Self-Assembled Monolayers on Gold Generated from Alkanethiols with the Structure RNHCOCH₂SH

This paper outlines a general synthetic route to alkanethiols useful in forming self-assembled monolayers (SAMs) starting from amines, RNH₂, and converting them, via α-chloroamides RNHCOCH₂Cl, to thiols having the structure RNHCOCH₂SH. The wettabilities (estimated from contact angles of water and hexadecane) and the thicknesses (as measured by ellipsometry and X-ray photoelectron spectroscopy, XPS) of SAMs having an amide moiety β to Au/thiolate were similar to those of SAMs of alkanethiols with similar backbone length on gold. The internal amide group present in SAMs prepared from CF₃CH₂-NHCOCH₂SH increases their stability against desorption or exchange with hexadecanethiol in ethanol relative to SAMs from CF₃(CH₂)₃SH. The desorption of SAMs from CF₃(CH₂)₃SH was first order in the alkanethiolate and had a half-life of ∼2 h at 10^-9 Torr; SAMs from CF₃CH₂NHCOCH₂SH, by contrast, showed no loss after 48 h at ∼ 10^-9 Torr. The rate of exchange of a SAM from CF₃CH₂NHCOCH₂SH with hexadecanethiol in ethanol was 10²-10³ times slower than the SAMs from CH₃(CH₂)₃SH or from CF₃(CH₂)₃-SH. The susceptibility of a SAM prepared from a short-chain alkanethiol (e.g., CH₃(CH₂)₃SH or CF₃-CH₂NHCOCH₂SH) to damage by UV was 10 times greater than that observed for a SAM prepared from hexadecanethiol (as measured by XPS). UV damage of SAMs derived from CF₃CH₂NHCOCH₂SH, followed by protection of the UV-damaged SAM by replacement with hexadecanethiol and etching, gave gold patterns with minimum feature sizes of 5 μm; these sizes were limited by the lithographic procedure used and do not reflect the true edge resolution of this photolithographic method.