STRUCTURAL EVOLUTION OF HEXADECANETHIOL MONOLAYERS ON GOLD DURING ASSEMBLY - SUBSTRATE AND CONCENTRATION-DEPENDENCE OF MONOLAYER STRUCTURE AND CRYSTALLINITY
Rh. Terrill et al., STRUCTURAL EVOLUTION OF HEXADECANETHIOL MONOLAYERS ON GOLD DURING ASSEMBLY - SUBSTRATE AND CONCENTRATION-DEPENDENCE OF MONOLAYER STRUCTURE AND CRYSTALLINITY, Langmuir, 14(4), 1998, pp. 845-854
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.