Formation of multilayers by self-assembly

Abstract

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 (0.1-hm scale), ordered, multilayer films. In recent years organized molecular systems have attracted growing attention. The techniques which are presently available for the construction of such systems include both Langmuir-Blodgett (LB)’ and self-assembly methods, by which ordered, monomolecular layers can be formed on hydrophilic surfaces. These systems are believed to have technological potential in both optical and molecular e1ectronics.l They allow the chemist to potentially design new organic materials at a molecular level by incorporating useful functional groups into such systems and controlling such variables as the spacing of these groups within and between monomolecular layers. Although the LB method has been studied intermittently for many years and has been found successful for the formation of an extremely wide variety of monolayer and multilayer films (including relatively thick films of even several hundred layers), the SA method offers important advantages for future applications in such areas as molecular electronics and optical applications. The use of derivatives of alkyltrichlorosilanes (e.g., octadecyltrichlorosilane, OTS) results in monomolecular layers which are durable, thermally stable! and resistant to degradation by a variety of strong reagents.%l6 The trichlorosilyl head group forms covalent bonds to the hydrophilic surface as well as cross-links to adjacent molecules via Si-0 bonds created upon hydrolysis with trace water. This method can be adapted to the formation of multilayers by designing monolayers containing terminal functional groups that can be treated with various reagents to unmask a fresh, hydrophilic surface upon which a succeeding monolayer can be adsorbed.’J’