Silicon bond distribution

Fig. 2. The distribution of silicon—oxygen—silicon bond angles in vitreous siUca (22,25). The function V(a) is the fraction of bonds with angles normalized to the most probable angle, 144°. This distribution gives quite a regular stmcture on the short range, with gradual distorting over a distance of 3 or 4 rings (2—3 nm). Crystalline siUca such as quartz or cristobaUte would have a narrower distribution around specific bond angles.

The selective cleavage of the silicon-silicon bond in the disilacyclopentane and -hexane is probably due to the concentration of internal angular strain at this bond in such smaller polygonal molecules. In case of the disilacyclo-heptane, however, it seems likely that the strain is smaller and uniformly distributed all over the ring, and hence cleavage occurs preferentially at the silicon-carbon bond by the accepted mechanism involving both an electrophilic attack on carbon and nucleophilic attack on silicon by the sulfuric acid molecule(s) 169). 

A physical model for the formation energies is more easily addressed for undoped a-Si H because the dopants do not have to be included. The origin of the distribution is the disorder of the silicon network. Defect generation is expected to occur when a silicon-silicon bond... 

Q From a consideration of the Pauling electronegativity values, predict the charge distribution in the carbon silicon bond. 

Typical results for a semiconducting liquid are illustrated in figure Al.3.29 where the experunental pair correlation and structure factors for silicon are presented. The radial distribution function shows a sharp first peak followed by oscillations. The structure in the radial distribution fiinction reflects some local ordering. The nature and degree of this order depends on the chemical nature of the liquid state. For example, semiconductor liquids are especially interesting in this sense as they are believed to retain covalent bonding characteristics even in the melt. 

Availability of Monomer. Under most solution conditions employed for gel formation, monomer is rapidly consumed to form a distribution of oligomeric species (12). Thereafter the only possible source of monomer is that which is produced by hydrolytic (or alcoholic) depolymerization of siloxane bonds (dissolution). According to Her (241 the dissolution of a-Si02 above pH 2 is catalyzed by OH ions which are able to increase the coordination of silicon above 4... 

The time-of-flight secondary ion mass spectrum of a thick film prepared from Si(OEt)4 on a hydrophilic silicon substrate (Fig. 1) reveals a distribution of masses up to 1200 amu. The observed formation of cationized oligomers with a distribution shown in Fig. 1 can be explained by bond cleavage within the uppermost monolayer of the polycondensate of TEOS as a result of primary Ar+ ion impact. 

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