Tunable anisotropy in ReS₂flakes achieved by Ar⁺ion bombardment probed by polarized Raman spectroscopy

Crystal-orientation-dependent properties in low symmetric two-dimensional material ReS2 have inspired its novel applications in photonics, electronics, and optoelectronics. In this work, Ar+ ion bombardment is used to treat ReS2 with different thicknesses, and different amounts of defect densities are introduced in ReS2 flakes. Angular dependent polarized Raman spectra is used to probe the change in anisotropy. The intensities of mode V exhibit maximum and minimum intensities with incident polarization along the b-axis and perpendicular. The intensity ratio Imin/Imax is used as a quantity to detect the change in anisotropy. It is found that with the same defect density, the change in anisotropy in 1-layer (L) ReS2 is the highest among ReS2 flakes; thus, 1L ReS2 is susceptible to ion bombardment. With an increase in the defect density in 1L ReS2, an increase in the Imin/Imax value is observed. A phenomenological model is established to understand the quantitative relationship between defect density and anisotropy, and the isotropic scattering range associated with defects is obtained. The results here provide ways of quantitatively understanding anisotropy by angular polarized Raman spectroscopy as well as ways of tuning anisotropy in two-dimensional anisotropic materials by ion bombardments.