Preliminary calculations based on a simple model give a good description of the molecular orientation and packing of alkanethiol monolayers on gold surfaces. These calculations suggest that alkanethiol molecules on gold have a total alkyl chain axis tilt of approximately 38°in a plane that bisects the methylene H-C-H angles, followed by a rotation about the alkyl chain axis of =46°. The alkyl chain tilt is a function of the sulfur-sulfur spacing in a hexagonal crystal lattice layer and maximizes the attractive interactions between neighboring molecules. These results are in agreement with the molecular orientations obtained from grazing angle FTIR experiments on dodecanethiol monolayers on a (111) gold surface.
Monolayer and multilayer films were formed by self-assembly of methyl 23-(trichlorosilyl)tricosanoate (1) from organic solution. In agreement with published results, this compound was found to form good quality, close-packed monolayers on silicon surfaces. We have, however, found that films significantly thicker than the three monolayers previously obtained can be formed from continued chemisorption of monolayers of this compound followed by reduction of the surface ester with LiAlH4 in THF to form an "alcohol surface". The quality of monolayer formation in the multilayer films was monitored in detail by ellipsometry, contact angle, and FTIR measurements, and, although generally increasing disorder can be detected, films of up to 25 discrete monolayers can be successfully made. These results indicate that self-assembly is a viable alternative to the Langmuir-Blodgett transfer technique for the construction of relatively thick (O.l-gm scale), ordered, multilayer films.
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