AN ORIENTATION ANALYSIS OF DIFFERENTLY ENDGROUP-FUNCTIONALIZED ALKANETHIOLS ADSORBED ON AU SUBSTRATES

The molecular orientation in self-assembled films of methyl-, hydroxyl -, and carboxylic acid-terminated alkanethiols of different chain leng ths (n-docosanethiol [DCT], 22-mercaptodocosanol [MDO], 22-mercaptodoc osanoic acid [MDC], n-hexadecanethiol [HDT], 16-mercaptohexadecanol [M HO], 16-mercaptohexadecanoic acid [MHC]) on Au substrates has been inv estigated by near edge X-ray absorption fine structure (NEXAFS) spectr oscopy and X-ray photoelectron spectroscopy (XPS). Whereas XPS-measure ments do not reveal changes in thickness upon replacing the endgroups, the degree of orientation as determined with NEXAFS exhibits signific ant differences. The two methyl-terminated thiols show the same alkyl- chain tilt-angle a of 39 degrees. For the short-chain OH-terminated fi lms no significant difference could be detected, but the monolayers fo rmed from the long-chain OH-terminated chains exhibit a slightly stron ger anisotropy and thus a smaller tilt-angle alpha than the correspond ing CH,(3)-terminated thiol. NEXAFS-spectra recorded for films from CO OH-terminated thiols reveal only a very small anisotropy which indicat es the absence of significant molecular orientation resulting from a h igh degree of disorder. (C) 1997 Elsevier Science S.A.