Photon-emitting material

Silicon is the most widely used material in the electronics industry. To develop silicon-based devices for optoelectronic applications, one would like to make silicon a photon-emitting material. Unfortunately, silicon is an indirect gap semiconductor and, thus, the efficiency of photon emission is extremely low since the radiative recombination of the electron-hole pair is not allowed without the assistance of a momentum-conserving phonon. Moreover, the existence of defects leads to an almost total quenching of this rather unlikely process. 

Light is emitted from the bulk material at random times and in all directions, such that the photons emitted are out of phase with each other in both time and space. Light produced by spontaneous emission is therefore called incoherent light. 

Although it is known that the colour of black body radiation is only dependent upon temperature, sparks have colours that are also dependent upon the type of emitting material. However, the form of the radiance curves does not relate exactly with known molecular energy transitions. This suggests that the mechanism of emission in excess of black body radiation is not yet fully established. It is possible that some emission bands only become active when the metal oxide particle is molten, or that the energy is dissipated simply via collisions with other molecules rather than the emission of photons. 

Radiative transfer plays a role essentially when the absorption band of the acceptor ion is allowed. A photon emitted by an ion is absorbed by an other ion before escaping from the material. This requires overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor. Radiative transfer between identical ions causes a modification of the spectral distribution. This is the case for the Ce + emission when the Stokes shift is small. The cerium emission originates from the lowest 5d state and consists of two bands because the ground state 4f is split by spin-orbit coupling into the states Fs/2 and F7/2 (Figme 5), the shorter-wavelength component 5d 4f( Fs/2) having the... 

Fluorescence intensities can be expressed in terms of quantum yield, i.e., the ratio of the number of photons emitted to those absorbed this is a function of the exciting wavelength and temperature. Quinine sulfate, yield about 50%, is commonly used as a standard, but the yield in other materials may be higher, and that of 9,10-diphenylanthracene is reported to be unity (R17). 

The LIBS technique basically involves four steps, namely laser-solid interaction, material removal, plasma formation (also called breakdown ) and analysis of the photons emitted by the plasma formed. The conditions used in each step are usually optimized in relation to the particular application. Two of the previous steps (viz. laser-solid interaction and material removal) are also present in laser ablation (see Section 9.2.2). 

Some of these carriers may recombine within the emissive layer yielding excited electron-hole pairs, termed excitons. These excitons may be produced in either the singlet or triplet states and may radiatively decay to the ground state by phosphorescence (PL) or fluorescence (FL) pathways (Fig. 1-2). An important figure of merit for electroluminescent materials is the number of photons emitted per electron injected and this is termed the internal quantum efficiency. It is clear, therefore, that the statistical maximum internal efficiency for an EL device is 25% as only one quarter of the excitons are produced in the singlet state. In practice, this maximum value is diminished further because not all of the light generated is visi-... 

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