Sixteen second-order Raman modes of monolayer ${\mathrm{MoSe}}_2$ are observed and analyzed by experiment and calculation, respectively, for four laser energies from 2.33–2.60 eV. The Raman peaks labeled by ${\mathrm{P}}_i$ ($i=1,2,3,4$) at $280\text{–}320{\mathrm{cm}}^{-1}$ and ${\mathrm{S}}_i$ ($i=1,2,3$) at $400\text{–}480{\mathrm{cm}}^{-1}$ are newly analyzed quantitatively by the first-principles calculation using qr²-code. The ${\mathrm{P}}_i$ bands are assigned to the second-order combination modes of TA (LA) and ${\mathrm{E}}_{\mathrm{TO}}^{\prime }\left({\mathrm{E}}_{\mathrm{LO}}^{\prime }\right)$, while the ${\mathrm{S}}_i$ bands are assigned to the 2LA mode with different phonon wave vectors, which is explained by the combination of two-phonon density of states and electron-phonon coupling calculations. The overlapped Raman peak of the first-order mode ${\mathrm{E}}^{\prime }$ and the second-order mode ${\mathrm{S}}_1$ can be identified separately using helicity-dependent Raman spectroscopy. The mode assignments are further confirmed by space-group analysis and helicity-dependent selection rule.

Close

PrevNext

View figure in article

• Electron-phonon coupling
• First-principles calculations
• Phonons
• Monolayer films
• Transition metal dichalcogenides
• Raman spectroscopy