A periodic mixed gaussians-plane waves DFT study on simple thiols on Au(111): adsorbate species, surface reconstruction, and thiols functionalization

Here we present DFT calculations based on a periodic mixed gaussians/plane waves approach to study the energetics, structure, bonding of SAMs of simple thiols on Au(111). Several open issues such as structure, bonding and the nature of adsorbate are taken into account. We started with methyl thiols (MeSH) on Au(111) to establish the nature of the adsorbate. We have considered several structural models embracing the reconstructed surface scenario along with the MeS center dot-Au-ad-MeS center dot type motif put forward in recent years. Our calculations suggest a clear preference for the homolytic cleavage of the S-H bond leading to a stable MeS center dot on a gold surface. In agreement with the recent literature studies, the reconstructed models of the MeS center dot species are found to be energetically preferred over unreconstructed models. Besides, our calculations reveal that the model with 1 : 2 Au-ad/thiols ratio, i.e. MeS center dot-Au-ad-MeS center dot, is energetically preferred compared to the clean and 1 : 1 ratio models, in agreement with the experimental and theoretical evidences. We have also performed Molecular Orbital/Natural Bond Orbital, MO/NBO, analysis to understand the electronic structure and bonding in different structural motifs and many useful insights have been gained. Finally, the studies have then been extended to alkyl thiols of the RSR' (R, R' = Me, Et and Ph) type and here our calculations again reveal a preference for the RS center dot type species adsorption for clean as well as for reconstructed 1 : 2 Au-ad/thiols ratio models.