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, RNH 2 , and converting them, via α-chloroamides RNHCOCH 2 Cl, to thiols having the structure RNHCOCH 2 SH. 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 CF 3 CH 2 -NHCOCH 2 SH increases their stability against desorption or exchange with hexadecanethiol in ethanol relative to SAMs from CF 3 (CH 2 ) 3 SH. The desorption of SAMs from CF 3 (CH 2 ) 3 SH was first order in the alkanethiolate and had a half-life of ∼2 h at 10 -9 Torr ; SAMs from CF 3 CH 2 NHCOCH 2 SH, by contrast, showed no loss after 48 h at ∼ 10 9 Torr. The rate of exchange of a SAM from CF 3 CH 2 NHCOCH 2 SH with hexadecanethiol in ethanol was 10 2 -10 3 times slower than the SAMs from CH 3 (CH 2 ) 3 SH or from CF 3 (CH 2 ) 3 -SH. The susceptibility of a SAM prepared from a short-chain alkanethiol (e.g., CH 3 (CH 2 ) 3 SH or CF 3 -CH 2 NHCOCH 2 SH) 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 CF 3 CH 2 NHCOCH 2 SH, followed by protection ofthe 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.