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CHAPTER 1
General Introduction
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1.1. Introduction
1.1.1 Lanthanide ions
The lanthanide elements (f-block elements) are the group of elements with
atomic number increasing from 57 (lanthanum) to 71 (lutetium). They are termed
lanthanide because the lighter elements in the series are chemically similar to
lanthanum. The lanthanides exhibit a number of features in their chemistry that
differentiate them from the d-block metals. In their electronic structure 4f orbitals are
gradually filled. Lanthanum has the electron configuration [Xe] 6s2 5d1 since, the 5d
subshell is lower in energy than 4f. As more protons are added to the nucleus, the 4f
orbitals contract rapidly and become more stable than the 5d (as the 4f orbitals
penetrate the ‘xenon core’ more), so that Ce has the electron configuration [Xe] 6s2
5d1 4f1 and the trend continues with Pr having the arrangement [Xe]6s2 4f3. This
pattern continues for the metals Nd–Eu, all of which have configurations [Xe]6s2 4fn
(n = 4–7). After europium, the stability of the half-filled f subshell is such that the
next electron is added to the 5d orbital, Gd being [Xe]6s2 5d14f7; at terbium, however,
the earlier pattern is resumed, with Tb having the configuration [Xe]6s2 4f9, and
succeeding elements to ytterbium being [Xe]6s2 4fn (n = 10–14). The last lanthanide,
lutetium, where the 4f subshell is now filled, is predictably [Xe]6s2 5d1 4f14[1].
Because of the nature of these 4f orbitals the chemistry of the lanthanides differs from
main group elements and transition metals. These orbitals are shielded from the
atom's environment by the 4d and 5p electrons. As a consequence of this, the
chemistry of these elements are largely determined by their size, which decreases
gradually from 102 pm (La3+) with increasing atomic number to 86 pm (Lu3+), the socalled lanthanide contraction. As the series La–Lu is traversed, there is a decrease in
both the atomic radii and in the radii of the Ln3+ ions, more markedly at the start of
the series. The 4f electrons are ‘inside’ the 5s and 5p electrons and are core-like in
their behaviour, being shielded from the ligands, thus taking no part in bonding, and
having spectroscopic and magnetic properties largely independent of environment.
The 5s and 5p orbitals penetrate the 4f subshell and are not shielded from increasing
nuclear charge, and hence because of the increasing effective nuclear charge they
contract as the atomic number increases[1]. All the lanthanide elements exhibit the
oxidation state of +3, whereas Ce3+ can lose its single f electron to form Ce4+ which is
resemble to the stable electronic configuration of xenon. And Eu3+ can gain an
2+
electron to form Eu
3
with the f configuration which has the extra stability of a half7
filled shell. Most of the lanthanide ions were discovered in the early 19th and some in
the 20th century, but since this fairly recent discovery, the technological importance of
the ions has been growing rapidly. The ions are abundant in the earths crust, but they
do not have the tendency to form concentrated ore deposits. A wide variety of
minerals, which can be found on a few places in the world, do contain rare earth
elements at relatively high concentration, in different compositions. The lighter ions
have a higher abundance in these ores and consequently have lower prices. The ions
have an extensive variety of technological importance in permanent magnet, catalysis,
batteries and optics [2]. The optical properties of lanthanide ions became important
due to its wide application such as Cathode Ray Tubes of computers and color
televisions [3] and in fiber optic telecommunications [4]. Moreover, lanthanide ions
doped materials have been of great interest due to their variety of applications
including phosphors, scintillators[5], solid state lighting, lasers, X-ray detectors and
optical data storage etc[5-10]. Lanthanide ions doped in inorganic host materials of
REPO4 (RE = Y, La, Gd, Lu), GdVO4, Y2O3, SnO2, Gd2O3, ZnGa2O4 etc. have been
extensively studied[11-21]. They have very high thermal and chemical stability[22].
Lanthanide ions doped nano particles are frequently used in luminescent and display
devices[23-30]. Reduction of the particle size in a crystalline system can result in
significant modification of their properties compared to those of bulk due to high
surface-to-volume ratio and quantum confinement effect[31-35]. It is especially this
luminescence property of lanthanide ions in nano size which is the subject of this
thesis.
1.1.2 Nano particles and nanotechnology
A catalyst for the development of the modern field of nanoscience and
technology is due to the work done by some renowned scientists such as J. C.
Maxwell, who in 1867 imagined in a thought experiment a tiny entity that could
manipulate individual molecules. The work of G. J. Stoney and J. J. Thompson led to
the discovery of electrons and to the development of the field of particle physics. This
work led to enquire into the nature and substance of small particles. In the 1920s,
Irving Langmuir introduced the concept of a monolayer, which is a layer of material
one molecule thick. Over the next half century, the development of various scanning
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microscopes enabled visualization and even manipulation of nano-sized structures.
Now, broadly defined, nanotechnology refers to technological study and application
involving nanoparticles. The term ‘nanotechnology’ was first used by Taniguchi et al
in 1974 [36] they defined it as the processing, separation, consolidation, and
deformation of materials by one atom or by one molecule. Nanoscience and
nanotechnologies have been defined by the Royal Society and Royal Academy of
Engineering [37,38] as follows: “Nanoscience is the study of phenomena and
manipulation of materials at atomic, molecular and macromolecular scales, where the
properties
differ
significantly
from
those
at
a
larger
scale”;
likewise,
“Nanotechnologies are the design, characterization, production and application of
structures, devices and systems by controlling shape and size at nanometer scale”.
Nanoparticles are microscopic particles with at least one dimension less than
100nm. It can be divided into three types as (i) one-dimension e.g. thin films whose
thickness is less than 100 nm, (ii) two-dimension e.g. nanowires and (iii) three
dimensions e.g. quantum dot. Nanoparticles or nanocrystals made of metals,
semiconductors, or oxides are of particular interest for their mechanical, electrical,
magnetic, optical, chemical and other properties. Nanoparticles have been used as
quantum dots and as chemical catalysts. Due to the reduction of size (less than 100
nm in at least one dimension) proportion of atoms in the surface and near surface
layers increased, thus quantum size effect i.e properties such as quantum confinement
in semiconductor particles, surface plasmon resonance in some metal particles and
superparamagnetism in magnetic materials[39-60] will vary with size and shape.
Nanoparticles are of great scientific interest as they are effectively a bridge
between bulk materials and atomic or molecular structures. A bulk material should
have constant physical properties regardless of its size, but at the nano-scale this is
often not the case. Nanoparticles exhibit a number of special properties relative to
bulk material. The properties such as melting point, color, ionization potential,
hardness, catalytic activity and selectivity[61-64] or magnetic properties such as
coercivity, permeability and saturation magnetization[65,66] changes with size and
shape. For example, ferroelectric materials smaller than 10 nm can switch their
magnetisation direction using room temperature thermal energy, thus making them
useless for memory storage. The bending of bulk copper (wire, ribbon, etc.) occurs
with movement of copper atoms/clusters at about the 50 nm scale. Copper
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nanoparticles smaller than 50 nm are considered super hard materials that do not
exhibit the same malleability and ductility as bulk copper. Nanoparticles often have
unexpected visual properties because they are small enough to confine their electrons
and produce quantum effects. For example gold nanoparticles appear deep red to
black in solution. Suspensions of nanoparticles are possible because the interaction of
the particle surface with the solvent is strong enough to overcome differences in
density, which usually result in a material either sinking or floating in a liquid. Such
behavior of nanoparticles can be classified into two types (i) 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 (ii) 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.
Presently nanoparticles are used in Magnetic Resonance Imaging for cancer tumor
[67-70], drug delivery and developing transistors, etc. Thus nanoparticles are
promising materials for wide range of industrial and technological applications.
1.1.3 Applications of nanotechnology
Recently, application of nanotechnology has increased in many fields,
including
electronics,
stain-resistant
clothing
manufacture,
and
cosmetics.
Nanotechnology has promising application in the field of human health and their wellbeing. Researchers have working repeatedly in nanotechnology and shown many
potential medical applications, such as in drug delivery, bioimaging, and new cancerfighting drugs. Molecular imaging of live cells and whole organisms is an important
tool for studying cancer biology and determining the efficacy of tumor therapies. The
development of fluorescent probes helped tremendously in this type of visualization
by the development of the so-called nanoparticles. Nanoparticles have been used in
living subjects to target tissue-specific vascular biomarkers [71] and cancer cells [7276] and to identify sentinel lymph nodes in cancer [77–80]. Another major area of
application is drug delivery. The goal is to improve contact between a drug and its
target, enabling the drug to combat the disease state more efficiently. Due to their
small size nanoparticles can pass through certain biological barriers. Also, they often
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allow a high density of therapeutic agent to be encapsulated, dispersed, or dissolved
within them.
1.1.4 Synthesis of nanoparticles
In general the synthesis of nanoparticles can be broadly grouped into two
categories: top-down and bottom-up. A top-down 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 bottom-up method of nanoparticles fabrication involves
condensation of atoms or molecular entities in a gas phase or in solution. The later is
more popular in the synthesis of nanoparticles.
1.1.5 Lanthanide-doped nanoparticles
Various preparation techniques have been reported for the preparation of
lanthanide-doped nanoparticles and novel properties of the luminescence of
lanthanide ions in these nanoscale materials. Some of the common methods reported
are sol-gel [81-85], hydrothermal [86-91], co-precipitation technique [92, 93] etc. The
reflux reduction of soluble metals’ salts in the presence of protecting polymers is the
most popular technique[94-97]. Some reducing agents used are sodium borohydride,
ascorbic acid, potassium bitartrate, etc. Various capping agents or reaction medium
such as ethylene glycol, tributyl phosphate (TBP), trihexylamine and dihexyl ether,
diethylene glycol (DEG) etc are also employed in order to control the particles size. In
this process the solutions of metal salt and capping agent under stirring is heated at
desired temperature and reducing agent is immediately added in order to hasten the
reduction reaction. Thus nucleation process becomes faster and small nanoparticles
are obtained. Earlier, nanoparticles are prepared at high temperature of 1000oC or
more[98-100]. Nanoparticles prepared in high temperature are not soluble in organic
solvent due to lack of solubilizing surface groups and the particles size are aggregated
to bigger size. Thus suitable capping agent or solvent has to be used for preparation of
nanoparticles.
1.1.6 Nucleation and growth from solutions
Precipitation technique for the synthesis of fine particles is one of the common
techniques recently employed. In this technique solid particles are obtained from a
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solution. In general soluble or suspended salts undergo reactions in solvent (aqueous
or non-aqueous). Once the solution becomes supersaturated with the product, a
precipitate is formed by either homogeneous or heterogeneous nucleation.
Homogeneous and heterogeneous nucleation refers to the formation of stable nuclei
with or without foreign species respectively. After the nuclei are formed, their growth
usually proceeds by diffusion. In diffusion-controlled growth, concentration gradients
and temperature are important factors in determining the growth rate. To form monodispersed particles, i.e. unagglomerated particles with a very narrow size distribution,
all the nuclei must form at nearly the same time, and subsequent growth must occur
without further nucleation [101] or agglomeration of the particles.
There are some factors which influenced the rate of reactions such as
concentration, temperature, pH and the order the reagents added to the solution.
Consequently, the rate of reaction affected the particle size, particle-size distribution,
amount of crystallinity, crystal structure and degree of dispersion of the prepared
particles. A multi-element material is often made by coprecipitation of the batched
ions. However, it is not always easy to co-precipitate simultaneously all the desired
ions, since different species may precipitate at different pH levels. Thus, special
attention is required to control chemical homogeneity and stoichiometry. Phase
separation may be avoided during liquid precipitation and homogeneity at the
molecular level can be improved by converting the precursor to powder form by using
spray drying or freeze drying [102, 103].
1.1.7 Stabilization of nano particles against agglomeration
Nanoscale particles have large surface areas and often agglomerate to form
either lumps or secondary particles, thus minimizing the total surface or interfacial
energy of the system. When the particles are strongly stuck together, these hard
agglomerates are called aggregates. Agglomeration of nanoparticles (fine particles)
can occur at the synthesis stage or during drying and subsequent processing of the
particles. Thus care must be taken at each step of particle production and powder
processing in order to prevent adverse agglomeration of the particles. Agglomeration
of fine particles is caused by the attractive van der Waals force and/or the driving
force that tends to minimize the total surface energy of the system. Repulsive
interparticle forces are required to prevent the agglomeration of these particles. Two
methods of stabilization are commonly used (i) dispersion using electrostatic
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repulsion. Its repulsion resulted from the interactions between a particle’s surface and
the solvent and (ii) stabilization using steric forces. The steric stabilization method is
particularly effective in dispersing high concentrations of particles.
1.1.8 Luminescence material
A luminescence material also known as phosphor is a material that emits
energy or radiation from an excited electron as light. The excitation of the electron is
caused by absorption of energy from an external source such as another electron, a
photon or an electric field. A schematic energy level scheme of the luminescent ion A
is shown in Fig.1.1. The system consists of a host lattice and a luminescent center,
often called an activator. In this thesis LaPO4 and LaF3 are used as host and Ln3+ (=
Dy3+, Eu3+, Tb3+ and Sm3+) as activator and Ce3+ as co-activator.
Fig.1.1. Schematic energy level scheme of luminescent ion A. asterisk indicates
the excited state, R the radiative return and NR the nonrediative return to
ground state.
The exciting radiation is absorbed by the activator, raising it to an excited state (A*).
The excited state returns to the ground state by emission of radiation (R). However,
every ion and every material may not show luminescence since the radiative emission
process has a competitor, viz. the nonradiative (NR) return to the ground state. In non
radiative process the energy of the excited state is used to excite the vibrations of the
host lattice, i.e. to heat the host lattice. There are various type of luminescence with
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their respective modes of excitation [104-105]. Some of the commonly known
luminescence are (i) photoluminescence (PL), (ii) electroluminescence (EL), (iii)
cathodoluminescence (CL), (iv) mechanoluminescence, (v) chemiluminescence and
(vi) thermoluminescence. A schematic illustration is shown in Fig.1.2. When an
insulator or semiconductor absorbs electromagnetic radiation i.e. a photon, an electron
may be excited to a higher energy quantum state by radiating a photon, the process is
called photoluminescence (PL).
Fig.1.2. Schematic illustrations of: (a) photoluminescence, (b) cathodoluminescence,
(c) electroluminescence.
When a material emits electromagnetic radiation as a result of application of an
electric field, the process is called electroluminescence (EL). Cathodoluminescence
(CL) is emission of light from a material that is excited by energetic electrons.
Fluorescence and phosphorescence are particular cases of photoluminescence.
Fluorescence is the emission of light by a substance that has absorbed light or 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.
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Normally, phosphorescence takes longer time and continues to emit light for few
microseconds, milliseconds, seconds, minutes, or even hours.
1.1.9 Luminescence of trivalent lanthanide ions
Lanthanide ions are characterised by an incompletely filled 4f shell. These 4f
electrons are shielded from the environment by the filled 5s and 5p shells. Since the
valence electrons are the same for all the ions, they all show very similar reactivity
and coordination behavior [1]. Luminescence of trivalent lanthanide ions occurs from
transition within 4f orbital. Since the 4f orbital lies inside the ion and is shielded from
the surroundings by the filled 5s2 and 5p6 orbitals, the influence of the surrounding (or
host lattice) on the absorption and emission of the ions is small. The electronic
configurations of trivalent rare-earth ions [106] in the ground states are shown in
Table1.1.
Table1.1. Electronic configuration of trivalent lanthanide ions in ground state.
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 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
the notation 2S+1LJ, (Russel-Saunders notation), where L represents S, P, D, F, G, H, I,
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K, L, M, .…........, corresponding to L = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ….........................,
respectively. Fig.1.3 shows the energy levels originating from the 4fn configuration as
a function of n for the trivalent ions. The width of the bars in figure gives the order of
Fig.1.3. Energy levels of the 4fn configurations of the trivalent lanthanide ions.
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magnitude of the crystal field splitting which is seen to be very small in comparison to
the metal ions. The transitions within the 4f state are parity forbidden, but due to
mixing with allowed transitions, like the 4f-5d transitions, they do occur. As a result
of the forbidden character, absorption coefficients are low and luminescence lifetimes
are long, ranging from microseconds up to several milliseconds. Three lanthanide ions
are not shown in the above figure and of these ions; La3+ and Lu3+ have a completely
empty and a completely filled 4f shell, respectively, and therefore have no optical
transitions and Ce3+ has one electron and one 4f level just above the ground state. Ce3+
has the lowest oxidation potential of the lanthanide ions making the allowed 4f-5d
transitions possible in the UV region. The crystal field has almost no effect on the
energy of the levels. For this reason, this energy level diagram can be used for
lanthanide ions in all sorts of host materials. In principle, this could lead to very
similar emission and absorption spectra for the same lanthanide ion in a range of
different hosts, but symmetry and quenching will have an effect on the emission
properties as discussed further in this chapter.
The selection rules for the different transitions are influenced by the symmetry
of the environment. The nature of the transitions varies from pure magnetic dipole
transitions to pure electric dipole transitions and mixtures of the two. The emission
spectrum of the Eu3+ ion is strongly influenced by the symmetry of the surroundings.
The main emissions of this ion occur from the 5D0 to the 7FJ (J = 0-6) levels. The
5
D0→7F1 transition is a pure magnetic dipole transition, which is practically
independent of the symmetry of the surroundings and the strength can be calculated
theoretically. The transitions to the 7F0,
3, 5
levels are forbidden both in magnetic and
electric dipole schemes and are usually very weak in the emission spectrum. The
remaining transitions to the 7F2, 4, 6 levels are pure electric dipole transitions and they
are strongly dependent on the symmetry of the environment. In a crystal site with
inversion symmetry the electric dipole transitions are strictly forbidden and the
5
D0→7F1 transition is usually the dominant emission line. In a site without inversion
symmetry the strength of the electric dipole transitions is higher. The 5D0→7F2
transition is usually the strongest emission line in this case because transitions with ∆J
= ±2 are hypersensitive to small deviations from inversion symmetry[107]. The
symmetry around the lanthanide ion can thus be obtained from the shape of the
emission spectrum of the Eu3+ ion. The other lanthanide ions have transitions that are
usually mixtures of electric and magnetic dipole transitions and the effects of the
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symmetry are less pronounced. The symmetry also has an influence on the radiative
lifetime of the 5D0 level. The radiative lifetime is the time for the luminescence to
drop to 1/e in intensity in absence of quenching. In the case of a Eu3+ ion without
inversion symmetry the rate of the forced electric dipole transition is higher than in
the case of a Eu3+ ion with inversion symmetry. This automatically means that the
radiative lifetime of a Eu3+ ion in a site with inversion symmetry is longer. Radiative
lifetimes of lanthanide ions have been calculated with several methods, of which the
Judd-Ofelt theory is the most popular[108,109]. In this theory the strength of the
electric dipole transitions are calculated from the absorption spectrum and these
strengths can be related to the radiative lifetime.
1.1.10 Luminescence quenching process
Every material may not show luminescence as well as every luminescent
material may not show prominent luminescence intensity since there is a radiative
emission process which competes with non-radiative return to the ground state. Some
of the quenching luminescence mechanisms are (i) Multi-phonon emission (ii) Crossrelaxation and (iii) energy transfer between lanthanide ions. The non-radiative return
to the ground state process, called multi-phonon emission[110-116] is possible when
the energy different/gap ∆E is equal to or less than 4-5 times the vibrational frequency
of the surroundings. The energy of the excited state is taken up by the surrounding in
the form of vibrational energy, often referred to as phonon emission. The
effectiveness of this process depends on the availability of high-energy vibrations in
the surrounding and the energy difference between the energy levels of the lanthanide
ion. The fundamental vibrations of the chemical bonds in the surrounding and the
energy of the vibration are determined by the reduced mass of a bond. Especially
bonds with hydrogen have a small reduced-mass and therefore high vibrational
energies. These bonds are therefore able to take up large amounts of energy and
effectively quench lanthanide ions with large separations between the energy levels.
In a cross-relaxation process two ions that are closely together, interact and
exchange energy[110-116]. Thus this quenching mechanism is associated with
exchange interaction between lanthanide ions. Such luminescence quenching occurs
when the concentration of lanthanide ions is large. i.e. if the Eu-Eu distance is shorter
than 5 Å, exchange interaction becomes effective. Another factor in the quenching of
lanthanide ions is the interaction between the lanthanide ions, of the same or different
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type. Two different lanthanide ions can transfer energy when they have similar
separations between the energy levels. Energy migration is another form of crossrelaxation between two ions of the same sort. The excitation state energy levels of two
identical ions are resonant, so the energy can be transferred to the neighboring ion by
cross-relaxation and travel through the material hopping from one ion to the other. An
increase in doping concentration leads to a faster energy migration through the
material, making the chance of meeting a quenching site higher. Due to this energy
migration and cross-relaxation, high doping concentration often leads to a decrease in
luminescence intensity and luminescence lifetime. However, a small mismatch in
energy can be compensated by the emission or uptake of a phonon. Energy transfer of
one lanthanide ion can be used to enhance luminescence of the other lanthanide ion.
1.2. Outlook
In this thesis an effort has been made in the synthesis of some lanthanide ions
3+
(Dy , Eu3+, Tb3+, Sm3+) doped in the inorganic host LaPO4 and LaF3 and its
physicochemical and structural properties have been studied with more emphasis on
luminescence properties. The lanthanide ions are prepared in water, ethylene glycol
(EG), dimethyl sulfoxide (DMSO) and water mixed with EG and DMSO at a relative
low temperature (150oC). The luminescence intensity of lanthanide ions varies with
respect to the solvents used. This is due to the presence of high-energy vibrations of
the organic bonds surrounding the lanthanide ion. The prepared samples are
characterized using X-ray diffraction, spectroscopy and electron microscopy.
Photoluminescence and decay process of the lanthanide ion have been discussed. In
addition, the re-dispersible behavior of the prepared sample is also discussed. The
prepared nanoparticles can be dispersed in polar solvent such as ethanol, methanol,
water, etc. Further, the prepared luminescence nanoparticles are incorporated in
polymer film such as PVA polymer.
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References
[1] S. Cotton; Lanthanides and Actinides;Wiley; England; (2006)
[2]
S. Cotton; Lanthanides and Actinides; Macmillan Education; Basingstoke;
(1991).
[3]
C. Feldmann; T. Justel; C.R. Ronda; P.J. Schmidt; Adv. Funct. Mater. 13 (2003)
511.
[4] P.C. Becker; N.A. Olsson; J.R. Simpson; Erbium doped amplifiers:
fundamentals and technology; Academic press: San Diego; (1999).
[5] G.J. Blasse; J. Chem. Mater. 6 (1994) 1465.
[6] G.J. Blasse; J. Chem. Mater.1 (1989) 294.
[7] K.A. Wickersheim; R.A. Lefever; J. Electrochem. Soc. 47 (1964) 111.
[8] A.H. Peruski; L.H. Johnson; L.F. Peruski; J. Immunol. Methods. 35 (2002) 263.
[9] G.Y. Adachi; N. Imanaka; Chem. Rev. 98 (1998) 479.
[10] J.A. Capobienco; J.C. Boyer; F. Vetrone; M.A. Speghini; M. Bettineli; J. Chem.
Mater. 14 (2002) 2915.
[11] J. Dexpert-Ghys; R. Mauricot; M.D. Faucher; J. Lumin. 69 (1996) 203.
[12] L.R. Singh; R.S. Ningthoujam; V. Sudarsan; I. Srivastava; S.D. Singh; G.K. Dey;
S.K. Kulshreshtha; Nanotechnology 19 (2008) 055201.
[13] C.P. Frank; K.L. Albert; Appl. Optics 5 (1966) 1467.
[14] N.S. Singh; R.S. Ningthoujam; L.R. Devi; N. Yaiphaba; V. Sudarsan; S.D.
Singh; R.K. Vatsa; R. Tewari; J. Appl. Phys.104 (2008) 104307.
[15] (a) N.S. Singh; R.S. Ningthoujam; N. Yaiphaba; S.D. Singh; R.K. Vatsa; J.
Appl. Phys. 105 (2009) 064303; (b) M.N. Luwang; R.S. Ningthoujam;
Jagannath; S.K. Srivastava; R.K. Vatsa; J. Am. Chem. Soc. 139 (2010) 2759.
[16] R.S. Ningthoujam; R. Shukla; R.K. Vatsa; V. Duppel; L. Kienle; A.K. Tyagi; J.
Appl. Phys. 105 (2009) 084304.
[17] C.M. Rao; V. Sudarsan; R.S. Ningthoujam; U.K. Gautam; R.K. Vatsa; A. Vinu;
A.K. Tyagi; J. Nanosci. Nanotech. 8 (2008) 5776.
[18] N. Yaiphaba; R.R. Ningthoujam; N.S. Singh; R.K.Vatsa; N. Rajmuhon; J.
Lumin. 130 (2010) 174.
[19] K. Srinivasu; R.S. Ningthoujam; V. Sudarsan; R.K. Vatsa; A.K. Tyagi; P.
Srinivasu; A.Vinu; J. Nanosci. Nanotechnol. 9 (2009) 3034.
[20] M. Ferhi; K. Horchani-Naifer; M. Ferid; J. Rare Earths 27 (2009) 182.
[21] M. Anitha; P. Ramakrishnan; A. Chatterjee; G. Alexander; H. Singh; Appl.
16
Phys. A 74 (2002) 153.
[22] U. Rambabu; N.R. Munirathnam; T.L. Prakash; S. Buddhudu; Mater.Chem. 78
(2002) 160.
[23] P. Andersson; D. Nilsson; P.O. Svensson; M.X. Chen; A. Malmstrom; T.
Kugler ; M. Berggren; Adv. Mater. 14 (2002) 1460.
[24] H. Meyssamy; K. Riwotzki; A. Kornowski; S. Naused; M. Haase; Adv. Mater.
11 (1999) 840.
[25] O. Lehman; K. Kömpe; M. Haase; J. Am. Chem. Soc. 126 (2004) 14935.
[26] K. Kömpe; H. Borchert; J. Storz; A. Lobo; S. Adam; T. Möller; M. Haase; Angw.
Chem. Int. Ed. 42 (2003) 5513.
[27] Y.H. Zhou; J. Lin; J. Alloys Compd. 408 (2006) 856.
[28] K. Riwotzki; M. Haase; J. Phys. Chem. B 102 (1998) 10129.
[29] G. Jia; Y. Song; M. Yang; Y. Huang; L. Zhang; H. You; Optical Mater. 31
(2009) 1032.
[30] W. Lu; H. Zhou; G. Chen; J. Li; Z. Zhu; Z. You; C. Tu; J. Phys. Chem. C 113
(2009) 3844.
[31] R.N. Bhargava; D. Gallagher; X. Hong; A. Nurmikko; Phys. Rev. Lett. 72
(1994) 416.
[32] L. Yu; H. Song; S.Lu; Z. Liu; L. Yang; X. Kong; J. Phys. Chem. B.108 (2004)
16697.
[33] A.P. Alivisatos; Science 271 (1996) 933.
[34] A.P. Alivisatos; J. Phys. Chem. 100 (1996) 13226.
[35] C.B. Murray; D.J. Norris; M.G. Bawendi; J. Am. Chem. Soc. 115 (1993) 8706.
[36] N. Taniguchi; On the basic concept of ‘nano-technology’ Proceedings of the
International Conference of Production Engineering; Society of Precision
Engineering; Tokyo; Japan (1974).
[37] P.J. Borm; D. Robbins; S. Haubold; T. Kuhlbusch; H. Fissan; K. Donaldson; R.
Schins; V. Stone; W. Kreyling; J. Lademann; J. Krutmann; D. Warheit; E.
Oberdorster; The potential risks of nanomaterials: a reviewcarried out for
ECETOC. Part. Fibre Toxicol. 3, 11 ( 2006).
[38] A. Dowling; R. Clift; N. Grobert; D. Hutton; R. Oliver; O. O’neill; J. Pethica; N.
Pidgeon; J. Porritt; J. Ryan; A. Seaton; S. Tendler; M. Welland; R. Whatmore;
Nanoscience and Nanotechnologies: Opportunities and Uncertainties; The
Royal Society and the Royal Academy of Engineering; London; UK (2004).
17
[39] E. L. Wolf; Nanophysics and Nanotechnology; Wiley-VCH; Verlag (2006).
[40] E. Ruduner; Nanoscopic Materials: Size Dependent Phenomena (The Royal
Society of Chemistry, London; (2006) 574.
[41] Al. E. Efros; A. L. Efros; Sov. Phys. Semicond. 16 (1982) 772.
[42] A. R. Kortan; R. Hull; R. L. Opila; M. G. Bawendi; M. L. Steigerwald; P. J.
Carroll; L.E. Brus; J. Am. Chem. Soc. 112 (1990) 1327.
[43] C.N. Chinnasamy; B. Jeyadevan; K. Shioda; K. Tohji; D.J. Djayaprawira; M.
Takahashi; R.J. Joseyphus; A. Narayanasamy; Appl. Phys. Lett. 83 (2003) 2862
[44] V. Singh; V. Srinivas; M. Ranot; S. Angappane; J.G. Park; Phys. Rev. B 82
(2010) 054417.
[45] A. Thurber; K.M. Reddy; V. Shutthanandan; M.H. Chou; C.J. O’Connor; L.
Spinu; J. Appl. Phys. 93 (2003) 7486.
[46] M. V. R. Krishna; R. A. Friesner; J. Chem. Phys. 95 (1985) 8309.
[47] C. R. Berry; Phys. Rev. 161 (1967) 848.
[48] A. P. Alivisatos; Endeavour 21 (1997) 56.
[49] S. V. Gaponenko; Optical Properties of Semiconductor Nanocrystals
(Cambridge University Press, Cambridge (1998).
[50] D. L. Klein; P. L. McEuen; J. E. Bowen Katari; A. P. Alivisatos; Appl. Phys.
Lett. 68 (1996) 2574.
[51] J. Li; J.B. Xia; Phys. Rev. B 62 (2000) 12613.
[52] J. Nanda; B. A. Kuruvilla; D. D. Sarma; Phys. Rev. B 59 (1999) 7473.
[53] S. Sapra; D. D. Sarma; Phys. Rev. B 69 (2004) 125304.
[54] G. Hodes; A. Albu-Yaron; F. Decker; P. Motisuke; Phys. Rev. B 36
(1987) 4215.
[55] H. Li; Z. Bian; J. Zhu; Y. Huo; H. Li; Y. Lu; J. Am. Chem. Soc. 129 (2007)
4538.
[56] L. E. Brus; J. Phys. Chem. 79 (1983) 5566.
[57] A. D. Yofee; Advances in Physics 42 (1993) 173.
[58] G. T. Einevoll; Phys. Rev. B 45 (1992) 3410.
[59] P. E. Lippens; M. Lannoo; Phys. Rev. B 39 (1989) 10935.
[60] R. Rossetti; R. Huli; J. M. Gibson; L. E. Brus; J. Chem. Phys. 82 (1) (1985) 552.
[61] P. Pawinrat; O. Mekasuwandumrong; J. Panprano; Catal. Commun. 10 (2009)
1380.
[62] J. Wu; C. Tseng; Appl. Catal. B 66 (2006) 51.
18
[63] F. B. Li; X. Z. Li; Appl. Catal. A 228 (2002) 15.
[64] S. Goldstein; D. Behar; J. Rabani; J. Phys. Chem. C 112 (2008) 15134.
[65] A. Thurber; K. M. Reddy; V. Shutthanandan; M. H. Engelhard; C. Wang; J.
Hays; A. Punnoose, Phys. Rev. B 76 (2007) 165206.
[66] L. D. Tung; V. Kolesnichenko; D. Caruntu; N. H. Chou; C. J. O’Connor; L.
Spinu; 626 J. Appl. Phys. 93 (2003) 7486.
[67] R. Popovtzer; A. Agrawal; N. A. Kotov; A. Popovtzer; J. Balter; T. E. Carey;
R. Kopelman, Nano Lett. 8 (2008) 4598.
[68] T. C. Yih; T. Chiming Wei; Nanomedicine: Nanotechnol. Biol. Medicine 1,
(2005) 191 .
[69] X. H. Huang; I. H. El-Sayed; W. Qian; M. A. El-Sayed; Nano Lett. 7 (2007)
1591.
[70] P. K. Jain; I. H. El-Sayed; M. A. El-Sayed; Nano Today 2 (2007) 18.
[71] X. Gao; Y. Cui; R.M. Levenson; L.W. Chung; S. Nie; Nat. Biotechnol. 22
(2004) 969.
[72] H. Tada; H. Higuchi; T.M. Wanatabe; N. Ohuchi; Cancer Res. 67 (2007) 1138.
[73] M.E. Akerman; W.C. Chan; P. Laakkonen; S.N. Bhatia; E. Ruoslahti; Proc.
Natl. Acad. Sci. U.S.A. 99 (2002) 12617.
[74] X. Montet; K. Montet-Abou; F. Reynolds; R. Weissleder; L. Josephson;
Neoplasia (Ann Arbor, MI, U.S.) 8 (2006) 214.
[75] W. Cai; D.W. Shin; K. Chen; O. Gheysens; Q. Cao; S.X. Wang; S.S. Gambhir;
X. Chen; Nano Lett. 6 (2006) 669.
[76] X. Michalet; F.F. Pinaud; L.A. Bentolila; J.M. Tsay; S. Doose; J.J. Li; G.
Sundaresan; A.M. Wu; S.S. Gambhir; S. Weiss; Science 307(2005) 538.
[77] Z.B. Li; W. Cai; X.J. Chen; Nanosci. Nanotechnol. 7 (2007) 2567.
[78] I.L. Medintz; H.T. Uyeda; E.R. Goldman; H. Mattoussi; Nat. Mater. 4 (2005)
435.
[79] E.G. Soltesz; S. Kim; R.G. Laurence; A.M. DeGrand; C.P. Parungo; D.M. Dor;
L.H. Cohn; M.G. Bawendi; J.V. Frangioni; T. Mihaljevic; Ann. Thoracic Surg.
79 (2005) 269.
[80] B. Ballou; L.A. Ernst; S. Andreko; T. Harper; J.A.J. Fitzpatrick; A.S.
Waggoner; M.P. Bruchez; Bioconjugate Chem. 18 (2007) 389.
[81] M. Wang; J. Wang; W. Chen; Y. Lui; L. Wang; Mater. Chem. Phys. 97 (2006)
219.
19
[82] M. Ferhi; K. Horchani-Naiten; M. Ferid; J. Lumin. 128 (2008) 1777.
[83] V. Sudarsan; S. Sivakumar; Frank C.J.M. van Veggel; Chem. Mater. 17 (2005)
4736.
[84] M. Yu; J. Lin; J. Fu; H.J. Zhang; J. Mater. Chem. 13 (2003) 1413.
[85] M.Yu; J.Lin; J.Fang; Chem. Mater. 17 (2005) 1783.
[86] Y.P. Fang; A.W. Xu; R.Q. Song; H.X. Zhang; L.P. You; J.C. Yu; H.Q. Liu; J.
Am. Chem. Soc. 125 (2003) 16025.
[87] R. Yan; X. Sun; X. Wang; Q. Peng; Y. Li; Chem. Eur. J. 11 (2005) 2183.
[88] Z. Huo; C. Chen; D. Chu; Y. Li; Chem. Eur. J. 13 (2007) 7708.
[89] L. Yu; D. Li; M. Yeu; J. Yao; S. Lu; Chem. Phys. 326 (2006) 478.
[90] L. Yu; D. Li; D. Yeu; Mater. Lett. 61 (2007) 4374.
[91] C. Li; Z. Hou; C. Zhang; P. Yang; G. Li; Z. Xu; Y. Fan; J. Lin; Chem. Mater 21
(2009) 4598.
[92] O. Lehmann; K. Kömpe; M. Haase; J. Am. Chem. Soc. 126 (2004) 14935.
[93] K. Riwotzki; H. Meyssamy; A. Kornowski; M. Haase; J. Phys. Chem. B 104
(2000) 2824.
[94] K. Okamoto; R. Akiyama; H. Yoshida; T. Yoshida; S. Kobayashi; J. Am.
Chem. Soc. 127 (2005) 2125.
[95] Z. Lu; G. Liu; H. Phillips; J. M. Hill; J. Chang; R. A. Kydd; Nano Lett. 1(2001)
683.
[96] A. Kiriy; S. Minko; G. Gorodyska; M. Stamm; Nano Lett. 2 (2002) 881.
[97] J. Hu; Y. Zhang; B. Liu; J. Liu; H. Zhou; Y. Xu; Y. Jiang; Z. Yang; Z.-Q. Tian;
J. 663 Am. Chem. Soc. 126 (2004) 9470.
[98] F. C. Palilla; A. K. Levine; M. Rinkevics; J. Electrochem. Soc. 112, (1965) 776.
[99] A. Bril; W. L. Wanmaker; J. Broos; J. Chem. Phys. 43 (1965) 311.
[100] M. Anitha; P. Ramakrishnan; A. Chatterjee; G. Alexander; H. Singh; Appl.
Phys. A 74 (2002) 153.
[101] V.K. La Mer; R.H. Dinegar; J. Am. Chem. Soc. 72 (1950) 4847; J. T. G.
Overbeek; Adv. Colloid Interface Sci. 15 (1982) 251; T. Sugimoto; Adv.
Colloid Interface Sci. 28 (1987) 65
[102] F. Nielsen; Manufact. Chemist 53 (1982) 38.
[103] M.W. Real; Proc. Br. Ceram. Soc. 38 (1986) 59.
[104] W. M. Yen; S. S. Shionoya; H. Yamamoto; Handbook Phosphor 2nd Edition
CRC 813 Press, Taylor and Francis Group, New York (2006) 814.
20
[105] H. W. Laverenz, An Introduction to Luminescence of Solids, Wiley and Sons,
New York (1950).
[106] W.T. Carnall; G.L. Goodman; K. Rajnak; R.S. Rana; J. Chem. Phys 90 (1989)
343.
[107] A.F. Kirby; F.S. Richardson; J. Phys. Chem. 87 (1983) 2544.
[108] B.R. Judd; Phys. Rev. 127 (1962) 750.
[109] G.S. Ofelt; J. Chem. Phys. 37 (1962) 511.
[110] G. Blasse; B. C. Grabmeir; Luminescent Materials; Springer Verlag, Berlin,
(1994) .
[111] G. Blasse; Prog. Solid State Chem. 18 (1988) 79.
[112] C. Ronda; J. Lumin. 129 (2009) 1824.
[113] G. Blasse; J. Lumin. 1 (1970) 766.
[114] M. J. Webe; Phys. Rev. 171 (1968) 283.
[115] C. B. Layne; M. J. Weber; Phys. Rev. B 16 (1977) 3259.
[116] J.M. F. Van Dijk; M. F. H. Schuurmans; J. Chem. Phys. 78 (1983) 5317.