Photoluminescence , model

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

There have been very few measurements made on the physical properties of Tg derivatives, their relative greater difficulty of preparation when compared with the Tg analogs has meant little interest in their properties. However, TglOSiMeslg has been found to show photoluminescence in the blue region of the spectrum, third-order nonlinear optical properties for TgMeg have been modeled, and electronic properties for and TgMeg have been calculated. 

When photoluminescence spectra were recorded for a Ti(OSi(CH3)3)4 model compound, upon excitation at 250 nm only one emission band was detected (at 500 nm), which was assigned to a perfect closed Ti(OSi)4 site. The excitation of these species is considered to be a LMCT transition, 02 Ti4+ —<> (0-Ti3+), and the emission is described as a radiative decay process from the charge transfer state to the ground state, O Ti3+ — 02 Ti4+. Soult et al. (94) also observed an emission band at 499 nm, which they attributed to the presence of a long-lived phosphorescent excited state. The emission band at 430 nm of TS-1 was tentatively assigned to a defective open Ti(OSi)3(OH) site (49). 

The results of infrared absorption measurements on GaAstMn prepared by the solid-state diffusion method are also in good agreement with the A°(d5+h) center model (Linnarsson et al. 1997). According to infrared spectroscopy and photoluminescence (PL) measurements for GaAstMn with a Mn concentration of 1018 cm-3, this acceptor level is located 113 meV above the top of the valence band (Chapman and Hutchinson 1967 Ilegems et al. 1975). Two photoluminescence (PL) lines observed by Liu et al. (1995) in... 

Octasilacubanes were used as a model in an attempt to understand the optical properties of porous silicon because both porous silicon and octasilacubane show a broad photoluminescence spectra and large Stokes shifts52. 16 for example, shows an absorption edge at ca 3.2 eV and a broad photoluminescence spectrum with a peak at 2.50 eV. 

Partially functionalized cyclopolysilanes recently attracted attention as model substances for siloxene and luminescent silicon. The yellow luminescent silicon is formed by the anodic oxidation of elemental silicon in HF-containing solutions and may be used for the development of silicon-based materials for light-emitting structures which could be integrated into optoelectronic devices77. Because the visible photoluminescence of... 

A complete and satisfactory characterization of quantum dots prepared by any of these methods requires many of the same techniques listed for metal nanoparticles described already (see above). In addition to critical electronic properties, photoluminescence spectroscopy is an extremely valuable tool to obtain preliminary information on size and size distribution of quantum dots, which can in many cases (i.e., for larger sizes and quasi-spherical shapes) be estimated from 2max and the full width at half maximum (fwhm) of the absorption or emission peak using approximations such as Bras model or the hyperbolic band model [113]. 

Ab initio and density functional calculations of potential energy surfaces for the ground and excited electronic states of model clusters simulating various point defects, impurities, and their combinations in nanosized silica and germania materials are reported. The accurate geometric and electronic structures of these clusters, calculated photoabsorption and photoluminescence (PL) energies, and predicted absorption and PL spectra are obtained. Our calculations reproduced the experimental excitation energy (1.9-2.0 eV)... 

Kageshima and Shiraishi directly calculated the momentum matrix elements from the wave functions at the T point, with the ultrasoft pseudo-potential method including a core-repair term [14], Their result shows that the momentum matrix elements of ZB and WZ GaN are almost the same and about 40% smaller than that of ZB GaAs. This trend is consistent with the result of Suzuki et al although the absolute values are about 30% smaller. Experimentally, the momentum matrix element was indirectly deduced from a fit to photoluminescence (PL) spectra and optical absorption by Im et al [15], Fanciulli et al measured the g-value by electron spin resonance (ESR) and indirectly determined it with the k.p model [16], The agreement between theoretical and experimental results is fairly good. 

A long time ago, Hong, Noolandi, and Street [16] investigated geminate electron-hole recombination in amorphous semiconductors. In their model they included the effects of tunneling, Coulomb interaction, and diffusion. Combination of tunneling and diffusion leads to an S(t) oc t 1/2 behavior. However, when the Coulomb interactions are included in the theory, deviations from the universal t /2 law are observed—for example, in the analysis of photoluminescence decay in amorphous Si H, as a function of temperature. 

The process shown in Fig. 90(b) is favoured by the known position of the band edges in alkali, the similarity with the electroluminescence induced by persulphate, and the energy of the [Fe(CN)6]3, 4 redox couple. Either mechanism would, however, be consistent with the observation that the onset of electroluminescence coincides, approximately, with the flat-band potential, as shown in Fig. 91. Interestingly, the photoluminescence reaches a maximum at more negative potentials, a result not predicted by either model, as shown in Fig. 91 [167],... 

The recombination is modified in a multilayer structiu e whose layer spacing is similar to the carrier tunneling distance and is observed in photoluminescence measurements (Tiedje 1985). Fig. 9.22(a) shows that the luminescence intensity of a-Si H/nitride multilayers decreases as the layer thickness drops below about 500 A. The interface states and bulk nitride defect states cause non-radiative recombination because the electron-hole pairs are never far from an interface. The model of non-radiative tunneling developed in Section 8.4.1 can be adapted for recombination in thin layers. When the layer thickness is less than the critical transfer radius, the luminescence efficiency is (see Eq. (8.52)). 

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