Electron-hole generation

In Section 7.4 we gave only a brief outline of the photoeffects caused by electron-hole generation by photons with an energy above that of... 

Fig. 1. Electron-Hole Generation upon Photoexcitation of a Semiconductor Electrode...

It has been suggested that the ambient ionising radiation field of a reprocessing stream may induce electron-hole generation in semiconductor particles/films in situ, so obviating the need for an external UV light source. If such were found to be the case, heterogeneous photo catalytic systems could be deployed more easily. 

Among the simple cycloalkanes, we first discuss electron transfer from the three-to eight-membered cycloalkane prototypes to electron holes generated by radiolysis in different matrices, giving rise to the simple cycloalkane radical cations. Because of the significant interest they have attracted, the electron-transfer reactions of cyclopropane and, to a lesser extent, cyclobutane derivatives will be treated separately. Finally, electron transfer from some bicyclic hydrocarbons and the resulting radical cations will be discussed in a separate section (Section 2.4). 

To facilitate a self-contained description, we will start with well-established aspects related to the semiconductor energy band model and the electrostatics at semiconductor electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation and charge separation at these interfaces. Finally, the steady-state and dynamic (i.e., transient or periodic) aspects of charge transfer will be considered. Nanocrystalline semiconductor films and size quantization are briefly discussed, as are issues related to electron transfer across chemically modified semiconductor electrolyte interfaces. 

The space-charge build-up process can be divided into two steps the electron-hole generation process followed by the transport of one carrier species. The creation of a correlated electron-hole pair (or exciton) after absorption of a photon can be followed by recombination. This process limits the formation of free carriers that can participate in the transport process and is, therefore, a loss for the formation... 

Consider a true coaxial detector, shown in Fig. 12.39a (see also Fig. 7.26). Since the electric field is radial, electrons and holes will follow a trajectory perpendicular to the axis of the detector. The maximum time required for collection of the charge corresponds to electron-holes being produced either at A or C. That time t is equal to I AC)/v, where 4C is the detector thickness and V is the speed of electrons or holes. For a detector bias of about 2000 V and the size shown in Fig. 12.39a, v 0.1 mm/ns = 10 m/s, which gives a maximum collection time of 120 ns. The best risetime corresponds to electron-holes generated at point B (Fig. 12.39a) and is equal to about 60 ns. 

FIGURE 1.1. Schematics of the electron-hole generation in a photocatalyst particle and some of the mechanisms involved a) Ray promotes the formation of the electron-hole and electron, b) electron-hole is used in the formation of the OH groups promoting oxidation processes, c) the electron is utilized in a number of reduction processes, d) electron and electron-hole can recombine contributing to process inefficiency. 

Three different but connected problems must be studied (i) the reaction kinetics model (ii) the development of the rate of electron-hole generation in a material particle of the solid suspension and (iii) the model for characterizing the radiation field to evaluate the local volumetric rate of photon absorption (LVRPA). Point (iii) has been already described in section 6.6.1 for quantum yield determinations. In the first part of this section, we will concentrate on problems (i) and (ii). 

Here, is the rate of electron-hole generation per particle (rg[=]mols particle ). From equation 6.106 we can extract [/ +] and substitute into equation 6.107. From the resulting quadratic equation in [OH ] one obtains... 

Notice that (i) excluding the evaluation of the rate of electron-hole generation (Ug), equation 6.110 has only two constants a and c and (ii) aj, is not an adsorption equilibrium constant. 

The rate of electron-hole generation (r ). We look for an expression of Ug valid for any material point in the reacting space i.e. a local value that is applicable to the suspension of the solid in the liquid (Ug [=] mols particle" ). Upon radiation absorption by the catalytic particle, electrons and holes are generated with a primary quantum yield equal to < A-... 

Part of this radiation may be reflected on the surface (scattered) and part may be absorbed. The rate of electron-hole generation at point P (x + p) on the differential surface dA of particle n at a time t and for a wavelength A (actually between A and A + dA) is... 

The space-charge build-up process can be divided in two steps the electron-hole generation process followed by the transport of generally one carrier species. 

Figure 5.7. Schematic illustration of the fabrication of Au-DNA probe modified Ti02 electrode and the detection of target DNA (A] and the photo-induced processes of electron-hole generation and charge transfer processes fB] [adapted from Ref 40 with permission]. See also Color Insert.

The interactions of luminescent sensors with analytes can generate or inhibit PET (Fig. 2d). If there is a suitably positioned occupied state at higher energy, the electronic hole generated by the excitation process can be filled by transfer of... 

Other applications in micro and nanoelectronics are been developed with the help of CNTs. One example is the development of sensors that are placed in situ in concrete for the evaluation of internal porosity [13], through the use of oriented nanotube membranes that vibrate when they are in the middle. For certain applications, the solution to this approach is the synthesis of films of aligned CNTs [14-17], enabling the manufacture of monitors and microwave generators. This is possible because of their ability to emit electron by field effect [18]. The applications of these films also include the development of polymeric solar cells the semiconducting properties of CNTs films with anisotropic morphology result in a route for the separation of pairs and conduction electrons/holes generated by photons [19]. 

It should be noted that in contrast to classical band theory, the doped electrons/ holes generated in conductive polymers are not fuUy delocalized. The reason for... 

For applications focusing on the sterilizing feature, photocatalysts are normally incorporated with antibacterial metals such as Cu and Ag to exert the antibacterial function even in the dark. Metal incorporation also improves the decomposition activity when the photocatalyst is exposed to light because it produces a more efficient charge separation of the electron and electron hole generated from photoexcitation. 

One of the strategies is to minimize the diffusion path of exitons (botmd states between an electron and electron hole) generated by photons hitting the active material in a photovoltaic cell. Once separated, holes and electrons should reach anode and cathode electrodes, respectively. During the process, recombination of holes and electrons lead to energy losses via thermal dissipation. In order to reduce such losses, minimizing the diffusion paths of exitons by means of BCP-derived nanostructures may be beneficial. To this end, a thin solid-state dye-sensitized solar cell (ssDSSC) with a 3D gyroidal titania network was fabricated (see Fig. 12) [37]. In contrast to typical disordered nanoparticle networks, the ordered mesoporous... 

---