Preparation Of MoS₂ Thin Films By Pulsed Excimer Laser And Its Application In Devices

In recent years,graphene,transition metal sulfide,black phosphorus and other two-dimensional layered materials have attracted extensive attention in academia and industry.Because of their atomic thickness and outstanding surface area ratio,they have broad application prospects in many fields such as electricity,optics,sensing,catalysis and energy.Transition metal sulfide has become one of the new semiconductor materials which are most likely to continue Moore's law.A novel pulsed excimer laser-induced fabrication technology is proposed in this paper,which can rapidly,selectively and extensively fabricate two-dimensional semiconductors such as MoS₂,which is compatible with CMOS process.In this thesis,the synthesis mechanism,synthesis experiment and device application are studied in detail.The innovation and research work mainly include the following aspects:(1)The technology of pulsed excimer laser induced MoS₂ thin film preparation is proposed,and the technology and the characteristics of MoS₂ thin film are systematically studied.The effects of different precursor parameters,spin coating parameters and laser processing parameters on MoS₂ film formation were studied.By adding ethylene glycol,spin coating PVP intermediate layer and adjusting the relationship between concentration,rotation speed,laser power and pulse number,high quality millimeter scale MoS₂ thin films were finally prepared,which confirms that large scale MoS₂ films can be prepared by pulse laser-induced technology.A series of characterizations of MoS₂ thin films were carried out.The E¹_2g and A_1g peaks of 2H semiconductor phase and the J₁,J₂ and J₃ peaks of 1T phase were observed by Raman spectroscopy;the 3d peak of Mo and the 2p peak of s in MoS₂were both composed of the peaks of 1T metal phase and 2H semiconductor phase by X-ray photoelectron spectroscopy;the triangular lattice of 1T metal phase and the hexagonal lattice of 2H semiconductor phase were observed by transmission electron microscopy It is proved that there are both 1T metal phase and 2H semiconductor phase.The results of AFM confirm that the thickness of MoS₂ film can be controlled by the ratio of different concentrations of precursor solution.(2)The technique of in situ doping induced by pulsed excimer laser is proposed.The doping process and the characteristics of MoS₂ films are systematically studied.The centimeter-scale MoS₂ films with Au,Pt and Pd doped have been successfully prepared by the optimized process.The maximum area of the films is 1.412cm×0.2cm.The characteristic peaks of Au,Pt and Pd were observed on X-ray photoelectron spectroscopy.The TEM showed that Au,Pt and Pd were successfully doped into MoS₂ films,which shows that the laser-induced doping technology can realize the synthesis and doping of MoS₂ films in one step.The first principle calculation results show that the doping of Au,Pd and Pt atoms will cause lattice distortion,the Au atom distortion is the largest,and the distortion of Pt atom is the smallest;the surface modification of Au will produce p-type doping energy level in MoS₂ and the substitution doping of Pt and Pd will enhance the n-type conductivity of MoS₂.(3)A reaction model of photochemical decomposition of(NH₄)₂MoS₄ to MoS₂ is proposed.Three kinds of reaction paths were analyzed,and the optimal reaction path of H⁺adsorption and HS·radical removal was obtained.Meanwhile,It is proposed that H₂ could improve the synthesis efficiency of MoS₂.Gaussian software is used to simulate the photochemical reaction in the whole laser process.By calculating the UV-Vis absorption spectra of the reactants MoS₄^2-,MoS₄^-,MoS₄H₂ and(NH₄)₂MoS₄,combined with the experimental results,it is proved that the reactants will produce light excitation under the irradiation of 248nm laser.The results show that when H⁺is sufficient,MoS₂ and a small amount of S will be deposited in the product,and NH₃ and H₂S will be released;When H⁺is insufficient,MoS₃ will be deposited in the product.The existence of MoS₃ impurity was confirmed by Raman spectrum and optical microscope,which verified the correctness of the reaction path.Through the simulation calculation of charge density distribution of all intermediate states in the optimal path of photochemical reaction,it is found that the charge mainly transfers between the p orbital of S atom and the p,d orbitals of Mo atom in the process of photochemical decomposition.(4)The n-channel field effect transistor based on MoS₂ and the back gate p-channel FET based on Au doped MoS₂ are designed and prepared.The results show that Au doping not only changes the conductivity of MoS₂ from n-type to p-type,but also improves the performance of FET.The switching ratio is increased from 10⁴ to 10⁵,and the carrier mobility is increased from 0.11 cm²V^-1s^-1 to 0.57 cm²V^-1s^-1.MoS₂ and Pt doped MoS₂prepared by laser technology are used for electrochemical hydrogen evolution.The results show that MoS₂ prepared by laser technology has good hydrogen evolution catalytic performance.When the exchange current density is 10 m A/cm²,the over potential is-0.39m V and taffel slope is 106m V/dec,and Pt doping further improves the hydrogen evolution catalytic performance of MoS₂.When the exchange current density is 10 m A/cm².The over potential is-0.26m V,and taffel slope is 87m V/dec.High performance photodetectors were fabricated by transferring MoS₂ prepared by CVD to MoS₂ prepared by laser process to form vertical homojunction.The photoelectric detection performance of2H-MoS₂ is enhanced because of the 1T@2H-MoS₂ intermediate transport layer.When the incident light is 2.35 m W/cm²,V_gs=6V,V_ds=5V,the photoresponse of the 1T@2H-MoS₂homojunction photodetector is 5.7 times that of the 2H-MoS₂ photodetector,the detection rate is 3 times,and the external quantum efficiency is 2.5 times.