1 CHAPTER - 1 INTRODUCTION Owing to the increase in global demand for solid-state lighting appliances, energy-saving light sources are evaluated together with the performance of the phosphors used in them. In the interest of reducing the energy demand of these appliances’, extensive research has been carried out in search of alternative light sources. In the recent years, researchers have explored methods for the preparation of new ceramic luminescent phosphors and the improvement of their luminescence properties through various experimental techniques. Because, luminescent phosphors also known as ‘light-bearing materials’ are widely used in various potential applications such as WLEDs, display devices, imaging systems, monitor screens, optoelectronics, therapeutics, lumino-magnetic applications and biological labeling, its demand increases day-by-day as novel devices keep emerging [1-6]. In this study, feasible routes have been envisaged for the synthesis of an excellent luminescent system, which has broad applications in WLEDs, display devices, and bi-functional luminescence and magnetic applications. In this chapter, the basic phenomena of luminescence, energy processes involved in a phosphor, selected synthesis methodologies, and scope and objective of the thesis are discussed. 1.1 Luminescence and Phosphor - An Introduction The concept of luminescence was introduced by Eilhardt Wiedemann [7] in 1888. According to International Union of Pure and Applied Chemistry (IUPAC) luminescence can be defined as “Spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment” [8]. Depending upon the source of excitation, luminescence may be classified into photoluminescence, cathodoluminescence, chemiluminescence, thermoluminescence, electroluminescence, radioluminescence, etc. In particular, photoluminescence (PL) is of great interest in this research in which higher-energy electromagnetic radiation (or light) is absorbed by a phosphor material and
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CHAPTER - 1 INTRODUCTIONshodhganga.inflibnet.ac.in/.../10603/49454/2/chapter_1.pdfThe trivalent lanthanide ions are most widely used as the light emitting center in phosphor materials,
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1
CHAPTER - 1
INTRODUCTION
Owing to the increase in global demand for solid-state lighting appliances,
energy-saving light sources are evaluated together with the performance of the
phosphors used in them. In the interest of reducing the energy demand of these
appliances’, extensive research has been carried out in search of alternative light
sources. In the recent years, researchers have explored methods for the
preparation of new ceramic luminescent phosphors and the improvement of their
luminescence properties through various experimental techniques. Because,
luminescent phosphors also known as ‘light-bearing materials’ are widely used in
various potential applications such as WLEDs, display devices, imaging systems,
monitor screens, optoelectronics, therapeutics, lumino-magnetic applications and
biological labeling, its demand increases day-by-day as novel devices keep
emerging [1-6]. In this study, feasible routes have been envisaged for the
synthesis of an excellent luminescent system, which has broad applications in
WLEDs, display devices, and bi-functional luminescence and magnetic
applications. In this chapter, the basic phenomena of luminescence, energy
processes involved in a phosphor, selected synthesis methodologies, and scope
and objective of the thesis are discussed.
1.1 Luminescence and Phosphor - An Introduction
The concept of luminescence was introduced by Eilhardt Wiedemann [7] in
1888. According to International Union of Pure and Applied Chemistry (IUPAC)
luminescence can be defined as “Spontaneous emission of radiation from an
electronically or vibrationally excited species not in thermal equilibrium with its
environment” [8]. Depending upon the source of excitation, luminescence may be
classified into photoluminescence, cathodoluminescence, chemiluminescence,
thermoluminescence, electroluminescence, radioluminescence, etc. In particular,
photoluminescence (PL) is of great interest in this research in which higher-energy
electromagnetic radiation (or light) is absorbed by a phosphor material and
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re-emitted as light photons having lower energy than the exciting radiation. The
absorption and emission of radiation is governed by the Franck–Condon principle
viz., “Electronic transitions are so fast (10-15 S) in comparison to the nuclear
motion (10-13 S) that immediately follow after the transition, that the nuclei have
nearly the same relative position and momentum as they did just before
transition.” Usually, most of the luminescence phenomena are concerned with the
radiative emission of visible light, which is a down-conversion (DC) process.
However, there are also instances in which a lower energy radiation has been
upconverted into higher energy radiation (for example, IR to visible) in an up-
conversion (UC) process. In a PL process, during the emission, some of the
electrons may return to the ground state through a non-radiative pathway by
releasing energy in the form of heat in addition to the radiative pathway. Thus, the
material may dissipate the absorbed energy either radiatively or non-radiatively.
The luminescence process can be explained schematically as shown in
Figure. 1.1.
Figure. 1.1: Schematic representation of a luminescence process.
Phosphor materials are optical transducers that yield luminescence when
excited by suitable electromagnetic radiations. Phosphors may be organic or
inorganic. Organic phosphors (for e.g., fluorescent organic dyes used in dye
lasers) do not have any specified activator center, whereas, inorganic phosphor is
generally characterized by specific activator centers. Hence, inorganic phosphors
mainly comprise of two sections:
(1) The host compound.
(2) The activator ion or the added metal cation (transition metal or rare-earth) or
the impurity ion [9].
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The host lattices of the inorganic phosphors may be semiconductors or insulators
and selecting a suitable host matrix with peculiar properties is important for
determining the emission properties. An activator ion is selected on the basis of its
optical activity and the stability of its valence state within the crystalline host. The
optical properties of the activator ion are determined by its electronic
configuration. Usually, the ions with an electronic configuration in which the spins
are not coupled to the phonon modes of the host lattice are selected. Hence the
activator must have a ground state with no coupled spins, no angular momenta,
and in which the sum of all electron moments J, is zero, i.e., its ground state must
have 1So electronic configuration. This means the ground state has a spherical set
of Eigen state energies, which are symmetrical to the crystal field [9].
Generally, the activator ions can be distinguished into two types: In the first
type, the activator ions strongly interact with the host lattice. This is the case when
‘d’ electrons are involved, for example, in Mn2+, Eu2+ and Ce3+ as well as for S2
ions like Pb2+ or Sb3+ ions interact with complex anions such as MoO42- or WO4
2-.
In the second type, the energy levels of the activator ion involved in the emission
process show only weak/strong interactions with the host lattice. Typical examples
are many of the lanthanide ions Ln3+ where the optical transitions take place solely
between 4f levels that were well shielded from their chemical environment by
outer electrons. Consequently, characteristic line emission spectra can be
observed. These types of activators are exemplified in phosphors like
CaMoO4:Eu3+, Y2O3:Eu3+, etc [10, 11]. Based on the type of activators involved,
inorganic phosphors can be classified into two main types:
(a) Self-activated, e.g., CaWO4 [12], CaMoO4 [13], ZnS [14], etc.