Theoretical Study On Resonant Raman Spectra Of The Janus MoSSe

Janus transition metal dichalcogenides(TMDs)emerge as a novel type of twodimensional materials with the transition metal layer sandwiched by two different chalcogen layers.Due to the absence of mirror symmetry,Janus TMDs have great potential for applications in stress regulation,photovoltaic effects and quantum information storage,etc.,and therefore have attracted more and more attention.The Raman characterization technology is widely used in experiments to study the structure,composition and physical properties of two-dimensional materials,owing to its advantages of being fast,non-destructive and highly sensitive in sample testing.Especially,the resonant Raman,including single resonant Raman and double resonant Raman,plays an indispensable role in materials science research since it addresses the shortcomings of weak signal and poor resolution in ordinary Raman.Although the resonant Raman has been widely used in experimental research,the relevant theoretical research has not developed enough to keep up with experiments,which seriously hinders the in-depth exploration of the microscopic mechanism under the novel physical phenomena due to the inability to reasonably interpret the resonant Raman signal.For the reasons given above,we calculate the resonant Raman spectra of Janus MoSSe monolayer based on first-principles,and systematically study the properties of the single resonant Raman and double resonant Raman under various geometric configurations and external stresses in conjunction with group theory analysis in this paper.In the single resonant Raman spectra,the E-mode Raman signal becomes stronger as the incident laser energy increases.It is found that the spin-orbit coupling suppresses the Raman signal of the E-mode,and a higher laser energy makes the electron transition deviate from the levels where the spin-orbit coupling are strong.The Raman intensity of the E mode in MoSSe monolayer can also be adjusted by applying external stress,as the applied stress will change the electronic band structure as well as the energy gap size,thus being able to vary the electron transition position even when the laser energy is fixed.For the double resonant Raman,an important work is to assign the double resonant Raman peaks.Different from the method of inspecting the phonon band used in the previous literatures,we achieve the assignments of the 11 double resonant Raman peaks in MoSSe monolayer completely from the numerical calculation in this paper,and quantitatively analyze the contribution of individual assignment to the double resonant Raman intensity.The calculated assignments satisfy the two-phonon Raman selection rules determined by group theory analysis,and can also self-consistently explain the variation of double resonant Raman peaks under various geometric configurations and external stresses,which further verify the rationality of our assignments.