Raman And Photoluminescence Investigation Of Monolayer And Multilayer MoS₂under High Pressure

Molybdenum disulfide (MoS2) is a layered semiconductor with acrystal structure similar to graphene. The optical and electronic propertiesof monolayer MoS2are quite different from the bulk material due toquantum confinement effect[1]. Monolayer MoS2has aroused wide concernin both physics and material science since monolayer MoS2is an idealmaterial which can be applied to many electronic devices.Both theoretical calculation and experimental research demonstratedthat high pressure will lead to a transition in both the crystal structure andelectronic structure on bulk MoS2[2,3]. Comparing to the bulk MoS2,monolayer MoS2is more valuable for application in optical, electronicalfield due to the strong photoluminescence intensity in monolayer MoS2.Now, there exist few researches in the crystal structure and electronicstructure of monolayer MoS2under high pressure, so it is necessary tostudy the crystal structure and band structure of monolayer MoS2underhigh-pressure condition.The paper mainly introduces the method of producing monolayerand multilayer MoS2and studies the evolution process of crystal structureon monolayer, multilayer and bulk MoS2under high pressure through insitu high-pressure Raman scattering measurements. In addition, the paperalso study the evolution process of electronic structure and differentproperties on monolayer MoS2under high pressure through in situ high- pressure photoluminescence measurements.1. Monolayer and multilayer MoS2were exfoliated from bulk MoS2to Si substrates using mechanical exfoliation technique. The method ofdeterming the layer number of MoS2were also introduced.2. Our in situ high-pressure Raman scattering measurements revealstructural distortions of monolayer MoS2, trilayer MoS2and quadlayerMoS2respectively starting from4.2GPa,6.46GPa,5.17GPa, which areevidenced by the splitting of A1gmode. For both monolayer andmultilayer MoS2, two new Raman vibrational modes emerge nearly at9.4GPa, and the Raman intensity of the two new Raman peaks becomestronger with the increasing pressure, which suggests that both monolayerand multilayer MoS2may undergo structure transitions. The pressuredependence of Raman frequency shift for monolayer, multilayer and bulkMoS2were obtained by respectively analyzing the Raman spectra of thesamples under high pressure at room temperature and the pressuredependence of FWHM（full width at half maximum）for monolayer,multilayer and bulk MoS2were also obtained.3.The pressure dependence of the bandgap value for monolayerMoS2was obtained by analyzing the photoluminescence measurements ofthe sample under high pressure at room temperature. Our experimentalresults reveal a linear blueshift for the bandgap value of monolayer MoS2with the increasing pressure. The photoluminescence intensity of mono- layer MoS2become weaker and weaker with the increasing pressure. Thereason why the photoluminescence intensity of monolayer MoS2becomeweaker maybe is there exist a transition for monolayer MoS2from adirect band gap semiconductor to an indirect band gap semiconductorunder high pressure.