Fingerprint Raman spectroscopy for two-dimensional MoS_2xSe_2(1-x) alloys

Two-dimensional transition metal dichalcogenides (TMDs) and alloys based on them, is a promising platform for creating opto- and nanoelectronic devices. For layered alloys, there is a strong need to theoretically determine the frequencies of vibrational modes and dependences of their energies on the stoichiometric composition. By comparing experimentally measured Raman modes with theoretical predictions, it becomes possible to determine the stoichiometric composition of the manufactured alloys. In this work, we investigated the vibrational properties of monolayer MoS_2x Se_2(1−x) alloys utilizing density functional theory method and confirmed them experimentally by Raman spectroscopy. The dependence of A₁ʹ and E¹ _2g ‘dactylographic’ modes on the stoichiometric composition of alloys has been calculated. When in pure MoSe₂ structure Se atoms are substituted by S atoms, the in-plane E¹ _2g (S–Mo), E¹ _2g (Se–Mo) and out-plane A₁ʹ (S–Mo) modes shifted linearly in x to higher frequencies, while the out-plane A₁ʹ (Se–Mo) mode did the same in nonlinear way. We also identified the E¹ _2g (Se–Mo–S) mode, which specific for the MoS_2x Se_2(1-x) alloys and does not appear in pure two-component TMDs.