Origin of the complex Raman tensor elements in single-layer triclinic ReSe2

Low symmetry 2D materials offer an alternative for the fabrication of optoelectronic devices which are sensitive to light polarization. The investigation of electron-phonon interactions in these materials is essential since they affect the electrical conductivity. Raman scattering probes light-matter and electron-phonon interactions, and their anisotropies are described by the Raman tensor. The tensor elements can have complex values, but the origin of this behavior in 2D materials is not yet well established. In this work, we studied a single-layer triclinic ReSe2 by angle-dependent polarized Raman spectroscopy. The obtained values of the Raman tensor elements for each mode can be understood by considering a new coordinate system, which determines the physical origin of the complex nature of the Raman tensor elements. Our results are explained in terms of anisotropy of the electron-phonon coupling relevant to the engineering of new optoelectronic devices based on low-symmetry 2D materials.