States deep-trap

Figure 7.5 Schematic presentation of photoactivation and relaxation processes in a CdSe quantum dot aggregate (a) surface-passivation of photoexcited quantum dots by solvent molecules or dissolved oxygen, (b) thermal activation followed by the formation ofa stabilized state, (c) the formation of deep-trap states, (d) non-radiative relaxation of deep-...

A group of scientists have studied current transients in biased M-O-M structures.271,300 The general behavior of such a system may be described by classic theoretical work.268,302 However, the specific behavior of current transients in anodic oxides made it necessary to develop a special model for nonsteady current flow applicable to this case. Aris and Lewis have put forward an assumption that current transients in anodic oxides are due to carrier trapping and release in the two systems of localized states (shallow and deep traps) associated with oxygen vacancies and/or incorporated impurities.301 This approach was further supported by others,271,279 and it generally resembles the oxide band structure theoretically modeled by Parkhutik and Shershulskii62 (see. Fig. 37). 

I5A. J. Kumnick, and H. H. Johnson, Deep Trapping States for Hydrogen in Deformed Iron, Acta Metall., 28(1), 33-39 (1980). 

Arkhipov VI, Heremans P, Emelianova EV, Adriaenssens GJ, Bassler H (2002) Weak-field carrier hopping in disordered organic semiconductors the effects of deep traps and partly filled density-of-states distribution. J Phys Condens Matter 14 9899... 

The rate of decay and the temperature dependence of the saturated voltage can be used to obtain the concentration and energy distribution of the deep traps responsible for the residual potential. Thus, provides a useful means of studying the nature of deep traps in amorphous semiconductors and has been successfully used to derive the energy distribution of deep localized states in the mobility gap of both a-Se and a-Si H [10,18],... 

Sulfates. The basic lattice of sulfate phosphors absorbs very short wavelength UV radiation. On excitation with X rays or radiation from radioactive elements, a large proportion of the energy is stored in deep traps. For this reason, CaS04 Mn is used in solid-state dosimeters. Of the glowpeaks which can be selected by thermoluminescence, more than 50 % fail to appear at room temperature because of a self selection of the shallow traps. Other activators, such as lead or rare-earth ions (Dy3 +, Tm3 +, Sm3+), stabilize the trapped electrons [5.399]—[5.401]. 

Concerning ices, it has been discussed that they must be amorphous (Smoluchowski 1983) in the interstellar medium and not crystalline. This implies that the adsorbed H atoms are localized in deep traps so that their wavefunctions have a limited spatial extent. This fact reduces significantly their mobility and hence the interaction with another H atom absorbed on another site is slow as compared to the residence time unless the two atoms happens to be localized near each other. This phenomenon reduces the rate of H2 formation by several orders of magnitude when compared to the situation on crystalline surfaces. Computational simulations on soft and hard ice model surfaces have shown that for a cross-section of 4,000 nm2 the reaction probability is 1 (Takahashi et al. 1999). Furthermore, the H2 formed, due to the high amount of energy liberated is rapidly desorbed in an excited state from the ice mantle in timescales of 500 fs (Takahashi et al. 1999). 

Even though the F+HCl reaction does not possess the deep trapping wells that are apparent for insertion or ion-molecule reactions, the sheer number of resonance states that exist for this system make it a complex-forming... 

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... 

As the trap is made deeper the intensity spreads more widely over the levels of the band. In treating deep traps it is convenient to begin with the hypothetical limit of the infinitely deep trap, based on the wave functions (17) of the residual host, and to consider transitions to them from the ground state. The transition... 

Furthermore, femtosecond diffuse reflectance spectroscopy with a white continuum probe pulse has been applied to detect the dynamics of hole transfer from photoexcited TiC>2 to adsorbed reactant molecules. As shown in Figure 18, at pH < 7 of the TiC>2 aqueous suspension with KSCN, ultrafast hole transfer takes place in less than 1 ps (Furube et al., 2001b). Subsequent structure stabilization of dimer anion radicals, (SCN)2, within a few picoseconds and slow hole transfer with a time constant of a few hundred picoseconds are clearly observed (Furube et al., 2001b). Fast hole transfer is caused by a surface-trapped state interacting strongly with adsorbed molecules. Slow hole transfer observed at pH values >7 is caused by deep trapped states with a Boltzmann distribution... 

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