An orientation analysis of differently endgroup-functionalised alkanethiols adsorbed on Au substrates

The molecular orientation in self-assembled films of methyl-, hydroxyl-, and carboxylic acid-terminated alkanethiols of different chain lengths (n-docosanethiol [DCT], 22-mercaptodocosanol [MDO], 22-mercaptodocosanoic acid [MDC], n-hexadecanethiol [HDT], 16-mercaptohexadecanol [MHO], 16-mercaptohexadecanoic acid [MHC]) on Au substrates has been investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). Whereas XPS-measurements do not reveal changes in thickness upon replacing the endgroups, the degree of orientation as determined with NEXAFS exhibits significant differences. The two methyl-terminated thiols show the same alkyl-chain tilt-angle a of 39 degrees. For the short-chain OH-terminated films no significant difference could be detected, but the monolayers formed from the long-chain OH-terminated chains exhibit a slightly stronger anisotropy and thus a smaller tilt-angle alpha than the corresponding CH,(3)-terminated thiol. NEXAFS-spectra recorded for films from COOH-terminated thiols reveal only a very small anisotropy which indicates the absence of significant molecular orientation resulting from a high degree of disorder. (C) 1997 Elsevier Science S.A.