Anisotropy Of Two-dimensional Materials Based On Raman Spectroscopy

The research is based on the method of Raman spectroscopy measurement,and the anisotropy of two-dimensional materials black phosphorus（BP）and molybdenum disulfide（MoS₂）was discussed from the perspective of Raman enhancement and resonance Raman,respectively.As a new type of two-dimensional optoelectronic material,black phosphorus has been widely used in the field of micro/nano optoelectronic devices,due to its excellent photoelectric characteristics and unique anisotropy.In the study of two-dimensional materials characterizations,the indirect and non-destructive method of Raman spectroscopy measurement is often used.However,in the actual spectral measurement,the thickness of black phosphorus thin layer is only nanometer and the interaction between light and material is weak.It is difficult to obtain clear Raman signal.So,the research proposes a method of surface-enhanced Raman spectroscopy（SERS）of BP,which enhanced the Raman signal by the surface modification with gold nanoparticles（Au NPs）.The anisotropy of BP was discussed by the enhanced Raman spectra,and solved the problem that Raman signal is too weak to observe in the study of BP anisotropy.After Au NPs surface modification,the Raman signal of black phosphorus was significantly enhanced.The effect of Au NPs with different particle diameters（50nm and 100nm）and concentrations（1×10⁹/cm² and 5×10⁹/cm²）on the enhancement was also discussed.Based on the enhanced Raman spectra,the anisotropic Raman spectra of BP was observed by changing the polarization angle of incident light.Molybdenum disulfide（MoS₂）is also a kind of typical transition metal dihalides two-dimensional material,which is used many fields,such as optical sensing,photocatalysis and son on.In the study of MoS₂ Raman spectra,only the anisotropy of non-resonance Raman spectra or the resonance Raman spectra were generally analyzed,and the anisotropy of MoS₂ resonance Raman was few studies.Therefore,the anisotropy of MoS₂ resonant Raman spectrum was discussed in the study.According to the bandgap of thin layer MoS₂（1.8e V）and the excitation principle of resonance Raman,458nm（2.71e V）and 633nm（1.96e V）lasers were selected as the excitation light source of non-resonant and resonant Raman spectra,respectively.The characteristic modes were identified by Lorentz fitting.The resonance Raman spectra of MoS₂ of different polarization angles were also tested.The anisotropy of resonance Raman spectra were observed and the polar diagram showed a bipolar spindle shape.Meanwhile,the mathematical analysis method of Raman tensor was used,and the experimental data of Raman intensity agreed well with the theoretical values calculated by Raman tensor.The non-resonant Raman spectra of MoS₂ in different polarization states were also experimentally measured for comparison with the resonant Raman spectra.The results show that the non-resonant Raman spectra of MoS₂ is also anisotropic,and the polar diagram showed the multi-pole shape with an interval of about 45°.