Signatures of the Dichalcogenide–Gold Interaction in the Vibrational Spectra of MoS₂ and MoSe₂ on Au(111)

Various atomic structures for the interface between Au(111) and monolayers of MoS 2 and MoSe 2 are investigated by means of first-principles calculations approximating van der Waals interactions by pairwise atomic interactions. Calculated bond lengths and interface energies are reported. The focus is on the calculation of vibrational spectra and their comparison to experimental data. The MoSe 2 monolayer, due to its almost perfect match with the Au(111) surface in the (√3 × √3) R30° superstructure, shows shifts of less than one wavenumber of the Raman-active A 1g and E 2g vibrational modes upon physisorption on Au(111). For MoS 2, we find that two structural models, an almost unstrained superstructure with large periodicity and a strained layer with (√3 × √3) R30° supercell, may coexist, as evidenced by their almost identical formation energy. Considerable mode softening in the strained MoS 2 layer is observed in both the E 2g (1) mode as a consequence of strain and the A 1g mode due to spill-over of charge from the Au(111) surface into the conduction band minimum of strained MoS 2 . The latter observation helps us to rationalize the experimentally observed satellite peak of the A 1g Raman signal from MoS 2 /Au(111) and other layered sulfides while this feature is absent in MoSe 2 .