Resonance Raman scattering in bulk 2H-MX2 (M = Mo, W; X = S, Se) and monolayer MoS2

We have performed a comparative study of resonance Raman scattering in transition-metal dichalcogenides 2H-MX2 semiconductors (M = Mo, W; X = S, Se) and single-layer MoS2. Raman spectra were collected using excitation wavelengths 633 nm (1.96 eV), 594 nm (2.09 eV), 532 nm (2.33 eV), 514 nm (2.41 eV), and 488 nm (2.54 eV). In bulk-MoS2 and WS2, the resonant energies appear to coincide with their exciton excitations. The resonance can be fine tuned by varying sample temperatures, which confirms its excitonic origin in both MoS2 and WS2. Temperature dependence of Raman intensities is analyzed in the context of resonance Raman theory, which agrees well with the existing absorption data. While in WSe2, the resonance has been observed in a wider range of excitations from 633 to 514 nm, which cannot be explained with its excitonic energies of 1.6 and 2.0 eV. It is considered that additional excitonic bands induced by band splitting are involved in the inter-band transitions and substantially extend the resonance energy range. The Raman resonance energy range remains unchanged in single-layer MoS2 compared with that in the bulk sample. However, most phonon modes in single-layer MoS2 are significantly broadened or strongly suppressed under resonance conditions. This change could be related to the modification of acoustic modes by the substrate. (C) 2014 AIP Publishing LLC.