Phonon dispersion of MoS$_2$

Abstract

Transition metal dichalcogenides like MoS2, MoSe2, WS2, and WSe2 have attracted enormous interest during recent years. They are van-der-Waals crystals with highly anisotropic properties, which allows exfoliation of individual layers. Their remarkable physical properties make them promising for applications in optoelectronic, spintronic, and valleytronic devices. Phonons are fundamental to many of the underlying physical processes, like carrier and spin relaxation or exciton dynamics. However, experimental data of the complete phonon dispersion relations in these materials is missing. Here we present the phonon dispersion of bulk MoS2 in the high-symmetry directions of the Brillouin zone, determined by inelastic X-ray scattering. Our results underline the two-dimensional nature of MoS2. Supported by first-principles calculations, we determine the phonon displacement patterns, symmetry properties, and scattering intensities. The results will be the basis for future experimental and theoretical work regarding electron-phonon interactions, intervalley scattering, as well as phonons in related 2D materials.

Thermal transport in MoS2 from molecular dynamics using different empirical potentials

“…Our predicted thermal conductivity values agree well with those measured on samples with relatively high quality [9][10][11]15]. We also compared the phonon dispersion curves calculated using the empirical potentials with available experimental data [64]. From all these comparisons, we identify the REBO-LJ potential as a transferable empirical potential for MoS 2 that can be applied to study thermal transport properties of MoS 2 in more complicated situations such as systems with the presence of defects, grain boundaries or specifically engineered nanoscale features, where the BTE approach is less practical.…”

“…5. By comparing with the experimental data [64], we see that all the three potentials (REBO-LJ, SW13, and SW16) describe the low-frequency acoustic phonons fairly well. The SW13E potential does not lead to reasonable description of the phonon dispersion, which is expected as it is an incorrect implementation of the SW13 potential.…”

“…A proper description of thermal transport without an adequate description of phonon dispersion is impossible. To this end, we also calculated the phonon dispersions of MoS 2 as described by the empirical potentials using the finite displacement method as implemented in the PHONOPY [63] package and compared with experiment data [64] determined by inelastic X-ray scattering. The required harmonic force constants are calculated by using a 8 × 5 rectangular supercell (240 atoms), which is large enough to take care of the long-range LJ potential in the REBO-LJ potential.…”

Resonance Profiles of Valley Polarization in Single-Layer MoS2 and MoSe2

“…The same holds for MoS 2 ; however, very recently, experimental data have been obtained by in-elastic x-ray scattering [26]. These measurements find E 2LA = 59.6 meV, while density-functional theory calculations find 58.5 meV [26]. In our measurements of MoS 2 , on the other hand, the decrease of the DOP ρ starts already well below 60 meV excess energy [Fig.…”

“…Therefore, the phonon energy of E 2LA = 39 meV can only be taken from calculations [25]. The same holds for MoS 2 ; however, very recently, experimental data have been obtained by in-elastic x-ray scattering [26]. These measurements find E 2LA = 59.6 meV, while density-functional theory calculations find 58.5 meV [26].…”