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 ext ernal reflection spectroscopy (FTIR-ERS). FTIR-ERS data provide eviden ce for an ongoing increase in SAM crystallinity and, in some cases, a decrease in average chain tilt angle with respect to the surface norma l during this long assembly time. SAM crystallinity, judged by line sh ape and position, increased more rapidly for more concentrated HDT sol utions but appears to converge on the same final value independent of HDT concentration in the range of 10(-6) to 10(-2) M. Computer-optimiz ed fits to the ensemble of C-H stretching bands between 2800 and 3000 cm(-1) made on similar to 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 th e average cant and twist angles were 20 +/- 2 degrees and 49 +/- 3 deg rees, respectively, and for Au on HF-etched Si[111] they were 22 +/- 2 degrees and 51 +/- 1 degrees, 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 h ave orthogonal backbone planes.