Investigation On Physical Properties Of Layered WSe₂ Under High Pressure

Transition-metal dichalcogenide?TMD?materials have attracted extensive attention because of their graphene-like structure and excellent optoelectronic properties.With the success in preparation of two-dimensional?2D?monolayer or multilayer TMDs,their remarkable properties have been discovered in recent years.For instance,TMD materials show suitable band gap and high carrier mobility,thus they offer the potential for applications in developing novel photodiodes and integrated electronic devices.Pressure can effectively decrease atomic distance,enhance electron orbital coupling,tune electron spin,transfer electronic structure,and even induce crystal structure phase transition.So far,pressure engineering of the TMD materials is still a relatively new field,especially there is lack of systematic research on monolayer and multilayer TMD materials under pressure.This research may provide a new route for exploring novel structure and properties of TMD materials.As a representative TMD material,tungsten diselenide?WSe₂?has attracted great attention in the prospective application including novel optoelectronic devices and new energy materials due to its strong photoluminescence?PL?emission,ambipolar transport,phototonus and photocurrent properties.WSe₂ has a“trilayer?TL?”unit composed of three atomic layers?Se-W-Se?linked by covalent bonds between them,and then the adjacent TLs are coupled via weak van der Waals?vdW?interactions.Pressure can easily modulates interlayer spacing,which in turn alters the crystal structure to affect interlayer and intralayer interactions.Consequentely,by carrying out the pressure studies on monolayer,bilayer and fixed-layer TMD materials,we can clearly obtain how the pressure affects microscopic crystal structure and interlayer-intralayer interactions,and further reveal how the pressure affects band structure,intramolecular interactions,etc.Hence,we have systematically investigated the pressure-dependent physical properties over WSe₂ with different thickness including monolayer,multilayer and bulk WSe₂.By exploring their changes in photoluminescence,lattice vibration and band structure,we have analysed the evolution rules of exciton,electronic structure and crystal structure under pressure.The main achievements are as following.Through the in situ PL and Raman measurements,the electronic band structure variation and lattice vibrational dynamics of monolayer WSe₂ under pressure have been investigated.Based on the peak evolutions of neutral exciton and negatively charged exciton,we have detected an electronic structure transition in monolayer WSe₂ at 3.8GPa.As the pressure increases,the energy of direct???interband transition increases,whereas the indirect???interband transition decreases,leading to a significant transition from direct to indirect band gap at 3.8 GPa.Then,with the increase of pressure,the intensity of direct???interband transition constantly decreases and finally vanishes at approximate 12.2 GPa.Based on the variation in Raman spectra of monolayer WSe₂ under pressure,we find the lattice disorder is enhanced under pressure,which is evidenced by the increasing intensity of LA?M?.The asymmetrical pressure triggers the lattice distortion,and further results in the lattice disorder.Through the XRD and Raman measurements,the crystal structure variation of bulk WSe₂ under pressure has been investigated.Based on the evolutions of lattice parameter a and c,we find that bulk WSe₂ underwents an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 40 GPa.On the basis of Raman measurements,the interlayer E²_2g and intralayer E¹_2g shear modes show no splitting.That is,compared with bulk MoS₂,the bulk WSe₂ maintains a better stability under pressure without any layer sliding transitions.Subsequently,we have studied the pressurized vibrational behaviors of multilayer WSe₂ from 2TL to 6TL by Raman spectroscopy.Based on the Raman data,we have calculated the force constants and elastic constants of multilayer WSe₂ under pressure.In addition,we have discovered that the pressure-induced splitting of intralayer E¹_2gg and interlayer S1 modes occurs in all the measuring multilayer WSe₂.Furthermore,with the layer number increases,the splitting pressure increases.More precisely,from2TL to 6TL WSe₂,the splitting pressure is observed to be 14.3 GPa,18.3 GPa,18.6GPa,21.6 GPa and 23.6 GPa,respectively.This finding indicates that multilayer WSe₂undergoes in plane deformation under pressure,resulting in the occurrence of shear vibration nondegeneration splitting.Meanwhile,with the layer thickness increases,the interlayer coupling interaction heightens,and eventually the in plane deformation is suppressed to occur at higher pressures.