CHAPTER 1

General Introduction

2 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 so-

called 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

3 electron to form Eu2+ with the f7 configuration which has the extra stability of a half-

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

4 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

5 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 well-

being. Researchers have working repeatedly in nanotechnology and shown many

potential medical applications, such as in drug delivery, bioimaging, and new cancer-

fighting 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 [72-

76] 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

6 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

7 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 mono-

dispersed 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

8 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.

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

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.

9 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).

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.

Fig.1.2. Schematic illustrations of: (a) photoluminescence, (b) cathodoluminescence, (c) electroluminescence.

10 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,

11 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.

12 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 5D0→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 5D0→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

13 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) Cross-

relaxation 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

14 type. Two different lanthanide ions can transfer energy when they have similar

separations between the energy levels. Energy migration is another form of cross-

relaxation 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

(Dy3+, 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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