CHAPTER‐1 

General Introduction 

 

1A. General Introduction

1A.1 Nanoscience and nanotechnology

During the last decade, new direction in the materials research broadly defined as

‘nano scale science and nanotechnology’ have emerged. Nano scale science i.e.

nanoscience is basically study of particles (metals, semiconductors, insulators etc.)

with size in the range of 1 to 100 nm at least in one of the three dimensions.

Nanotechnology on the other hand literally means any technology performed on a

nanoscale that has applications in the real world. Nanotechnology encompasses

the production and application of physical, chemical, and biological systems at

scales ranging from individual atoms or molecules to submicron dimensions, as

well as the integration of the resulting nanostructures into larger systems.

Nanotechnology is likely to have a profound impact on our economy and society

in the early twenty-first century, comparable to that of semiconductor technology,

information technology, or cellular and molecular biology.1-4

The nanoparticles can be broadly divided according to their degree of

confinement i.e. the degree of restriction of charge carriers (a) 1-dimension, e.g.

thin films whose thickness is less than 100 nm, (b) 2-dimensions, e.g. Nanowires

and (c) 3-dimensions i.e. confined in all three spatial dimensions, e.g. quantum

dot. Because of this reduced size (> 100 nm) with any mentioned degree of

confinement leads to increase in proportion of atoms in the surface and near

surface layers thus quantum size effect.2,4,5-25 In this size regime the properties

such as melting point, colour (i.e. bandgap and wavelength of optical transitions),

ionisation potential, hardness, catalytic activity and selectivity,26-40 or magnetic

properties such as coercivity, permeability and saturation magnetisation,41-48

which we are used to thinking of as constant, vary with size and shape. The

optical bandgap and photoluminescence emission of binary II-VI semiconductors

can be tuned in the entire visible range of light spectrum with the variation of

size.12,49-60 Due to this variation in novel physical and chemical properties of

nanoparticles with size will lead to unique applications. The idea of the limiting

size scale of a miniaturized technology is fundamentally interesting for several

reasons. As sizes approach the atomic scale, the relevant physical laws change

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from the classical to the quantum-mechanical laws of nanophysics. The changes in

behaviour from classical, to ‘mesoscopic’, to atomic scale, are broadly understood

in contemporary physics, but the details in specific cases are complex and need to

be worked out. While the changes from classical physics to nanophysics may

mean that some existing devices will fail, the same changes open up possibilities

for new devices.

Nanoscale materials frequently show behaviour which is intermediate between

individual atom/molecule and bulk solid system. Such behaviour of particles in

the nanoscale range can be broadly divided into two types (a) Scalable effects:

Surface atoms are different from bulk atoms. As the particle size increases, the

surface to volume ratio decreases proportionally to the inverse particle size. Thus,

all properties which depend on the surface to volume ratio change continuously

and extrapolate slowly to bulk values and (b) Quantum effects: When the

molecular electronic wave function is delocalised over the entire particle then a

small, molecule-like cluster has discrete energy levels so that it may be regarded

like an atom (sometimes called a super atom). The quantum effect is more

pronounced with small particle system. The quantum size effect of nanoparticles

can only be explained with the laws of quantum mechanics.4

In fact naoparticles will be the next generation materials for the industrial and

technological developments for the mankind. To quote few of the applications of

nanoparticle in the technology include: Iron oxide nanoparticles can be used to

improve MRI (Magnetic resonance imaging) images of cancer tumors. The

nanoparticle is coated with a peptide that binds to a cancer tumor, once the

nanoparticles are attached to the tumor the magnetic property of the iron oxide

enhances the images from the Magnetic Resonance Imagining scan. Magnetic

nanoparticles that attach to cancer cells in the blood stream may allow the cancer

cells to be removed before they establish new tumours. Gold nanoparticles can be

used for drug delivery and will be able to allow heat from infrared lasers to be

targeted on cancer tumors.61-65 Titanium oxide nanoparticles are used for cleaning

the drinking water.66 Gold nanoparticles combine with organic molecules to create

a transistor known as a NOMFET (Nanoparticle Organic Memory Field-Effect

Transistor). Zinc oxide nanoparticles dispersed in industrial coatings to protect

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wood, plastic and textiles from exposure to UV rays. Silver nanoparticles in fabric

can kill bacteria making clothing odour-resistant.3

1B. Synthesis methods of nanoparticles

Synthesis methods for nanoparticles are typically grouped into two categories:

‘top-down’ and ‘bottom-up’. The first involves division of a massive solid into

smaller portions. This approach may involve milling or attrition, chemical

methods, and volatilization of a solid followed by condensation of the volatilized

components. The second, ‘bottom-up’, method of nanoparticle fabrication

involves condensation of atoms or molecular entities in a gas phase or in solution.

The latter approach is far more popular in the synthesis of nanoparticles. In this

section some synthesis methods of later category will only be focussed for metal

and semiconductor nanoparticles.

1B.1 Synthesis of metal and metal oxide nanoparticles

Metal nanoparticles such as Fe, Co, Ni, Pd, Au, Ag etc. with good dispersibility

have been synthesized through several methods, among which most adopted

synthetic route is the reflux reduction of soluble metals’ salts in the presence of

protecting polymers.67-71 The reducing agents commonly used are sodium

borohydride, ascorbic acid, potassium bitartrate etc. In the reduction method of

preparation the metal nanoparticles are obtained by refluxing the solutions of

metal salt with reducing agent and capping agent for controlling the size of

nanoparticles. In this process the solutions of metal salt and capping agent under

stirring is heated at desired temperature and reducing agent is quickly added in

order to hasten the reduction reaction. In the process the nucleation becomes faster

and small nanoparticles are obtained. These nanoparticles are also can be prepared

using organomettalic precursors and other organic media. In such preparation,

some typical high-boiling point organic solvents including oleic acid,

trioctylphosphine, ethylene glycol, etc., are well introduced to reduce the reaction

rate of the system, and limit mass transfer in both the initial nucleation and growth

steps of the nanocrystals based on the solvents’ good viscous properties or low

reducibilities.72-76 Normally in such process of nanoparticle synthesis the reaction

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temperature exceeds 100 °C. Metal nanoparticles can also be prepared using

polyvinyl pyrolidone (PVP), thiols etc. as capping agents77 and by photochemical

method.78 Metal oxides (Fe2O3, Fe3O4, etc.) and metal alloys (CoFe, NiCoFe, etc.)

are also prepared using different capping agents such as oleic acid, ethylene

glycol, sodium oleate etc.79-88

1B.2 Synthesis of binary semiconductor nanoparticles

Nanoparticles and quantum dots of binary semiconductors are normally prepared

using wet chemical method which is one of the easiest and cheapest methods of

preparation. Generally, in this method, some organic capping agents are used

during the reaction in order to control the particle size and its distribution. There

are many reports of wet chemical method in aqueous medium to prepare the

nanocrystals of this materials.89-100 In this method of preparation, the salts of

cation and anion are made solutions in water. Then the solutions are slowly mixed

together in the presence or absence of some organic compounds/solvents (capping

agents). The organic compounds/solvents are used to control the growth of

particle. By adding organic compounds/solvents a thin layer of organic compound

is adsorbed/bonded on the surface of nanoparticles thereby agglomeration of

particles is hindered. The commonly used organic compounds/solvents are

L-Glutathione, α-methacrylic, mercaptoethanol, polyvinyl alcohol, sodium citrate,

polyvinyl pyrrolidone, polynucleotide, thiophenol, etc. Another frequently

adopted method of preparation is reverse microemulsion route for its simplicity of

operation, the potential on controlling the number of nanocrystallines

encapsulated and adjusting for the core size and the shell thickness.101-105 In this

method the preparation is carried out in an isotropic liquid mixtures of oil, water

and surfactant, frequently in combination with a cosurfactant. The two basic types

of microemulsions are direct (oil dispersed in water, o/w) and reversed (water

dispersed in oil, w/o). The commonly used surfactants are dioctyl sulfosuccinate

sodium salt (AOT), Triton-X100, sodium dodecyl sulfate (SDS), cetyl

trimethylammonium bromide (CTAB), etc. Sometimes sol-gel technique is also

used for the preparation of semiconductor nanocrystals.106-109 In this process, the

sol (or solution) evolves gradually towards the formation of a gel like network

5

containing both a liquid phase and a solid phase. Typical precursors are metal

alkoxides and metal chlorides, which undergo hydrolysis and polycondensation

reactions to form a colloid.

Though above mentioned preparation techniques are simple, easy and cost

effective, the frequent problems in controlling the desired particle size and size

distribution lead to low photoluminescence (PL) quantum yield with broad

emission peak. To overcome such problems Murray et al.12 in 1993 developed

organomettalic route using long chain Tri-n-octylphosphine (TOP) and Tri-n-

octylphosphine oxide (TOPO) as cappants and solvents to prepare high quality

II-VI semiconductor quantum dots. Since then the high quality quantum dots of

CdSe, CdS etc. have been prepared successfully.57,110-116 Quantum dots prepared

using this route yields nearly monodispersed with desire size. And their optical

properties can be tuned successfully with high quantum yield PL emission. The

synthesis protocol of this route is as follows: Two stock solutions of cation and

anion are made in TOP or TBP (tributyl phosphine). The cation containing stock

solution is made hot with TOPO at 240-300 °C under inert atmosphere, then anion

stock solution is swiftly injected and quickly withdrawn after desired duration of

reaction. However, this synthesis route is very expensive due to costly solvents

such as TOP/TBP and TOPO. In addition, TOP/TBP and TOPO are hazardous,

not environment friendly solvent and unstable in open atmosphere which requires

inert atmosphere during the preparation. Recently there are few reports of

preparing high quality monodispersed nanoparticles of CdSe, CdS using cost

effective and non toxic chemicals such as liquid paraffin and oleic acid as solvents

and cappants. 53,117-119 This method has the advantage over the former since the

latter can be used to prepare in open to air atmosphere.

There are many techniques of thin films preparation of binary and ternary

semiconductors such as RF-sputtering, sol-gel, pulse laser deposition, chemical

vapour deposition, electrodeposition, spray pyrolysis, vacuum evaporation, laser

ablation, molecular beam epitaxy, chemical bath deposition (CBD), successive

ionic layer adsorption reaction (SILAR) etc.120-128 Among these methods, CBD is

most commonly used technique for deposition of nanocrystalline binary and

ternary thin films.122,129-137 This method easily gives nanocrystalline film with

6

ease. This method is simple and cost effective and can be carried out in open

atmosphere in wide range of temperature. Essentially CBD process takes place

through (i) ion by ion reaction which takes place by sequential ionic reaction, (ii)

hydroxide (cluster) mechanism through either colloidal metal hydroxide or as an

adsorbed species on the substrate but not in the bulk of the solution.

The binary II-VI semiconductor quantum dots having different optical

bandgaps and PL emissions of entire visible to near infra red (NIR) have been

synthesized using the different mentioned routes. These binary semiconductors

belong to both cubic zinc blende and wurtzite (hexagonal) structure. Figure 1

shows the typical unit cell of cubic zinc blende and wurtzite (hexagonal) structure.

A brief list of their physical and structural properties is given in the Table I. With

the successful synthesis of these semiconductor nanoparticles they will be useful

in many promising applications in display devices and photovoltaic and

solar cells. However, they have the limitations is display applications due to very

short luminescence lifetime. The lifetime of these nanoparticles ranges from

several nanoseconds to 1 μs. Again, the Cd based semiconductor nanoparticles

Figure 1 Typical unit cell of cubic zinc blende (left) and wurtzite (hexagonal) structures (right) of CdS and CdSe respectively.

which are very useful for photovoltaic, light emitting diodes for flat panel displays

and solar cells applications have environmental concerns due to their toxicity.

Despite the optical tunability of these luminescent quantum dots there is frequent

problem of quenching the luminescence due to surface defects and dangling

bonds. Thus surface atoms are not fully coordinated and eventually low quantum

yield. The use of long chain organic compounds with strong coordinating ability

with surface atoms can passivate the surface defects to certain extent resulting

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decrease in non-radiative recombination confining wavefunction of electron hole

pairs to the interior of the crystal. More efficient ways of increasing the

luminescence quantum yield include (a) adding a small amount of second

component during the synthesis of the nanoparticles. For instance, doped nano-

Table I A brief list of physical properties of some binary semiconductor nanoparticles

Semiconductor Bandgap (eV)

Type Emission wavelength region (nm)

Description

CdSe 1.74 Direct 450-650 It is an n-type, use in optoelectronics. Tested for high-efficiency solar cells.

CdS 2.42 Direct 400-450 Used in photoresistors and solar cells; CdS/Cu2S was the first efficient solar cell. Used in solar cells with CdTe. Crystals can act as Electroluminescent.

CdTe 1.49 Direct 520-700 Used in solar cells with CdS. Used in thin film solar cells and other cadmium telluride photovoltaics. Fluorescent at 790 nm.

ZnSe 2.7 Direct 370-420 Used for blue lasers and LEDs. Common optical material in infrared optics.

ZnS 3.5/3.9 Direct -- Bandgap 3.5 (cubic), 3.9 eV (hexagonal). Common scintillator/ phosphor when suitably doped.

ZnO 3.37 Direct -- Window layer coatings transparent in visible and reflective in infrared region and as conductive films in LCD displays and solar panels as a replacement. Resistant to radiation damage. Possible use in LEDs and laser diodes. Can have the novel magnetic properties as well as photocatalytic properties.

particles such as ZnS:Mn2+ 138,139 show an increase in luminescence quantum

efficiency, (b) epitaxially growing an inorganic shell of material with larger

bandgap over a smaller bandgap core which is known as core shell.58,-140-143 In

these core shell nanoparticles, the hole is confined to the core of the particles

surface while the electrons can travel over the whole particle. Because of the

absence of hole at the particle surface the quantum yield is increased. The later

method proves to have dramatic improvement in the luminescence quantum yield

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of II-VI semiconductors nanomaterials such as CdS/Cd(OH)2, CdSe/ZnSe,

CdSe/ZnS, CdSe/CdS etc.

In thin film form the II-VI semiconductors are quite useful many

optoelectronic and solar cell applications. n-type semiconductors CdSe and CdS is

useful in solar cells as window layer by making p-n junction with p-type CdTe

because near lattice match. Transparent ZnO can replace indium tin oxide (ITO)

which is a transparent conducting oxide. CdS and CdSe thin films will be quite

useful for transistors, electronic switches and optical detectors.

1B.3 Synthesis of lanthanide ions doped rare earth vanadate nanoparticles

Rare earth vanadates such as GdVO4, YVO4, LaVO4 etc. are useful host materials

for many applications in laser, cathode ray tubes and display devices when doped

with lanthanide ions (trivalent lanthanide ions emission will be discussed later).

With the evolution of nanoscience and nanotechnology and owing to their

promising applications there has been great interest in the synthesis and

luminescence studies during the last decade or so. Several synthesis methods have

been utilized during the last decade for preparation of nanoscale lanthanide ions

doped rare earth vanadate (rare earth vanadate means doped with lanthanide ions

in the remaining part of the chapter otherwise stated). Earlier, nanoparticles were

made via solid state reaction at high temperature of 1000 °C or more.144-146 This

method of synthesis has the drawback of producing aggregated bigger size of

nanoparticles as well as not soluble in organic solvents due to absence of

solubilising agents on the surface of particle. In this method of reaction

stoichiometric amounts of rare earth oxides or oxalates and lanthanide oxides with

V2O5 or NH4VO3 are thoroughly mixed together and fired at ~1000 °C for several

hours. In recent times ‘bottom up’ approach using several chemical methods have

become more popular which includes sol-gel, hydrothermal, polyol method using

urea or sodium hydroxide (NaOH) as hydrolyzing agent, precipitation, etc.147-162

In sol-gel method of preparation, stoichiometric amounts of precursors are

dissolved in water-ethanol mixture using chelating agents such as citric acid or

sodium citrate or acetic acid with a cross linking agent like polyethylene glycol

(PEG). The reaction solution which can be in either acid or basic solution is

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stirred for few hours. The solution first forms the sol showing complete mixing of

precursors and eventually to gel. The gel is finally separated for the

characterization. Hydrothermal technique is one of the versatile methods for the

preparation of crystalline nanoparticles which are dispersible in organic solvents.

In the hydrothermal process, the crystallization of particle takes place under high

temperature and vapour pressure. In a typical synthesis, required precursors are

dissolved in water and put in a tightly closed teflon lined stainless autoclave and

maintained at desired temperature for different durations of time. Thus

crystallized rare earth vanadates are obtained. Another very simple and cost

effective chemical method of synthesis is polyol method. In this method cost

effective ethylene glycol/glycerol which have very short chain is used both as

capping agent and reaction solvent. Urea or NaOH can be used as hydrolyzing

agent. Short hydrocarbons which can control the nanoparticle size is very

important for luminescence since very long hydrocarbon capping agents have high

tendency of quenching the luminescence efficiency. The synthesis protocol is very

simple which consists of putting altogether the requisite precursors of rare earth,

lanthanide ions and vanadate sources in the ethylene glycol/glycerol and reflux at

130-160 °C with urea/NaOH for 3-4 hrs. The precipitate formed is separated with

centrifugation for characterization.

Both GdVO4 and YVO4 belong to zircon type tetragonal structure having

space group I41/amd. Figure 2 shows the typical unit cell of YVO4 crystal.

Figure 2 Typical unit cell representation of YVO4 crystal

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Similar atomic positions exist with the replacement of Y by Gd in the GdVO4

tetragonal crystal. Single crystal and polycrystalline form of GdVO4 and YVO4

are important host material for laser applications. Most commonly use vanadate

crystal laser is YVO4:Nd or GdVO4:Nd or LuVO4:Nd.163-168 There are also

vanadate crystals doped with other rare earth ions, e.g. with ytterbium, erbium,

thulium or holmium etc. Due to similar size, yttrium, gadolinium or lutetium ions

can be replaced with laser-active rare earth ions without strongly affecting the

lattice structure. This is important e.g. for preserving high thermal conductivity of

the doped materials. Moreover, vanadate crystals are naturally birefringent, which

eliminates thermally induced depolarization loss in high-power lasers. Also, the

laser gain is strongly polarization dependent. Another most important application

of rare earth vanadate crystals is use as host material for lanthanide ions emission.

The lanthanide ion emissions in the visible light spectrum are used in many

applications (which are already mentioned above). Crystalline GdVO4 and YVO4

have the advantage of excellent chemical and thermal stability. The most

important factor of using these materials as host materials is their ability to act as

both holding the activators (lanthanide ions) as well as sensitizing the

luminescence by energy transfer. In the process most of the absorption energy

from vanadate is transferred to the excited states of lanthanide ions along with the

vanadate emission.

1C. Luminescence of materials

A material can become a generator or origin of light (radiation) after absorbing

suitable extraneous primary energy by two processes. In one process the absorbed

energy is converted into low quantum energy heat that diffuses through the

material and emits as thermal radiation. In the other process an appreciable

amount of absorbed energy is temporarily localized as relatively high quantum

energy excitation of atoms or small group of atoms which then emit non thermal

radiation called luminescence. The current investigation will only be dealt with

the later emission process of material. There are various types of luminescence

according to mode of excitation (Table II).169,170

Fluorescence and phosphorescence are particular cases of photoluminescence.

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Table II Various types of luminescence with their respective mode of excitation

Luminescence phenomenon Mode of excitation Photoluminescence (fluorescence, phosphorescence, delayed fluorescence

Absorption of light (photons)

Radioluminescence Ionizing radiation (X-rays, α, β, γ rays)

Cathodoluminescence Cathode rays (electron beams)

Electroluminescence Electric field

Thermoluminescence Heating after prior storage of energy (e.g. radioactive irradiation)

Chemiluminescence Chemical process (e.g. oxidation)

Bioluminescence Biochemical process

Triboluminescence Frictional and electrostatic forces

Sonoluminescence Ultrasounds

Figure 3 Schematic illustrations of: (a) photoluminescence; (b) electroluminescence; and (c) Cathodoluminescence.

The mode of excitation is absorption of a photon, which brings the absorbing

species into an electronic excited state. The emission of photons accompanying

deexcitation is then called photoluminescence (fluorescence, photoluminescence

or delayed fluorescence), which is one of the possible physical effects resulting

from interaction of light with matter. Figure 3 shows the schematic representations

of (a) photoluminescence; (b) electroluminescence; and (c) Cathodoluminescence.

Fluorescence is the emission of light by a substance that has absorbed light or

12

other electromagnetic radiation of a different wavelength. The time between intial

absorption and return to the ground state takes place in the order of 10-8 sec.

Quantum mechanically, fluorescence occurs between singlet states. However,

phosphorescence is a transition between triplet state in the excited state and singlet

ground state. Normally, phosphorescence takes longer time and continues to emit

light for few microseconds, milliseconds, seconds, minutes, or even hours.

1D. Luminescence emission from lanthanide ions

The rare-earth elements usually comprise 17 elements consisting of the 15

lanthanides from La (atomic number 57) to Lu (atomic number 71), of Sc (atomic

number 21), and of Y (atomic number 39). The electronic configurations of

trivalent rare-earth ions in the ground states are shown in Table III. As shown in

Table III Electronic configurations of trivalent Rare-Earth Ions in the ground state

the table, Sc3+ is equivalent to Ar, Y3+ to Kr, and La3+ to Xe in electronic

configuration. The lanthanides from Ce3+ to Lu3+ have one to fourteen 4f electrons

added to their inner shell configuration, which is equivalent to Xe. In the ground

state, electrons are distributed so as to provide the maximum combined spin

angular momentum (S). The spin angular momentum S is further combined with

13

the orbital angular momentum (L) to give the total angular momentum ( J) as

follows J = L – S, when the number of 4f electrons is smaller than 7, J = L + S,

when the number of 4f electrons is larger than 7. An electronic state is indicated

by notation 2S+1LJ, where L represents S, P, D, F, G, H, I, K, L, M, .…..................,

corresponding to L = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, …..............................., respectively.

Figure 4 Energy level diagram of 4fn states of lanthanide ions.

14

More accurately, an actual electronic state is expressed as an intermediate

coupling state, which can be described as a mixed state of several 2S+1LJ states

(Russel-Saunders notation) combined by spin-orbit interaction. Ions with no 4f

electrons, i.e., Sc3+, Y3+, La3+, and Lu3+, have no electronic energy levels that can

induce excitation and luminescence processes in or near the visible region. In

contrast, the ions from Ce3+ to Yb3+, which have partially filled 4f orbitals, have

energy levels characteristic of each ion and show a variety of luminescence

properties around the visible region. Many of these ions can be used as

luminescent ions in phosphors, mostly by replacing Y3+, Gd3+, La3+, and Lu3+ in

various compound crystals.

The current investigation will focus on the luminescence of some of the

lanthanide ions (Dy3+, Eu3+, Sm3+ and Tm3+) in GdVO4 and YVO4 nanoparticles.

The 4f electronic energy levels of lanthanide ions are characteristic of each ion.

Since 4f electrons are shielded by the outer 5s2 and 5p6 electrons, the levels are not

affected much by the environment or crystal field. The transitions within the 4f

state are parity forbidden. Figure 4 presents the 4fn energy levels of lanthanides.171

Though the transitions are forbidden, due to the mixing with allowed transitions,

like 4f-5d transitions, can occur. As a result of the forbidden nature, the absorption

cross section is small with long luminescence lifetimes ranging from

microseconds to several milliseconds. Each energy level designated by J in Figure

4 can split into a number of sublevels by the Stark effect due to crystal field. The

number of split sublevels is at most (2J+1) or (J+1/2) for J of integer or J of half

integer. Electrons in the excited states always relax via two competitive pathways:

one by light emission and other by phonon emission. The rate of phonon emission

w depends on the number of phonons emitted simultaneously to bridge the energy

gap and is expressed as,169

(1)

where ΔE is the energy gap to the nearest lower level and hνmax is the maximum

energy of phonons coupled to the emitting states.

Luminescence originating from 4f levels of lanthanides is predominantly due

to electric and magnetic dipole interactions with the wavefunctions of higher and

maxexp( / )w k E hν= − Δ

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lower energy levels. Electric dipole f-f transitions in free 4f ions are parity-

forbidden, but become partially allowed by mixing with orbitals having different

parity because of an odd crystal field component. The selection rule for transition

is ΔJ = ±2, ±4 and ±6. Magnetic dipole f-f transitions are not affected much by the

site symmetry because they are parity-allowed. The selection rule for transition is

ΔJ = 0, ±1 (except 0 0).172,173 For example, Eu3+ emission in GdPO4 occurs with

the dominant emission due to magnetic dipole transition, 5D07F1. This is

because of the fact that Eu3+ occupies in a crystal site having inversion symmetry,

however, the emission due to electric dipole transitions of Eu3+ in is forbidden. In

a crystalline site without inversion symmetry, the electric dipole transition 5D0

7F2 is dominant because transition with ΔJ = ±2 is hypersensitive to small

deviations from inversion symmetry. For instance Eu3+ emission corresponding to

electric dipole transition is dominant in crystalline GdVO4 or YVO4.

Since the absorption cross section of the lanthanide ions (activators) is small

due to forbidden nature of f-f transitions (α = 0.1-10 cm-1), it is required to

sensitize for obtaining efficient emission from them such as by vanadate ion

whose absorption cross section is large (α = 103-105 cm-1). Essentially there are

two types of sensitization: impurity-sensitization and host-sensitization. Among

these types of energy transfer only later type will be briefly discussed. Resonant

energy transfer can be obtained between allowed transition in the sensitizer and

forbidden activators (lanthanide). In general the transition in the host will be

allowed electric dipole transition. So, the transfer mechanism from sensitizer to

sensitizer will usually be dipole-dipole (dd) process. Depending on the degree of

forbiddeness of the transition in activator, the transfer mechanism is either a dd

process, a competition between dipole-quadrupole (dq) and exchange, or

exchange from sensitizer to activator.174 The probability per second of energy

transfer from sensitizer to activators is given by,169,174

(2)

and (3)

4 4

5 4 6 4

3 ( ) ( )( )4

A D Add

D

c h f E F EP R dEn R E

σπ τ

= ∫8 9

8 6 8 8

135 ( ) ( )( )4

A D Adq

D A

c h f E F EP R dEn R Eα σ

π τ τ= ∫

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with α = 1.266. Here, R is the separation between sensitizer to activator, n is the

refractive index of the crystal (host), αA is the absorption cross-section of

activator, αD and αA are radiative lifetime of sensitizer and activator. fD(E) and

fA(E) represent the shape of the emission and absorption spectra from sensitizer

and activator respectively, which are normalized. The integrals of above two

equations are, therefore, the overlapping ratios of these two spectra, which is a

measure of the resonance condition. Figure 5 shows the typical energy transfer

mechanism from vanadate (VO43-) excited states to excited states of lanthanide

ions (Eu3+).175,176 It is highly important to transfer some excited electrons to the

excited states of the lanthanide ions. Sensitizing elements such as Ce, Yb etc. are

used in many cases. But host material sensitized phosphors have advantage for

their ease

Figure 5 Energy transfer scheme from vanadate (VO43-) to Eu3+ in YVO4:Eu3+

in preparation. In this regard, GdVO4 or YVO4 are good choice as host materials

for lanthanide ions emission. They also have similar ionic radii and making ease

in replacing the lattice sites of Gd/Y by lanthanide ions. In these systems there is

strong absorption due to charge transfer from oxygen ligands to the central

vanadium atom inside the [VO4]3- ion which has Td symmetry with V-O distance

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of 1.72 Å. According to the molecular orbital theory, this excitation band arises

from the 1A2(1T1) ground state to the 1A1(1E) and 1E(1T2) and relaxes

nonradiatively to the lowest excited state 1A1(1A1).175 Most of these electrons at

the lowest excited state are transferred to the excited states of lanthanide ions.

1E. Luminescence quenching processes

As already mentioned in the above, there are various mechanisms which compete

with radiative transitions of excited states to the ground states of lanthanide ions.

It is important to discuss atleast some of the mechanisms for better understanding

of the luminescence of lanthanides.

1E.1 Cross-relaxation

This process is fundamentally the same as energy transfer, however after the

energy transfer process both ions are in excited states. But not all excited energy is

transferred, if only part of it is transferred, is called cross-relaxation.176-179

Thus this quenching mechanism is associated with exchange interaction between

lanthanide ions. Such luminescence quenching occurs when the concentration of

lanthanide ions is large. Figure 6 shows the typical quenching mechanism due to

cross-relaxation. For example if the Eu-Eu distance is shorter than 5 Å, exchange

Figure 6 Energy level schemes of Eu3+ and Sm3+ showing possible pathways of 5D2, 5D1emission of Eu3+ and 4G5/2 emission of Sm3+ due to cross-relaxation.

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interaction becomes effective. In case of Sm3+ also 4G5/2 emission can be

transferred by exchange interaction to the low lying energy levels of another ion

so that orange emissions from 4G5/2 are quenched. Similar mechanism is also

prevailed in Dy3+ emissions. The following cross-relaxations may occur in Eu3+

and Sm3+ to quench the higher energy level emissions due to high doping

concentration.176,177

(4)

1E.2 Multi-phonon emission

Luminescence emissions of lanthanide ions can also be quenched due to non-

radiative transition processes due to phonon vibrations of surroundings which

often refer to phonon emission. The non-radiative return to the ground state is

possible if certain conditions are fulfilled, viz. the energy difference/gap ΔE is

equal to or less than 4-5 times the vibrational frequency of the surroundings. In

such case, this amount of energy can simultaneously excite a few high-energy

vibrations, and is often lost for the radiative process. This non-radiative process is

called multi-phonon emission.176,177-182 The rates of multiphonon emission for rare

earths in crystals and oxide glasses have been found to exhibit an approximately

exponential dependence on energy gap ΔE to the next lower level of the

form,181,182

(5)

where W0 and α are phenomenological parameters. Although the energy gaps of

Eu3+ is large between the emissive 5D0 level of 12150 cm-1 , but still these ions are

substantially quenched when the environment of ions are surrounded by water of

its fourth overtone vibration (vibrational energy of OH, 3500 cm-1). However,

quenching effect is less in Tb3+ whose energy gap is large, 15000 cm-1 because its

fifth overtone of OH bond vibration is required for extensive quenching of Tb3+

emission in water. Phonon vibration of host lattice also plays a role in the

luminescence quenching mechanism. For example in classic oxide glasses the rare

3 5 3 7 3 5 3 71 0 0 3

3 4 3 6 3 45/2 5/2 9/2

( ) ( ) ( ) ( )

( ) ( ) 2 ( )

Eu D Eu F Eu D Eu F

Sm G Sm H Sm F

+ + + +

+ + +

+ → +

+ →

0 exp( )MPW W Eα= − Δ

19

earth ions do not emit efficiently, since their high vibrational frequency (~1000-

1200 cm-1) surrounds the rare earth ions. Such luminescence quenching of host

lattice vibrations are only important when the energy gap is less than about five

times the highest vibrational frequency of the host lattice and it is independent of

the concentration of activators.

1F. Luminescence from binary semiconductors

Luminescence from the binary semiconductors comes when exited electrons are

relaxed radiatively at some lower energy states. The electrons from the valence

band are excited through the bandgap by absorption with a suitable energy at the

empty quantum states of conduction band. In photoluminescence, excitation is

through light photon absorption. In the process, electron and hole possess high

energies due to transitions from ground state to an excited state and forms exciton.

The electron may recombine with the hole and relax to a lower energy state,

ultimately reaching ground state. The excess energy resulting from recombination

and relaxation may be either radiative (emits photon) or non-radiative (emitting

phonons or Auger electrons). Figure 7 depicts the two recombination processes.

Some of the radiative recombination processes consisting of band edge or near

band edge transitions, or from defect and/or activator quantum states is discussed.

Figure 7 Radiative and non-radiative processes occur during photoluminescence (e: electron, h: hole)

20

1F.1 Band edge or near band edge emission

Band edge or near band edge (exciton) emission is the most common radiative

relaxation process in the intrinsic semiconductors and insulators. The

recombination of an excited electron in the conduction band with a hole in the

valence band is called band edge emission (Figure 7A).58,110,141,183 Electron and

hole is bound by few meV to form exciton. So, radiative recombination leads to

near band edge emission at slightly lower than bandgap. Depending on the path

required to relax, radiative emission may also be characterized as either

fluorescence or phosphorescence. In a typical PL process, an electron in a

phosphor is excited by absorption of an electromagnetic wave, hv from its ground

state to an excited state. Through a fast vibrational (non-radiative) process, the

excited electron relaxes to its lowest energy excited vibrational state. For

electronic relaxation in molecules, nanoparticles or bulk solids, the emitted photon

is red shifted relative to the excitation photon energy/wavelength (i.e. Stokes shift

as discussed below) because of the presence of vibrational levels in the excited

state as well as the lower energy (e.g. ground) states (Figure 7A).

1F.2 Defect emission

Radiative recombination may also occur at the localized impurity/defect and/or

activators quantum states in the bandgap (Figure 7B). Defect state can act as a

donor (has excess electrons) or an acceptor (has a deficit of electrons) which is

dependent on the type of defect or impurity. These defects states can be

categorized into either shallow or deep levels, where shallow level defect states

have energies near the conduction band or valence band edge. In most cases, a

shallow defect exhibits radiative relaxation at temperatures sufficiently low so that

thermal energies (kT) do not excite the carriers out of the defects or trap states.

Deep levels, on the other hand, are so long-lived that they typically experience

non-radiative recombination. Despite many passivation processes, in many cases

of quantum dots these defect states are more due to large surface atoms. These

surface states act as traps for charge carriers and excitons, which generally

degrade the optical and electrical properties by increasing the rate of non-radiative

recombination. However, in some cases, the surface states can also lead to

21

radiative transitions, such as in the case of ZnO which gives green/yellow-red

emission due to oxygen related defects states.183,184-187

1F.3 Activator emission

Luminescence from intentionally incorporated impurities (activators) is called

extrinsic luminescence. These activators perturb the band structure by creating

localized quantum states within the bandgap. The predominant radiative

mechanism in extrinsic luminescence is electron-hole recombination, which can

occur via transitions from conduction band to acceptor state, donor state to

valance band or donor state to acceptor state. In some cases, this mechanism is

localized on the activator atom center. In many cases, transition and lanthanide

ions are doped in semiconductors for achieving different colours of

luminescence.87,138

1G. Outlook of the thesis

In the present investigation an effort has been made in the synthesis of lanthanide

(Dy3+, Eu3+, Sm3+ and Tm3+) doped GdVO4 and YVO4 nanoparticles through

simple polyol method. The samples so prepared were characterized using X-ray

diffraction, spectroscopy and electron microscopy. Photoluminescence and its

decay process of the lanthanide ions emission in these nanoparticles have been

discussed. PL behaviour of re-dispersible nanoparticles in polar solvents and

incorporation polymer based films has been discussed. Quantum dots and thin

films of CdSe and CdS binary semiconductors were prepared by chemical method

and characterized.

22

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