Study Of ReS₂ Anisotropy Based On Polarization Raman Spectroscopy

Two-dimensional rhenium disulfide（ReS₂）material is an important member of layered transition metal chalcogenides The metallic Re atom in the middle and two layers of nonmetallic S atoms in the upper and lower layers form a low symmetric octahedral coordination（1T′）structure,which makes the coupling between the layers weak,and the bandgap value does not change with the number of layers.Low symmetry of the crystal lattice structure makes ReS₂ has in-plane anisotropy of physical properties,such as field effect transistor based on less layer ReS ₂,two major axis direction of carrier mobility of anisotropy ratio can reach 3.1,based on the less layer ReS₂ photoelectric detector,wide band range of anisotropic absorption of polarized light detection is achieved,the excellent physical properties of ReS ₂ has excellent application prospects in the fields of electronics and optoelectronics.Raman spectroscopy is widely used in the detection and research of two-dimensional material characterization,biological imaging and other fields due to its advantages such as fast detection speed,rich spectral information and simple experimental operation.Polarized Raman spectroscopy is developed rapidly on the basis of Raman spectroscopy.It can be used in the detection of polarizing Raman tensor of two-dimensional anisotropic materials,birefringence phenomenon and determination of crystal axis direction of anisotropic materials.Polarized Raman spectroscopy has become a conventional method to quickly identify the crystal axis of two-dimensional anisotropic materials,which promotes the research and application related to crystal orientation.In this paper,polarization Raman spectroscopy was used to systematically study the thermal properties of ReS₂ with different layers.When the incident light is parallel to the a-axis and b-axis,it was observed that the Raman peak of mode V of 1L ReS₂was redshifted by about 3 cm^-1 in the temperature range of 90 K to 570 K,which is mainly due to the thermal expansion effect of ReS₂ and the non-harmonic vibration of interatomic potential energy.In addition,a semi-quantitative model was introduced,and the thermal expansion coefficients of different layers of ReS₂ crystals along the b-axis and a-axis were obtained based on the change of Raman peak position with temperature,taking into account the crystal volume effect.Thus,the thermal anisotropy of ReS₂ was detected based on polarization Raman spectroscopy.In addition,based on phonon attenuation,the relationship between half-height and half-width of Raman model and temperature is also analyzed quantitatively.Part of the research work will help to promote the understanding of the thermal anisotropy of ReS₂ and lay a foundation for exploring the thermal stability of two-dimensional ReS₂materials in optoelectronic devices.The low symmetry of the crystal orientation of ReS₂ enables it to have optoelectronic properties,which shows new applications in electro nics and optoelectronics,such as the preparation of polarized phase modulator.However,there is currently a lack of quantitative regulation of the ReS₂ anisotropy.In this paper,the vacancy defects were introduced in ReS₂ by the Ar⁺ion bombardment method,and the defect density was calibrated based on a single-layer graphene sample,achieving the introduction of defect density in the range of 7.1×10¹¹ cm^-2 to 7.7×10¹² cm^-2into the ReS₂ crystal.And the change of ReS₂ anisotropy was quantitatively detected based on polarization Raman spectroscopy.When the polarization direction of incident light was along the b-axis and perpendicular to the b-axis,the Raman intensity of mode V presented the maximum(I_max)and minimum(I_min)intensity values,respectively.Therefore,the mode V intensity ratio of I_min/I_maxcould be used to explore the change of ReS₂ anisotropy.When the same defect density(3.6×10¹² cm^-2)was introduced into the 1,2,and 8L of ReS₂,it was found that the I_min/I_max ratio of 1L ReS₂ increased from 0.048 to 0.184,while the I_min/I_max ratio of 8L ReS₂ increased from 0.029 to 0.032,indicating that the anisotropy of 1L ReS₂ was more easily regulated by defects under the same defect density.In addition,the quantitative change of ReS₂ anisotropy with defect density in a single layer was systematically studied.As the defect density increased from 7.1×10¹¹ cm^-2 to 7.7×10¹² cm^-2,the I_min/I_max ratio was observed to increase from 0.078 to 0.286.Then,a quantitative model was established to explore the relationship between defect density and anisotropy.Considering that the introduction of defects would cause the scattering of carrier isotropy,the isotropic scattering area would increase with the increase of defect density,which would lead to the decrease of ReS₂ anisotropy.Finally,by fitting the experimental results,the isotropic scattering range associated with the defect is obtained,which is 2.86 nm.The results of this study will facilitate the quantitative detection of the anisotropy of two-dimensional anisotropic materials by polarization Raman spectroscopy and provide a method to regulate the anisotropic properties.