Some Orthophosphate Phosphors

Abstract

The phenomena which involve absorption of energy and subsequent emission of light are classified generically under the term luminescence. Phosphors are luminescent materials that emit light when excited by radiation, and are usually microcrystalline powders or thin-films designed to provide visible color emission. After decades of research and development, thousands of phosphors have been prepared and some of them are widely used in many areas. Excitation by absorbance of a photon leads to a major class of technically important luminescent species which fluoresce or phosphoresce. In general, fluorescence is " fast " (ns time scale) while phosphores-cence is " slow " (longer time scale, up to hours or even days). For convenience, the topic of photoluminescence (PL) will be broadly divided into that based on rela-tively large-scale inorganic materials, mainly exhibiting phosphorescence, and that of smaller dye molecules and small-particle inorganic (" nanomaterials "), which can either fluoresce or phosphoresce. Their applications differ. For many of the derived technical applications, it is irrelevant whether the luminescence is fluorescence or phosphorescence. Either way the current range of applications is extensive, and in one case has been recognized by the award of a Nobel Prize, in 2008. 2.1 Excitation and Emission Spectra Figure 2.1 shows a typical spectrum of the excitation and emission of a fluorochrome. These spectra are generated by an instrument called a spectrofluorimeter, which comprised two spectrometers: an illuminating spectrometer and an analyzing spec-trometer. First, the dye sample is strongly illuminated by a color of light that is found to cause some fluorescence. A spectrum of the fluorescent emission is obtained by scanning with the analyzing spectrometer using this fixed illumination color. The analyzer is then fixed at the brightest emission color, and a spectrum of the excita-tion is obtained by scanning with the illuminating spectrometer and measuring the variation in emission intensity at this fixed wavelength. For the purpose of designing filters, these spectra are normalized to a scale of relative intensity.