Controllable Preparation And Electrical Performance Of Two-dimensional MoS₂

Molybdenum disulfide(MoS₂)has received extensive attention due to the continuous in-depth research on two-dimensional layered materials in recent years.MoS₂ with different crystal phase structures has different physical properties,such as metallic 1T phase and semiconductor 2H phase.Recently The multi-phase MoS₂with coexistence of 2H and 1T phases has attracted research interest because of its unique properties.Therefore,the large-area controllable preparation of MoS₂with different crystal phase structures is not only helpful for in-depth understanding of the growth mechanism of 2D materials,but also promotes MoS₂.The basis for applications in the fields of electricity,optoelectronics and electrocatalysis.At present,the preparation research of MoS₂mainly focuses on the controllable preparation of 2H phase,and there are few researches on the preparation of other crystalline phases of MoS₂,especially the preparation of large-area multiphase MoS₂ still faces many challenges.In this thesis,the controllable preparation of MoS₂ thin films with different crystal phase structures was investigated by low-pressure chemical vapor deposition.On the one hand,the controllable preparation of centimeter-scale single-layer and double-layer2H-phase MoS₂ films was achieved,and the results of Raman and photoluminescence spectroscopy showed that the evaporation temperature of molybdenum source and sulfur source would significantly affect the quality of 2H-phase MoS₂films,which is Since the evaporation temperature of the molybdenum source and the sulfur source will determine the respective concentrations when the molybdenum source and the sulfur source meet and react,evaporating the molybdenum source(sulfur source)too early(late)will make the molybdenum source(sulfur source)insufficient during the reaction,and then lead to defects.On the other hand,the controllable preparation of multiphase MoS₂with the coexistence of 2H and 1T phases in centimeter-scale monolayers was achieved.The results of Raman and photoluminescence spectroscopy showed that in the extremely sulfur-rich growth environment,the 1T phase would begin to form and interact with 2H.The phases constitute a multiphase coexistence structure.Based on the preparation of MoS₂ thin films with different crystal phase structures,their electrical properties were studied.On the one hand,the 2H-phase MoS₂transistor shows its typical n-type semiconductor characteristics.Further research found that the water molecules adsorbed by the MoS₂ film in the atmosphere will do p-type doping on it,increasing the Schottky potential of MoS₂ in contact with the electrode.In addition,compared with single-layer MoS₂,double-layer MoS₂has a smaller work function,which makes its Schottky barrier with the electrode smaller,so double-layer MoS₂transistor has better performance.The performance of the prepared monolayer MoS₂transistor is 1.1×10^-1 cm~2/V·s and 1.9×10~5,and the mobility and switching ratio of the bilayer MoS₂ transistor are 1.8×10^-1cm~2/V·s and 5.7×10~5.On the other hand,the polyphase MoS₂ with coexistence of monolayer 2H and 1T phases shows metallicity when the current remains unchanged under gate voltage modulation,while its current decreases with the decrease of temperature(negative temperature coefficient of resistance),showing semiconducting properties.This unique semi-metallic property may explain the variable-range hopping of electrons in two-dimensional planes through the localized metallic 1T phase surrounding the semiconducting 2H phase for multiphase MoS₂.