Structural Evolution of Hexadecanethiol Monolayers on Gold during Assembly: Substrate and Concentration Dependence of Monolayer Structure and Crystallinity

The structures of hexadecanethiol (HDT) self-assembled monolayers (SAM) on gold assembled for up to 11 days from ethanolic HDT solutions are reported. Gold|HDT films were periodically removed from 0.001, 0.1, 1, or 10 mM HDT solution and analyzed by Fourier transform infrared external reflection spectroscopy (FTIR-ERS). FTIR-ERS data provide evidence for an ongoing increase in SAM crystallinity and, in some cases, a decrease in average chain tilt angle with respect to the surface normal during this long assembly time. SAM crystallinity, judged by line shape and position, increased more rapidly for more concentrated HDT solutions but appears to converge on the same final value independent of HDT concentration in the range of 10 - 6 to 10 - 2 M. Computer-optimized fits to the ensemble of C−H stretching bands between 2800 and 3000 cm - 1 made on ∼50 different SAMs which were prepared under a wide range of conditions return surprisingly consistent average tilt and twist angles. For Au on Cr-primed glass the absolute values of the average cant and twist angles were 20 ± 2° and 49 ± 3°, respectively, and for Au on HF-etched Si〈111〉 they were 22 ± 2° and 51 ± 1°, respectively. A clear trend in SAM cant angles, decreasing with increasing time, is observed for the glass|Cr|Au|HDT system. Two structural models were compared: one in which all chains are identical and one in which alternating alkanethiol chains have orthogonal backbone planes.