Abstract
We show that the adsorption site basis of the (2√3 × 3)rect. phase of n-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au-S interfaces are used to explore the minimized energy alkyl-chain packing of n-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecules per unit cell; thus, differences are due to the headgroup spacing within the 4-molecule basis as opposed to the average surface density. We demonstrate for the first time the 4-molecule-basis twist structure driven by the packing of alkanethiol molecules in a large simulation box (100 molecules, 25 unit cells) using molecular dynamics. Our results validate the prediction put forward by Mar and Klein that to achieve the 4-molecule-basis twist symmetry observed by the experiment, the headgroups must deviate from the high-symmetry (√3 × √3)R30° sites. The key structural parameters: tilt, twist, and end-group height, as well as their spatial order, are compared with experimental results, which we show is a highly sensitive approach that can be used to vet proposed Au-S interfacial models.