Accurate assignment of double resonant Raman bands in Janus MoSSe monolayer from first-principles calculations

Janus transition metal dichalcogenides (TMDs) structures, as a new type of two-dimensional layered materials, have drawn increasing research efforts mostly by the Raman characterization technique since their successful synthesis. First- and second-order resonant Raman spectra (RRS) have been reported by experiments. But, unlike much interest paid to the first-order RRS, there has been so far no much discussion dedicated to the second-order double resonant Raman (DRR) bands and band assignments of Janus TMDs, which nevertheless is indispensable but hampered by the difficulty of calculations. In this work, we calculate the DRR spectra of Janus MoSSe monolayer within the first-principles framework and succeed in achieving accurate assignments of the DRR bands. The assignments are in agreement with our group theoretical analysis. Moreover, taking advantage of its strain-sensitive feature, we calculate the DRR spectra under biaxial strain, and further verify the rationality of our assignments by analyzing strain-induced shift of the DRR bands. Our present study supplies an efficient strategy for quantitative understanding on the electron-phonon coupling in the Janus structures.