Electron detrapping

Shkrob lA, Sauer MC. (2005) Photostimulated electron detrapping and two-state model for electron transport in nonpolar liquids. / Chem Phys 122 134503. 

All our observations can be interpreted in terms of electron detrapping. If electrons are trapped during irradiation either in physical defects in the solvent or by attachment to biphenyl molecules, they can be liberated by illumination in the corresponding absorption bands ... 

Although the electron trap ability of Ti02 can be very useful for hole-mediated photochemistry, electrrai trapping inhibits some processes relying on rapid electron transport (such as photovoltaic application). Thus, rapid electron detrapping is more important in these applications. Electron detrapping usually occurs in two ways ... 

The proposal that holes are detrapped at lower temperatures than the excess electrons is based on the observations discussed above in Very Shallow Traps and Shallow Traps (Sec. 4.3.2). One expects that the activation energy needed to detrap the hole from Gua in duplex DNA is relatively small, an order of magnitude less than that needed to detrap the electron. This fits well with the observation that upon warming 4 K irradiated crystalline DNA to 77 K, 10-30% of the radicals anneal out, i.e., at least one of the trapping sites [fide infra Gua(N3-H) ] is very shallow. 

Fluorescence spectra of semiconductor particles originate from the recombination of charge carriers in either a trapped or exdtonic state [578, 579], The former manifests itself in the appearance of a broad and Stokes-shifted band. In contrast, the spectrum, due to exdtonic fluorescence, appears as a sharp band near the absorption onset and is considered to arise from the detrapping of the trapped electrons [579]. The highly environmentally dependent position of the semiconductor emission maxima has been related to semiconductor sizes and size quantization [579]. 

By optically creating carriers with a pulse of above band-gap illumination, then monitoring the subsequent current transient due to thermal detrapping, Hurtes et al (1978) and Fairman et al (1979) were able to apply the DLTS method to bulk, high-resistivity materials. This method, however, is unable to distinguish between electron and hole traps, and the calculation of trap densities is difficult. 

It is assumed that deeply trapped holes, h+ff, are chemically equivalent to surface-bound hydroxyl radicals. Weakly trapped holes, on the other hand, that are readily detrapped apparently posses an electrochemical potential close to that of free holes and can therefore be considered to be chemically similar to the latter. Their shallow traps are probably created by surface imperfections of the semiconductor nanocrystals. From these traps the charge carriers recombine or they are transferred by interfacial charge transfer to suitable electron acceptors or donors adsorbed at the surface of the semiconductor. 

Direct hole transfer was also observed when dichloroacetate (DCA) was used as the electron donor [26]. While it was obvious that deeply trapped holes /zt+rd do not react with dichloroacetate, it was observed that the /zt+rd concentration is reduced considerably in the presence of DCA", either the free holes, h+, can be directly transferred to adsorbed DCA" molecules (reaction 19) or shallowly trapped holes, ht+r s, are detrapped (reaction 11) to react with DCA" in the nanosecond time scale via reaction 22. 

The above observations have been interpreted within the framework of two distinct models, one involving trapping/detrapping of the photogenerated electrons [345, 346] and the other based on electron diffusion (or field-assisted diffusion) not attenuated by electron localization [347, 348]. The millisecond transit times also mean that the transit times are very long compared with equilibration of majority carriers in a bulk semiconductor or electron-hole pair separation within the depletion layer of a flat electrode. The slow transport is rationalized by a weak driving force and by invoking percolation effects [338]. 

Alternatively, the dynamics of trapping and detrapping of electrons localized in intra-bandgap states can control the overall reaction kinetics, which would not depend upon the sole interfacial electron transfer rate. 

The emission-lifetime measurements in ns-time region were also carried out for the two emission bands. Multi-exponential decay behavior was observed for both the emission bands. Fast decay component at >.=480 nm less than the order of ns was attributed to the recombination of electrons and holes. Slow decay component at >,=480 nm in the order of a few ns was attributed to thermal detrapping of the electron from the surface states to the conduction band since such thermal activation could enhance the lifetime at the band-edge emission. The emission lifetime at >.=480 nm increased as excess Cshallow trap sites. 

Time dependent fluorescence depolarization is influenced by the exciton annihilation which occurs in confined molecular domains . Photoemission results from singlet exciton fusion as shown by the excitation intensity dependence which occurs in anthracene crystals. Reabsorption of excitonic luminescence is an effect which has been shown to occur in pyrene crystals. The dynamics of exciton trapping in p-methylnaphthalene doped naphthalene crystals involves phonon assisted detrapping of electronic energy. Ps time resolved spectroscopy was the experimental technique used in this work. 

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