Study On CVD Growth And Spectroscopic Properties Of Two-dimensional MoSe₂

The monolayer MoSe₂,with a direct band gap of 1.52 eV,is located in infrared light wave band with adjustable band gap size,whose photoelectric conversion efficiency is significantly higher than that of the bilayer and few layers,showing optical characteristics different from the corresponding bulk material.The weakening of Coulomb shielding effect in two-dimensional MoSe₂ leads to a significant increase in exciton binding energy,causing the formation of multi-particle systems including neutral excitons,trions,bound excitons,and biexcitons at room temperature,thus exhibiting rich optical properties.The low-energy electrons and holes of the monolayer MoSe₂ at the corner of the first Brillouin zone?K-point?have the same effective mass,and their wave functions are constrained by the Mo atomic layer strength within the range of 0.2 nm in the direction perpendicular to the plane.Therefore,monolayer MoSe₂ is an ideal nanomaterial for exploring exciton physics in the two-dimensional limit.As a potential material for the application of visible light-emitting optoelectronic devices,the optical properties of two-dimensional MoSe₂are worthy of further study.Since the carrier concentration of the semiconductor affects its exciton state and photoluminescence characteristics,in this paper,using the method of spectroscopy,the intrinsic mechanism of MoSe₂ photoluminescence was studied by introducing p-type dopant Nb to regulate the carrier type and concentration in MoSe₂.There are many parameters in the CVD system,and the regulation of each parameter will significantly affect the MoSe₂ nanosheets.Therefore,this paper can control the growth of MoSe₂ by adjusting various growth parameters to obtain MoSe₂ nanosheets with different morphology,thickness and high quality.Increasing the concentration of MoO₃ will grow thick,large-sized MoSe₂ nanosheets,and the difference in concentration will result in different topography;If the growth temperature is too high,the H₂ ratio is too large,and the pressure is too high,the size of MoSe₂ will decrease.Because solid MoO₃ will melt with NaCl to form low melting Mo-oxychloride and Na-molybdate at 600°C,NaCl can effectively reduce the melting point of Mo source,increase the vapor pressure of Mo source,and significantly improve the deposition rate of thin film.Therefore,the addition of NaCl as a catalyst in MoO₃ can shorten the growth time by nearly 7 times,and obtain MoSe₂ nanosheet of several hundred microns in large size.Further studies have also confirmed that NaCl can effectively improve the crystal quality.The fluxing action of NaCl eliminates the huge difference in the growth temperature between MoSe₂ and NbSe₂,and Nb was successfully incorporated into NoSe₂ at 800?by CVD method for the first time.The effective doping of Nb changes the crystal structure and electronic structure of MoSe₂.The vibration of the Mo-Se bond overlaps with that of the Nb-Se bond,which weakens the peak strength of MoSe₂ A_1g,and causes redshift and broadening.The blue shift of 31 meV at peak position was observed in PL spectrum for the first time.The introduction of Nb can explain this phenomenon:First,it affects the interaction between Mo-Se atoms and the interaction between MoSe₂-substrate;Second,it forms lattice defects and produces bound excitons;Third,it increases the number of holes,resulting in the decrease of negative excitons and the increase of neutral excitons.Finally,it forms the impurity level and produces the local electron state,which affects the formation of the electron local bands.