CVD Preparation And Photoelectric Properties Of Sub-millimeter Two-dimensional WX₂（X=S,Se）

WS₂ and WSe₂ are typical transition metal dichalcogenides（TMDs）and have a layered crystal structure.The transition metal atoms and the chalcogenide atoms are combined by covalent bonds to form a sandwich monolayer,and the van der Waals force is used to interact between the layers.As semiconductor materials,two-dimensional TMDs have attracted extensive attention due to their unique layered structure and tunable band gap,which have great application prospects in the fields of micro-nano optoelectronic devices and quantum devices.With the deepening of researches,a variety of methods have been developed to prepare two-dimensional TMDs.However,it is still a challenge to prepare two-dimensional TMDs with large size and large area only atomic thickness with high efficiency,high quality,low cost and controllable.The solution of this problem has important scientific significance and application value for the development of new optoelectronic devices based on two-dimensional TMDs and their practical applications.In this work,based on the principle of chemical vapor deposition（CVD）,the key factors and corresponding conditions for controlling the growth of two-dimensional TMDs by CVD were systematically found by using tungstic acid and chalcogenide as precursors through self-built vapor deposition equipment.The size,morphology,distribution and quality of two-dimensional WS₂ and WSe₂ were characterized and analyzed by optical microscope、PL、Raman、AFM、EDS、SEM and photoelectric test,which laid a technical and experimental foundation for the future large-scale construction of new heterostructure optoelectronic devices based on two-dimensional WS₂ and WSe₂.The main work is listed as following:（1）A chemical vapor deposition（CVD）system was built based on a two-temperature zone tube furnace.The effects of carrier gas concentration,W/S mass ratio,holding time,carrier gas flow rate,precursor temperature,H₂ withdrawal temperature and different substrates on the preparation of two-dimensional WS₂ were systematically explored.As a result,the two-dimensional WS₂ with size more than 1 mm was achieved.Optical microscope（OM）,fluorescence spectroscopy（PL）,Raman spectroscopy（Raman）,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）and atomic force microscope（AFM）were used to characterize and analyze the size,morphology,distribution and quality of the prepared two-dimensional WS₂.The results show that under the conditions of 850°C in high temperature zone and 180°C in low temperature zone,the mass of H₂WO₄and S are 120 mg and 220 mg,the concentration of H₂ is 5%,the carrier gas flow is 80 sccm,and the holding time is 10 min,the two-dimensional（monolayer）WS₂ with the size of more than 1 mm can be prepared.（2）The effects of carrier gas concentration,W/Se mass ratio,holding time,carrier gas flow rate,precursor temperature,H₂ removal temperature and different substrates on the preparation of two-dimensional WSe₂ were also systematically explored based on the self-built CVD.The results show that when the temperature is 900°C in the high temperature region and 350°C in the low temperature region,the mass of H₂WO₄ and Se are 140 mg and220 mg,the concentration of H₂ is about 10%,the carrier gas flow is about 80 sccm and the holding time is 5 min,the two-dimensional WSe₂ with the size of more than 2 mm can be achieved.A series of characterizations and tests showed that we successfully prepared single-layer WSe₂ with small surface roughness（Ra<0.15 nm）and high quality.（3）Based on the successful preparation of two-dimensional WX₂（X=S,Se）,their basic electrical behavior and photoelectric response characteristics were investigated.The results show that the average effective resistance of the 2D WS₂ device decreases with the increase of laser wavelength under darkness and laser irradiation at different wavelengths（450 nm,532 nm and 635 nm）,and the device is most sensitive to blue light with wavelength of 450 nm,and the relative sensitivity S_450nm is 133.33;for two-dimensional WSe₂,the average effective resistance of the device under white light condition is greater than that under dark condition.The I-t response of the two devices irradiated by 450 nm,532 nm and635 nm laser is tested.Both have good signal-to-noise ratio and the waveform is regular square wave.Under the same wavelength laser,by changing the laser power,it can be found that the recovery time of the two devices does not increase significantly with the increase of power,and their response time increases with the increase of power,but both have faster photoelectric response characteristics.It was found that the preparation process of V_ds and electrodes had an important influence on the electrical properties of the device.The electrode prepared by laser direct writing has better contact with the material,which makes WS₂ show better electrical properties.The response time of the device is reduced to 0.17 s under 635nm laser irradiation.The electrical properties of WS₂ transistor fabricated by laser direct writing were also tested.From its output characteristics,it can be seen that the gate voltage has a modulation effect on the carrier,and the source and drain electrodes have good contact with the material.It can be seen from the transfer characteristics that the transistor switching ratio reaches about 3.8×10³,and shows p-type transistor characteristics.In summary,two-dimensional WS₂ with a size of more than 1 mm and two-dimensional WSe₂ with a size of more than 2 mm were prepared by a self-built dual-temperature CVD system using tungstic acid and chalcogenide as precursors.The prepared two-dimensional WX₂ had good optical and electrical properties.It is worth noting that the electrode preparation process has an important influence on the contact between the two-dimensional WX₂ and the electrode,which will directly affect the test of the electrical properties of the device.It has important reference significance for the large-scale preparation of two-dimensional WX₂ and its application in optoelectronic devices in the future.