| AVS 58th Annual International Symposium and Exhibition | |
| Surface Science Division | Monday Sessions |
| Session SS2-MoA |
| Session: | Molecular Ordering and Electrochemical Interfaces |
| Presenter: | Quanmin Guo, University of Birmingham, UK |
| Authors: | L. Tang, University of Birmingham, UK F. Li, University of Birmingham, UK & Northwestern Polytechnical Univ., P.R. China W. Zhou, Northwestern Polytechnical Univ., P.R. China Q. Guo, University of Birmingham, UK |
| Correspondent: | Click to Email |
Surface structures of self-assembled methylthiolate and ethylthiolate monolayers on Au(111) have been imaged with STM. For saturation coverage at room temperature, the well-known (√3×√3)R30° phase routinely observed for longer chain alkanethiolates does not appear under any conditions for adsorbed methylthiolate and ethylthiolate. Instead, both thiolate species organize themselves into a well-ordered 3×4 structure with a coverage equal to 0.33 ML. We thus conclude that the stable structure for saturation coverage of methylthiolate/ethylthiolate on Au(111) at RT is 3×4, not (√3×√3)R30° as generally believed.
We performed experiments by imaging methylthiolate and ethylthiolate monolayers in ultra-high-vacuum (UHV) using high-resolution STM. The methylthiolate monolayer was prepared by exposing a gold single crystal in vacuum to 10-8 mbar of dimethyl disulfide (DMDS) vapour at RT for 15 minutes. It is known that DMDS adsorb dissociatively on Au(111) at RT by forming methylthiolate. Ethylthiolate monolayer was prepared by exposing a (111) oriented gold film to 5×10-5 mbar of ethanethiol vapor at room RT for 2 hours. The much higher exposure required for ethanethiol is due to the very low dissociation probability of this molecule.
When the 3×4 phase is subjected to a gentle thermal annealing to 320 K, partial desorption takes place and a striped phase with coverage equal to 0.27 ML appears. The striped phase for ethylthiolate resembles that observed for propylthiolate and other long chain thiolates, but the striped phase for the methylthiolate takes a unique structure of its own. Our findings suggest that the general belief that all alkanethiols form at least one common structural phase is not true and new theoretical modeling is urgently required to address the chain-length dependent property of alkanethiol monolayers.