Electron-hole trapping

Corresponding results were obtained in a recent study [39] using a very pure CoO single crystal. Measurements at 1000-1300 °C temperature and pO led to a model in which free electron holes are considered and electron holes trapped by cation vacancies. The latter configuration corresponds to the singly charged vacancy its lifetime could be estimated to be twenty times larger than the residence time of an electron hole on a cation site. 

Color center Electron or electron hole trapped in a vacancy 0... 

The energ E2 shown in Fig. 1, results of the same case when a charge pair is formed. In that case, instead of the energies of the polarisation P of the polymer by a single trapped hole and the depth q of an electron trap, the Coulomb energ>" Q(r) of the charge pair (distance r) and the depth of the electron-hole-trap q j have to be taken into account. 

In addition to the structural defects, crystals also contain electronic defects, i.e. electrons and electron holes that are relatively free to move in the crystal. The electronic defects may either be formed through internal excitation of valence electrons or they may be formed in association with point defects. If these electronic defects are localised (trapped) at regular sites in the structure, the electronic defects are termed polarons or - from a chemical point of view - valence defects. Defect electrons or electron holes trapped at point defects often make otherwise transparent materials coloured, and composite defects involving point defects and trapped electronic defects are termed colour centres. 

Examine as an example uranium dioxide (UO. The prevalent defect consists of an oxygen ion in interstitial position, accompanied by two electron holes trapped on uranium(lV) ion transforming it to uranium(Vl) ion. Then the electric relation of neutrality and the distance from stoichiometry in oxygen are... 

The addition of a foreign element with a valence different from that of the basic con nents will allow controlling the electronic defect in a non-stoichiometric solid. Take as an example a sohd of Wagner with cation vacarKies such as the FeO arrd dope it by lithium, by using the vapor of this metal. We have seen earlier (see section 2.3.2.4) that in FeO, the electric corrqrensation of the anion vacancies was due to electron holes trapped on iron of the lattice. The lithium irrtroduction into the iron vacancies causes a decrease in the number of trivalent ions, that is, the nrrrrrber of electron holes (Figure 2.8). The reaction of introduction is as follows ... 

We wish to model the oxidation of metal M and some of its alloys by oxygen. The samples are presented in the form of sheets with a thickness 2 mm and an area So. As soon as the reaction occurs, all the sheets are already covered with a very fine layer of oxide MO. This oxide, the only formed phase in all the cases, which we will study, is a p-type semiconductor whose prevalent defects are cation vacancies and electron holes trapped per pairs on cations of the network. Oxidation is carried out under conditions of pressure and temperature very far away from the equilibrium conditions between metal, oxygen, and oxide the fractional extent reached at the end of the experiment is very low. 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

Figure 9-19. Bund diagram of LPPP with hole traps and gold electrodes with Va<- vacuum level. Ec conduction band, Eva valence band. E, Fermi level. . baudgup energy. and , " trap depths. ,( ) trap distribution, X electron affmity, and All work function of the gold electrodes.

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

Absorption of a light quantum leads to an electron-hole pair Eq. (19). The electron reacts with an adsorbed oxygen molecule Eq. (20), and the hole semi-oxidizes a sulfide anion at the surface Eq. (21). Further oxidation of the sulfide anion occurs by O and O2 Eq. (22). The number of Cd ions formed equals that of the sulfate anions The oxidation of illuminated CdS powders was investigated by measuring the consumption and by detecting the superoxide radical,, by an ESR spin trapping method... 

With the site-selective hole injection and the hole trapping device established, the efficiency of the hole transport between the hole donor and acceptor, especially with respect to the distance and sequence dependence, were examined. Our experiments showed that hole transport between two guanines was extremely inefficient when the intervening sequence consisted of more than 5 A-T base pairs [1]. Hole injection into the DNA n-stack using photoexcited dCNBPU was accompanied by the formation of dCNBPU anion radical. Therefore, hole transport would always compete with the back electron transfer (BET). To minimize the effect of BET, we opted for hole transport between G triplets, that are still lower in oxidation potential than G doublet. With this experimental system, we researched the effect of the bridging sequence between two G triplets on the efficiency of hole transport [2]. 

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