Luminescence properties

Bulk silicon is a semiconductor with an indirect band structure, as schematically shown in Fig. 7.12 c. The top of the VB is located at the center of the Brillouin zone, while the CB has six minima at the equivalent (100) directions. The only allowed optical transition is a vertical transition of a photon with a subsequent electron-phonon scattering process which is needed to conserve the crystal momentum, as indicated by arrows in Fig. 7.12 c. The relevant phonon modes include transverse optical phonons (TO 56 meV), longitudinal optical phonons (LO 53.5 meV) and transverse acoustic phonons (TA 18.7 meV). At very low temperature a splitting (2.5 meV) of the main free exciton line in TO and LO replicas can be observed [Kol5]. 

Because of its indirect bandgap, bulk crystalline silicon shows only a very weak PL signal at 1100 nm, as shown for RT and 77 K in Fig. 7.9. Therefore optoelectronic applications of bulk silicon are so far limited to devices that convert light to electricity, such as solar cells or photodetectors. The observation of red PL from PS layers at room temperature in 1990 [Cal] initiated vigorous research in this field, because efficient EL, the conversion of electricity into light, seemed to be within reach. Soon it was found that in addition to the red band, luminescence in the IR as well as in the blue-green region can be observed from PS. 

The formation of luminescent PS requires HF to be present in the electrolyte, while the presence of water is not essential [Pr7]. The intensity as well as the peak energy of the PL emission increases with the PS formation current density J for a fixed electrolyte concentration. If various electrolytes are compared, the ratio between formation current density J and the critical current density JPS is more relevant than the absolute value of J. Because the porosity itself depends on J/JPS, in many studies 

Surprisingly it was found that PS luminescence excited by polarized light emerges from the sample preferentially with the same sense of polarization. This memory effect has been observed despite the fact that the electron-hole pair loses energy in the order of 1 eV in elastic processes with lifetimes in the order of 

The polarization memory has been ascribed to elongated crystals in the porous network which act as dipole radiators. This idea is supported by experiments in which micro PS was formed under illumination with polarized light. In this case luminescence excitation with unpolarized light led to a partly polarized PL. These investigations point to extended states as recombination centers, in a sense that wave functions occupy a major part of the microcrystallite and depend therefore on their shape and orientation. For local defect states of atomic dimension no polarization memory would be expected [Roll]. 

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Bawendi M G ef a/1992 Luminescence properties of CdSe quantum crystallites resonance between interior and surface localized states J. Chem. Phys. 96 946... 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

It has been established, that both DN and Ibp form complex compounds with ions Eu(III), Sm(III), Tb(III) and Dy(III), possessing luminescent properties. The most intensive luminescence is observed for complex compounds with ion Tb(III). It has been shown, that complexation has place in low acidic and neutral water solutions at pH 6,4-7,0. From the data of luminescence intensity for the complex the ratio of component Tb Fig was established equal to 1 2 by the continuous variations method. Presence at a solution of organic bases 2,2 -bipyridil, (Bipy) and 1,10-phenanthroline (Phen) causes the analytical signal amplification up to 250 (75) times as a result of the Bipy (Phen) inclusion in inner coordination sphere and formation of different ligands complexes with component ratio Tb Fig Bipy (Phen) = 1 2 1. 

Shock Luminescence. Some transparent materials give off copious amounts of light when shocked to a high pressure, and thus they can serve as shock arrival-time indicators. A technique used by McQueen and Fritz (1982) to measure arrival times of release waves is based on the reduction of shock-induced luminescence as the shock pressure is relieved. Bromoform, fused quartz, and a high-density glass have been used for their shock luminescence properties. 

In optoelectronic materials and devices, it is the luminescence properties that are of practical importance. 

Photolysis, luminescent properties, and laser activity of coumarin derivatives 97MI27. 

The active layer consists of a polymer having electronic conductive, ionic conductive, and luminescent properties, is blended with an ionic salt [48]. The polymer with the required properties can be realized by a blend of a conjugated and an ionic conductive polymer [481 or by specially designed polymers [71-73],... 

A large number of the analogues of coelenterazine have been synthesized (see Section 4.1.7). The absorption spectra of coelenterazine analogues are very similar to that of the original coelenterazine, except for e-coelenterazines. The properties of e-coelenterazines differ significantly from those of coelenterazines, particularly in their luminescence properties. The absorption spectra of e-coelenterazine (Amax 442 nm, s 9,600) and e-coelenteramide (Amax 347nm, s 19,500), both in methanol, are shown in Fig. 5.2. 

Polynuclear dendrimer complexes of this type can undergo redox reactions at the metal centre and have luminescent properties. They have been proposed as molecular photochemical devices, although no practical examples have yet been produced. 

Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1-94 Bronstein LM (2003) Nanoparticles Made in Mesoporous Solids. 226 55-89 Bronstrup M (2003) High Throughput Mass Spectrometry for Compound Characterization in Drug Discovery. 225 275-294... 

Blasse G (1976) The Influence of Charge-Transfer and Rydberg States on the Luminescence Properties of Lanthanides and Actinides. 26 43-79 Blasse G (1980) The Luminescence of Closed-Shell Transition Metal-Complexes. New Developments. 42 1-41... 

Blauer G (1974) Optical Activity of Conjugated Proteins. 18 69-129 Bleijenberg KC (1980) Luminescence Properties of Uranate Centres in Solids. 42 97-128 Boca R, Breza M, Pelikan P (1989) Vibronic Interactions in the Stereochemistry of Metal Complexes 71 57-97... 

Xiong, S. Huang, S. Tang, A. and Teng, F. (2007). Synthesis and luminescence properties of water-dispersible ZnSe nanocrystalls. Mater. Lett, 61, 5091-5094. 

Recent Studies on the Scintillation Luminescence Properties of Borophosphates... 

The Preparation and X-ray-excited Intrinsic Scintillation Luminescence Properties of Ba3BP30i2, BaBPOs and Ba3BP07 [14, 15]... 

The X-ray-excited Luminescence Properties of Ce -activated Ba3BP30i2, BaBPOs and Ba3BP07... 

Figures 21.16, 21.17 and 21.18 show the X-ray-excited luminescence properties of Ce -activated Ba3BP30i2, BaBPOs and Ba3BP07 respectively. They all show a broad emission band with peak center at about 351 nm for Ba3BP30i2 Ce, ...

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