Resonance Raman effects in transition metal dichalcogenides

Raman spectroscopy is broadly used in the studies of transition metal dichalcogenides to determine the number of layers or other structural parameters. However, unlike the case of graphene, the Raman spectrum varies greatly depending on the excitation energy, and many unusual effects have been reported. The optical absorption spectrum has many features related to exciton states due to the strong coulomb interaction in these materials, and dramatic resonance effects occur when the excitation energy matches one of these exciton states. Several forbidden Raman modes and some unexplained peaks appear near resonance, and Davydov splitting of some Raman modes is observed. Furthermore, the polarization dependence of the some Raman modes also shows excitation energy dependence. In this review, recent progress in resonance Raman studies on transition metal dichalcogenides and some unresolved issues are reviewed. Copyright (c) 2017 John Wiley & Sons, Ltd.