Semiconducting phenomena

Ultrafast photoinduced electron transfer in semiconducting polymers mixed with controlled amounts of acceptors this phenomenon has opened the way to a variety of applications including high-sensitivity plastic photodiodes, and efficient plastic solar cells ... 

TCNQ-Polyphosphazene Systems. Tetracyanoquinodimethane (XX) salts crystallize in the form of stacked arrays that allow electrical semiconductivity (42). Although this phenomenon has been studied in many laboratories, it has not been possible to fabricate conductive films or wires from these substances because of the brittleness that is characteristic of organic single crystals. However, it seemed possible that, if the flexibility and ease of fabrication of many polyphosphazenes could be combined with the electrical properties of TCNQ, conducting polymers might be accessible. 

The corrosion behavior of semiconductors can, in principle, be described within the framework of the same concepts as for metals (see, for example, Wagner and Traud, 1938), but with due account for specific features in the electrochemical behavior of a solid caused by its semiconducting nature (Gerischer, 1970). One of the main features is photosensitivity related to a change in the free-carrier concentration under illumination. Photosensitivity underlies the phenomenon of photocorrosion. 

For every electron excited to the antibonding conduction bund, there will remain behind a hole, or vacancy, in the valence band. The electrons in both the valence band and the conduction band will be free to move under a potential by the process shown in Fig. 7.22b. but since the number of electrons (conduction band) and holes (valence band) is limited, only a limited shift in occupancy from left-bound states to right-bound states can occur and the conductivity is not high as in a metal. This phenomenon, known as intrinsic semiconduction, is the basis of thermistors (temperature-sensitive resistors). 

Doping. In the case of semiconducting catalysts, a small amount of foreign material dissolved in the original catalyst may modify the rate of a particular reaction. This phenomenon is sometimes called doping by analogy with the effect of similar materials upon semiconductivity. 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

Nuclear magnetic resonance (NMR) is a similar type of paramagnetic phenomenon. Like ESR, NMR involves the generation of a signal by the application of an external magnetic field, except that in NMR, shifts in the orientation of the nucleus, rather than of the unpaired electrons, are measured with respect to the external field (cf. Dyer 1965). The major point for our discussion, which would indicate that ESR rather than NMR would function as the modulated carrier-wave, is that unpaired electrons are characteristic of charge-transfer reactions and semiconduction, and as we have seen previously, serotonin and many of its analogs can function as power-... 

Heterogeneous layers based on semiconducting metal oxides are known to show gas-sensing features different as compared to single oxides [1,2], The mentioned synergetic phenomenon can be provided by two effects i) the presence of active centers with diverse adsorption and catalytic behavior ii) the efficient separation of the sensor functions, receptor and transducer, between different oxide phases. 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

As already pointed out, the most direct consequence of a reduction in the nanocrystallite size on the electronic structure of semiconducting materials is a pronounced increase in the band gap due to the quantum confinement effect. While there are several ways to quantitatively understand this phenomenon from a theoretical standpoint, the experimental determination of the band gap variation as a function of size is most directly performed by ultraviolet-visible absorption spectroscopy, with the experimental absorption threshold corresponding to the direct band gap in the material. As the band gap shifts to higher energy, the blue-shift in the absorption edge signals the formation of progressively smaller sized nanocrystals. Therefore, UV-Vis absorption spectroscopy has played an immensely important role in the study of these systems and we discuss the essential aspects in Section 11.3. 

In the photoresistors and photodiodes use is made of the internal photoelectric phenomenon and of specific properties of semiconducting materials. Photons impinging on the photosensitive element generate an electrical current, which flows through the photoconductor and is amplified by the effect of a small applied voltage. The increase of the current intensity is proportional to the intensity of photons that strike the photosensitive element. 

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