A phosphor is a material that emits light as a result of it absorbing energy (Fig. 1.1). Luminescence is the term given to the emission of this light.Hundreds of thousands of phosphors have been synthesized, each one having its own characteristic color of emission and period of time during which light is emitted after excitation ceases. Most phosphors are composed of a transparent microcrystalline host (or a matrix) and an activator, i.e., a small amount of intentionally added impurity atoms distributed in the host crystal. Therefore, the lumi-nescence processes of a phosphor can be divided into two parts: the processes mainly related to the host, and those that occur around and within the activator. The interaction between the host and the activator is not explicitly discussed in this section; in this sense, the host is treated only as a medium for the activator. The interaction processes such as the transfer of the host excitation energy to the activator will be discussed for each phosphor in Chapter 4.The activators are primarily responsible for the luminescence. 1.1 Luminescence Classification Luminescence, whatever may be said about it in quantum physics, is well known to everyone. You meet luminescence every day in
your office, laboratory, supermarket, on a street, and at home. You see luminescence when you switch on a fluorescent light, look at the screen of a computer or electron microscope, print on a paper, see bright traffic safety and advertising signs, deal with a document with an invisible marking, select an ideally white cotton, or relax in front of your TV set. In all these cases you see the emission of light. However, all is not luminescent that shines. When you heat a solid to a temperature in excess of about 600°C, it also shines and emits infrared radiation, but this is a thermal radiation. Lumines-cence (from the Latin luminis for light) is emission of light due to other causes than high temperature. Of course, during lumines-cence the temperature of a solid is raised; however, a luminescent material converts certain type of energy into electromagnetic ra-diation over and above thermal radiation. Besides, the decay time of luminescence (t) exceeds a period of light oscillations of molecules (T = 10-14-10-15 s). 1.1.1 Types of ExcitationA luminescent material, also called a phosphor, is a solid which con-verts the incoming radiation into visible light. Sometimes, such con-version can also be in other spectral regions, such as the ultraviolet or infrared, but usually it is a visible light (Fig. 1.1). Luminescence can occur as a result of many different kinds of excitation, excluding
thermal radiation. In practice, most often we observe the next types of energy excitation and corresponding luminescence: Electromagnetic (ultraviolet) radiation → PhotoluminescenceX-ray → X-ray luminescenceBeam of energetic electrons → cathodoluminescenceElectric current or voltage → ElectroluminescenceMechanical energy → TriboluminescenceEnergy of chemical reaction → ChemiluminescenceChemical reactions in living things → Bioluminescence Photoluminescence. Photoluminescence (PL) is the spontane-ous emission of light from a material under optical excitation. Opti-cally stimulated luminescence is excited by electromagnetic (often ultraviolet) radiation or by visible light or infrared. In this case ultra-violet (UV), vacuum ultraviolet (VUV), or infrared (IR) light is only a trigger for release of previously stored energy. X-ray luminescence. X-ray luminescence (XL) can be defined as luminescence from materials which absorb X-rays and convert the
absorbed energy efficiently into luminescence, in practice often ul-traviolet or visible emission. Cathodoluminescence. When excitation is done by electron beams generated at the electrical cathodes, the emission produced is called cathodoluminescence (CL). The screens of cathode ray tubes (CRT) and television tubes glow by this kind of emission. In cathode ray tubes zinc and cadmium sulphide phosphors often are used. Pro-duction of phosphors for TV screens is a very specialized technique, which requires a variety of colors and their appropriate persistence
to smooth out the flicker of the scan. Electroluminescence. Electroluminescence is luminescence
caused by electric current or a strong electric field. Examples of electroluminescence are neon lights, the auroras and lightning flashes. This should not be mistaken for what occurs with the ordinary incan-descent electric lights, in which electricity is used to produce heat, and it is the heat that in turn produces light. There is another type of electroluminescence known as injection luminescence. In this elec-tron are injected from an external supply across a semiconductor p-n junction. On applying a direct current voltage across the junction, such that the electrons flow to the p region, luminescence is produced
by the electron-hole recombination in that region. The light emitting diodes (LEDs), which are now commonly used as display devices in many scientific instruments, are based on this principle.