defects within a material. As the system size decreases, the ability to support such

defects becomes increasingly more difficult and mechanical properties are altered

accordingly. The properties of nanoparticles arise as a consequence of the huge

fraction of surface atoms in the total amount of atoms and increasing influence of

the wave-like property of electrons i.e. quantum mechanical effects [29] which have

been explained briefly in the following section.

(a) (b) (c)

Figure 1.1: Classification of Nanomaterials.

1.1.2.1 Surface Area to Volume Ratio

Surface of nanomaterials plays an important role in determining their

properties. Materials in nanosize have high surface area to volume ratio. If we

assume that these nanoparticles are spherical having radius r, the surface area to

volume ratio of material is equal to

Chapter-1 Introduction

5

Therefore, S/V α 1/r which implies that decreasing the particle radius increases the

surface area to volume ratio. For example, for a cube of volume 1 cm3, the

percentage of surface atoms will be only 10-5

%. When the cube is divided into

smaller cubes with an edge of 10 nm, the percentage of surface atoms will increase

to 10%. In a cube of 1 nm3, almost every atom will be a surface atom. Such a drastic

increase in ratio of surface to volume atoms in nanomaterials is responsible for

change in their physical and chemical properties. As the surface of the particle is

involved in chemical reactions, large surface area can make materials more active

[34].

1.1.2.2 Quantum Confinement Effects

In a bulk crystal, the properties of the material are independent of size and are

dependent only on chemical composition. As the size of the crystal is reduced to the

nanometer range, the electronic structure is altered from the continuous electronic

bands to discrete or quantized electronic levels.

Figure 1.2: The variation of density of states g(E) with energy (E) for the infinite

bulk solid, a quantum film, a quantum wire and a quantum dot.

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6

As a result, the continuous optical transitions between the electronic bands

becomes discrete and the properties of the nanomaterials become size dependent

[35-36]. The density of states changes as one goes from the bulk to quantum films

that are confined in one dimension to quantum wires confined in two dimensions

and finally, the quantum dots that are confined in all the three dimensions. The

variation of density of states g(E) versus energy (E) is shown schematically in

Fig.1.2. This variation of density of states versus energy for confined systems does

not follow the typical E-1/2

dependence, as in the case of an infinite solid.

Important among these nanoscale materials are nanocomposites, in which the

constituents are mixed at nanometer length scale. They have unique properties that

are different from their bulk counterparts. The study of nanocomposite materials

requires a multidisciplinary approach involving novel synthesis techniques and an

understanding of physics and surface science. Therefore, in depth studies related to

nanocomposite materials is of immense significance both from fundamental as well

as applied point of view.

1.2 Nanocomposites

Nanocomposites are composites containing different compositions or

structures, where at least one of the constituent is in the nanoscale regime in other

words nanocomposites are materials that are created by introducing nanomaterials

(often referred to as filler) into a macroscopic sample material (often referred to as

the matrix) [37-39]. After adding nanomaterials to the matrix material, the resulting

nanocomposites not only exhibit enhanced electrical and thermal conductivities but

also distinctive optical and dielectric properties due to their quantum size effects and

surface effects. Nanocomposite materials are emerging as suitable alternatives to

overcome limitations of micro composites. They are reported to be the materials of

21st century as they possess unique design and property combinations that are not

found in conventional composites. The general understanding of these properties is

yet to be reached [40], even though the first inference on them was reported about

thirty years back [41].

1.2.1 Classification of Nanocomposites

Nanocomposites can be classified according to their matrix materials into three

different categories i.e. metal matrix nanocomposites, ceramic matrix

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7

nanocomposites and polymer matrix nanocomposites. Nanocomposite of insulating

materials such as glasses, ceramics or polymers with embedded metal nanoparticles

are under focus because of their special structural, mechanical, electrical, linear and

nonlinear optical properties.

Among the nanocomposites, metal-glass nanocomposite materials exhibit

interesting novel properties which include nonlinear optical behaviour, increased

mechanical strength, high refractive index, electrical resistivity etc. Such

nanocomposites containing metal nanoparticles dispersed in glass matrices have

also drawn attention because of their second order non-linear effects and have

applications in developing high speed and low power optical devices for future

communication systems [42-43]. Thus the aim of the present work was to synthesize

silver-soda glass nanocomposites and to investigate structural, optical, mechanical

and electrical properties of fabricated nanocomposites.

1.2.2 History of Metal-Glass Nanocomposites

Although nanoparticles are often considered as the innovation of modern

technology, the use of metal nanoparticles for “artistic” applications is not new at

all. In fact nanocomposites formed by incorporating transition metal nanoparticles

in glass matrices display peculiar optical properties that have made them famous

since literally the millennia. The first glasses containing metal nanoparticles were

fabricated by Roman glassmakers in the fourth century AD. One example of this

technology has been preserved till date in the form of Lycurgus Cup as shown in

Fig. 1.3 [44-45]. This cup is made of a soda-lime glass containing very minute

amount of silver and gold. This extraordinary cup is the only complete example of a

very special type of glass, known as dichroic, which changes colour in the presence

of light. The opaque green colour of the cup turns to a glowing translucent red when

light is put inside it. This peculiar colour display is due to the presence of tiny

amounts of colloidal gold and silver embedded inside the glass [46]. Medieval

cathedral windows demonstrate great varieties of beautiful colours due to the

nanosized metal particles embedded in the glass matrix. The first attempt to explain

the nature of the colour induced in the glasses by metal nanoparticles can be

attributed to Michael Faraday [47]. Since then, studies dedicated to the optical

properties of metal-glass nanocomposites have been developing continuously.

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8

Nonlinear optics has provided further motivation for the development of preparation

and characterization methods for metal-glass nanocomposites.

Figure 1.3: Roman Lykurgos goblet (fourth century AD) made of soda-lime glass

containing silver and gold particles. Colour changes from opaque

green to strong red when the light source is put inside [44].

In 1905 Maxwell Garnett formulated the first theoretical approach for

understanding the interaction of light with metal nanoparticles [48-49]. The

Maxwell-Garnett articles discusses the optical properties of a medium containing

metal nanoparticles, showing that the colors of metal nanoparticles could be

explained using the theory developed by Lorentz in 1880 [50] for non-homogeneous

optical systems. Gustav Mie observed that the Maxwell-Garnett explanation was not

Chapter-1 Introduction

9

suitable for the colours observed in different experimental conditions of dilute

solutions with metal nanoparticles. Mie was able to explain it theoretically in 1908

by solving Maxwell’s equations [51-52]. He proposed that the interaction of light

with metal nanoparticles can give rise to collective oscillations of the free electrons

commonly known as surface plasmons.

1.2.3 Surface Plasmon Resonance

The surface plasmon resonance (SPR) is the coherent motion of the

conduction band electrons caused by interaction with an electromagnetic field [3,

29, 53-56]. Fig. 1.4(a) depicts the systematic representation of interaction between

an incoming electromagnetic field and metallic nanoparticles. In a classical

description, the electric field of an incoming light wave induces polarization of

electrons with respect to the much heavier ionic core of a spherical nanoparticle as

shown in Fig. 1.4(a). As a result, net charge difference appears on the surface of the

nanoparticle, which in turn acts as a restoring force. Consequently, this net charge

difference oscillates with the incident electric field known as plasmon oscillation.

Figure 1.4 (a): Schematic diagram illustrating the collective oscillations of free

electrons under the effect of an electromagnetic wave.

Chapter-1 Introduction

10

Figure 1.4 (b): Schematic diagram of plasmon oscillation for a sphere, showing

the displacement of the conduction electron charge cloud relative

to the nuclei.

Thus the final properties of the resulting material can be easily control by

taking care of these factors. Different metal nanoparticles produce different light

interactions and therefore different colours. As an example, bulk gold looks

yellowish in reflected light, while thin gold films look blue in transmission. This

blue colour steadily changes to orange as the particle size is decreased to ~ 3 nm.

1.2.4 Metal-Glass Nanocomposites

Metal-glass nanocomposite is an exciting emerging area of research. The

reasons for the present excitement in the metal-glass nanocomposite research are

due to their several inherent advantages over other dielectric nanocomposites,

exceptional properties, unique functions in nanoscience and future nanotechnology

[42, 60].

Metal-glass nanocomposite is a class of materials in which metallic

nanoparticles are embedded in a glass matrix in order to dramatically improve the

properties of the glass as compared to their traditional bulk composites. At this scale

large surface area of nanoparticles even at very low concentration can noticeably

Chapter-1 Introduction

11

change the macroscopic properties of the glass and contribute many new

characteristics to the glass. Properties which have been shown to undergo

substantial improvements include [7, 61-66]:

Surface appearance (colour)

Optical properties (refractive index)

Linear and nonlinear optical behaviour

Electrical conductivity

Mechanical properties (surface hardness)

Corrosion resistance

The incorporation of metal nanoparticles into the glass matrix allows the

construction of devices to utilize their advantageous properties. The host matrix not

only forms the structure of device but also protects the nanoparticles and prevents

agglomeration. These nanocomposite materials have a huge range of potential

scientific applications such as in the fields of optical data storage media, optical

waveguides, optical switches based on their nonlinear optical properties,

photochromatic and colour glass recycling industry, solid-state lasers, sensors,

coloured glasses, dichroic polarizers, display devices, enhanced fluorescent

materials, modified refractive index materials, solar cells, optoelectronic materials

etc. [8, 43, 67-76]. In all these applications size, shape, number density and

distribution of the nanoparticles critically determine performance and properties of

the metal-glass nanocomposites.

1.3 Selection of Host Matrix: Soda Glass

Glass is an amorphous (non-crystalline) solid material that possesses no long-

range atomic order. It is formed by fusion of mixture of silica, basic oxides and

some other compounds and is solidified from the liquid state without crystallization

[1-2].

A wide range of substances can form glasses, including covalent, ionic,

metallic, van der wall and hydrogen bonded materials. These glass-forming oxides

can be relegated into three types: network or glass formers, conditional glass-

formers and network modifiers. The most important glass formers SiO2 form glasses

on their own when cooled from their molten state. However, the conditional glass-

formers (e.g. BeO, ZnO, Al2O3, etc.) do not form a glass network on their own and

Chapter-1 Introduction

12

need to be melted along with a suitable oxide to form glass. These intermediate

oxides have coordination numbers and bond strengths between the network-formers

and network-modifiers. The network-modifiers (e.g. CaO, BaO, MgO, Na2O, K2O,

Li2O etc.), having large coordination numbers and relatively weak bonds and they

are distributed throughout the holes in the network. They alter the glass-forming

network by replacing stronger bridging oxygen (BO) bonds with weaker,

nonbridging oxygen (NBO) bonds. Especially, sodium ions depolymerize the silicon

oxygen continuous random network by breaking the Si-O-Si bonds and randomly

reside in the structural interstices thus created. These network modifiers are

generally added to the glass to change its properties like softening point, fluidity,

resistivity, thermal expansion coefficient, chemical durability etc. [77].

Figure 1.5: Two dimensional schematic representation of the random network of

silicate glass [79].

Chapter-1 Introduction

13

As properties of glass depend upon its chemical composition and atomic

structure, thus the understanding of their structure is important from fundamental

point of view.

Fig. 1.5 shows the structure of silicate glass. The basic structural unit of glass

network is the silicon-oxygen tetrahedron in which a silicon atom is connected to

oxygen atoms through each corner. The oxygen atoms shared between two

tetrahedral are called bridging oxygen (BO). Those not shared are referred to as

nonbridging oxygen (NBO). X-ray and neutron diffraction studies indicate that the

Si-O distance in the tetrahedron is 1.61 Å and the shortest O-O distance is 2.65 Å,

same as that of crystalline silica [78]. The inter-tetrahedral (Si-O-Si) bond angle

distribution is centered at approximately 143°, which is much broader than that of

crystalline silica; hence do not show long-range order, as shown schematically in

Fig. 1.5. The lower density of pure silica can be attributed to the existence of defects

and holes in the network. Ion-exchange in pure silica takes place through these

defects and holes at relatively higher temperatures and applied electric field. The

temperature to obtain pure silica is very high and thus makes it expensive.

When network-modifiers (alkali ions) are added to silicate glasses, they fill the

gaps and holes existing therein by raising the concentration of nonbridging oxygens,

which, in turn, lowers the connectivity of a structure. This decrease in the

connectivity of SiO4 tetrahedral network causes a subsequent decrease in the

transition temperature and melts viscosity as well as an increase in thermal

expansion coefficient, density and ionic conductivity. The resulting modified-

random-network has alkali-rich regions surrounded by presilicate network. Since

alkali ions exhibit lower density and higher mobility through interstices, they

contribute much to the ionic conductivity. The concentration of alkali considerably

changes the ionic and thermal properties of an oxide glass, and thus, this behavior is

exploited in the realization of doping with foreign atoms for optical waveguide

fabrication [80]. Soda glass is an important and widely used example of such

glasses.

A typical composition of a soda glass is 69-74 wt% SiO2 (silicon dioxide), 10-

16 wt% Na2O (Sodium Oxide) and 5-14 wt% CaO (calcium oxide) with much

smaller amounts of various other compounds [81]. Network modifiers such as Na2O

and CaO are added to silica to alter the network structure by replacing Si-O-Si

Chapter-1 Introduction

14

bonds with Si-O- Na

+ or Si-O

- Ca

2+ bonds. This separates the SiO2 tetrahedral from

each other, which makes the mixture more fluid and therefore more likely to form a

glass after it has been melted and then cooled. The modified-random-network is

shown schematically in Fig. 1.6. Small amounts of other compounds (Al2O3, MgO,

CaO etc.) are added for tailoring other properties of glass such as durability,

refractive index, expansion coefficient, melting point etc. It is the most common

type of glass and is used in the construction industry.

Figure 1.6: Two dimensional schematic representation of modified-random-

network of glass [79].

Chapter-1 Introduction

15

In order to increase applications of glass in various fields, glass materials are

combined with the emergent field of nanotechnology via the incorporation of

nanoparticles into glass to produce novel materials. The selection of suitable glass

matrix for the synthesis of metal-glass nanocomposites by using the vacuum

deposition and ion-exchange techniques play an important role in controlling the

features of nanoparticles such as size, shape and distribution. Depending on the

nature and possibility to utilize the structural network variety of glasses can be

selected for incorporating various metal nanoparticles. The alkali content must be

high enough (Na+) to perform a suitable diffusion.

In the present study, we have chosen soda glass as a host matrix for

embedding nanoparticles because they have a high Na+ content and are easily

available. Moreover, soda glass is transparent, inexpensive, chemically stable,

reasonably hard and extremely workable. These exceptional properties of soda glass

make them widely used engineering material [82-83]. Soda glass have wide spread

applications in diverse disciplines such as photonic devices, optical sensors,

biosensors, optical waveguides, light emitting diodes, integrated optics for

communication as well as in many other optical components [84-90].

It is excellently suited as a matrix for fundamental research on nanocomposites

as it involves minimal chemical interaction between the nanoparticles and the host

matrix. Moreover, glass matrices provide long-term stability to metal nanoparticles.

In glass even the smallest metal nanoparticles can be stabilized and investigated [91-

92].

1.4 Choice of Filler

1.4.1 Silver (Ag) Nanoparticles

Recently metals with free electrons (essentially Au, Ag and Cu) are being

investigated extensively as they exhibit plasmon resonances in the visible spectrum

and have wide spread applications in diverse disciplines. Although silver and gold

share many similar properties and applications, interestingly, gold nanoparticles

have been exploited to a much larger extent than silver nanoparticles for potential

applications despite the fact that silver nanoparticles exhibit higher efficiency of

plasmon excitation which in turn leads to enhanced properties including catalysis

[93], magnetic and optical polarizability [94], electrical conductivity [95] and

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16

antimicrobial activity [96]. The reason for the considerable interest in the use of

silver nanoparticles can best be summarized by quoting the following statement [97-

98]:

Of the three metals (Ag, Au, Cu) that display plasmon resonances in the

visible spectrum, silver exhibits the highest efficiency of plasmon excitation.

Moreover, optical excitation of plasmon resonances in nanosized silver particles is

the most efficient mechanism by which light interacts with matter. A single silver

nanoparticle interacts with light more efficiently than a particle of the same

dimension composed of any known organic or inorganic chromophore. The light

interaction cross-section for silver can be about ten times that of the geometric cross

section, which indicates that the particles capture much more light than is physically

incident on them. Silver is also the only material whose plasmon resonance can be

tuned to any wavelength in the visible spectrum.

From this statement the obvious advantages of silver, and the reason for its

choice as a model nanoparticle for the novel synthesis of nanocomposite materials is

clear. Silver nanoparticles have applications in many areas including biomedical,

materials science, catalysis etc. Some of these are briefly discussed below:

1.4.2 Applications of Silver Nanoparticles

(a) Optical Sensors

Silver nanoparticles show a unique peak in absorption due to the SPR effect.

This effect is caused by a collective excitation of the conduction band electrons of

the nanoparticle during their interaction with the incident electromagnetic radiation

which is already discussed in section 1.2.3. The value of maximum wavelength of

this plasmon resonance peak depends upon the size and shape of nanoparticles as

well as the host matrix. Due to this extraordinary optical characteristic, silver

nanoparticles have large number of applications in photonics, sensors, colour filters

etc. [43, 99-103].

(b) Catalyst

Another possible application of silver nanoparticles is their use as a catalyst.

High surface area and high surface energy predetermine metal nanoparticles for

being effective catalytic medium. Growing small particles of silver have been

observed to be more effective catalysts than stable colloidal particles [104-105].

Chapter-1 Introduction

17

(c) Antimicrobial Agent

Since ancient times silver is considered as a non-toxic, safe inorganic

antibacterial agent capable of killing microorganisms that cause diseases. According

to the mechanism reported, silver nanoparticles interact with the outer membrane of

bacteria, and arrest the respiration and some other metabolic pathway that leads to

the death of the bacteria. It has a significant potential for a wide range of biological

applications such as antibacterial agents for antibiotic resistant bacteria, preventing

infections, water filters to clean infected water and prevent diseases as well as

wound dressings [106-107].

(d) Surface Enhanced Raman Scattering

Silver nanoparticles exhibit a phenomenon known as surface enhanced raman

scattering (SERS) [108]. The SERS technique is a powerful analytical tool in the

fields of surface science, biology, analytical chemistry, biochemistry, catalysis and

materials research. The excellent sensitivity and selectivity of SERS allow for the

determination of chemical information from single monolayer on planar surfaces

and extend the possibilities of surface vibrational spectroscopy to solve a wide array

of problems. The aggregation of silver nanoparticles is prerequisite for stronger

SERS enhancement.

1.5 Synthesis of Nanocomposites

Material scientists are conducting research to develop novel materials with

better properties, more functionality and lower cost than the existing ones. Several

physical, chemical and biological synthesis methods have been developed to

enhance the performance of nanocomposites displaying improved properties with

the aim to have a better control over the particle size, distribution and morphology

[109-111]. Synthesis of nanocomposites to have a better control over the particle

size, distribution morphology, purity, quantity and quality by employing

environment friendly economical processes has always been a challenge for the

researchers [112]. Nanocomposites can be synthesized by various methods like low

energy ion-beam mixing, sol-gel, direct metal-ion implantation, ion-exchange,

vacuum deposition, field-assisted ion diffusion etc. [11-17, 62, 113-115]. Among

the different methods for fabrication of silver-soda glass nanocomposites, vacuum

deposition and ion-exchange are promising methods. Both these methods are easy to

Chapter-1 Introduction

18

handle, economical & consume less time, have minimal requirements for sample

preparation, ease of adaptation to automated operation, have no residual solvents as

in wet chemical synthesis processes.

1.5.1 Vacuum Deposition Method

Now a days, vacuum deposition technique is routinely being used to form

optical interference coatings, mirror coatings, decorative coatings, permeation

barrier films on flexible packaging materials, electrically conducting films, wear

resistant coatings and corrosion protective coatings [116]. This method can be

effectively used for the synthesis of metal-glass nanocomposites. In this method a

thin film of the metal intended to form nanostructures is grown on the host matrix

by thermal evaporation.

Advantages of Vacuum Deposition Method:

1) Extreme versatility in composition of deposit. Almost any metal, alloy,

refractory or intermetallic compound, some polymeric type materials and their

mixtures can be easily deposited.

2) The ability to produce unusual microstructures and new crystallographic

modifications.

3) Good adhesion can be achieved between thin film and substrate.

4) The substrate temperature can be varied within very wide limits.

5) The ability to produce coatings at high deposition rates with high purity.

6) This technique is relatively inexpensive compared to other physical vapour

deposition techniques such as electroplating.

1.5.2 Ion-Exchange Method

Ion-exchange is a well-known technique proposed and developed since 1970s,

to modify the electrical and optical properties of glass by embedding metal

nanoparticles [14-15, 117-118, 119-152]. In this process, monovalent alkali ions on

the surface layers of the glass are replaced by the ions of the same valence from the

surrounding medium. Consequently, this replacement can change the refractive

index of the host material.

Chapter-1 Introduction

19

Advantages of Ion-Exchange Method

1) Ion-exchange as a fabrication process promises simplicity, economy and optical

fiber compatibility, not requiring complicated manufacturing equipment.

2) It allows for batch processing and also flexibility of process and glass choices, so

it can be adapted for many applications such as optical limiters, optical sensing

devices etc. [117-118].

Vacuum deposition and ion-exchange methods have been discussed in detail in

chapter 2.

1.6 Modified Properties of Silver-Glass Nanocomposites and

Related Literature

Intense research in the field of synthesis and study of composite materials

containing metal nanoparticles is motivated by the rise of their various potential

applications in diverse disciplines of science and technology [7-8, 43, 67-75].

Growth of nanoparticles under the surface of bulk material is a key technology in

order to improve the quality and desired properties of the host glass matrix such as

colour, refractive index, corrosion resistance, wear resistance, surface hardness etc.

[3, 61-66]. Addition of metal nanoparticles in glass is responsible for the change in

their structural, optical, electrical and mechanical properties. Based on these

interesting aspects, synthesis of metal nanoparticles in glass has been studied by

many research groups [5, 11-17, 113-115, 119-152]. In the literature, there are

numerous reports paying more attention to the synthesis of silver metal

nanoparticles in glass by ion-exchange and vacuum deposition methods followed by

ion-irradiation, annealing in reduced atmosphere or in a high vacuum atmosphere

[5, 11, 14-16, 76, 119-152]. A brief description of literature on synthesis methods is

presented here.

Synthesis of silver nanoparticles in ion-exchanged soda glass followed by

annealing in hydrogen reducing atmosphere have been studied by A. Miotello et al.,

C. Mohr et al., E. Borsella et al. and some other authors [119-125]. P.

Gangopadhyay et al. and S. Bera et al. have reported synthesis of silver

nanoparticles in glass matrix by ion-exchange and annealing in vacuum [126-128].

P. Magudapathy et al. and some other authors have made silver nanoparticles in

glass by the combined use of ion-exchange and subsequent ion irradiation [15, 76,

129-135]. Formation of silver nanoparticles in ion-exchanged soda-lime glass in the

Chapter-1 Introduction

20

presence of Ar+ laser beam has been studied by M. D. Niry et al. and A. Nahal et al.

[14, 136]. H. Hofmeister et al. used ion, electron and laser irradiation of soda glass

containing silver nanoparticles and discussed the obtained structure in terms of

radiation effects [137]. Some authors reported growth of silver nanoparticles in ion-

exchanged soda glass during laser irradiation [138-147].

P. Gangopadhyay et al. [11, 16] have discussed the growth of silver

nanoparticles in soda glass matrix after irradiation with argon ions on silver thin

films depositing by thermal evaporation. Xia Wu et al. [148] have studied optical

properties of Ag-Bi2O3 nanocomposite films prepared by co-sputtering method

followed by annealing at different temperatures. Formation of gold nanoparticles

embedded in silica films using RF-magnetron sputtering technique with subsequent

thermal treatment have been reported by A. Belahmar et al. [149]. Effect of air

annealing on optical and structural properties of silver films prepared by thermal

evaporation have been investigated by Jing Lv et al. [150-151]. A. Serrano et al.

[152] have reported the formation of gold nanoparticles in soda glass by thermal

evaporation method followed by annealing in air.

1.6.1 Optical Properties

Knowledge of optical characteristics (refractive index, dielectric constant,

photoluminiscence etc.) of metal-glass nanocomposites is of immense importance

due to their applications in fabrication of optical fibers, optical sensors, waveguides,

integrated optics for communications etc. [42, 43, 67-75]. These applications of

glasses can be further improved by modifying their properties [61-66].

Modifications of the glass properties can be achieved by modifying the bulk glass

composition or by modifying the glass surface, affecting the whole performance of

the glass product. Glass surface modification can be performed in different ways

such as by adding materials to the original surface by vacuum deposition and ion-

exchange techniques. Both techniques are relatively easy to perform and are used to

alter the glass surface properties. Some previous studies have reported the tailoring

of optical properties of glasses by vacuum deposition and ion-exchange techniques.

Following section briefly describes some of the optical properties studied in the

present endeavor.

Chapter-1 Introduction

21

(a) Colour

Optical properties of glass are characterized by the interaction of glass with

electromagnetic radiation. Most types of glasses, partially absorb, reflect and

transmit the incident light. The chemical composition of the glass matrix and its

additives determine these properties. Most oxide glasses are coloured due to

excitation of d-orbital electrons of transition metallic ions to a higher energy level.

On the other hand, the mechanism of yellow colouration involves the scattering and

absorption of incident light by the presence of “metallic” nanoparticles in the matrix

[3, 34]. Mie theory explains that the presence of silver nanoparticles in the glass

matrix can affect the transmission of light resulting in yellow colouration [29]. The

introduction of silver nanoparticles into glass matrix strongly affects its optical

properties. The change in optical properties can be attributed to the quantum

confinement of electrons within nanoparticles and the surface plasmon resonance

which has been discussed earlier in section 1.1.2.2 & 1.2.3 respectively.

Literature contains some reports on tuning of optical properties of silver-glass

nanocomposites synthesized by the combined use of ion-exchange and subsequent

thermal annealing in vacuum [126-127]. However, there are few reports available

where annealing has been carried out in air but at very high temperatures and for

long durations [153-154]. They have shown no shift in the surface plasmon

resonance peak [126, 154-155]. Of practical relevance is the influence of annealing

atmosphere on silver deposited glass substrate, a subject currently under discussion.

In this regard, several authors have reported the vanishing of SPR peak of silver

nanoparticles when composited are exposed to air and ascribed it to oxidation of

nanoparticles [156-160]. They have also reported that annealing of silver doped

films in air atmosphere at 450°C yielded colorless films containing silver oxide.

These films turned yellow when heated in H2–N2 (reducing atmosphere) due to the

formation of silver nanoparticles.

(b) Refractive Index

Refractive index is one of the most important optical constant of a material,

which in general depends on the wavelength of the interacting electromagnetic

wave, through a relationship called dispersion. Optical properties of nanocomposites

are directly related to the refractive index ‘n’ which is a measure of the ability of the

Chapter-1 Introduction

22

glass to refract or bend light as it passes through the glass. In materials where an

electromagnetic wave loses its energy during its propagation, the refractive index

becomes complex. The complex refractive index, n*, is defined by n

*=n-ik.

The real part is usually the refractive index ‘n’ and the imaginary part is

called the extinction coefficient ‘k’. The optical constants n and k can be determined

by measuring the reflectance from the surface of a material as a function of

polarization and the angle of incidence. From normal incidence, the reflection

coefficient, r, is obtained as [161]:

The reflectance R is then defined by:

To obtain refractive index equation (3) can be solved:

Extinction coefficient is related to the absorption coefficient (α) and the wavelength

(λ) by [161-163]:

Further, refractive index is related to the relative dielectric constant εr by using the

standard result derived from Maxwell’s equations:

where ε1 and ε2 are, respectively, the real and imaginary parts of εr Equation (6)

gives [162]:

Chapter-1 Introduction

23

In relevance to optics, the real part of dielectric constant is closely related to the

refractive index and the imaginary part is to the extinction coefficient which

represents the losses of photon energy when optical wave propagates through the

media. The evaluation of refractive indices of optical materials is of considerable

importance for applications in integrated optical devices such as switches, filters and

modulators etc., where the refractive index of a material is the key parameter for

device design.

The method of vacuum deposition and ion-exchange are effective tool for

modifying the refractive index of glasses and can be successfully applied to form

waveguide structures [164-165]. The ion-exchange technique has been utilized to

modify optical constants like refractive index, dielectric constant of glass. J. R.

Hensler et al. [166] have observed a method to increase the refractive index of

alkali-silicate glass by diffusing ions of silver, copper, thallium etc. Changes in the

refractive index of ion-exchanged glasses have also been observed by S. Ruschin et

al., J. Albert et al., R. Oven and other authors [117, 167-171]. Previously some work

has been done on silver-glass nanocomposites to study the increase in value of

refractive index using ion-exchange technique but the changes observed in

refractive indices were very small [117, 172].

Literature contains number of reports on photoluminescence of silver-glass

nanocomposite glasses. E. Borsella et al. [125] have investigated spectral

luminescence of silver in ion-exchanged soda-lime glass. They have also reported

structural rearrangement of the Ag+

ion environment with increasing silver

nanoparticle concentration in glass [173]. Photoluminescence of silver nanoparticle

glasses prepared by ion-exchange followed by annealing have been reported by O.

Veron et al. [174]. Work on photoluminescence study of silver nanocomposite

glasses is still in progress.

1.6.2 Surface Hardness

The most widely accepted definition for hardness is, “the ability of a material

to resist permanent penetration. A hard material is generally defined as one which is

not easily indented by a rigid body or as one which is difficult to scratch. The

measurement of hardness of brittle materials is usually carried out with the help of

Vickers or Knoop indentation at various loads. In general, an indenter is pressed

Chapter-1 Introduction

24

into the surface of the sample material to be tested under a specific load for a

definite time interval and a measurement is made of the size or depth of the

indentation. The force and size of the indentation can be related to a hardness which

can be objectively related to the resistance of the material to permanent penetration

[175].

Glass has a unique combination of desirable properties for various engineering

applications such as transparency, hardness, durability and low cost. However, its

use has always been limited due to its low tensile strength and resistance against

fracture, low hardness etc. For metal-glass composites, mechanical properties,

namely its hardness, can be improved significantly when the size of embedded metal

particles is reduced to less than a few nanometers. Microhardness is a complex

property, composed of mechanical characteristics and chemical bonding; it could

serve as a valuable guide in many engineering applications of the investigated

material. Glass with improved microhardness is used in military, motor, locomotive,

electronic and architectural sectors, information recording media application which

includes magnetic disks, optical magnetic disks and high density optical disks.

Different treatments can be applied to glass for modifying their mechanical

properties. Among different techniques, vacuum deposition and ion-exchange both

are simple and attractive techniques which provide the possibility to improve the

hardness of glass.

Literature contains only a small number of reports to investigate the surface

hardness of silver-glass nanocomposites. K. J. Berg et al. have measured the

changes in Vicker’s microhardness of silver exchanged glass [64]. M. Suszynska et

al. have studied the changes in Vicker’s microhardness of the ion-exchanged soda

glass [176]. They have also reported microhardness of copper doped soda lime silica

glass [177].

1.6.3 Electrical Conductivity Behaviour

The electrical properties of the nanocomposites are strongly influenced by the

metal filling factor and changes in the microstructure [178]. A critical filler loading

must be incorporated to transfer the composite from the insulating state into the

conducting state. At this critical concentration, which is known as the percolation

threshold, the electrical conductivity of the composite suddenly increases by several

Chapter-1 Introduction

25

orders of magnitude. Often at the percolation threshold, the filler forms a continuous

network inside the host dielectric matrix and further increase in the filler loading

usually has little effect on the composite electrical resistivity. However, if a

remarkable decrease in the composite’s electrical resistivity is noticed with

increasing the filler loading above the percolation threshold, this means that the

three dimensional conductive network has not yet been formed at the percolation

concentration, and thus the composites conductivity is due to tunneling in addition

to direct contact between the particles. In some cases, tunneling could be the

dominant mechanism. Tunneling conduction occur when the distance between the

filler particles are close enough, roughly less than 10 nm. Investigating the current-

voltage (I-V) relationship gives an indication whether the composite conductivity is

due to tunneling or direct contact between the particles [179-180]. Very few reports

are available in literature on electrical conductivity of silver-soda glass

nanocomposites, nevertheless in one report P. Magudapathy et al. [129] have

studied the variation in resistivity of silver nanoparticles embedded in glass with

temperature.

1.7 Justification and Aim of the Present Work

It has been clearly revealed from the existing literature [7-8, 61-66] that the

fabrication of nanocomposites of glass consisting of metal nanoparticles have

recently received considerable interest as advanced technological material because

of its drastically improved properties. As a result, their usage has increased

tremendously for a variety of applications particularly in the fabrication of photonic

devices, data storage systems, biosensors, waveguides, solar cells, aerospace,

microelectronics etc. [43, 67-75].

Metal-glass nanocomposites can be synthesized by various techniques such as

vacuum deposition, ion-exchange, low energy ion-beam mixing, sol-gel, direct

metal-ion implantation, field-assisted ion diffusion etc. [11-17, 113-115]. Further, it

is well established that vacuum deposition and ion-exchange techniques for

synthesizing such nanocomposites are the most powerful and promising techniques

as these are very less time consuming, economical and commercially viable. In the

existing literature, reports are present on fabrication of metal-soda glass

nanocomposites by using these techniques followed by long term heat treatment at

high temperatures either in reducing atmosphere or in a high vacuum or by laser or

Chapter-1 Introduction

26

ion irradiation [119-152]. However, nanocomposites synthesized using these

methods hardly show any shift in the surface plasmon resonance (SPR) band [126,

153-154, 181]. It is imperative to mention here that tuning of SPR band is essential

for most of optical phenomenon like optical filters and surface enhanced

fluorescence [182]. Besides this, many properties like optical absorption, reflection,

transmission, refractive index, photoluminescence, dielectric, conduction

mechanisms and surface hardness of such nanocomposites still remain unknown

even despite numerous studies have been carried out in the past few years. Many

researchers have studied some of the properties of metal-soda glass nanocomposites

synthesized as above [119-152] by taking different metal nano filler for example

silver, copper, gold etc. But most of these studies have been carried out in different

contexts and in random manner. Moreover, synthesis of metal-soda glass

nanocomposites by vacuum deposition method followed by thermal annealing in air

and properties of thus formed nanocomposites have rarely been discussed.

Keeping above facts and prospects into consideration an effort has been made

in the present research work to synthesize the silver-soda glass nanocomposites by

using vacuum deposition and ion-exchange techniques both followed by thermal

annealing in air and to investigate their structural, optical, mechanical and electrical

properties. Here soda glass has been chosen as a host matrix due to its variety of

applications in diverse fields, transparency and excellent matrix for growing small

metallic particles. Silver metal as nano filler have been taken into consideration as

they display surface plasmon resonance in the visible region besides their high

conductivity and easy availability.

In the present work, we have synthesized nanocomposites consisting of silver

nanoparticles dispersed into a soda glass matrix by vacuum deposition technique

and ion-exchange technique followed by thermal annealing in air. The effect of

nanoparticles on structural, absorption, reflection, transmission, colour, refractive

index, photoluminescence, dielectric, surface hardness and electrical conductivity

behaviour of soda glass has been studied. The structural properties of prepared

nanocomposites have been studied using transmission electron microscopy (TEM)

and scanning electron microscopy (SEM). The optical properties of the synthesized

nanocomposites have been characterized by UV-Visible spectroscopy and

photoluminescence spectroscopy. Surface hardness measurements of silver-soda

Chapter-1 Introduction

27

glass nanocomposite samples carried out using Knoop microhardness technique.

The electrical conductivity behaviour of nanocomposites have been studied by two

probe I-V measurement technique.

The area of glass-mediated assemblies of nanoparticles will definitely open up

several new avenues for efficient and flexible nanofabrication with unique and

varying combinations of properties and having high potential for successful

commercial development.

1.8 Layout of Chapters

In addition to chapter 1 (introduction) the rest of the thesis is divided into

following chapters:

Chapter 2: Materials and Experimental Techniques

This chapter gives a brief description of the experimental methods which we

have used in the synthesis and characterization of silver-soda glass nanocomposites.

In this chapter, we have given description of vacuum deposition and ion-exchange

methods which have been used as synthesis methods in present research work. Also

the description of the characterization techniques such as UV-Visible spectroscopy,

Scanning electron microscopy, Transmission electron microscopy,

Photoluminescence spectroscopy, I-V measurements, Microhardness measurements

have been included to get information regarding optical, structural, electrical and

mechanical properties.

Chapter 3: Results and Discussion

This chapter presents the results and discussion of the research work that has

been carried out. It describes the preparation methods and studies related to the

synthesis of silver-soda glass nanocomposites by vacuum deposition method and

ion-exchange method followed by thermal annealing in air. These synthesized

nanocomposites were characterized by various techniques such as UV-Visible

spectroscopy, Scanning electron microscopy and Transmission electron microscopy

to confirm the formation of silver nanoparticles in soda glass matrix. Different

optical parameters like reflection, transmission, refractive index, real and imaginary

parts of dielectric constant have been calculated and presented in details. The

photoluminescence spectra, surface hardness and electrical conductivity behaviour

Chapter-1 Introduction

28

of these nanocomposites have also been discussed.

Chapter 4: Summary, Conclusions and Scope of the Future Work

This chapter includes brief summary, major outcomes of the present

investigations and conclusions along with suggestions for future scope of present

work in this exciting area of research.

Chapter-1 Introduction

29

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