| Graphene-like molybdenum disulfide (MoS2), which is composed of a monolayer or fewlayers of MoS2, is a new two-dimensional (2D) layered material that has attractedconsiderable attention recently because of its unique structure and optical and electronicproperties. Compared to graphene, MoS2have a brighter future in the field of optoelectronicdevices, due to its inherent band gap.Raman spectroscopy is an important tool to study graphene-like2D layered materials.With decreasing number of layers, the Raman-active mode A1ghas a redshift and theRaman-active mode E12ghas a blueshift. This can be used to identify the number of the layersof MoS2. The Raman spectrum of graphene-like MoS2is also affected by external conditions,such as thermal and pressure effects. The studies of the dependence of Raman spectra ofMOS2under different thermal and pressure environments provide further insights into basicphysical properties of such materials.Here we use a chemical vapor deposition method to prepare the monolayer and bilayerMoS2. We comparatively study the thermal dependence of Raman spectra of the differentlayers of MoS2under532nm and633nm laser excitation. We also study the pressure effecton MoS2powders and single crystal by using diamond Anvil cell （DAC）. Our results of thestudies are as follows:1. Under excitation of different laser, the Raman spectra have a different number ofpeaks with different relative intensity. More additional vibration modes appear in the Ramanspectra when633nm (1.96eV) lasers are applied. The Raman resonance at633nm can bewell understood as facilitated by excitonic excitations in the vicinity of A (1.88eV) exciton.2. Both the A1gand E12gRaman peaks shift toward lower frequencies with increasinglaser power，as well as its Raman peak FWHM increases. The A1gmode has a greater rate ofchange than E12gmode. This is because a thermal effect can be generated in the sample bylaser induced. The thermal effect can be enhanced with the increasing of laser power. It willcause the change between phonons interaction. In addition, the thermal expansion of thelattice is also one of the important reasons. This will lead to the non-harmonic changes in theatomic potential energy. The A1gmode is more easily affected by the thermal effect. 3. Both A1gand E12gpeaks of the single crystal and powders of MoS2shift toward higherfrequencies with increasing pressure. The A1gpeak has a greater change rate. The volume andintermolecular distance of the sample reduce drastically under high pressure, which producenon-harmonic damping vibrations. In addition, a structural lattice distortion followed by anelectronic transition from a semiconducting to metallic state will be produced. Out-plane A1gmode is more susceptible to the effects of pressure. |
