Studies On The CVD Growth And Photoelectric Characteristics Of Two-Dimensional MoX₂?X=S,Se?

MoS₂ and MoSe₂ are typical transitional metal dichalcogenides?TMDCs?with layered structures.At room temperature,TMDCs are usually stable in a hexagonal phase structure,with transition metal atoms and sulfur group atoms alternately forming a hexagonal honeycomb structure which is similar to graphene.The transition metal atoms are sandwiched between two layers of sulfur group atoms,forming a cell layer through the action of ionic covalent bond.The band structure of TMDCs is obviously dependent on the number of layers,and the band gap gradually increases with the decrease of the number of layers.When reduced to a single layer,the band gap reaches its maximum and changes from an indirect band gap to a direct band gap.At the same time,the stress,defects,surface modification and heterogeneous interface effects have a regulatory effect on the band gap.Hence,TMDCs have great application prospects in the fields of electronics,optoelectronics and the conversion of energy etc.However,the controllable,efficient and large-scale synthesis of TMDCs still remains a challenge.Especially the controllable preparation of large-area continuous two-dimensional TMDCs still faces many difficulties,such as defects difficult to control,surface pollution and high cost.Therefore,this paper aims to explore the key factors and parameters of the synthesis of TMDCs based on chemical vapor deposition?CVD?,and attempts to realize the controllable preparation of large-sized two-dimensional TMDCs,providing experimental and technical support for the practical application of two-dimensional TMDCs in the fields of electronics and optoelectronics.In this article,MoS₂ and MoSe₂?MoX₂,X=S or Se?were taken as the synthesis objects,and the main factors and parameters of two-dimensional MoX₂ growth in two sets of CVD systems under atmospheric pressure were compared and analyzed.The key experimental parameters for the reliable preparation of large-sized two-dimensional MoX₂ were obtained,and the morphology,luminescence performance of the sample and the electrical and photoelectrical properties of the device were characterized and tested.The main work and results of this paper are as follows:?1?The CVD system was built based on the ordinary tube furnace,and the effects of growth temperature,carrier gas composition and the mass of sulfur powder on the growth of two-dimensional MoX₂ were mainly investigated.Finally,MoS₂ with transverse size up to 80?m was obtained.MoSe₂ was grown by this CVD system on the basis of MoS₂growth,and MoSe₂ with the maximum size of about 60?m was also successfully obtained.Two-dimensional MoX₂ was characterized by Raman spectroscopy and photoluminescence spectroscopy,and the causes of the changes in photoluminescence intensity and the movement of Raman peak and fluorescence peak were explained.The monolayer and a few layers of MoX₂ were successfully grown which was confirmed by atomic force microscopy.?2?For further improving the growth size and quality of two-dimensional MoX₂,we built another CVD system based on the rapid heating furnace with large pipe diameter,the effects of factors such as insulation time,heating rate and the mass of MoO₃ on the growth of two-dimensional MoX₂ were mainly explored.We obtained the large-size MoS₂ of different shapes.Among them,the longitudinal size of continuous MoS₂ film can reach 20mm,the transverse size can reach 2 mm,the size of a single multilateral shape MoS₂ can reach to 500?m,and the size of a single irregular triangular MoS₂ can reach to 450?m.The longitudinal size of continuous MoSe₂ film can reach to 20 mm and the transverse size can reach to 1 mm in this CVD system.The results of Raman and photofluorescence spectra show that the grown MoX₂ has good quality and similar photoluminescence characteristics to the stripped one,both with powerful fluorescence under 532 nm laser excitation.?3?The electrical and photoelectric response characteristics of the MoS₂ were investigated.First,the volt-ampere characteristics of the wide-channel single-layer MoS₂device are tested under the dark and 532 nm laser.The results show that the device is sensitive to 532 nm laser,and the relative sensitivity is about 9.87×10⁵.The characteristics of the narrow-channel MoS₂ device were also tested.In the I-V and I-t characteristics of changing 532 nm laser power,the photocurrent increases with the increase of laser power and the photocurrent response is stable.In the I-V and I-t characteristics with different laser wavelengths,the device is most sensitive to the laser of 450 nm,and the photocurrent response and recovery time are 0.42 s and 2.34 s respectively.For its phototransistor,the photocurrent increases with the increase of laser power in the output characteristic,and the gate voltage V_bg almost loses its control over I_ds in the transfer characteristic.Finally,the characteristics of back gate transistor based on triangle MoS₂ are tested.The current of I_dss increases with the increase of the gate control voltage V_bg in the output characteristic.In the transfer characteristics of the device,it shows obvious switching characteristic,the order of magnitude of the turn-off current is 10^-12,the turn-on current is about 0.27 nA,the on/off ratio is about 1.10×10²,and the carrier mobility is about 0.18 cm²V^-1s^-1.The main reason for the low on/off ratio and carrier mobility may be that the defect of the material itself or the schottky barrier between the electrode and the channel.According to the above research results,by exploring different experimental factors and parameters,we were able to grow large-size two-dimensional MoX₂ in a controllable way.Moreover,it has the characteristics of good photoluminescence performance and fast photoelectric response,which has certain reference significance for the commercial preparation of two-dimensional MoX₂ and the application of photoelectric devices.