Positive hole transport

Other indirect evidence for fast hole transport in 3-methyloctane at low temperatures (127 K) comes from pulse radiolysis studies (Gillis et al., 1977). Electrons and parent cations are characterized by their optical absorption spectra. While the cations absorb at 625 nm, the trapped electrons absorb at 2100 nm. They disappear after the radiation pulse by recombination. The temporal decay of the absorption spectrum was faster in the pure liquid as compared to a solution of 3-methyloctane and triethylamine, a hole scavenger. In the latter case, the resonant positive hole migration is terminated by formation of the triethylamine positive ion which has a much lower mobility. 

The resulting positive TMSi-ion has a much lower mobility (see Section 3.4). The transport of the hole in IXe can be rationalized by the model of the small polaron (see Section 7.3). 

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Most of the materials used as charge transport materials (CTMs) in commercial photoconductors work by a positive hole-transport mechanism, as indicated in Figure 4.10. Hole transport p-type semi-conductors are materials that are electron rich and give up their electrons readily. Because of this property they are easily oxidised by air and hght, and a balance must be obtained between environmental... 

Fig. 14. Possible sign combinations involving the sign of the interfacial charge at the oxide—oxygen interface and the sign of the charge of the field-driven mobile species originating at the oxide—oxygen interface, together with schematic diagrams of the concentraion profiles for the mobile species, (a) Field-driven positive-hole transport (b) field-driven anion interstitial (or cation vacancy) transport.

Charge transporting materials include a positive hole-transporting material and an electron transporting material. 

Examples of the positive hole-transporting material are PVK and its derivatives. Further materials are ... 

The photoreceptor has a light-sensitive layer containing a binder resin, a chargegenerating agent, and a positive hole-transporting material of stilbene derivative I. 

The relative contributions of ions, electrons and positive holes to the conductivity is described drrough the transport number which is related to a partial conductivity defined by... 

For definiteness, the oxidation of copper to copper(l) oxide may be considered. Our picture of the process is that cation vacancies and positive holes formed at the Cu O/Oj interface by equation, 1.166 are transported to the Cu/CujO interface where they are destroyed by copper dissolving in the non-stoichiometric oxide. We require an expression for the rate of oxidation. 

This equation can be obtained in another way which may be more instructive. Assume that the slow step in the oxidation is the transport of cation vacancies. The positive holes may then be considered to take up their equilibrium distribution, defined by Boltzmann s equation... 

Our picture of the transport process in these thick oxide layers is that there is a uniform concentration gradient of defects (cation vacancies and positive holes) across the layer. But it is important to notice that the oxidation flux is exactly twice that to be expected if diffusion alone were responsible for the transport of cation vacancies. The reason for this is, of course, that the more mobile positive holes set up an electric field which assists the transport of the slower-moving cation vacancies. 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

This almost distance independent hole transfer over (A T)n sequences where adenines are charge carriers is very surprising. Maybe the transfer of a positive charge between adenines of an (A T)n sequence is extremely fast, as recent calculations of M.D. Sevilla predicted [20], One could also speculate that the positive charge is delocalized over more than one A T base pair so that polaron hopping, which is discussed in this volume by G.B. Schuster as well as E.N. Conwell, might make the hole transport in oxidized (A T)n sequences very efficient. 

The resolution of this apparent contradiction to the thermodynamic expectations for this transfer is that the ionic membrane will always contain a small electron/positive hole component in the otherwise predominantly ionic conductivity. Thus in an experiment of very long duration, depending on the ionic transport number of the membrane, the eventual transfer would be of both oxygen and sulphur to the manganese side of the membrane. The transfer can be shown schematically as... 

In the overall scheme of the photosynthesis of green plants the electron transport cycle starts with the excitation of chlorophyll a in photosystem 2. The excited electron then follows a downward electron acceptor chain which eventually reaches the chlorophyll a of photosystem 1 (P700) in which it can fill the positive hole left by electronic excitation. The energy released in the electron transport chain which links photosystems 2 and 1 is used for other biochemical processes which are thereby related to photosynthesis. One of these is the process of photophosphorylation which is the production of molecules with phosphate chains used as energy transfer agents in many biochemical reactions. 

In all cases there is a need for the rapid transfer of electrons in a controlled direction. It is to be expected that ligands will have mobile, conjugated electronic systems to allow electrons to be transported away rapidly from the metal centre to avoid recombination with the positive hole. In addition to movement of electrons through the binding system, the possibility of electron tunnelling must be considered. 

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