Signatures of 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 MoS2 and MoSe2 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 MoSe2 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 A1g and E2g vibrational modes upon physisorption on Au(111). For MoS2, 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 MoS2 layer is observed in both the E2g(1) mode as a consequence of strain and the A1g mode due to spill-over of charge from the Au(111) surface into the conduction band minimum of strained MoS2. The latter observation helps us to rationalize the experimentally observed satellite peak of the A1g Raman signal from MoS2/Au(111) and other layered sulfides while this feature is absent in MoSe2.
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