Study On Raman And Photoluminescence Spectra Of 2D Gallium Selenide

The two-dimensional Gallium Selenide（GaSe）material is a member of the group Ⅲ-Ⅵ layered metal chalcogenide compound MX（MX:M=Ga,In;X=S,Se,Te）.GaSe crystals are constructed by stacking four Se-Ga-Ga-Se basic cells with van der Waals forces.The large birefringence of GaSe and its low ab sorptivity for Terahertz light make it suitable for the fabrication of terahertz sources.GaSe is usually a P-type semiconductor with a band gap of～2.0 e V.Based on the GaSe photodetector,the photocurrent changes significantly with the light intensity.G a Se ultrathin layers present good ON/OFF ratio,the current responsivity reaches 2.8AW^-1,and the quantum efficiency reaches 1367%.Field effect transistor based on a single-layer GaSe,exhibits switching ratio of 10⁵,and carrier mobility of 0.6cm²V^-1s^-1.These excellent properties make GaSe important in the fields of optoelectronics,nonlinear optics,hertz generators,etc.As the number of the GaSe layers increases,its band structure changes from an indirect bandgap to a direct bandgap.For GaSe flakes with more than seven layers,GaSe becomes a direct bandgap semiconductor and exhibits a very high fluorescence emission quantum efficiency,which is prominent for the fabiricaiton of ultra-thin,ultra-light,and low-power light-emitting devices in the future.Raman spectroscopy is widely used in the characterization of materials,pesticide detection,virus diagnosis,bio-imaging and other fields due to its targeted,fast and non-destructive properties.The development of SRES chips with high sensitivity will greatly promote the practical application of SERS technology.Since graphene has been found to have a very good SERS enhancement effects and fluorescence quenching effects,two-dimensional materials have attracted widespread attention as SERS substrates.It is necessary to develop new two-dimensional SERS substrates to meet the diverse detection needs of Raman spectroscopy.Experimentally,we prepared 1-200 layers of GaSe by micromechanical exfoliation method and transferred to Si O₂（300nm）/Si substrate.With an increasing number of layers（NL）,the color of GaSe flakes changes periodically as observed by an optical microscope,which is determined by multi-beam interference.Brilliant cresyl blue（BCB）was used as the probed molecule.The molecules are adsorbed on the GaSe sheet by the solution(solution concentration of BCB molecular is 5x10^-4 M)soaking method.The SERS enhancement effects with increasing number of GaSe layers are systematically studied.It is found that the Raman intensity of the probed molecule on the multilayer GaSe（more than 30 layers）is determined by the interference effect,while the thin layer GaSe（1-30 layers）exhibits a higher enhancement effect.The Raman intensity(vibrational mode at 1654 cm^-1)of BCB molecules absorbed on a single-layer GaSe is 16 times higher than that of on 100 layers GaSe.This is because the thin layer of GaSe is unstable in the air and is easily oxidized,thus a lot of conical-shaped oxide formed on the originally smooth two-dimensional GaSe surface The density of the conical-shaped oxide can be controlled by oxidation time.After exposing the multi-layer GaSe（～30 layers）sample to air,as the oxidation time increased from 12 h to 60 h,the atomic force microscopy results showed that the density of conical-shaped oxide increased from 0.06μm^-2 to 0.125μm^-2,The corresponding BCB 1654 cm^-1 Raman signal was increased by 3 times.This uneven surface increases the adsorption concentration of BCB molecules,thereby enhancing the Raman signal of the absorbed molecules.The unique oxidation characteristics of thin-layer GaSe provide a new way to prepare SERS chips.In this thesis,we also systematically studied the change of the Photoluminence（PL）spectrum of GaSe flakes（1-200 layers）with the increasing number of layers.It was experimentally found that the PL peak of GaSe was observed at～2.0 e V under the excitation of a 532 nm laser.Through low-temperature PL spectroscopy,it was confirmed that the emission peak was originated from free exciton emission.Experimentally,it was found that the PL intensity of GaSe flakes changes periodically with the increasing of the number of layers.The first interference enhancement peak appears at 13 layers,and the second interference peak appears at120 layers.The theoretical calculation of the PL intensity of GaSe is carried out by the Fresnels’equations,and the theortical results match the experimental observations very well.Compared with monolayer and bilayer Mo S ₂,multilayer GaSe has stronger PL intensity and higher PL emission quantum yield,which make GaSe as an ideal material for preparing light-emitting devices.In order to fully utilize the incident light and effectively collect the fluorescence of GaSe,We theoretically constructed periodically alternating optically thinner medium（Si O2,nd=1/4λ）and optically denser medium（Si,nd=1/4λ）as the multilayer high reflectors（period from n=1 to n=4）.The enhanced reflection coefficient reduces the light incident into the substrate,thereby increasing the PL intensity of the GaSe sheet.When n increases from 0 to 4,the enhancement factor（normalized relative to the intensity of n=0）increases from 2.85,3.42,3.56 to 3.56.Moreover,those multilayer high reflectors not only increase the PL intensity of GaSe,but also are suitable for enhancing the PL intensity of other two-dimensional materials such as Mo S₂sheets.Since GaSe materials are unstable in air and are easily oxidized,covered with a chemically inert protective layer is needed for the future optoelectronic device fabrications,The protecting layer such as polydimethylsiloxane（PDMS）,boron nitride（BN）,Aluminum oxide（Al₂O₃）,etc.,could avoid the degradation of GaSe in the air.Theoretically it is found that the PL intensity of GaSe flakes showed a periodic change with the increase of the thickness of the protective layer.For example,when the PDMS thickness is increased from 29.5um to 31.5um,the maximum and minimum intensity of 100L（200L）GaSe PL can be increased by 4.8times（5.3 times）.The effect of the thickness of the protective layer,Si O₂ and Si on the GaSe PL intensity is also calculated theoretically.The systematic investigation on GaSe PL intensity provides theoretical and experimental guidance for the future optoelectronic devices fabrication based on GaSe materials.