Tuning Properties Of The Layered MoS₂ And WS₂ Via Hydrostatic Pressure

Layered transition metal chalcogenide compounds（TMDs）are a class of two-dimensional materials whose chemical formula is MX₂,in which M is the transition metal element and X is the chalcogenide element.Due to different atomic arrangements and layer stacking modes,TMDs have multiple structural phases,among which the stable 2H phase is currently the focus of research.TMDs have a layered structure similar to graphene,and interlayer coupling relies on van der Waals interactions,which allows for tunable physical properties with the number of layers.These exceptional characteristics make TMDs a"star"candidate material for the next generation of optoelectronic and photonic devices.Diamond Anvil Cell（DAC）pressure is an important means of exploring and discovering novel physical phenomena.Therefore,this thesis takes 2H-phase layered Mo S₂and WS₂as the research objects.We use the using mechanical exfoliation to prepare layered Mo S₂and WS₂.Using DAC to control the lattice structure and exciton properties of single-layer,bilayer,and bulk Mo S₂,as well as single-layer and bulk WS₂materials,we have investigated the changes of TMDs material structure and exciton luminescence properties under pressure,our findings provide experimental and theoretical support for application of TMDs in photoelectric devices and new field effect transistors.In this thesis,four important results were mainly obtained shown as following:1.During the process of increasing pressure from 0 GPa to 30 GPa,the band gap of single-layer Mo S₂continuously becomes larger.Compared with the bilayer Mo S₂,upon the external pressure the in-plane compression has a greater response than that of the out-of-plane compression in the single-layer Mo S₂.2.Around 6 GPa,the redshift rate and intensity change trend of the fluorescence peak A in the bilayer Mo S₂PL spectrum changed.It is accompanied by the excitation of the E_1gand B_2g¹modes in the Raman spectrum,indicating an electronic structure phase transition in the bilayer Mo S₂,corresponding to the transition from direct band gap to indirect band gap.At around 16 GPa,the E_1gmode in the bilayer Mo S₂Raman spectrum undergoes a jump,while the A_1gmode remains unchanged.The position and intensity of the fluorescence peak A are abnormal,indicating an interlayer slip in the bilayer Mo S₂,corresponding to the transition from 2Hc to 2Ha.The bilayer Mo S₂does undergo metallization and has an electronic structure similar to 2Hc.3.Around 2.6 GPa,the fluorescence peak A in PL spectrum of single-layer WS₂first shows a blue shift and then a red shift,indicating that the electronic structure of the single layer WS₂undergose a phase transition,with the conduction valley changing from the original K point to theΛpoint,which corresponds to the electronic structural phase transition from a direct band gap to an indirect band gap.4.At around 18.5 GPa,the E_2g¹and A_1gRaman vibration modes of multilayer WS₂undergo splitting,indicating that the interlayer slip of multilayer WS₂leads to the change of stacking mode,corresponding to the structural phase transition from 2Hc to2Ha.