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
15
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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