Study On The Physical Properties Of Van Der Waals Selenides Based On Raman Spectroscopy

Two-dimensional materials have attracted much attention due to their special layered structure,excellent physical properties and great research potential and application prospect in many fields such as photoelectric applications.However,two-dimensional semiconductor materials with stable and high performance in practical device applications need to be further explored by researchers.Based on the above background,in this thesis,we chose van der Waals selenidesγ-InSe and FePSe₃ as the starting point,and study the environmental stability of thin layer InSe and spin-phonon coupling effect in FePSe₃ single crystal based on Raman spectra.The main research results of this thesis are as follows:1、The microstructure ofγ-InSe single crystal was systematically characterized,and the environmental stability of thin layer InSe was comprehensive studied.Bridgman method was used to grow InSe crystals.X-ray diffraction and scanning electron microscopy showed that the InSe crystals had high quality and layered structure.Laser-induced experiments showed that the thin layer of InSe degraded rapidly under laser irradiation of 1 m W power,and the oxidation products were In₂O₃ and In₂Se₃.Based on the environmental stability experiment through Raman spectrometer,four layers ofγ-InSe were found to be very unstable,and we speculated that the thinner layer of InSe might degrade immediately after peeling.It was observed by atomic force microscope that the thickness of thin InSe layer increased after 5 h.X-ray photoelectron spectroscopy results showed that InSe reacts with oxygen and water in the air.In Raman spectrum studies on thermal stability experiment,it was found that when the temperature increased,the Raman characteristic peak moved to the low-frequency direction,and high temperature heating would destroy the stability of InSe.2、The microstructure of FePSe₃ single crystal was systematically characterized,and the spin-phonon-coupling effect was studied by Raman spectrometer.The single crystal FePSe₃ was grown by CVT method.X-ray diffraction and scanning electron microscopy showed that high quality single crystal FePSe₃ had obvious lamellar structure.Ultraviolet diffuse reflection experiments show that FePSe₃ crystal has obvious absorption in the range of 200-1000 nm.The band gap of FePSe₃calculated by Tauc Plot and first-principles is about 1.27 e V.X-ray photoelectron spectroscopy showed that the single crystal FePSe₃ reacts with oxygen and water in the air after prolonged exposure.The experiment of superconducting quantum interferometer shows that FePSe₃ single crystal changes from paramagnetic to antiferromagnetic at about 115 K under 500 Oe magnetic field,and shows that FePSe₃ single crystal has magnetic anisotropy.Among variable temperature experiment,the E_g peak displacement changes linearly with temperature above Neel temperature,but does not change linearly below Neel temperature,which is presumed to be caused by spin-phonon coupling effect.3、The electron and phonon properties ofγ-InSe were calculated by using MS and VASP software.First,the band gap of InSe crystal and monolayer InSe were calculated and the contribution of different orbitals was analyzed.Then,the band gap of monolayer InSe under-5%to 5%biaxial strain is calculated.When the compression strain is above-3%,the direct band gap can be observed at theΓpoint.Indirect band gaps were observed under-2%compression strain.The results show that under-3%and-2%compressive strains,monolayer InSe begins to transform from direct band gap to indirect band gap.The path of the conduction band isΓ-M and the path of the valence band is K-Γ.The effective electron mass at the conduction band is calculated to be m*=0.161m₀,which is consistent with the experimental value.Finally,phonon spectra under-1%,0%and%1 biaxial strain are calculated.It is found that the²₁₂)mode changes the most obvious under biaxial strain,and 1%strain corresponds to 5 cm^-1 frequency shift.The corresponding Grunneisen parameters are calculated and it is found that there is a specific balance between the van der Waals force and the atomic pair force in InSe.