Monolayer MoSe₂ And WSe₂:Synthesis And High Pressure Investigation

Transition Metal Dichalcogenides?TMDs?are graphene-like two-dimensional materials,which are composed of transition metal and chalcogenide atoms.They have unique physical properties and potential applications in field effect transistors,light-emitting diodes and photodetectors.There is indirect-to-direct bandgap transition from bulk to monolayer TMDs,leading to different optical properties.Monolayer MoSe₂ and WSe₂ became the research hotspots among transition metal chalcogenides due to their special optical,magnetic and mechanical properties,rendering themselves promising applications as transistors,logic circuits,sensors or optoelectronic devices.We takes MoSe₂ and WSe₂ as examples to explore the larger-scale synthesis of monolayer TMDs by Physical Vapor Deposition?PVD?method,and further investigated their pressure-dependent mechanics,optical properties by Diamond Anvil Cell?DAC?and Raman spectroscopy,PL spectroscopy and Absorption spectroscopy.The main results are as followed.Firstly,using PVD method the monolayer MoSe₂ and monolayer WSe₂ was grown on two different substrates including Si/SiO₂ plate and sapphire plate,the optimal condition for monolayer growing is found.Due to possessing similar hexagonal crystallographic plane,it's much easier to get extensive and homogeneous TMDs monolayer on sapphire.The monolayer MoSe₂ and monolayer WSe₂ grown on the Si/SiO₂ substrate give more negative excitons than neutral excitons at ambient condition,while those grown on the sapphire substrate give an opposite proportion,it indicates there are different doping effects from the substrate to the sample.Secondly,the high pressure Raman spectra of monolayer MoSe₂ shows that all the Raman peaks of monolayer MoSe₂ exhibit a blue shift with pressure increasing,and all vibrational modes present a softening at around 11 GPa,it is related to the phase transition of silicon substrate.There is a Raman-inactive A₂^''mode appears at around 4.84 GPa,combined with the analysis of absorption spectroscopy it was confirmed that,the appearance of A₂^''mode is related with the pressure-induced structural distortions in monolayer MoSe₂.The high pressure PL spectra of monolayer MoSe₂ shows that the PL peaks show a blue shift under pressure,and quench rapidly at 7.43GPa,the distortion of the Si/SiO₂ substrate under pressure may suppress the transition probability and finally cause disappearance of PL peaks.Finally,the high pressure Raman spectra of monolayer WSe₂ shows that all the vibrational modes exhibit a blue shift with the pressure increasing.Because the out-of-plane vibration is more susceptible to pressure,its pressure coefficient of A₁^'vibrational mode is digger than that of E^'mode.For the monolayer WSe₂ obtained by traditional mechanical exfoliation method,its A₁^'mode decreased and disappeared gradually under pressure,while the intensity of 2LA?M?mode increased greatly.On the other hand,for the monolayer WSe₂ grown by PVD method,both of the A₁^'and2LA?M?of decreased simultaneously.It means that the interaction between monolayer and substrate may affect their distinct vibrational behaviors of monolayer TMDs.