Controlled Synthesis And Characterization Of Two-dimentional MoSe₂ Nanosheets And In₂Se₃/MoSe₂ Heterostructures

Two-dimensional(2D)layered semiconductors,including transition metal dichalcogenides(TMDs)(MX2,where M = transition metals such as Mo or W,and X=S,Se,or Te)and ?-? group layered chalcogenides(MX or M2X3,where M = Ga or In,and X = S,Se,or Te),have attracted broad attention for applications of next-generation electronic and optoelectronic devices.Particularly,it is of tremendous significance to form novel 2D heterostructures for not only fundamental research,such as long-lived interlayer excitons in MoSe2/WSe2 heterostructure owing to type-?junction band alignment and ultrafast charge separation,but also many device applications including photovoltaics,light-emitting diodes and photodetectors.In this work,we focus on exploring suitable conditions to grow monolayer MoSe2 and developing a versatile chemical vapor deposition(CVD)method to synthesize TMDs and ?-? group layered chalcogenides.Using this method,we produce In2Se3(2L)/MoSe2(1L)van der Waals heterostructures successfully.Structures and properties of the heterostructure are carefully investigated through various characterizations.The contents of dissertation are outlined as following:In chapter one,we first presented a systematical review on the properties of 2D TMDs,including crystal structure,electrical structure,and optical properties.We then summarized briefly the applications of 2D TMDs in the areas of field effect transistor,photodetector,environmental sensor and valleytronics.Finally,we introduced two main synthesis strategies including "top-down" and "bottom-up".In chapter two,we investigated the effect of various parameters of CVD growth on the products of MoSe2 nanosheets.The parameters include the amount of reaction sources,locations of substrate,carrier gas flux,and growth time.Beased on the systematic trials,we proposed an improved method to overcome the difficult that the MoO3 source in an open environment is consumed too fast to maintain the stable reaction.This modified method is not only suitable for the growth of monolayer MoSe2,but also can be used to synthesize monolayer MoS2 and In2Se3,indicating its versatility.In addition,we carried out a series of characterizations of monolayer MoSe2,including AFM,TEM,Raman,PL and electrical measurement,by which the good structure and properties of 1L MoSe2 can be identified.At the same time,several techniques which are vital for the characterizations,such as the transfer of 2D materials and the preparation of electrodes,have also been carefully developed.In chapter three,we first introduced the status of 2D-2D hetrostructures,including the preparation methods,physical properties,and applicaitons for devices.We then reported a two-step CVD growth method for creating 2D vertically-stacked heterostructure consisted of In2Se3 and MoSe2.To our best knowledge,this is the first report of vertically-stacked 2D In2Se3/MoSe2 heterostructures obtained by van der Waals epitaxial growth.The vertical heterostructures were verified by the characterizations of atomic force microscopy(AFM)and Raman spectroscopy.The investigation of transmission electron microscopy(TEM)reveals the well-aligned lattice orientation between In2Se3 and MoSe2.In addition,owing to the interlayer interaction,the apparent quenching and small red shift of the photoluminescence(PL)can be observed.More importantly,due to the unique band alignment of the two different materials,the fabricated device exhibits both the current rectification behavior and the photovoltaic effect.The surface contact potential difference of the heterostructure and the formed n?-n junction is further characterized by using Kevin probe force microscopy(KPFM).In chapter four,we prospected the existing challenges as well as the opportunities for the future researches of the 2D materials and 2D-2D heterostructures.