Trapping center

Fig. 15a-c. Effect of TPMP+ on (a) light-induced potential changes (Left ordinate in % of uninhibited control), (b) on photophobic accumulations in light traps (Center ordinate in % of uninhibited control) and (c) resting potential (Right ordinate in mV). Abscissa TPMP+ concentration in mol (after Ha-der48))... 

Fig. 17.9 Sketch of a typical setup for ion trap experiments on lasing microdroplets. The oscillating field between the inner and outer ring electrodes forms the trapping potential, and gravitational forces can he opposed by static electrical fields to move the droplet to the trap center with no micromotion...

It seems to be natural to suspect if these excimer forming sites were the effective trapping center also for hole carriers. 

It should also be briefly recalled that semiconductors can be added to nanocarbons in different ways, such as using sol-gel, hydrothermal, solvothermal and other methods (see Chapter 5). These procedures lead to different sizes and shapes in semiconductor particles resulting in different types of nanocarbon-semiconductor interactions which may significantly influence the electron-transfer charge carrier mobility, and interface states. The latter play a relevant role in introducing radiative paths (carrier-trapped-centers and electron-hole recombination centers), but also in strain-induced band gap modification [72]. These are aspects scarcely studied, particularly in relation to nanocarbon-semiconductor (Ti02) hybrids, but which are a critical element for their rational design. 

If adatom-impurity atom interaction is attractive, then the impurity atom can act as a trapping center. A diffusing adatom may be trapped. In heterogeneous catalysis, the reaction rate may be changed by the trapping effect of impurities as also by lattice defects and lattice steps and so on. 

The last three involve the capture of a charged carrier at an oppositely charged center. In all of these events except the free-hole trapped-electron recombination, the free carrier is the electron and the trapping center has a charge of +e/2. The key assumption is that the cross section for electron capture is determined by the coulombic attraction. On this basis, Hamilton derived an equation that includes one term to cover low-intensity reciprocity failure and another which is a first-order approximation of high-intensity reciprocity failure. Its predictions were in good accord with experimental data on the effects of sulfur sensitization. 

It is, however, not clear at present whether such a model can be applied to metal solutions. It is worth mentioning again at this point that the specific conductivity of a saturated solution of lithium in methyl-amine (10) (concentration 5.5M) has been found to be 28 ohm l cm."-1, which is two orders of magnitude lower than that for a corresponding saturated metal-ammonia solution. This experimental result seems to indicate that the overlap between electron trapping centers may not be... 

The presence of a trapping center is very important since Eq. (19) indicates that the steady-state strength of the photoinduced space-charge field depends on the number density of the deep traps. Nevertheless, the nature of the traps in organic PR materials is the least studied of all the elements for the PR effect. The main reason is the lack of structural information of the trapping centers. The amorphous nature of these materials warrants the existence of a variety of trapping centers, such as energy levels localized at impurities or structural defects. However, one can differentiate between deep traps, which are localized... 

The bipolar single-trap model assumes that both electrons and holes share identical trap centers. Since sequential trappings of the electrons and holes by the identical centers mean the neutralization of the electric charge, the effective space-charge field will depend on the relative power (i.e., the mobilities) of electron and hole transports. The expressions for the writing and erasing diffraction efficiency are [100] ... 

Figure 2. (a) - Potential curves of M-STE and A-STE. (b,c) - Simplified geometrical structure of M-STE and A-STE. (c) - Scheme of the population of molecular neutral and charged trapped centers in atomic cryocrystals. 

Complementary studies of neutral [20] and charged [16] intrinsic trapped centers, comparison of cathodoluminescence [21] and thermoluminescence data [12] with results of analysis of photoelectron scattering [13] and pump-probe experiments [14] allow us to extend the energy relaxation scheme (Fig.2d, dotted arrows) including electron-hole recombination channels. The formation of H-band emitting centers (R2+) occurs through the excitation of STH by an exciton. The bulk recombination of trapped holes with electrons populates the (R2 ) states with subsequent M-band emission [22], After surface recombination of STH with electrons the excited dimers escape from the surface of the crystal with subsequent IF-band emission. 

The general behavior of the excitation spectra of H- and IF-bands (Fig.lb) reproduces all surface (S) and bulk (transverse = 1,2,3 and longitudinal L) features of photoabsorption. The opposite behavior of the M-band underlines the branching of competing channels of population neutral and charged trapped centers [16],... 

Many publications are devoted to the EPR study of the hole trapped centers in Ti02 nanoparticles after low temperature irradiation [36, 37,44-46,49, 63-72], The interpretations of the observed EPR signals and assignments of the photogenerated hole species are less clear than electron traps, and several contradictions can be found in the literature. As it was shown recently [36, 37,46, 54, 55, 57] the main reason for the controversy is that holes are localized in the surface region of the nanoparticles and the structure of the hole centers strongly depends upon surface treatment and modification. 

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