CHAPTER 1 General Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/39970/7/07_chapter 1.pdf · In his lecture he considered the possibility of manipulating materials

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Synthesis, Characterization, Photoluminescence and Magnetic Properties of Zinc Oxide Nanoparticles. Ph. D. Thesis: Ningthoujam Surajkumar Singh

CHAPTER 1

General Introduction

2 Chapter 1: General Introduction


1.1. Introduction to Nanoscience and Nanotechnology

“The principles of physics, as far as I can see, do not speak against the possibi-

lity of maneuvering things atom by atom. It is not an attempt to violate any laws; it is

something, in principle, that can be done; but in practice, it has not been done

because we are too big.” The above words, by Richard Feynman in his talk entitled,

There’s plenty of room at the bottom, during a presentation to a meeting of the Ameri-

can Physical Society in 1959 at Caltech, is widely accepted as the spark that initiated

the present ‘nano’ age [1]. In his lecture he considered the possibility of manipulating

materials at atomic scale and envisions the whole volumes of the Britannica Encyclo-

pedia written in a pin head [2, 3]. The term "nanotechnology" was first used by Norio

Taniguchi in 1974, though it was not widely known. Inspired by Feynman's concepts,

K. Eric Drexler independently used the term "nanotechnology" in his 1986 book

Engines of Creation: The Coming Era of Nanotechnology, which proposed the idea of

a nanoscale "assembler" which would be able to build a copy of itself and of other

items of arbitrary complexity with atomic control [4]. Since then lot of research on

nanoscience and nanotechnology has carried out throughout the globe [5]. And it has

resulted into the discovery of new types of materials which possess physical and che-

mical properties which are not observed in their bulk counterparts [6-9]. Harold Kroto

and co-workers discovered in 1985, a new allotrope of carbon, fullerene (C60) [7].

Iijima burst into the scene in 1990s with the discovery of another allotrope of carbon,

called carbon nanotubes, and phenomena of superconductivity and ferromagnetism

were found in C60 [2, 8]. Novoselov et al. in 2004 discovered graphitic films called

graphene [9].

As the need often arises, the national nanotechnology initiative (NNI), USA

established a general working definition of nanotechnology [10]. Nanotechnology is

thus defined as possessing the following features;

• Nanotechnology involves research and technology development at the 1 nm to

100 nm range.

• Nanotechnology creates and uses structures that have novel properties because

of their small size.

• Nanotechnology builds on the ability to control or manipulate at the atomic

scale.



The most common working definition of nanoscience and nanotechnologies as

given by the Royal Society & The Royal Academy of Engineering, UK are as the

following: “Nanoscience is the study of phenomena and manipulation of materials at

atomic, molecular and macromolecular scales, where properties differ significantly

from those at a larger scale” and “nanotechnologies are the design, characterisa-

tion, production and application of structures, devices and systems by controlling

shape and size at nanometre scale” [11].

Nanoscience now deals with the science of materials and technologies having

size scale in the range of ~ 1-100 nm. One nanometre (1 nm) being equal to 10-9

metre. The prefix “nano” which is derived from the Greek word for dwarf, is referred

to the length scale of one billionth of a metre (10-9) [2, 12, 13]. To put this idea of

scale in perspective, we may take up this example; the average strand of a human hair

is roughly 75,000 nm in diameter, or from the other extreme 1 nm is the length of 10

hydrogen atoms lined up end to end. This means, the nanoscience deals with a few

hundred to a few thousand atoms or atomic clusters, whereas the microscopic world is

made out of trillion of atoms or molecules. As the grain sizes become so small; a

significant volume fraction of the atoms resides in grain boundaries and the materials

possess a large number of interfaces [14-18]. Nanoscience and nanotechnology is thus

made up of a broad umbrella covering interdisciplinary research on fabrication of

nanomaterials, tuning their properties and applications of these novel properties.

The world of nanotechnology is implanting its footprint in the present decade

very rapidly. According to a study by Global Industry Analysts Inc. [19], the global

market for products incorporating nanotechnology is projected to grow at a compound

annual growth rate of 11.1% between 2010 and 2015 and will reach the impressive

sum of US $ 2.4 trillion, that is, roughly 1/25 of the current world gross domestic

product [20]. Presently, the nanoscience and technology represents the most active

discipline all around the world and is considered as the fastest growing technology

revolution, the human history had ever seen. This intense interest in the science of the

nanomaterials, which confined within the atomic scales, stems from the fact that these

nanomaterials exhibit fundamentally interesting unique properties with great potential

of next generation technologies in electronics, computing, optics, biotechnology,

medical imaging, medicine, drug delivery, structural materials, aerospace, energy, etc.



1.2. Nanomaterials

Generally, nanomaterials have structured components with at least one dimen-

sion less than 100 nm (1 nm = 10-9m) and often exhibit distinctly different physical

and chemical properties in comparison to their micron sized counterparts [21]. In

nanoparticles, the various material properties such as electrical, mechanical, optical,

magnetic etc. can be selectively controlled by engineering the size, morphology and

composition of the particles [22]. It is possible to produce nanostructure materials,

using a variety of synthesis methods, in the various forms like thin films, powder,

quantum wires, quantum wells, quantum dots, etc. Generation of carbon nanostruc-

tures, which are related to the famous Bucky ball, is also of considerable interest.

Conventional materials have grains varying in size anywhere from hundreds of

microns (µm) to millimetres (mm). A nanocrystalline material has grains on the order

of 1-100 nm. The average size of the atom is of the order of 1 to 2 Å in radius. One

nanometre comprises of 10 Å and hence in one nanometre (nm), there may be 3-5

atoms, depending on the atomic radii [23, 24].

The most intuitive ‘nano-size’ effect is produced due to the dominance of sur-

face atoms in the nanomaterials. There are atoms which have dangling bonds or dis-

order of atomic arrangement at the outermost surface and this is compensated within a

crystal by strained lattice parameters of the surface atoms with their penultimate

atoms. As bulk lattice bonds are much abundant compared to the strained surface

bonds, this ‘tweaking’ usually goes unnoticed in most materials. If a chunk of a crystal

is broken into two, its exposed surface area would increase, thereby slightly increasing

the ratio of strained surface bonds to the bulk bonds. If the crystal is kept breaking up

into smaller units a situation will come when the surface bonds will dominate over the

bulk bonds. Theoretical calculations and experiments found that such a situation

occurs when dimensions of the matter is hundred nanometres or less [25, 26]. The new

properties of the material are now determined by strained bonds of the surface atoms

[27]. A lower melting point for nanomaterials is one of the several manifestations of

the effect of dominance of the strained surface bonds [28]. It also leads to an enhanced

chemical reactivity and for this reason, aluminium metal in the form of a fine powder

acts as fuel for rocket engines, including the booster stages that were used in space

shuttle launches. The change in the quantum mechanical states of the electrons in a

nanomaterial is another distinct nano-scale effect. There are distinct energy states for

Synthesis, Characterization, Photoluminescence and Magnetic Properties of Zinc Oxide Nanoparticles.

single atoms that the electrons can occupy. When lot of atoms come together to form

bulk lattice, due to inter

now available for an excited electron to form a continuum called conduction band.

The corresponding ground states merge to form the valence band as s

tically in Figure 1.1.

enough atoms, the availability of energy states for electrons become di

apart (Figure 1.1).

Figure 1.1: Schematic

in a single atom, atoms in a bulk crystal

Such nanocrystals are characterized as atomic clusters and are cal

confined systems [29]. The requirement for quantum confinement is that the size of

the nanocrystals should be smaller than the exciton

The band gap energy and the separation between available states for an excited

electron in a nanocrystal become larger with decreasing size.

nano-size effect, wherein, below a certain material dependant critical size, th

trons in a nanocrystal becomes ‘quantum confined’ leading to novel size dependant

interactions of the valence electrons to specific energies of excitation (notably

and electric field) [31].

respect to energy (per unit volume of the material) is called the ‘density of states’. For

quantum confined nanocrystals

sensitively dependant on the size of the nanocrystal (along the confi

[32]. When the radius of nanocrystals is less than the Bohr radius of excitons (elec

tron-hole pairs) in bulk materials, the charge carriers become spatially confined and

the continuous energy bands of bulk semiconductor will split into

vels like molecules. This spatial confinement can be in one dimension (1D), two di

Chapter 1: General Introduction

Synthesis, Characterization, Photoluminescence and Magnetic Properties of Zinc Oxide Nanoparticles.Ph. D. Thesis: Ningthoujam Surajkumar Singh

single atoms that the electrons can occupy. When lot of atoms come together to form

bulk lattice, due to interatomic interactions, several closely placed en


ding ground states merge to form the valence band as s

. However, for very small crystal (nanocrystal), due to

enough atoms, the availability of energy states for electrons become di

chematic representations of excited states available to valance electrons

e atom, atoms in a bulk crystal and atoms in a nanocrystal.

Such nanocrystals are characterized as atomic clusters and are cal

]. The requirement for quantum confinement is that the size of

the nanocrystals should be smaller than the exciton Bohr radius of the mate


electron in a nanocrystal become larger with decreasing size. This is another distinct

size effect, wherein, below a certain material dependant critical size, th


interactions of the valence electrons to specific energies of excitation (notably

]. The availability of allowed states for an excited e


quantum confined nanocrystals (NCs), the band-gap and the density of states become

sensitively dependant on the size of the nanocrystal (along the confi

When the radius of nanocrystals is less than the Bohr radius of excitons (elec

hole pairs) in bulk materials, the charge carriers become spatially confined and

the continuous energy bands of bulk semiconductor will split into

This spatial confinement can be in one dimension (1D), two di



single atoms that the electrons can occupy. When lot of atoms come together to form a

atomic interactions, several closely placed energy states are


ding ground states merge to form the valence band as shown schema-

However, for very small crystal (nanocrystal), due to lack of

enough atoms, the availability of energy states for electrons become discrete and far

of excited states available to valance electrons

s in a nanocrystal.

Such nanocrystals are characterized as atomic clusters and are called quantum

]. The requirement for quantum confinement is that the size of

Bohr radius of the material [30].


This is another distinct

size effect, wherein, below a certain material dependant critical size, the elec-


interactions of the valence electrons to specific energies of excitation (notably photons

The availability of allowed states for an excited electron with


nd the density of states become

sensitively dependant on the size of the nanocrystal (along the confined direction)

When the radius of nanocrystals is less than the Bohr radius of excitons (elec-

hole pairs) in bulk materials, the charge carriers become spatially confined and

the continuous energy bands of bulk semiconductor will split into discrete energy le-

This spatial confinement can be in one dimension (1D), two di-



mensions (2D), or in all the three dimensions (3D). Nanostructures can thus be classi-

fied into three groups depending upon the confinement of particles in a particular

crystallographic direction within a structure. The three groups are [33]:

1. Zero-dimensional (0D) nanostructure: the materials that confine electrons in

three dimensions or the structure do not permit free particle motion in any

direction. Semiconductor quantum dots (QDs), nanoparticles and colloidal

particles are some examples to include in this group.

2. One-dimensional (1D) nanostructure: the materials that confine electrons in two

dimensions or the structure do not permit free particle motion in two

dimensions. Some examples are nanorods, nanowires, nanotubes and nanofila-

ments etc.

3. Two-dimensional (2D) nanostructure: the materials exhibit a confinement of

electrons in one dimension or the structure does not permit free particle motion

in one dimension, such as nano discs or platelets, thin film on a surface and

multilayered material.

The values of energy levels are ultimately determined by the size of NCs. As a

result, the optical and electronic properties are also dependent on the size of NCs. As

shown in Figure 1.2, the energy of the bulk materials is continuous in three-

dimensional space. When the system is transited from bulk to QDs, the density of

states will be gradually reduced. When the charge carriers are confined by the quan-

tum confinement in one-dimensions, the energy is only continuous in the two-dimen-

sional space, so the materials are termed quantum wells or quantum films. Similarly,

when the carriers are confined in the two-dimensional space and the energy is only

one-dimensionally continuous, the materials are quantum wires or quantum rods

(QRs). When the carriers are confined in all three spatial dimensions, like the move-

ment of carriers limited in a ‘small box’, these are quantum dots (QDs), whose energy

is completely quantized. With the size decreasing of the materials resulting in

quantum effects, the structure and property will shift from macro to micro. QDs have

more obvious quantum effect than quantum wells and quantum wires, resulting from

the confinement of three-dimension.

The term, quantum confinement effect, was introduced to explain a wide range

of mechanical, electrical and optical properties of nano-sized materials in response to

changes in dimensions or shapes within nano-scales [34-38]. Excitons have an average


physical separation between the electron and hole, r

radius, this physical distance is different for each material. In bulk, the dimensions of

the semiconductor crystal are much larger than the Exciton Bohr radius, allowing

exciton to extend to its natural limit.

becomes small enough that it approaches the size of the material's Exciton Bohr

radius, then the electron energy levels can no longer be treated as continuous

must be treated as discrete, meaning that there is a small and finite separation between

energy levels.

Figure 1.2: The density of energy states of bulk materials, quantum wells, quantum

rods and quantum dots show the process of energy bands splitting

This situation of discrete energy levels is called qua

conditions, the semiconductor material ceases to resemble bulk and instead can be

called a quantum dot. This has large repercussions on the absorptive and emissive

behaviour of the semiconductor material.

changes in colour in the range of the visible spectrum,

ZnS appear in the ultraviolet region. Similar findings are

nanoparticles, exhibiting red shifts of the UV

less than 7 nm in size [39

1.3. II - VI Semiconductor nanoparticles

A compound semiconductor

or more different groups of the



physical separation between the electron and hole, referred to as the Exciton Bohr

adius, this physical distance is different for each material. In bulk, the dimensions of

the semiconductor crystal are much larger than the Exciton Bohr radius, allowing

exciton to extend to its natural limit. However, if the size of a semiconductor crystal


radius, then the electron energy levels can no longer be treated as continuous


The density of energy states of bulk materials, quantum wells, quantum


from bulk to quantum dots.

This situation of discrete energy levels is called quantum confinement and under these



r of the semiconductor material. For example, CdSe

r in the range of the visible spectrum, while confinement effects in

ZnS appear in the ultraviolet region. Similar findings are also observed with ZnO

nanoparticles, exhibiting red shifts of the UV-Vis absorption peaks for

less than 7 nm in size [39].

VI Semiconductor nanoparticles

compound semiconductor is a semiconductor composed of

or more different groups of the periodic table. II- VI semiconductors are thus a type of



eferred to as the Exciton Bohr

adius, this physical distance is different for each material. In bulk, the dimensions of

the semiconductor crystal are much larger than the Exciton Bohr radius, allowing the

However, if the size of a semiconductor crystal


radius, then the electron energy levels can no longer be treated as continuous - they


The density of energy states of bulk materials, quantum wells, quantum


ntum confinement and under these



example, CdSe nanocrystals show

while confinement effects in

also observed with ZnO

peaks for the particles

composed of elements from two

VI semiconductors are thus a type of



compound semiconductors composed of group II and VI elements. Nanoparticles of

these types of semiconductors, which have wide and direct band gap structures, are

very important in many fields, due to their tunable electrical and optical properties.

Their physical properties depend on their crystallite sizes and they show size depen-

dent electrical or optical properties in the quantum size regime. Due to the fundamen-

tal as well as technological importance, the modification in the energy band gap of

semiconductors is the most attractive property. Semiconductors which possess such

property of tunable energy band gap are considered to be the materials for next gene-

ration flat panel displays, photovoltaic, optoelectronic devices, laser, sensors, photonic

band gap devices, etc.

Some of the well known II - VI semiconductors are: ZnO, ZnSe, ZnS, ZnTe,

CdS, CdSe, CdTe, HgS, HgSe, HgTe, etc. In the present work, technologically

important ZnO semiconductor nanoparticles are dealt with the consideration of its

synthesis, structural, luminescence and magnetic properties. The properties and poten-

tial applications of some of the above semiconductors are given in Table.1.1 [40, 41].

Table 1.1: Properties and potential applications of Semiconductors

Group Name Bandgap (eV)

Properties and Potential applications

II -VI Zinc

Oxide

(ZnO)

3.37 Possess direct band gap, tunable from 3-4 eV

by doping with Magnesium and Cadmium.

Intrinsic n-type, p-type doping is difficult.

Used as window coatings transparent in visible

and reflective in infrared region. Possible use

in LEDs and laser diodes and in LCD displays.

Can have novel magnetic properties as well as

many pharmaceutical applications.

II – VI Zinc

Selenide

(ZnSe)

2.7 Direct band gap. Used for the development of

lasers, LEDs, sensors and fluorescent biologi-

cal labelling. Easy to n-type doping, p-type

doping is difficult. Common optical material in

infrared optics.



II – VI Zinc

Sulfide

(ZnS)

3.54 /

3.91

Direct band gap. 3.54 eV (cubic), 3.91 (hexa-

gonal). Can be doped both n-type and p-type.

Common scintillator / phosphor when suitably

doped.

II – VI Zinc

Telluride

(ZnTe)

2.26 Direct band gap. Used in solar cells, micro-

wave generators, blue LEDs and lasers. Toge-

ther with lithium niobate used to generate tera-

hertz radiation.

II – VI Cadmium

Selenide

(CdSe)

1.74 Direct band gap. Intrinsic n-type, difficult to

dope p-type, but can be p-type doped with

nitrogen. Possible use in optoelectronics.

Tested for high-efficiency solar cells.

II – VI Cadmium

Sulfide

(CdS)

2.42 Direct band gap. Used in Photoresistors and

solar cells; CdS/Cu2S was the first efficient

solar cell. Common as quantum dots. When

doped, can act as a phosphor.

II – VI Cadmium

Telluride

(CdTe)

1.49 Direct band gap. Used in solar cells with CdS.

Used in thin film solar cells and other

cadmium telluride photovoltaics; less efficient

than poly silicon but cheaper. Fluorescent at

790 nm.

1.4. Diluted Magnetic Semiconductors

Diluted magnetic semiconductors (DMS) are a class of magnetic semiconduc-

tors in which a fraction of the cations are substitutionally replaced by magnetic transi-

tion metal ions like Cr, Mn, Fe, Ni, or Co (having net spin) into a semiconducting host

such as III–V (GaAs, GaN, etc.) and II–VI (ZnTe, ZnO, etc.) compound semiconduc-

tors [42-51]. The diluted magnetic semiconductors (DMSs), which have both ferro-

magnetic and semiconducting properties, are a unique type of promising materials for

the fast emerging field of spintronics, where conventional charge-based electronics

could be replaced with devices possessing both spin and charge functionality. The po-

ssibility to manipulate the spin of the electron, as well as the charge, opens up fasci-

nating routes for processing information and data storage [52]. The spin-dependent



magnetic phenomena can be manipulated in the low-dimensional tailored magnetic

DMSs thin films for various spin-based devices to unprecedented capabilities [53, 54].

These DMSs have recently attracted increasing attention because of their potential use

in spintronic devices [43, 55-59].

As a consequence of the substitution of 3d transition-metal ions (some species

of magnetic ions, i.e., ions bearing a net magnetic moment) for the cations of the host

semiconductors, the electronic structure of the substituted 3d transition-metal impuri-

ties in semiconductors is influenced by two competing factors: strong 3d-host hybridi-

zation and strong Coulomb interactions between 3d-3d electrons. The later is respon-

sible for the multiplet structures observed in d-d optical-absorption spectra. On the

other hand, as specifically shown for the Mn-doped systems the hybridization between

the transition-metal 3d and the host valence band gives rise to the magnetic interaction

between the localized 3d spins and the carriers in the host valence band [53].

The main challenge in the practical applications of the DMS materials is the

attainment of ferromagnetism (FM) above room temperature (RT) to be compatible

with junction temperatures. Dietl et al., [60] predicted the existence of high-tempera-

ture FM in some magnetically doped wide band gap semiconductors [59, 60]. The

ferromagnetism in DMSs due to interactions between local magnetic moments of

doped magnetic ions and the spins of charge carriers in host semiconductors is known

as carrier-induced ferromagnetism [61]. The interaction among the doped magnetic

ions (mediated through holes or electrons) leads to ferromagnetic order at relatively

low temperatures [62-64]. The pair exchange interactions exhibit a strong directional

dependence and exponentially damped with increasing distance between doped

magnetic atoms [65]. Among wide band gap semiconductors, ZnO has been consi-

dered as one of the promising candidates for fabricating DMS due to its high solubility

for transition metals (TM) and superior semiconductor properties [59]. Despite large

number of studies reported on ZnO-based DMSs, there is no clear agreement about the

nature and origin of the magnetic properties of samples prepared by different methods

and different groups [57]. Some reports suggested segregation and the formation of

Co clusters as the origin of FM signal [66], but more recent results seems to indicate

the existence of intrinsic FM [67-69]. Ueda and Kawai reported FM with a Curie tem-

perature higher than RT for the Co-doped ZnO films grown by the pulsed laser depo-

sition technique [70]. RT and high temperature FM has been reported on Co doped



ZnO thin films by several groups [71-73]. Rao and Deepak observed the absence of

ferromagnetism in Co doped ZnO powder fabricated by the low-temperature decom-

position of acetate solid solution [74]. These controversial results between research

groups suggest that the magnetic properties of DMS materials seem to be very sensi-

tive to the preparation method and the structure of materials [75]. The RT FM in Co

doped ZnO are sensitive to the synthesis conditions, and are still debatable, thus re-

search work in this field is still in its infancy and stalk of RT FM semiconductors,

which are the basis for spintronics, are still in exigency. Potential applications for

ferromagnetic semiconductors and oxides include electrically controlled magnetic

sensors and actuators, high-density ultralow-power memory and logic, spin-polarized

light emitters for optical encoding, advanced optical switches and modulators, and

devices with integrated magnetic, electronic, and optical functionality. Challenges are

formidable in that in addition to coherent spin injection, the device dimensions must

be comparable if not less than the spin-coherence lengths.

1.5. ZnO nanoparticles

As far as nanomaterial is concerned, Zinc oxide (ZnO) is one of the materials

which have been attracting attention due to its numerous interested properties. Re-

cently, it has attracted much attention within the scientific community as a ‘future ma-

terial’. In fact ZnO has however, been widely studied since 1935 [53, 76], with much

of our current industry and day-to-day lives critically reliant upon this compound.

ZnO is an important II–VI compound semiconductor material due to its direct energy

gap and large excitonic binding energy at room temperature. Compared to other com-

pounds of group II-VI materials, the bonding in ZnO is largely ionic (Zn2+-O2-) with

the corresponding radii of 0.74 Å for Zn2+ and 140 Å for O2- and space group of ZnO

is P63mc or [77]. The crystal structures for ZnO could be wurtzite (B4), zinc

blende (B3), and rocksalt (B1), as schematically shown in Figure 1.3. The zinc blende

structure can be stabilized only by growth on cubic substrates, and the rocksalt (NaCl)

structure may be obtained at relatively high pressures [78, 79]. Generally, at ambient

conditions, it has been found to crystallize in the hexagonal wurtzite-type (B4) struc-

ture in which the Zn or O atoms are tetrahedrally coordinated to 4-O (or Zn) atoms

[77, 80, 81]. The schematic diagram of tetrahedrally coordinated hexagonal wurtzite

structure of ZnO is shown in figure 1.4. ZnO has the unique optical and electrical pro-


perties which can be used in a variety of applications, such as high transmittance

ductive oxide coatings for solar cells, gas sensors, UV photodetectors, and bulk acous

tic wave resonators.

Figure 1.3: Stick and ball representation of ZnO crystal structures:

(a) hexagonal wurtzite (

Figure 1.4: Tetrahedrally coordinated hexagonal wurtzite structure of ZnO

The renewed interest in this material has arisen out of the

growth technologies for the fabrication of high quality single crystals and epitaxial

layers, allowing for the realization of ZnO

vices. ZnO, which is a direct band gap semiconductor with a wide band ga

eV having a large exci

good piezoelectric characteristics, chemical stability and biocompatibility, suggest a



perties which can be used in a variety of applications, such as high transmittance

ductive oxide coatings for solar cells, gas sensors, UV photodetectors, and bulk acous

Stick and ball representation of ZnO crystal structures:

hexagonal wurtzite (B4), (b) cubic zinc blende (B

and (c) cubic rocksalt (B1)

Tetrahedrally coordinated hexagonal wurtzite structure of ZnO

The renewed interest in this material has arisen out of the


layers, allowing for the realization of ZnO-based electronic and optoelectronic de

vices. ZnO, which is a direct band gap semiconductor with a wide band ga

xcitonic binding energy of 60 meV [82-87], at room temperature,




perties which can be used in a variety of applications, such as high transmittance con-

ductive oxide coatings for solar cells, gas sensors, UV photodetectors, and bulk acous-

Stick and ball representation of ZnO crystal structures:

B3),

Tetrahedrally coordinated hexagonal wurtzite structure of ZnO.

The renewed interest in this material has arisen out of the development of


based electronic and optoelectronic de-

vices. ZnO, which is a direct band gap semiconductor with a wide band gap of 3.37

], at room temperature,




host of possible practical applications, notably in the areas of ultraviolet/blue emission

devices [88]. Together, these properties of ZnO make it an ideal candidate for a

variety of devices ranging from sensors through to ultra-violet laser diodes and nano-

technology-based devices such as displays.

At nanoscale the size not only directly impacts the defect contents but also alters

the optical properties of materials [89]. As a matter of fact, unlike the luminescence

properties of bulk crystals which have been well characterized the photoluminescence

(PL) spectra of nanostructured ZnO largely depend on synthesis methods, crystallite

size and structure, and probably the most important, the defect contents in core and

surfaces [90-94]. Group II–VI semiconductor materials have many new properties that

attract both fundamental and technological interest to many researchers. The develop-

ment of advanced display and lighting technology such as field-emission displays and

plasma display panels require phosphors which has a high efficient and low degrada-

tion [95, 96]. Since sulphide is known to easily degrade at high current densities, the

research and development of oxide-based phosphors have become very important. In

order to design the electrical, optical and magnetic properties of ZnO for the practical

applications, the control of shape and crystal structure are very important, and the

synthesis of novel nanostructures is highly desired [97]. However, it has been realized

that tuning the band gap only by changing the morphology or size of nanocrystals is

not well suited for some applications such as fluorescent imaging and nano-electronics

[98, 99].

Doping ZnO with selective element has become an important route for

enhancing and controlling its optical, electrical, and magnetic performance, which is

usually crucial for their practical applications [97]. Rare earth (RE) ions incorporated

into ZnO nanoparticles result in remarkable changes in the optical properties of ZnO

[100, 101]. Of many RE ions, Tb3+ ion is an important dopant element for green emi-

ssion band [97, 102]. ZnO:Tb3+ nanoparticles are assumed to be new type of green

phosphor with interesting Photoluminescence (PL) properties [97, 102]. ZnO-based

diluted magnetic semiconductors have been predicted to have ferromagnetic proper-

ties with Curie temperature (TC) above room temperature [60, 103, 104]. Rath et al.

[105] also studied the magnetic property of ZnO:Co nanoparticles and reported that

the material exhibits paramagnetic behaviour. Rao et al. [106] investigated the struc-

tural, optical and electrical properties of ZnO thin films prepared by spray pyrolysis



technique. ZnO, because of its potential applications, makes one of the most encoura-

ging materials for probable applications in spintronics and as (DMSs) material.

Although much experimental research has been focused in this area, the origin of

ferromagnetism in DMSs materials is still under debate [67].

1.6. Photoluminescence Spectroscopy

Photoluminescence (PL) is the emission of light from a material under optical

excitation. It is one of the kinds of the more general phenomenon of luminescence,

namely the emission of optical radiation resulting from various types of excitation:

chemical or bio-chemical, electrical energy, subatomic motions, reactions in crystals,

or stimulation of an atomic system. Accordingly, one can speak about bio-, chemi-,

electro-, thermo-, radio-luminescence, and so on. So, depending upon the source of

excitation, there are different types of luminescence [107]. The following are types of

luminescence and their corresponding mechanism of production along with the

sources of excitations [5, 41, 95, 108, 109]:

1. Photoluminescence, a result of absorption of photons.

a. Fluorescence, photoluminescence as a result of singlet-singlet electronic

relaxation (typical lifetime: nanoseconds).

b. Phosphorescence, photoluminescence as a result of triplet-singlet

electronic relaxation (typical lifetime: milliseconds to hours).

2. Chemiluminescence, a result of a chemical reaction.

a. Bioluminescence, emission as a result of biochemical reaction by a living

organism.

b. Electrochemiluminescence, a result of an electrochemical reaction.

3. Crystalloluminescence, produced during crystallization.

4. Electroluminescence, a result of an electric current passed through a substance.

a. Cathodoluminescence, a result of a luminescent material being struck by

the electrons.

5. Mechanoluminescence, a result of a mechanical action on a solid.

a. Triboluminescence, generated when bonds in a material are broken when

that material is scratched, crushed, or rubbed.

b. Fractoluminescence, generated when bonds in certain crystals are broken

by fractures.


c. Piezoluminescence

6. Radioluminescence

7. Thermoluminescen

heated after prior absorption of energy (e.g. radioactive irradiation)

8. Sonoluminescence

in a liquid when excited by sound.

The theme of the thesis is mainly compr

transition metal ion doped ZnO nanopartcles by photoluminescence and hence photo

luminescence spectroscopy is discussed here in detail.

tronic excitations are created w

Eventually, the electrons return to the ground state: if

emitted light is the photoluminescence signal.

cence or phosphoresce

many minerals and metallic compounds, in some organic compounds, and in some

living organisms such as marine fauna and insects (the most familiar one being the

firefly, whose light flashes are produced by biochemiluminescence).

Figure 1.5: Jablonski di



Piezoluminescence, produced by the action of pressure on certain soli

Radioluminescence, a result of bombardment by ionizing radiation

Thermoluminescence, the re-emission of absorbed light when a substance is

after prior absorption of energy (e.g. radioactive irradiation)

Sonoluminescence, the emission of short bursts of light from imploding bubbles

in a liquid when excited by sound.

The theme of the thesis is mainly comprised of characterization of rare

transition metal ion doped ZnO nanopartcles by photoluminescence and hence photo

oscopy is discussed here in detail. Photons are absorbed and elec

tronic excitations are created when light of sufficient energy is incident on a

Eventually, the electrons return to the ground state: if this relaxation

ight is the photoluminescence signal. Often one can also refer to fluores

scence: the latter is a type of luminescence that occurs naturally in


such as marine fauna and insects (the most familiar one being the


Jablonski diagram showing the phenomenon of Fluorescence

and Phosphorescence



, produced by the action of pressure on certain solids.

, a result of bombardment by ionizing radiation.

emission of absorbed light when a substance is

after prior absorption of energy (e.g. radioactive irradiation).

bursts of light from imploding bubbles

ised of characterization of rare earth and

transition metal ion doped ZnO nanopartcles by photoluminescence and hence photo-

Photons are absorbed and elec-

hen light of sufficient energy is incident on a material.

this relaxation is radiative, the

Often one can also refer to fluores-

nce: the latter is a type of luminescence that occurs naturally in


such as marine fauna and insects (the most familiar one being the


Fluorescence



Phosphorescence is distinguished from fluorescence for two main reasons: (a) in phos-

phorescence there is a longer time period between the excitation and the emission of

light; (b) phosphorescence may continue for some time (even hours) after the exciting

source has been removed, while fluorescence ceases when excitation is off [107]. A

Jablonski diagram showing the phenomenon of fluorescence and phosphorescence is

depicted in figure 1.5.

In the figure, S0, S1, S2 and T1 represent ground, first, second singlet excited and

triplet excited states respectively of a luminescence material. Upon absorption of light

by the material, it is excited to higher energy states, S1 or S2. If excited to S2, it is

rapidly relaxed to the lowest energy level, S1. This is called internal conversion and

occurs within 10-12 s or less. Emission from the excited state occurs at a lower energy

than absorption due to this internal conversion process as the excitation at higher

energy level has already been come down as a result of the internal conversion [5, 95,

110]. The emission can be occurred from S1 in two different ways. If S1 jumps to S0

directly, the type of photoluminescence is called fluorescence. On the other hand, if S1

jumps to S0 through the triplet state, T1, then it is called phosphorescence. Transition

of S1 to T1 is called intersystem crossing. Transition from T1 to the singlet ground

state is spin forbidden, as a result the lifetime of fluorescence and phosphorescence

differ several times. Typical lifetime for fluorescence is 10-8 s and that of phosphores-

cence ranges from millisecond to hours [40, 95-109].

1.6.1. Radiative Transition

There are several possibilities of returning an excited state to the ground state.

The observed emission from a luminescent centre is a process of returning to the

ground state radiatively. Such transitions in which there is emission of photons are

called radiative transitions. Figure 1.6 shows the configurational coordinate diagram

in a broad band emission. Upon excitation, the electron is excited in a broad optical

band and brought in a high vibrational level of the excited state. The centre thereafter

relaxes to the lowest vibrational level of the excited state and give up the excess ener-

gy to the surroundings. This relaxation usually occurs nonradiatively. The electron

then returns to the ground state by means of photon emission from the lowest vibra-

tional level of the excited state. Therefore, the difference in energy (called the Stokes

shift) between the maximum of the excitation band and that of the emission band is



observed [111]. The radiative transfer consists of absorption of the emitted light from

a donor molecule or ion by the acceptor species. In order that such transfer takes

place, the emission of the donor has to coincide with the absorption of the acceptor.

Translational symmetry leads to the formation of electronic energy bands in the bulk

of a crystalline material.

Figure 1.6: Configurational coordinate diagram in a luminescent centre.

Defects and impurities break the periodicity of the lattice and perturb the band

structure locally. The perturbation usually can be characterized by a discrete energy

level that lies within the bandgap. The state then acts as a donor or acceptor of excess

electrons in the crystal depending on the defect or impurity. Donors and acceptors

have different binding energies because electrons and holes have different effective

masses. When the temperature is sufficiently low, carriers will be trapped at these

states. If these carriers recombine radiatively, the energy of the emitted light can be

analyzed to determine the energy of the defect or impurity level [112]. Shallow levels,

which lie near the conduction or valence band edge, are more likely to participate in

radiative recombination, but the sample temperature must be small enough to

discourage thermal activation of carriers out of the traps [112]. Deep levels tend to

facilitate nonradiative recombination by providing a stop-over for electrons making

their way between the conduction and valence bands by emitting phonons [112].



Several intrinsic and impurity transitions are illustrated in Figure 1.7. Surfaces

and interfaces usually contain a high concentration of impurity and defect states. The

surface represents a drastic interruption of the material itself resulting in the forma-

tion of dangling bonds. Dangling bonds often provide numerous midgap states that

facilitate rapid nonradiative recombination.

Figure 1.7: (a–c) Radiative recombination paths: (a) band-to band;

(b) donor to valence band; (c) conduction band to acceptor;

(d) Nonradiative recombination via an intermediate state.

1.6.2. Nonradiative Transition

The energy absorbed by the luminescent materials which is not emitted as

radiation is dissipated to the crystal lattice. Transitions associated with such dissipa-

tions which are not emitted as photon are called nonradiative transitions. Nonradia-

tive transitions compete with the radiative transitions and it is crucial to suppress such

radiationless processes. In order to understand the physical processes of nonradiative

transitions in an isolated luminescent centre, the configurational coordinate diagrams

are presented in Figure 1.8. In Figure 1.8 (a), there is a Stokes shift between the

ground state and the excited state. The relaxed-excited state may reach the crossing of

the parabolas if the temperature is high enough. It is possible for electrons to return to

the ground state in a nonradiative manner via the crossing. The energy is given up as

phonon vibrations to the lattice during the process [95]. In Figure 1.8 (b), the parabo-

las of ground state and excited state are parallel. If the energy difference is equal to or

less than four to five times the higher vibrational frequency of the surrounding, it can

simultaneously excite a few high-energy vibrations, and therefore is lost for the radia-



tion of phonons. This is called multiphonon emission. In a three-parabola diagram as

shown in Figure 1.8 (c), both radiative and nonradiative processes are possible. The

parallel parabolas (solid lines) from the same configuration are crossed by a third

parabola originated from a different configuration. The transition from the ground

state to the lower excited state (solid line) is optically forbidden, but it is allowed to

transit to the upper excited state (dash line). Excitation to the transition allowed para-

bola then relaxes to the relaxed excited state of the second excited parabola. There-

after, emission occurs from it.

Figure 1.8: Configurational coordinate diagram representing nonradiative transitions.

The nonradiative processes competing with luminescence are creation of phonon

vibrations of surrounding atoms and energy transfer to electronic states of atoms,

which may be resonant or phonon assisted. Special cases of energy transfer are cross-

relaxation, where the original system loses the energy (E2 − E1) by obtaining the lower

state E1 (which may also be the ground state E0) and another system acquires the

energy by going to a higher state. Cross relaxation may take place between the same

lanthanide (being a major mechanism for quenching at higher concentration in a given

material) or between two differing elements which happen to have two pairs of energy

levels separated by the same amount [95, 113].

Nowadays, photoluminescence is mostly exploited for materials’ characteriza-

tion: from the analysis of the PL signal one can derive much information on the

emitting material, and PL may be particularly useful in surface analysis, because the

phenomenon often originates from the surface layers of the material. A noticeable ad-



vantage of PL analysis is that it is a simple, versatile, and non-destructive technique.

Obviously, PL depends much on the nature of the optical excitation, as the excitation

energy selects the initial photoexcited state and governs the penetration depth of the

incident light.

1.6.3. Luminescence Quenching

The phenomenon of decrease in intensity of fluorescent compound at its λmax is

generally termed luminescence quenching [40, 95, 110,114] Luminescence quenching

can be occurred due to various mechanisms, such as multi-phonon emission, cross

relaxation, up-conversion, energy transfer between same luminescent centres, energy

transfer between different luminescent centres, etc [95, 110, 114, 115]. Energy trans-

fers between luminescent centres are unavoidable to doped nanocrystal. For a doped

nanocrystal, large concentration of the dopant and small crystal size plays a great role

in quenching [114, 116, 117].

1.6.4. Luminescence Emission from Lanthanide ions

In display application of luminescence, rare earth ions are mostly used by

doping in inorganic solids. Basically, there are four important parameters for the

mechanism of these display materials, viz. excitation type and spectrum, relaxation to

emitting state and the decay time, and emission intensity and emission spectrum. RE

spectra were observed extremely sharp (line-spectra). The above-mentioned four

factors vary from one-host materials to another [113]. RE (lanthanide) ions are

characterized by [Xe].4f12.6s2 electronic configuration; all of them have the same

outer-shell configuration, namely 5s25p66s2. The most stable ionisation state is the

trivalent one, with the 5s and 5p electrons remaining untouched and acting to screen

the energy levels of the 4f electrons from the effect of the surrounding environment.

Ions corresponding to configurations 4f 0(La3+), 4f 7(Gd3+) and 4f 14(Lu3+) are stable.

The RE element next to these three tends to exchange electron and acquire this stable

configuration. The energy levels of the RE ions are shown in figure 1.9 [107]. The

electronic energies were determined on the basis of published data [118].

The Judd-Ofelt theory [119, 120] is usually adopted to calculate transition

probabilities from the data of absorption cross-sections of several f-f transitions.

According to this theory, the strength of an f-f transition may be expressed by the sum

of the products of three intensity parameters Ωi (i = 2, 4, 6) times the squared matrix



elements U(i) between the initial J-states and the terminal J’-state. Once the phenome-

nological parameters Ωi have been calculated, it is possible to derive the strength of

any absorption or emission transition, as well as the stimulated emission cross-section,

the fluorescence branching ratio from level J to J’, and the radiative lifetime of an

excited level. For understanding the luminescent properties of rare earth ions, it is

necessary to know their key energy levels. The energy level may be divided into three

categories, those corresponding to 4f n configuration, 4f n−15d configuration, and those

corresponding to charge transfer involving the neighbouring ions [113].

Figure 1.9: Energy levels of trivalent rare-earth ions.

1.6.4.1. Discrete f-f Transition

The transitions within 4f shells are strictly forbidden, because the parity does not

change. The forbidden transitions are observed due to the fact that the interaction of

RE ion with crystal field or with the lattice vibrations can mix state of different pari-



ties into 4f states. Coupling of 4f electrons with transient dipoles induced in the

ligands by the radiation field leads to an amplification of the even parity multipolar

transition amplitudes for transitions within 4f shell. These transitions are called as in-

duced electric dipole transition. Quite often, the transition corresponding to selection

rules (∆S = 0, L ≤ ±2 and J ≤ ±2) shows large variations in oscillator strengths depen-

ding upon the surrounding environment. These have been termed as the hypersensitive

transitions. Table 1.2 lists the various hypersensitive transitions for different RE3+

ions.

Table 1.2: Hypersensitive transitions of rare earths

Rare earth Excited state Ground state

Ce – –

Pr 3H5, 3F2 3H4

Nd 4G5/2, 2G7/2,

4G7/2 4I9/2

Pm 5G12, 5G3 5I4

Sm 4H7/2, 6F1/2,

6F3/2 6H5/2

Eu 7F2 7F1, 7F0

Gd – –

Tb 7F5 7F6

Dy 6F11/2, 6H13/2,

6H11/2 6H15/2

Ho 5G6, 3H6 5I8

Er 2H11/2, 4G11/2 4I15/2

Tm 3F4, 3H4,

3H5 3H6

Yb – –

The transitions that are not allowed as electric dipole may take place as magnetic

dipole. The magnetic dipole transitions obey the selection rules ∆L = 0, ∆S = 0, ∆J = 0

and ∆J = 1(0 → 0 excluded). Spin orbit coupling weakens the selection rule on ∆L and

∆S. Interaction of RE ions with lattice vibrations also can mix the state of different

parities into 4f states [113].

1.6.4.2. f-d Transition

4f n−15d levels may be understood as formed by the electron in the 5d orbital

interacting with 4f n−1 core. As a consequence of this strong crystal field effect on the



5d electron, 4f n−15d configurations of RE ions in solids are very different from those

of free ions. 4f n → 4f n−15d absorption of most of the RE3+ and RE2+ ions exhibits two

features. First, they consist of strong bands corresponding to the components of 5d

orbital split in the crystal field. Consequently, their spectra are similar when ions are

embedded in same type of host. Second, the structures of 5d bands can be fitted to

energy differences in the ground multiplets of the 4f n−1 configurations [113]. Figure

1.10 shows the emissions given by radiative transitions in Terbium (Tb3+) ions.

Figure 1.10: Radiative Transitions in Tb3+ ions

1.7. Synthesis of nanoparticles

The methods of synthesizing the nanomaterials are broadly classified into two

types:

(i) Top-down process and (ii) Bottom-up process

(i) Top-down process

In this process, the bulk materials are broken into nano sized particle. Semi-

conductor technology uses this method. It is a process in which etching and deposition

techniques are used to sculpt a substrate. In this process, the materials continuously

shrink to smaller dimension. Top-down approach is an example of solid-state

processing of materials.



(ii) Bottom-up process

In this process, nanomaterials are produced by building of atom by atom. In this

approach, the powder components are formed first and they are packed into the nano-

structured material. Thus the individual atom and molecules are used to construct

useful nanostructured materials.

There are few widely known methods to produce nanomaterials. Using these

methods, it is possible to produce nano-phase materials in the form of nano - powders,

nano - crystals, nano - films, nano - wires, nano - tube, nano - dots, etc. The methods

are the following:

A. Physical Methods

i) Ball milling

One of the top-down approaches is ball milling method. In ball milling, small

hard balls are allowed to rotate inside a container and then it is made to fall on a solid

with high force to crush the solid into nano crystal. Ball milling is also known as

mechanical alloying or crushing. The hardened steel or tungsten carbide balls are put

in a container along with powder of particles (< 50 µm) of a desired material. The

container is closed with tight lids. When the container is rotating around the central

axis, the material is forced to press against the walls. The milling balls impart energy

on collision and produce smaller grain size of nano particle. Few milligrams to several

kilograms of nanoparticles can be synthesized in a short time. This technique can be

operated at large scale.

ii) Plasma arc method

Plasma is an ionized gas. To produce plasma a potential difference is applied

across two electrodes. The gas at low pressure gives up its electrons and gets ionized.

Ionised gas (plasma) conducts electricity and an electric arc is maintained between the

two electrodes. A typical plasma arcing device consists of two electrodes. When an

arc is set up between two electrodes, the material evaporates from anode as positively

charged ions. These positive ions are attracted towards the other electrode (cathode)

where they pick up the electrons and they are deposited to form nanoparticles.

iii) Vapour phase deposition

Vapour phase deposition technique is used to fabricate thin films, multilayers,

nanotubes, nanofilaments and nanosized particles of different materials. These mate-

rials can be organic or inorganic. There are generally two types of vapour phase depo-



sition techniques used in electronic industry such as, Physical Vapour Deposition

(PVD) and Chemical Vapour Deposition (CVD). PVD involves the direct deposition

of gaseous phase on the substrate surfaces. CVD on the other hand involves diffusion

with chemical reactions at the substrate surfaces. CVD is complex process than PVD.

B. Chemical Methods

(1) Sol-gel method

This method involves two types of materials or components ‘sol’ and ‘gel’ . In

solutions, the molecules of nanometre size are dispersed and move around randomly

and hence the solutions are clear. In colloids the molecules are suspended in a solvent.

When they are mixed with a liquid, colloids look cloudy or even milky. A colloid that

is suspended in a liquid is called as sol i.e., sols are solid particles in a liquid. A

suspension that keeps its shape is called a gel. Gels are nothing but a continuous

network of particles with pores filled with liquid (or polymers containing liquids).

Sol-gel method involves formation of ‘sols’ in a liquid and connecting the sol

particles (or some sub-units capable of forming a porous network) to form a network.

By drying the liquid, it is possible to obtain powders, thin films or even monolithic

solid.

In sol - gel formation, first sol can be obtained by the following methods.

(a) Hydrolysis

(b) Condensation and polymerization of monomers to form particles

(c) Agglomeration of particles

After this, the formation of network which extends throughout the liquid

medium is obtained to form a gel. Synthesis of sol - gel in general involves hydrolysis

of precursors, condensation followed by polycondensation to form particles, gelation

and drying process by various routes. The precursors (starting chemicals) are to be

chosen such that they have a tendency to form gels. The rates of hydrolysis and

condensation reactions are governed by various factors such as pH, temperature,

molar ratio, nature, concentration of catalyst and process of drying. Under proper

conditions spherical nanoparticles are produced.

ii) Electro-deposition Method

This technique is used generally in electroplating and in the production of nano-

films. In this technique, two electrodes are immersed inside the electrolyte [aqueous

solutions of salt, acids etc]. When the current is passed through the electrodes, certain



mass of substance is liberated from one electrode and is deposited on the surface of

the other electrode and hence forms a thin nano - film on the surface of the electrode.

The thickness of the nano-films can be adjusted by controlling the current and the time

of deposition. These films are mechanically robust, highly flat and uniform.

iii) Wet Chemical Method

The best method for synthesizing mono dispersed nanoparticles is wet chemical

synthesis which is also called as table top method. Wet Chemical synthesis has a

further advantage of tunable surface properties of the synthesized nanoparticles,

offered by the adsorbed ions (for electrostatic stabilization) or the passivating poly-

mer. Stable colloidal nanoparticles find many futuristic applications, for example

semiconductor and metallic nanoparticles can be used to make futuristic electronic

and optoelectronic devices. Wet chemical synthesis method enables the viability for

large scale production. Moreover, among the other methods, wet chemical approach is

ease processing and inexpensive technique unlike CVD, PVD and MOCVD methods.

In this research, wet chemical method is adopted for the preparation ZnO nanopar-

ticles.

1.8. Applications of nanoparticles

Consolidated nanocomposites and nanostructures enable production of ultra-

high strength, tough structural materials, strong ductile cements, and novel magnets.

Significant developments are occurring in the sintering of nanophase ceramic mate-

rials and in textiles and plastics containing dispersed nanoparticles [121]. Since nano-

particles can literally be particles from any substance, they are also versatile enough

that they can be used in many types of technological applications, from delicate elec-

tronics to revolutionary medical procedures. Surface properties of carbon nanotubes

are being explored for catalytic applications, especially after deposition of metal nano-

particles on the surface. Main applications of fields of metal oxide nanoparticles are

electronics, pharmacy/medicine, cosmetics as well as chemistry and catalysis.

Some universal applications of the nanoparticle include [122]:

Optical: Nanoparticles could be engineered and used for anti-reflection product

coatings, producing a refractive index for various surfaces, and also providing light

based sensors for use in diagnosing cancer. Functional devices based on quantum

confinement would be of use in photonic switching and optical communications.



Magnetic: Nanoparticles have the potential to increase the density of various

storage media, and also when magnetized they can improve the detail and contrast of

MRI images.

Thermal: Specifically engineered particles could improve the transfer of heat

from collectors of solar energy to their storage tanks. They could also enhance the

coolant system currently used by transformers in these types of processes.

Mechanical: Nanoparticles could provide improved wear and tear resistance for

almost any mechanical device. They could also give these devices previously unseen

anti-corrosion abilities, as well as creating entirely new composites and structural

materials that are both lighter and stronger than those we use today.

Electronic: Because of their tiny size, nanoparticles are inherently poised to aid

in the production of high performance delicate electronics; they may provide not only

materials with a high rate of conductivity, but also sleeker parts for small consumer

electronics like cell phones. Potential applications of carbon nanotubes are many

[123,124]. Carbon nanotubes are being used as tips in scanning microscopes and also

as efficient field emitters for possible use in display devices. Since SWNTs can be

metallic or semiconducting, we would expect many applications exploiting the

electronic structure of these materials [125]. Thus, the supercapacitance of the

nanotubes can be used for applications in various ways, such as electrochemical

actuators. Field-effect transistors have been fabricated using nanotubes.

Energy: Nanoparticle batteries would be longer-lasting and have a higher energy

density than those we use today. Nanostructured electrode materials could improve the

capacity and performance of the Li-ion batteries [122]. Metal nano-particle clusters

could also have revolutionary applications for hydrogen storage; they could also

produce extremely efficient fuel cells by acting as electrocatalysts for these devices.

Nanoparticles may also pave the way for practical and renewable energy; they have

already demonstrated an ability to improve solar panel efficiency many times over.

Not only that, but when nanoparticles are used as catalysts in combustion engines,

they have shown properties that render the engine more efficient and therefore more

economic.

Biomedical: Medical applications of nanoparticles are rapidly advancing. The

pathogen-sized proportions of nanoparticles naturally make them prime candidates for

the fight against various unwanted invaders of the human body; they can be injected



into the bloodstream to fight viruses and bacteria. Nanoparticles can be equipped with

sensors and cameras as well as cancer-killing drugs. They would then be able to swim

through the bloodstream, using their sensors to locate the exact site of the cancer

where it grows. Their cameras could beam back images so that they can be located on

both optical imaging devices and MRI. One could track these tiny particles as they

make their way through the human system and deliver doses of anti-cancer drugs to

the cancer site, killing off every last molecule of the tumour without painful side

effects or unnecessary damage. This would not only make cancer treatment much less

uncomfortable for patients, but also faster and more effective.

It may soon find that wounds are dressed with antibacterial coatings of silver

nanoparticles. Nanoparticles have also been used to produce “quantum dots,” which

can detect diseases, as well as interactive foods and drinks that change flavour and

colour based on ones tastes, or in some cases may even alter their nutrient content

based on person’s state of health. Colloidal gold particles attached to DNA strands can

be employed to assay specific complementary DNAs. Drug and gene delivery will

become increasingly more effective with the use of nanoparticles and nanocapsules

[122].

Chemical: Chemical and biochemical sensors have been fabricated with

nanotubes. There are many examples where semiconductor or metal nanocrystals or

quantum dots have been tagged for use as biological sensors. Nanotechnology has

given rise to house cleaning chemicals that appear to have miraculous effects; the

nanoparticles inside these cleaning fluids have been engineered on the molecular level

so that when they encounter unwanted dirt or grime, they “eat” it. Self-cleaning

fabrics has been manufactured. The nanoparticles inside these materials have been

similarly engineered to “eat” stains; in others nano-hairs have been applied in a thin,

invisible layer over the fabric itself so that stains cannot penetrate and the resulting

fabric is either extremely stain-resistant or virtually impossible to soil.

Cosmetics/Pharmaceuticals: In the range of cosmetics the most economic

relevant application are nanoparticle-based sunscreens. Here nanoparticulate titania

and zinc oxide are used as UV light absorbing components, which are transparent due

to their small size and provide an effective protection. As a result, TiO2 and ZnO are

finding increasing application in sensitive skin and baby products and daily-wear skin

lotions.



1.9. Outlook of the Thesis

In the present work an effort has been made in the synthesis of undoped ZnO

nanoparticles using different simple techniques based on low temperature chemical

precipitation method to obtain various sizes of nanoparticles. Synthesis of rare earth

ion, Terbium (Tb) doped ZnO nanoparticles and Magnesium (Mg) doped ZnO nano-

particles have also been carried out. ZnO has also been doped with transition metal

ions Cobalt (Co) and Manganese (Mn). The synthesized nanoparticles are characte-

rized by XRD, TEM, HRTEM, SAED, EDS, UV-Vis, PL spectroscopy and VSM, etc.

The influences on the synthesis and size of the nanoparticles by the presence of water

during synthesis process have been taken into consideration in this work. Size depen-

dent property of PL emission for undoped ZnO is also studied. The structutal and PL

properties of ZnO:Tb and ZnO:Mg nanoparticles are investigated here. The structural,

magnetic and optical properties of ZnO:Co and ZnO:Mn nanoparticles are also invest-

tigated in this work. Each chapter of the thesis contains introductions and motivations

to the basic subject and under study. The chapters are self contain and can be read

independently.



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CHAPTER 1 General Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/39970/7/07_chapter 1.pdf · In his lecture he considered the possibility of manipulating materials