Crystal Orientation Independent Strong TERS Response in Anisotropic ReS ₂ on Gold

ABSTRACT Rhenium disulfide (ReS 2 ) crystallizes in a distorted 1T′ lattice with triclinic symmetry arising from Re─Re dimerization, which produces in‐plane anisotropic properties and a dense set of Raman‐active modes with mixed atomic displacements. Prior conventional Raman microscopy studies in this material showed strong dependence of absolute and relative intensities of some Raman bands on the mutual orientation of the optical electric field and the in‐plane crystalline axii. In the presented study, we show strongly enhanced (at least 10 times over the far field background) tip‐enhanced Raman scattering (TERS) response from 1–4 L ReS 2 on gold, which not only enables high spatial resolution in the TERS maps, but also remains, to a great extent, indifferent to the in‐plane orientation of the ReS 2 crystals. We attribute this orientation indifference to the fact that in the gap mode TERS configuration, the optical electric field is normal to the crystal plane. Additionally, we demonstrate in our TERS spectra the presence of low‐frequency bands, the spectral position of which correlates with the local crystal layer number and matches very well previously published conventional Raman data, which allows a straightforward nanospectroscopic assessment of the layer number even in sub‐micron‐sized crystals.