Edge Modes of MoS₂ via Indirect Double Resonant Raman Spectroscopy

We report a set of resonantly enhanced defect modes in the Raman spectrum of molybdenum disulfide (MoS2) which are fully analogous to the D mode of graphene, allowing sensitive defect quantification and differentiation of zigzag and armchair edge structures. These modes become active at edges under indirect resonance conditions at 785 nm excitation due to strict backscatter constraints in real and reciprocal space, which exclusively select phonon wavevectors perpendicular to the edge direction. We assign features to single LA(K), TA(K), and TA(Q) phonons along the direction of the Brillouin zone and identify a separate preresonant enhancement of the whole spectrum along the direction, which is lost in thinner material as the band gap increases. In addition, we identify a clear incident polarization dependence of the K and M phonons, which suggests the presence of an inhomogeneous optical absorption analogous to that of graphene. We anticipate this indirect double resonance Raman technique will provide a powerful tool for materials characterization, allowing quantification of active sites in catalytic MoS2 and characterization of the structure-dependent properties of MoS2 nanomaterials. In addition, the intervalley scattering pathways provide a sensitive probe of the low energy landscape of the conduction band and may reveal a wealth of electronic information and scattering dynamics important for spin-valley coupled systems. We anticipate similar modes can be found in other transition metal dichalcogenide systems, given the appropriate excitation energy.