Research On The Preparation And Photoelectric Properties Of TMDCs And Their Heterostructure

The layered transition metal dichalcogenides（TMDCs）)such as molybdenum disulfide（MoS₂）have great application prospects in the fileds of flexible devices and photodetectors due to their unique atomic thickness structure,suitable band gap,good thermal stability and excellent optoelectronic and mechanical properties.However,the traditional chemical vapor deposition（CVD）method based on silicon dioxide（SiO₂）substrate has limitations in the large-scale and controllable preparation of TMDCs.Meanwhile,there are still great challenges in the controllable growth of TMDCs heterojunction with large size.In view of these problems,a pre-deposition CVD method based on soda-lime glass was proposed in this thesis.The growth of group VI TMDCs heterojunctions was studied in detail by using one-step growth method and two-step growth method.We finally achieved controllable growth of MoS₂-WS₂,Mo Se₂-WSe₂,Mo_1-xW_xSe₂-WSe₂heterostructures.Characterization measurements including optical microscopy（OM）,Raman（Raman）,photoluminescence（PL）,atomic force microscopy（AFM）,X-ray Photoelectron Spectroscopy（XPS）and transmission electron microscopy（TEM）confirmed that the as-grown samples possess good uniformity and lattice structure.The photodetectors based on the as-grown samples were constructed to deeply study their optoelectronic properties.The main research contents of the paper are as follows:1.Millimeter-scale single crystal MoS₂monolayers were grown by pre-deposition CVD method based on soda-lime glass.In this method,the molten substrate formed by the melting of soda-lime glass at high temperature plays an irreplaceable role in improving the growth of TMDCs.The continuous irregular movement of thermions inside the glass and the flat surface at atomic level greatly inhibit the nucleation rate and diffusion barrier,and finally realize the low density and high speed growth of MoS₂.By adjusting the growth parameters,the effects of gas flow rate,predeposition film and sulfur precursor distance on the growth were investigated in detail,and finally the controllable growth of MoS₂with certain layer number and size was realized.In order to further study the electrical and opotoelectronic propertiers of the as-grown MoS₂,MoS₂-based field effect transistors were fabricated by transfer electrode technology.The devices show good output characteristics with mobility up to 5 cm²V^-1s^-1and on-off rate up to 10⁵.The responsivity can reach 0.85 A/W under 520 nm light irradiation.In this thesis,WO₃powder was introduced into the pre-deposition CVD system,and MoS₂-WS₂lateral heterojunctions were successfully grown on molten glass.This may provide a foundation for further exploration of the growth of the heterojunction on molten glass.2.High-quality Mo Se₂-WSe₂lateral heterojunctions were successfully synthesized on amorphous SiO₂substrates by one-step growth method,and a series of characterization methods were used to prove that the grown samples have good homogeneity and heterojunction structure.During the growth process,the morphology and layer number of the heterojunction can be controlled by adjusting the distance of the chalcogen precursor and the growth temperature.The growth mechanism of Mo_xW_1-xSe₂,Mo_xW_1-xSe₂-WSe₂and Mo Se₂-WSe₂in the one-step growth process was studied in detail by adjusting the distance between metal sources.The Mo Se₂-WSe₂heterojunction photodetectors were constructed by using the transfer electrode method and their photoelectric properties were studied in detail.The results show that the heterojunction devices have good detection ability for visible light（447,520,637 nm）in applied bias voltage mode or self-driven mode.The responsivity can reach up to 216 A/W and the corresponding detection rate is 3×10¹⁰Jones under 520 nm light illumination in applied bias voltage mode.,In self-driven mode,the responsivity reaches 45.1A/W,and the lower dark current makes the device show excellent detection sensitivity,with a detection rate up to 1.76×10¹¹Jones.3.High-quality AA-stacked and AB-stacked Mo Se₂bilayers were successfully synthesized by setting the CVD heating curve with high and low temperature difference.Furthermore,Mo Se₂/WSe₂vertical heterojunctions were successfully synthesized by switching hydrogen flow on or off during the growth process.Raman spectroscopy was used to analyze the optical and structural characteristics of the as-grown Mo Se₂and its heterojunctions.The relationship between the number of heterojunctions and the WSe₂E¹_2gpeak strength was investigated,proving the synthesis of high-quality uniform TMDCs materials.The photodetectors of Mo Se₂and Mo Se₂/WSe₂heterojunctions were fabricated by transfer electrode technology and their photoelectrical properties were investigated.The results show that the switching ratio of the Mo Se₂device exceeds 10⁶,and the rectification ratio of the Mo Se₂/WSe₂vertical heterojunction device is up to 236.Under 520 nm light irradiation,the responsivity of Mo Se₂device can reach 0.08 A/W,and that of the heterojunction device can reach 1.45 A/W.