Probing Vibrational Properties at MoSe2 Edges and Grain Boundaries Using Tip-Enhanced Raman Spectroscopy.

Edges and grain boundaries of monolayer transition metal dichalcogenides (TMDs) exhibit unique physical properties, but yet their associated vibrational properties and Raman characteristics remain largely unexplored. Here, using tip-enhanced Raman spectroscopy combined with scanning tunneling microscopy, we identify additional Raman modes at approximately 244 cm-1 at zigzag edges and grain boundaries of monolayer MoSe2 on Au(001) and Au(111). Density functional theory calculations suggest that these modes can be attributed to edge-localized phonon modes or edge-activated Raman features originating from the combination of two out-of-plane acoustic phonons at the M point of the Brillouin zone of the basal plane. These findings suggest that the Raman modes at approximately 244 cm-1 can serve as spectroscopic signatures of edges, grain boundaries, or possibly other one-dimensional defects with zigzag orientations in monolayer MoSe2.