Anomalous polarization pattern evolution of Raman modes in few-layer ReS 2 by angle-resolved polarized Raman spectroscopy.

Low-symmetry of ReS 2 has not only in-plane but also out-of-plane anisotropic light scattering, which is complicated, yet interesting with intrinsic strong electron-phonon coupling. In such a case, the Raman tensor also gets sophisticated with nine non-zero elements, which is layer-dependent for different Raman modes. Herein, we systematically investigated the polarization pattern evolution of both in-plane and out-of-plane Raman modes of few-layer ReS 2 by angle-resolved polarized Raman spectroscopy. We found that in-plane Raman modes with less layer-dependence could be used to determine the crystal orientation (Re-chain direction) due to the weak electron-phonon interaction between layers. However, the out-of-plane and mixed vibration Raman modes demonstrate much evident layer-dependence due to the obvious electron-phonon interaction between layers. As such, the polarization patterns for the out-of-plane vibration Raman modes are distorted with layers in not only petal types but also maximum Raman intensity directions. This distortion is mainly due to the phase difference between Raman elements, which are complex values due to the near bandgap excitation laser. The results reveal that deep insights into anisotropy in low-symmetry two-dimensional materials could afford not only rich physics but also potential polarized optoelectronic devices.