Alkyl Silanes on Oxides

Monolayers of organic molecules may be covalently bound to inorganic solid substrates. Three main classes of such monolayers may be distinguished alkyl thiols on noble metals, alkyl silanes on oxides, and alkenes on hydrogen-terminated silicon. 

Upon immersing the substrates in the appropriate (organic) solution, the chemical bonds are formed, if necessary by exposure to heat or light. The layers thus obtained are often referred to as SAMs. Because the preparation of such covalent organic monolayers belongs to the domain of organic chemistry, we only briefly indicate the main principles and features. 

When ultraclean gold, silver, or (nonoxidized) copper are immersed in a, say, 10 M solution of alkane thiols, R - SH (where R is the alkyl chain), in an organic solvent a monolayer of the R - SH molecules at the Au surface is formed within a few minutes up to a few hours. The R - SH molecules become chemically linked to the Au surface by virtue of commensurate fitting of the -SH group in the Au(lll) crystal lattice. The resulting value for is 0.22 nm. Considering that the molecular ctoss section of a hydrocarbon chain is 0.18 nm, it is inferred that a close packed condensed monolayer is obtained. 

Alkyl silicium compounds of the types RSiXj, RiR2SiX2, and RiR2R3SiX, where R is an alkyl group and X a halogen atom (usually Cl) or an alkoxy group (usually methoxy or ethoxy), react with hydroxylized surfaces, according to 

Silica surfaces contain about five to eight silanol groups per nm. For an average of two bonds per alkyl silicium molecule, is in the range of 0.25-0.40 nm. Both for the alkane thiols and the silanes, the alkyl chain length may vary between, say, 1 and 18 C-atoms. Furthermore, the alkyl chain may be functionalized with, for example. 

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