Controlled Synthesis Mechanism And Characteristics Of MoSe₂ By Reverse-flow Chemical Vapor Deposition

With the intensive research of two-dimensional materials,Graphene has become a research focus due to its unique crystal structure and excellent characteristics.However,due to its extremely low switching ratio,it cannot be applied to field effect devices on a large scale.Transmission metal dichalcogenide?TMDCs?are different from graphene,whith are two-dimensional semiconductors with bandgap and have broad application prospects in the field of optoelectronics.As a member of TMDCs,molybdenum diselenide has attracted the attention of researchers due to its unique band structure,excellent electrical characteristics and other properties.However,the preparation of molybdenum diselenide with large-area,few nucleation points and good uniformity is still a huge challenge.In this paper,large-scale MoSe₂ single crystals can be synthesized by controllable reverse flow CVD strategy.SEM/TEM,AFM,Raman/PL,XRD/XPS and other methods were used to characterize the physical properties.The effects of growth parameters such as growth temperature,time and flow on the quality of the samples were analyzed.Physical properties and growth mechanism of the bilayer MoSe₂ with AA/AB stacking structure were systematically analyzed in the study.The main contents are as follows:?1?Controlled growth and physical properties of high-quality,large-area single-layer MoSe_2.Exploratory growth of MoSe₂ nanosheets and continuous films was synthesized with an improved reverse-flow device.Firstly,the effect of the time when the reverse-flow turns positive,flow,temperature,and reaction time on the quality of the growing sample were studied.Finally,the optimal growth conditions of high-quality monolayer MoSe₂ were optimized.The growing samples were characterized by a variety of characterization methods,indicating that the samples grown by the reverse airflow method have a good uniformity surface,high coverage of monolayer and high crystal quality.Finally,the optoelectronic devices were prepared and their electrical properties has been tested.The non-linear light absorption experiments of MoSe₂films grown on sapphire substrates were performed,which proved as-grown samples with good non-linear light absorption characteristics.This provides reference for future generations to prepare and study the growth and application of other single-layer transition metal sulfides.?2?Controlled growth and physical properties of high-quality,large-area bilayer MoSe_2.The growth time of two layers of MoSe₂ was accuratelly controlled for 5 minutes by multi-stage reverse-flow CVD growth method and bilayer MoSe₂ in different stacking sequences can be controllably systhesized by a specific temperature programe.The AA stacking morphology appeared at 810?,the AB stacking morphology easily appeared at 860?,the growth mechanism was further explored.Then a variety of characterization methods were used to systematically characterize the growing samples.The characterization results show that the as-grown bilayer MoSe₂ has excellent characteristics such as high yield and large-size area.The atomic structure,surface uniformity,and fluorescence properties of the two stacking structures were compared.Subsequently,field-effect devices were fabricated using bilayer MoSe₂ with different structures?AA and AB stacking?.The differences in electrical performance between single-layer and AA,AB stacking bilayer MoSe₂were compared.It can be concluded as follows:output current,ON/OFF ratio and conductivity:AA stacking bilyer MoSe₂ FFT>AB stacking MoSe₂ FFT>monolayer MoSe₂ FFT.This provides a feasible method for the controlled growth of other bilayer transition metal sulfides.