Metal Selenides and Tellurides

Lead sulfide is a photoconductor with an exceptionally high response in the near IR-region. It has been prepared by atomic layer epitaxy using lead halides, lead acetate or lead bis(/3-diketonate) compounds as the lead source and H2S as the sulfur source [132]. Another route to PbS is atomic layer epitaxy using Pb40(0-r-Bu)f, or polymeric [Pb(0-f-Bu)2] and H2S as precursors [133]. 

Amorphous AS2S3 is a chalcogenide semiconductor. Like GeSi, it shows structural transformations when exposed to light and is employed in optical imaging and storage devices [134]. 

Arsenic sulfide has been prepared by the chemical vapor decomposition of a mixture of arsine (ASH3) and hydrogen sulfide [135a]. 

Several metal selenides, particularly group 12 element selenides and tellurides, display readily exploitable electrical and optical properties. The properties and preparation of 12/16 (former II-VI) semiconductors are described elsewhere (Chapter 4). This section will focus on a few main group element selenides and tellurides which show interesting properties (Table 7-1) and can be prepared by CVD. 

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These closely resemble the corresponding sulphides. The alkali metal selenides and tellurides are colourless solids, and are powerful reducing agents in aqueous solution, being oxidised by air to the elements selenium and tellurium respeetively (cf. the reducing power of the hydrides). 

The induced co-deposition concept has been successfully exemplified in the formation of metal selenides and tellurides (sulfur has a different behavior) by a chalcogen ion diffusion-limited process, carried out typically in acidic aqueous solutions of oxochalcogenide species containing quadrivalent selenium or tellurium and metal salts with the metal normally in its highest valence state. This is rather the earliest and most studied method for electrodeposition of compound semiconductors [1]. For MX deposition, a simple (4H-2)e reduction process may be considered to describe the overall reaction at the cathode, as for example in... 

Metal selenides and tellurides are formed by some metals especially those of group 12 (the zinc group) and these are generally semiconductors which have photocatalytic activity. Some are hydrolysed by water or dilute acids to give H2Se(or H2Te) and the metal hydroxide or salt. The two hydrides have acidic properties and unpleasant smells. 

All three elements combine readily with most metals and many non-metals to form binary chalcogenides. Indeed, selenides and tellurides are the most common mineral forms of these elements (p. 748). Nonstoichiometry abounds, particularly for compounds with the transition elements (where electronegativity differences are minimal and variable valency is favoured), and many of the chalcogenides can be considered... 

The selenides and tellurides of the coinage metals are all metallic and some, such as CuSe2, CuTc2, AgTe. 3 and Au3Tc5 are superconductors at low temperature (as also are CuS and CUS2). 

Selenium and Tellerium Tantalum is attacked by selenium and tellurium vapours at temperatures higher than 80°C. Only slight attack is observed on the metal by liquid selenides and tellurides of ytirum, the rare earths, and uranium at temperatures of 1300 to 2100°C, and tantalum is considered to be a satisfactory material in which to handle these intermetallic compounds. 

Draganjac M, Rauchfuss TB (1985) Transition metal polysulfides Coordination compounds with purely inorganic chelate ligands. Angew Chem Int Ed Engl 24 742-757 DuBois MR (1989) Catalytic applications of transition metal complexes. Chem Rev 89 1-9 Ansari MA, Ibers JA (1990) Soluble selenides and tellurides. Coord Chem Rev 100 223-266... 

More quantitatively, it appears to a first approximation that the unit cell contraction of a compound containing Mn2+, Co2+, Ni2+, or Fe2+, relative to the isomorphous Mg+2 compound, is a linear function of the Ax of the metal-ligand bond if we neglect selenides and tellurides. Inclusion of these more covalent compounds indicates a greater dependence on Ax. 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

Selenides, tellurides andpolonides. Se, Te and Po react easily with most metals and non-metals to form binary compounds (selenides and tellurides are common mineral forms of these elements). Non-stoichiometry is frequently observed in the compounds with the transition elements many of these compounds may be described as metallic alloys. The compounds of the metals of the first two groups may be considered the salts of the acids H2Se, H2Te, etc. The alkali metal selenides... 

However, for chalcogenide compounds (metal sulfides, selenides and tellurides etc.) the potential across the compact layer is determined by the concentration of hydrated chalcogenide ions so that the flat band potential does not necessarily depend on pH. 

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