Grey Selenium

Grey selenium exhibits both photovoltaic and photoconductive properties, which make it useful in the production of photocells and solar cells. Moreover, 

50-150 nm wide crystallites consisting of many tiny crystals grow above this layer (a). A 500 nm thick silicon layer consists of coherent spherical crystallites (b). 

The deposited Se is partly dissolved at A3. The shoulders C] and C2 and their anodic counterparts Ai and A2 might be related to two different UPD processes. The pair C5 and A5 are likely to be due to the reduction of the deposited Se to Se2-. 

In our opinion the electrodeposition of selenium is quite promising for a variety of applications. For example, the possibility to deposit grey selenium, indium, and copper in one ionic liquid at variable temperatures might be regarded as the first step in making selenium-containing compound semiconductors like CIS by electrochemical means. 

In this chapter we have summarized selected literature data on the electrodeposition of semiconductors in ionic liquids. It has been demonstrated that elemental silicon, germanium, and selenium can be elecrodeposited in ionic liquids. Furthermore, it is shown that compound semiconductors like InSb, AlSb, CdTe and others can be made, especially at elevated temperatures where kinetic barriers are easier to overcome, even allowing the exclusive electrodeposition of grey selenium. In this context ionic liquids are very promising for semiconductor electrodeposition. Both wide electrochemical and thermal windows allow processes which are impossible in aqueous or organic solvents. 

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Figure 16.1 Structures of various allotropes of selenium and the structure of crystalline tellurium (a) the Seg unit in a- fi- and y-red selenium (b) the helical Se chain along the c-axis in hexagonal grey selenium (c) the similar helical chain in crystalline tellurium shown in perspective and (d) projection of the tellurium structure on a plane perpendicular to the c-axis.

It ignites when heated in air [1], A new and safer synthesis from red phosphorus and grey selenium has been described [2],... 

A number of benzo- or dibenzo-fused seven membered phosphorus heterocyclic systems have also been studied. These include the benzo-fused oxa-bridged phosphaalkene 76 prepared by thermolysis of 2,3-diphenylindenone 23-epoxide (as a source of the carbonyl ylide 1,3-dipole intermediate) in the presence of /-butylphosphaalkyne. This bridged phosphaalkene is unusually stable even without inert gas blanketing . Reaction of 76 with sulfur or grey selenium stereoselectively affords the thia- or selenaphosphiranes 77 (X = S, Se respectively). <00T6259>... 

Grey selenium (mp 494 K, metallic) is the stable form. It may be obtained crystalline from hot solutions of Se in aniline or from the melt. The structure, which has no sulfur analogue, contains infinite, spiral chains of selenium atoms. Although there are fairly strong single bonds between adjacent atoms in each chain, there is evidently weak metallic interaction between the neighboring atoms of different chains. Selenium is not comparable with most true metals in its electrical conductivity in... 

Pittman R. W. (1953), The photochemistry of selenium. Part III. Photogalvanic effects with red selenium Part IV. Photogalvanic effects with grey selenium , J. Chem. Soc. 855-860, 3888-3893. 

Selenium possesses several allotropes. Crystalline, red monoclinic selenium exists in three forms, each containing Seg rings with the crown conformation of Sg (Figure 15.6c). Black selenium consists of larger polymeric rings, and the thermodynamically stable allotrope is grey selenium. 

Tellurium has only one crystalline form which is a silvery-white metallic-looking solid and is isostructural with grey selenium. The red allotropes of 8e can be obtained by rapid cooling of molten 8e and extraction into C82. The... 

Tellurium has only one ctystalline form which is a silvery-white metallic-looking solid and is isostructural with grey selenium. The red allotropes of Se can be obtained by rapid cooling of molten Se and extraction into CS2. The photoconductivity of Se (see Box 16.1) and Te arises because, in the solid, the band gap of 1.66 eV is small enough for the influence of visible light to cause the promotion of electrons from the filled bonding MOs to the unoccupied antibonding MOs (see Section 6.8). 

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