Basic Mechanisms of Photoluminescence
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 powde
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Basic Mechanisms of Photoluminescence
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 phosphorescence 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 relatively 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 spectrometer. 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 excitation 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. K. N. Shinde et al., Phosphate Phosphors for Solid-State Lighting, Springer Series in Materials Science 174, DOI: 10.1007/978-3-642-34312-4_2, © Springer-Verlag Berlin Heidelberg 2013
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2 Basic Mechanisms of Photoluminescence
Fig. 2.1 Generic excitation and emission spectra for a fluorescent dye
These color spectra are described quantitatively by wavelength of light. The most common wavelength unit for describing fluorescence spectra is the nanometer (nm). The colors of the visible spectrum can be broken up into the approximate wavelength values [1]: Violet and indigo Blue and aqua Green Yellow and orange Red
400−450 nm 450−500 nm 500−570 nm 570−610 nm 610 to approximately 750 nm
On the short-wavelength end of the visible spectrum is the near-ultraviolet (nearUV) band from 320 to 400 nm, and on the long-wavelength end is the near-infrared (
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