Effects of dimensionality and excitation energy on the Raman tensors of triclinic ReSe₂

In this work, we present measurements of angle-resolved polarized Raman spectra of single-layer (1L) and bulk ReSe₂ recorded with excitation energies of 1.92 eV (647.1 nm) and 2.34 eV (530.8 nm). The Raman tensors for all modes were obtained by fitting simultaneously the angular dependence of the parallel (I_‖) and crossed (I_⊥) polarized intensities. We observed that the tensor elements are, in general, complex numbers, and their magnitudes and phases depend on both the dimensionality of the sample (1L or bulk) and the excitation energy. Results are discussed by considering the intrinsic contribution of a single layer to the tensor elements and the macroscopic contribution coming from the stacking of several layers. We show that the different behaviour of angle-resolved polarized Raman spectra for different excitation energies is due to the resonant Raman effect, which affects both the real and imaginary parts of the Raman tensor elements. Our work highlights the importance of understanding the fundamental physics of low symmetry 2D materials that can be used to fabricate devices sensitive to the direction of light polarization or electrical current.