First-principles Studies On The Structure, Electronic Properties, Raman Spectra And Thermal Conductivity Of Monolayer WX₂ (X=S,Se,Te)

In recent years,two-dimensional transition metal bihalogen WX₂ has attracted the attention of many scientists due to its unique electrical and optical properties.When WX₂changes from multilayer to single-layer,its band structure changes from indirect band gap to direct band gap,which greatly improves the photon emission efficiency and exhibits very high charge mobility.These characteristics make WX₂ materials an ideal choice for electronic and optoelectronic fields.These unique electrical and optical properties have advanced the applications of optoelectronic devices in information transmission,computers and health monitoring.Monolayer WX₂ has excellent physical properties and can change the structure and properties of the material by simple strain.The strain is mainly linear strain and angular strain.The linear strain is mainly used in this paper.The thermal transfer properties play an important role in the effective implementation of two-dimensional materials in electronic devices.Because an important problem in device development is dealing with heat dissipation of two-dimensional materials.In this study,we apply first principles,which is a calculation method based on density functional theory(DFT),to study the structure and electronic properties of monolayer WS₂,WSe₂ and WTe₂.The stability of three monolayer structures under biaxial tension and compression strain,the variation of Raman spectra and the shift of Raman peaks under strain were studied.We also study the lattice vibration mode of WX₂ and the thermal transfer properties of the crystal,as well as the factors that can affect the thermal conductivity:phonon group velocity and phonon lifetime.Specific research results are as follows:(1)In this paper,a single-layer WX₂ structure is firstly established,and the energy bands of three single-layer structures are calculated,all of which are direct bandgap semiconductors.Under the control of strain not only does the bandgap changes,but also the band properties shift between the direct bandgap and the indirect bandgap.After the spin-orbit coupling effect,the bands of WX₂ are split.By changing the strain degree,bond length and bond angle also change,at the same time,they have opposite trends.(2)We also studied phonon spectra of WX₂ under diverse strains.We find that the three structures exhibit good thermodynamic stability at the tensile strain(1%-10%),while at the compression strain(2%),an acoustic lattice wave of WS₂ and WSe₂ appears virtual frequency near the?point,indicating that the structures become unstable.The blue shift occurs with the tensile strain and the red shift with the compression strain.(3)Phonon spectrum is the key to calculate phonon thermal properties.The phonon spectra of our three monolayer structures are similar,and the frequencies of all phonon modes from WS₂ and WSe₂ to WTe₂ show a decreasing trend.Several lattice vibration modes of WS₂are also studied.And the phonon group velocity can be expressed by the slope of the phonon spectrum.It is found that the relationship between the phonon group velocity of the three materials is WS₂>WSe₂>WTe₂.(4)In addition to phonon group velocity phonon relaxation time is very important for the change of thermal conductivity.We also studied the relaxation time of the three substances.Through calculation,it is found that single-layer WTe₂has the lowest thermal conductivity among the three materials,which is 40.63 Wm^-1K^-1 at the temperature of 300K.Monolayer WS₂ has the highest thermal conductivity of 195.33 Wm^-1K^-1 at 300K.The thermal conductivity of the three materials is consistent with the group velocity.