Electrical properties, semiconductive

These considerations lead to the assumption that the practical aspects of the problem lie in the possibility of obtaining PCS-based thermally resistant materials, catalysts for some chemical reactions, antioxidants, stabilizers, photochromic substances, and materials combining valuable mechanical properties with special electrical (particularly semiconductive) properties. 

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 eight-coordinate vanadium complex V(S2CMe)4 contains both dodecahedral and square prismatic eight-coordinate molecules in the same crystal.322 Of particular interest is the chain-like, mixed valence platinum complex [Pt2(S2CMe)4( i-I)]A, which displays unusual electrical properties metallic conduction between 300 and 340 K and semiconducting properties below 300 K,323 whereas the analogous nickel complex, [Ni2(S2CMe)4(p-I)]x is a semiconductor.324... 

A comparative study of oxides which were closely related, but had different electrical properties, showed that both n- and p-type semiconduction promoted the oxidation reaction, forming CO as the major carbon-containing product. In a gas mixture which was 30% methane, 5% oxygen, and 65% helium, reacted at 1168 K the coupling reactions were best achieved with the electrolyte Lao.9Sro.1YO 1.5 and the /i-lype semiconductor Lao.sSro MntL A and the lily pe semiconductor LaFeo.sNbo.2O1 a produced CO as the major oxidation product (Alcock et al., 1993). The two semiconductors are non-stoichiometric, and the subscript 3 — x varies in value with the oxygen pressure and temperature. Again, it is quite probable that the surface reactions involve the formation of methyl radicals and O- ions. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

In its present stage of development, the electron theory of catalysis deals with catalysts which by their electrical properties belong to the class of semiconductors. Catalysis on semiconductors, as is well known, is extremely widespread, far more so than might appear at first sight. This is due to the circumstance that in most cases a metal is enclosed in a semiconducting coat and the processes which apparently take place on the surface of the metal actually take place on the surface of this semiconducting coat, whereas the underlying metal frequently takes practically no part in the process. 

It has been shown in Section 1.3.7 that in semiconductors or insulators the lattice defects and electronic defects (electrons and holes), derived from non-stoichiometry, can be regarded as chemical species, and that the creation of non-stoichiometry can be treated as a chemical reaction to which the law of mass action can be applied. This method was demonstrated for Nii O, Zr Cai Oiand Cuz- O in Sections 1.4.5, 1.4.6, and 1.4.9, as typical examples. We shall now introduce a general method based on the above-mentioned principle after Kroger, and then discuss the impurity effect on the electrical properties of PbS as an example. This method is very useful in investigating the relation between non-stoichiometry and electrical properties of semiconductive compounds. 

Thus, it has been shown that the electrical properties of semiconductive compounds depend on the chemical composition, viz. non-stoichiometry, and therefore the control of the composition is indispensable in the control of the semiconductive properties of the compounds. 

Semiconductive elements Si and Ge (Group IVB or 13 in the periodic table) have become very important electronic materials since development of a purification method. The electronic properties of semiconductive elements of high purity can be controlled by the species and concentration of defects and impurity elements. On the other hand, in the case of semiconductive compounds, that is, III-V and II-VI compounds, we have to consider not only control of the purity of constituent elements but also the nonstoichiometry, both of which have much influence on the electronic properties. In this sense, control of the electrical properties of semiconductive compounds is more difficult than that of semiconductive elements. 

While the study of the conventional semiconducting materials has progressed rapidly, the study of organic materials has received much less attention until the past few years. In particular the electrical properties of polymers have been much neglected and little authoritative work exists in the literature. In view of current developments in theories of the electrical properties of organic molecular crystals, it seems profitable to take stock of the situation as far as charge transfer in polymers is concerned. 

To fabricate the integrated circuit (IC), layers with various electrical properties must be introduced into or deposited onto the substrate. These layers may consist of insulating, semiconducting, and conducting films. The construction of the layers in only the desired areas relies on a series of patterning steps which is briefly illustrated in Figure 1. Light sensitive... 

It is to note that some of the issues touched upon in this chapter still require a more detailed experimental study and theoretical interpretation. In particular, the model considered above, which makes it possible to relate an increase in the catalytic activity to appearance of charge on nanoparticles, undoubtedly needs refinement and a more thorough experimental verification. A clear understanding of the important issue of how the electrical properties of conducting supports affect the catalytic activity of deposited structures requires that, in the first place, experiments with a larger number of reactions and a wide variety of metallic and semiconducting substrates... 

The theory outlined above was developed for group IY semiconducting elements such as silicon and germanium some of the compounds of group III and Y elements, the III-V compounds, are also covalently bonded and have similar electrical properties which can be described in terms of a band model. The best known semiconducting III-V compound is GaAs, which is exploited for both its photonic and semiconducting properties. 

A. In fact, low-spin d2 Re pairs would lead to similar magnetic and electrical properties of ReSe2. Whereas semiconducting ReTe2 has a different orthorhombic structure the equally non-metallic dichalcogenides of Tc all seem to crystallize in the ReSe2 structure or a closely related structure (33). 

It should not be supposed that crystal defects enter into the picture only as nuisances which the chemist seeks to avoid or eliminate. Actually, certain optical and electrical properties of oxides, sulfides, and halides have been found to depend strongly on the nature and extent of crystal defects. Indeed, semiconductivity, fluorescence (absorption of radiation and emission of less energetic radiation), and phosphorescence (delayed fluorescence) of some salts may be spectacularly increased, not only by a small stoichiometric excess of one of the constituents, but also by addition of very tiny quantities of a foreign ion. Perhaps the best known example is the case of zinc sulfide which, when precipitated from aqueous solution and dried at low temperatures, shows negligible fluorescence upon exposure to ultraviolet light. When the sulfide is heated to... 

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