Selenium, 4, 6, 50 amorphous

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

Iodine cataly2es the conversion of amorphous selenium to the black, semiconducting metallic modification, and is used for this purpose in the manufacture of photoelectric cells and electric rectifiers (see Seleniumand selenium compounds). 

G. Faivre, J. L. Gardissat. Viscoelastic properties and molecular structure of amorphous selenium. Macromolecules 79 1988-1996, 1986. 

Graham AK, Pinkerton HL, Boyd HJ (1959) Electrodeposition of amorphous selenium. J Electrochem Soc 106 651-654... 

To conduct experiments of this kind it is very convenient to make use of disorder adsorbent provided by a film of amorphous selenium. During deposition under vacuum conditions, the pressure being no higher than lO Torr, the amorphous modification of selenium is being formed [38]. There are two forms of amorphous selenium which differ in coordination numbers and radii of coordination spheres. The first form is... 

The experiment with films of amorphous selenium with adsorbed ethyl radicals [39] showed that during heating of these films in vacuum... 

If heating of amorphous selenium with absorbed ethyl radicals is conducted in presence of nitrogen or inert gas with pressure 100 Torr no signal is picked up from the sensor. This implies that in this case the signals of the sensor are controlled by adsorption of ethyl radicals on its surface. Consequently, the heating of amorphous selenium with adsorbed radicals and resulted crystallization lead to emission of radicals. 

In the Boliden process, developed by Boliden Kemi Company in Sweden, filter beds containing amorphous selenium are used to filter the gases. Mercury in the gas is deposited in the filter as mercury selenide (HgSe). Alternatively, the gases can be scrubbed with a slurry of amorphous selenium, whereupon mercury is precipitated as HgSe. Activated carbon filters are also useful for absorbing mercury vapor from the gas. 

Phosphorus pentaselenide prepared from red, amorphous selenium effects cyclization of 1,4-diketones (22) to substituted selenophenes (23) in moderate... 

Bindu, K. Lakshmi, M. Bini, S. Sudha Kartha, C. Vijayakumar, K. P. Abe, T. Kashiwaba, Y. 2002. Amorphous selenium thin films prepared using chemical bath deposition Optimization of the deposition process and characterization. Semicond. Sci. Technol. 17 270-274. 

Amorphous selenium glasses, semiconductivity in, 12 587 Amorphous semiconductors, 22 127-141. See also Amorphous silicon (a-Si) amorphous silicon growth for,... 

Black amorphous selenium, 22 74 Black-and-white instant films, 19 282 Black-and-white instant imaging processes, 19 279-282... 

Powdered amorphous selenium and liquid bromine are shaken together in dry methanol. After 30 minutes a saturated solution of H2S in methanol is added and a part of the solvent is distilled off. Upon slow cooling yellow to orange crystals of n 8-n appear with n amounting to approximately 1 or 4 depending on the initial ratio of selenium and hydrogen sulfide... 

Amorphous forms exhibit two colors, occurring as a red powder of density 4.26g/cm3 that has a hexagonal crystal structure and a black vitreous solid of density 4.28g/cm3. The red amorphous selenium converts to the black form on standing. Amorphous selenium melts at 60 to 80°C insoluble in water reacts with water at 50°C when freshly precipitated soluble in sulfuric acid, benzene and carbon disulfide. 

Selenium dioxide is reduced by hydrazine to black amorphous selenium Se02 + N2H4 Se + N2 + 2H2O... 

Hydroxylamine hydrochloride reduces selenium dioxide to reddish-hrown amorphous selenium ... 

Removal of the bulky, amorphous selenium is accomplished with the aid of a 6-in. Buchner funnel. The selenium is returned to the reaction flask and extracted with 300 ml. of boiling 95% ethanol for 1 hour (Note 4). The solution, obtained by decantation from the compact gray selenium, is combined with the above filtrate in a 4-1. distilling flask. Distillation under reduced pressure gives two fractions. The lower-boiling fraction (25-60°/16 mm.) consists mainly of ethanol, water, dioxane, and cyclohexanone the higher-boiling one (60-90°/16 mm.) contains cyclohexanone and 1,2-cyclohexanedione with traces of water and dioxane. The yield of crude product is approximately 322 g. 

Figure 2.4 Variation in the TSDC for various heating rates vp in amorphous selenium vp is 0.08, 0.17, 0.23, and 0.44K/s (curves 1-4, respectively). The inset shows the Arrhenius plot of TSDC for curve 1.

The addition of arsenic to amorphous selenium—even in a relatively low amount ( 1 at.%), as for glassy compounds—causes a change in (he shape of TSDC curve. As one... 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

A metal cylinder is mounted to rotate about a horizontal axis. This is called the drum. The end-to-end dimension is the width of print to be produced plus a generous tolerance. The drum in the copiers originally developed by Xerox Corporation was manufactured with a surface coating of amorphous selenium (more recently, ceramic or organic... 

Returning to the depletion discharge, it should be stressed that with thick films and a good blocking contact between a-Se and the preoxidized aluminum substrate, the depletion discharge process dominates. There are several reasons that amorphous selenium possesses good dark decay characteristics ... 

There are not many deep locahzed states in the mohihty gap of pnre amorphous selenium. 

If blue light is used for the discharge process, then the absorption is very close to the charged surface, and one can assume that the discharge process involves the transport of photogenerated holes through the bulk. Trapping of these holes in the bulk then results in the observed first residual potential Vri. In the case of amorphous selenium photoreceptor films, it has been found that Ki is predicted by the Waiter expression [16]... 

Let us first consider the carrier drift in pure amorphous selenium. Both the electron and hole drift mobility can be measured in a-Se by the TOF technique outhned earlier. At temperatures above 200 K, a well-defined transit pulse is observed. The transient... 

---