Density functional theory studies on the adsorption of methanethiol molecule on Au(111) surface at different coverage

By applying the first-principles method based on density functional theory, we have studied the adsorptions of non-dissociative CH₃SH molecule and dissociated CH³S group on Au(111) surface at five different coverage (1/12, 1/8, 1/6, 1/4, 1/3). By performing the total energy calculations for both CH₃SH and CH³S at four possible adsorption sites at every studied coverage, we have found the stable non-dissociative adsorption configuration, as well as the dissociated adsorption for CH₃SH from low to high coverage. Especially, the effect of van der Waals interaction on the adsorption configurations and energies have been studied by the DFT-D2 method. Our results suggest that the top site is most stable site for non-dissociative CH₃SH molecular at all the studied coverage, while the tilt angle of the adsorbed molecule is 70°±2°. The adsorption energy decreases with the increasing of the coverage, that is 0.33~0.35 eV at low coverage region of 1/12~1/8. On the other hand, the dissociated CH³S group prefers to adsorb on bri-fcc site at all coverage with the tilt angle of 48.3°~ 58.5°. For the dissociated CH³S, the adsorption energy also decreases with the increasing of the coverage, that is 2.07~2.08 eV at low coverage region of 1/12~1/8. At the saturated coverage of 1/3, the effect of van der Waals interaction was investigated for the adsorption of CH₃SH/Au(111) and CH³S/Au(111) systems. The calculation results indicate that the van der Waals force makes the adsorbates closer to the substrate and the adsorption energy larger. The respective adsorption energies for CH₃SH and CH³S are about 0.59 eV and 2.27 eV at coverage of 1/3. The calculation result by DFT-D2 method for CH₃SH agrees with the experimental conclusion, but that for CH³S is away from the experimental observation. Moreover, our calculation results on the electronic structure show us that charge is transferred from the adsorbed CH₃SH and CH³S to the Au substrate. And the amount of the transferred charge decrease with the increase of the coverage for both the CH₃SH and CH³S, which imply that the interaction between the adsorbed CH₃SH/CH³S and Au substrate decrease with the increasing of the coverage.