Charge separation, in semiconductors

Figure 12 Principle of charge separation in semiconductor heterostructures (a) capped (or Core-Shell) geometry and (b) coupled geometry. Electrons accumulate at the conduction band (CB) of Sn02 while holes accumulate at the valence band (VB) of Ti02.

The theory of photoelectric emission from semiconductors was developed by Kane in 1962. The semiconductor/vacuum system is more complicated than the metal/vacuum case. This is mainly because there is pronounced charge separation in semiconductors. Additionally, surface states may play an important role, and in many cases where potential is applied, the space charge region within the electrode has to be considered. Kane s theory may be divided into two categories volume (or bulk) processes and surface processes (Table IV). For a direct, volume transition the unity power law is predicted and has been found experimentally for clean surfaces. It will be developed in some detail below. The remainder of the power laws can be studied from the original paper. ... 

Fig. 10-4. G neration of electron-hole pairs by photoexdtation and their recombination or separation in semiconductor (a) generation and recombination of photoexdted electron-hole pairs in a flat band state, (b) generation and separation of photoexdted electron-hole pairs in a space charge layer.

Assuming that we have such a situation favorable for charge separation, we have to consider what factors influencing the efficiency of charge separation in an illuminated semiconductor electrode are affected by crystal orientation or crystal imperfections. Five such factors are listed in the following table ... 

Dielectric Constant The dielectric constant of material represents its ability to reduce the electric force between two charges separated in space. This property is useful in process control for polymers, ceramic materials, and semiconductors. Dielectric constants are measured with respect to vacuum (1.0) typical values range from 2 (benzene) to 33 (methanol) to 80 (water). The value for water is higher than that for most plastics. A measuring cell is made of glass or some other insulating material and is usually doughnut-shaped, with the cylinders coated with metal, which constitute the plates of the capacitor. 

Charge Separation in Titanium Oxide Nanocrystalline Semiconductors Revealed by Magnetic Resonance... 

The mechanism of the charge separation in the colloidal Ti02 particles was studied at low temperature by direct time-resolved and light-modulated EPR techniques [91]. The recombination kinetics in the nanocrystalline semiconductor particles usually is very fast, on... 

Composite semiconductor nanoclusters can be classified into two categories, namely, capped- and coupled-type heterostructures. The capped nanoclusters essentially have a core-shell geometry while in a coupled system two semiconductor nanoclusters are in contact with each other. The principle of charge separation in capped and coupled semiconductor systems is illustrated in Fig. 12. 

Fig. 16.12 Idealized photoinduced charge separation in a Sn02/Ti02 semiconductor film [Reprinted with permission from Vinodgopal et al. (1996). Copyright (1996) American Chemical Society]...

In single-crystal electrodes the electric field in the depletion layer of the semiconductor separates the charges and decreases the probability of charge-pair recombination [30]. However, in small-particle colloids such a depletion layer does not exist due to the nanometer particle size, and there is no electrical field to separate the charges [36]. Due to the large recombination rate in small-particle colloids, the lifetime of charged pairs is very short, and only very fast reactions with adsorbed species can lead to efficient charge separation. In order to facilitate chemical pro-... 

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