Semiconductors energetic levels

Photoconductivity is one of the most informative phenomena in semiconductor physics. The light beam enables us to exite definite energetic levels strongly and obtain information about photogeneration, recombination and transport processes of the charge carriers. The history of photoconductivity shows the every essential step in research and application has been linked with the appearance of new photosensitive materials with specific physico-chemical properties. 

The semiconductor nanocrystallites work as electron acceptors from the photoexcited dye molecules, and the electron transfer as sensitization is influenced by electrostatic and chemical interactions between semiconductor surface and adsorbed dye molecules, e.g., correlation between oxidation potential of excited state of the adsorbed dye and potential of the conduction band level of the semiconductor, energetic and geometric overlapping integral between LUMO of dye molecule and the density of state distribution of the conduction band of semiconductor, and geometrical and molecular orbital change of the dye on the... 

In this chapter we report on properties of nanometer-sized semiconductor particles in solution and in thin films and thereby concentrate on the photochemical, photophysical, and photoelectrochemical behavior of these particles. We shall, very briefly, describe the energetic levels in semiconductors and the size quantization effect. The bottleneck in small-particle research is the preparation of well-defined samples. As many preparative aspects are already reviewed in several actual assays, we present here only the preparative highlights of the last two years. In Section IV we describe the fluorescence properties of the particles. We report on different models for the description of the very complex fluorescence mechanism and we show how fluorescence can be utilized as a tool to learn about surface chemistry. Moreover, we present complex nanostructures consisting of either linked particles or multiple shells of different nanosized materials. The other large paragraph describes experiments with particles that are deposited on conductive substrates. We show how the combination of photoelectrochemistry and optical spectroscopy provides important information on the electronic levels as well as on charge transport properties in quantized particle films. We report on efficient charge separation processes in nanostructured films and discuss the results with respect to possible applications as new materials for optoelectronics and photovoltaics. 

Energetic levels of a semiconductor (a) intrinsic semiconductor (b) n-type semiconductor (c) p-type semiconductor. 

The notion of energetic levels of electrons in soUds can be extended to the case of an electrolytic solution containing a redox system (Gerischer, 1970). The occupied electronic levels correspond to the energetic states of the reduced species whereas the unoccupied ones correspond to the energetic states of the oxidized species. The Fermi level of the redox couple, ii redox, corresponds to the electrochemical potential of electrons in the redox system and is equivalent to the reduction potential, Vq. In order to correlate the energetic levels of a semiconductor to those of a redox couple in an electrolyte, two different scales can be used. The first is expressed in eV, the other one in V (Fig. 6.7a). The difference between the two scales is due to the fact that in solid state physics zero is the level of the electron in vacuum, whereas in electrochemistry the reference is the potential of the normal hydrogen electrode (NHE). The correlation between the two scales can be calculated from the value of potential of NHE which is equal to -4.5 eV when it is referred to that of the electron in vacuum (Lohmann, 1967). 

Scheme of the energetic levels at the semiconductor-electrolyte interface for an n-type semiconductor (a) at equilibrium (b) flat band potential. 

The photocatalytic properties of a semiconductor depend on the position of the energetic levels, on the mobihty and mean lifetime of the photogenerated electrons and holes, on the hght absorption coefficient and on the nature of the interface. Moreover, the photoactivity depends on the methods of preparation of the powders which allows varying many physico-chemical properties of the semiconductor as the crystalline structure, the surface area and the distribution of the particle size. 

A semiconductor can be described as a material with a Fermi energy, which typically is located within the energy gap region at any temperature. If a semiconductor is brought into electrical contact with a metal, either an ohmic or a rectifying Schouky contact is formed at the interface. The nature of the contact is determined by the workfunction, (the energetic difference between the Fermi level and the vacuum level), of the semiconductor relative to the mclal (if interface effects are neglected - see below) 47J. 

Other methods of excitation are effective or necessary for certain gain media. For example, certain energetic chemical reactions produce molecules in excited states. These excited molecules may then comprise the upper laser level of an inverted-population system. A specific example is the hydrogen fluoride "chemical laser" wherein excitation is provided by the reaction of hydrogen gas with atomic fluorine. Another method of excitation is simply the passage of an electric current through a semiconductor device. This serves as the exciter for diode lasers. 

Artificial satellites, which are now used for communication, broadcast, weather forecast, etc., are equipped with a variety of semiconductor devices, which are often exposed to the high levels of radiation found in space. Such energetic particles, called cosmic rays, cause the degradation and malfunction of semiconductor devices, which lowers both the mission lifetime and reliability of satellites. Using ion beam irradiation facilities at TIARA, which have been uniquely adapted for simulating the radiation environment of space, we have... 

Electron transfer is a fast reaction ( 10-12s) and obeys the Franck-Condon Principle of energy conservation. To describe the transfer of electron between an electrolyte in solution and a semiconductor electrode, the energy levels of both the systems at electrode-electrolyte interface must be described in terms of a common energy scale. The absolute scale of redox potential is defined with reference to free electron in vacuum where E=0. The energy levels of an electron donor and an electron acceptor are directly related to the gas phase electronic work function of the donor and to the electron affinity of the acceptor respectively. In solution, the energetics of donor-acceptor property can be described as in Figure 9.6. 

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