Brillouin light scattering studies of transition metal dichalcogenides MoS₂, MoSe₂, and WS₂

Abstract

Brillouin Light Scattering has been used for the first time to probe surface and bullk acoustic phonons in the transition metal dichalcogenides MoS₂, MoSe₂ and WS₂. Rayleigh surface and transverse bulk acoustic phonon velocities were obtained from the Brillouin peak frequency shifts. Rayleigh velocities were 1690 ± 70 m/s, 1630 ± 40 m/s, and 1430 ± 50 m/s for MoS₂, MoSe₂, and WS₂, respectively. The Rayleigh surface phonon velocities are similar to those found for other transition metal dichalcogenides. The transverse bulk acoustic modes propagating approximately along the z direction, were observed for WS₂ and for MoS₂ and were calculated by measuring the average value of the frequency shift. The transverse velocities were found to be 2430 ± 70 m/s and 3450 ± 100 m/s, respectively. The bulk transverse velocities for MoS₂, and WS₂ were found to be ~ 21 % and ~ 44 % lower, respectively, than those of single layer samples determined using first-principle calculations. Moreover, from transverse acoustic phonon velocities the elastic constants, C44 was determined as 44 ± 1 GPa and 60 ± 3 GPa for WS₂, and MoS₂, respectively. In general, obtained elastic constants are higher than those obtained from other bulk and single layer studies. Also, Brillouin peaks due to bulk modes were not seen for either sample of MoSe₂ so it was not possible to obtain the bulk velocities and elastic constants for this material. The extinction coefficient for MoSe₂ is much larger than those for WS₂ and MoS₂. This could be the reason why no bulk mode was observed in the Brillouin spectra of MoSe₂.