The Metal Chalcogenides

This complex may be reduced either directly to the metal chalcogenide or to the zero-valent metal, which then reacts chemically with selenosulfate adsorbed at the cathode to form MSe. 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

Thus, it was established that adsorption of metal hydroxide species on the surface of the substrate provides a nucleation layer which is chemically converted to the metal chalcogenide. The forming metal chalcogenide layer acts then as a catalytic surface for subsequent anion and cation adsorption. 

Particularly desirable among film deposition processes are solution-based techniques, because of the relative simplicity and potential economy of these approaches. However, the covalent character of the metal chalcogenides, which provides the benefit of the desired electronic properties (e.g., high electrical mobility), represents an important barrier for solution processing. Several methods have been developed to overcome the solubility problem, including spray deposition, bath-based techniques, and electrochemical routes, each of which will be discussed in later chapters. In this chapter, a very simple dimensional reduction approach will be considered as a means of achieving a convenient solution-based route to film deposition. 

The techniques for construction of LB films containing Q-state metal chalcogenide particles can be divided into two broad categories, distinguishable by at which stage in the film construction the metal chalcogenide is produced. 

FTIR has been mainly used to obtain structural details of films and to monitor intercalation of metal ions into the film structure and the subsequent reactions of the films with dihydrogen chalcogenides. Both transmission (FTIR-T) and reflection-absorbance (FTIR-RA) modes have been utilized. For the most part these studies have involved films of fatty acids with divalent metal ions. The key features of the FTIR spectra of these films include the asymmetric and symmetric stretching modes of the carboxylate group vs(C02-) and va(C02 ), associated with the M2+/carboxylate complex, and the carbonyl stretching mode v(C=0) of the proton-ated fatty acid. The disappearance of the v(C02 ) (1500-1600 cm-1) and appearance of the v(C=0) bands (—1700 cm-1), concurrent with the formation of the metal chalcogenide and regeneration of the fatty acid, have been used to evaluate... 

The rate-limiting step in CD for the first two mechanisms is almost always formation of the chalcogenide ion. This reaction should be slow otherwise fast, homogeneous precipitation of the metal chalcogenide will occur with little fihn formation. (Even rapid precipitation can lead to a film however, this film will be extremely thin and in most cases not visible.) Almost all the literature on CD is limited to sulfides (mostly), selenides, and oxides (including hydrated oxides and hydroxides). Anion-forming reactions are described in this section. 

The C/fb for n- and p-Si5) and that for metal chalcogenides such as n-CdS, n-CdSe and CdTe4,6) do not obey the above law, remaining nearly constant in a range of pH lower than about 6 for Si and about 10 for n-CdS. This is most probably because the semiconductor surface has no OH group in this pH range. It shoud be mentioned that the U for the metal chalcogenide semiconductors... 

Most of the metallic chalcogenides are found as crystalline materials, often with 2-D character and only a very limited number of them are able to form glasses from the melt. 

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