Resonance Raman, inelastic neutron scattering and lattice dynamics studies of [formula omitted

Abstract

2H-WS 2 is an interesting simple layered material with an indirect-gap semiconducting behaviour; its lattice structure (P6 3/mmc) is well established and its band structure is well documented. However, the assignment and location in the Brillouin zone (at Γ, A, M or K) of two direct low energy transitions, characterized by A and B exciton peaks, are still open to discussion. We have thus investigated in detail the resonance Raman spectra of 2H-WS 2 single crystals and demonstrated the activity of second-order Raman (SOR) processes due to coupling phonons with non-zero momentum. In addition, we have determined by inelastic neutron scattering (INS) techniques the transverse acoustic (TA) and longitudinal acoustic (LA) dispersion curves in the (001), (100) and (110) directions: we conclude that the more intense SOR band is due to the 2 × LA (K) overtone, while other SOR peaks can be assigned to combinations of sum or difference bands involving phonons at the K point coupled to the LA (K) mode. Consequently, we have made use of all these data to fix some unknown parameters in complete valence force field lattice dynamics calculations which reproduce correctly the INS dispersion curves and lead to consistent values for the elastic constants (C 11, C 33, C 44). We thus definitively conclude, in agreement only with recent band-structure calculations, that the A and B excitons in 2H-WS 2 correspond to the smallest gap at K and that the SOR scattering mechanisms are governed by the K 3 ⇒ K 5 (A) and K 1 ⇒ K 5 (B) electronic transitions.