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

Alrashdi, Ameera (2016) Brillouin light scattering studies of transition metal dichalcogenides MoS₂, MoSe₂, and WS₂. Masters thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (40MB)

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.

Actions (login required)

View Item