Controllable Synthesis And Electronic Performance Of New Type Two-dimensional Materials And Van Der Waals Heterostructures

Two-dimensional(2D)transition metal chalcogenide(TMDs)(such as MoS 2)have a wide and tunable band gap range(1-2 eV)and rich physical properties,such as a transition from indirect band gap to direct band gap when the thickness changes from bulk down to monolayer.For the features of atomically thickness,free off short-channel effect,valley free electron,flexibility,large specific area,and low cost,2D-TMDs have aroused widespread researches in recent years.2D vdW integration allows to combine highly distinct materials with unprecedented flexibility for their dangling-bonded-free surface,producing a series of new-generation van der Waals heterostructures(vdWHs)at the atomic scale with the potention to creat new functional devices.Existing researches have proven the exciting potential of 2D-TMDs and vdWH in ultra-short channel transistors,vertical tunnel transistors,new optoelectronic devices and flexible devices.The controllable synthesis of new 2D materials and vdW heterostructures,vdW heterostructure arrays in large scale is the key to develop the full potential of 2D materials and realize the practical applications of 2D materials.The main contents of this article are as follows:(1)In addition to semiconductor TMDs(s-TMDs),metallic TMDs(m-TMDs)have gradually aroused widespread research interests due to their unique physical properties,such as superconductivity,charge density waves,and ferromagnetism.Here,for the first time,single crystal,ultra-thin metallic MTe2(M=V,Nb,Ta)nanplates were controlledly synthesized through atmospheric pressure chemical vapor deposition method.Atomic force microscope studies have shown that the thickness of ultra-thin MTe2 nanoplates can be as low as 3 nm.X-ray diffraction(XRD),high-resolution transmission electron microscopy(TEM),and selected area electron diffraction(SAED)studies have shown that the obtained nanoplatelets are high-quality single crystals,and VTe2/Nb Te2/Ta Te2 are hexagonal/hexagonal/monoclinic phases,respectively.The electrical study has proven the metallic behaviors of MTe2(M=V,Nb,Ta).Hysteresis(MH)measurements show that VTe2 and Nb Te2 exhibit ferromagnetism at 10 K,while Ta Te2 exhibits paramagnetism.The preparation of magnetic metallic MTe 2(X=V,Nb,Ta)nanomaterials opens up exciting opportunities in the fields such as spintronics,sensors,and magneto-optical electronics.(2)A new type of high-quality 2D Bi I3 semiconductor material was successfully synthezied on a SiO2/Si substrate by a one-step physical vapor transport and deposition(PVD)method under ambient pressure.Transmission electron microscopy and electron diffraction studies of a single nanoplate confirmed the single crystal structure of Bi I3nanoplates.Raman studies show that Bi I3 nanoplates have a distinct characteristic peak of out-of-plane vibration at about 115 cm^-1,and show a slight red-shift with decreasing thickness.Photoluminescence(PL)studies show that the position of the PL spectral peak is in the range from 650 to 690 nm and a systematic blue shift of PL peak occurs with the decrease of the thickness of Bi I3 nanoplates.Electrical trasport studies show that Bi I3 nanoplates is n-type semiconductor and have a significant optical response.Make a step further,we proved that Bi I3 can be readily grown on other 2D s-TMDs(such as WS2,WSe2)to form a kind of van der Waals heterostructure through an improved two-step physical vapor deposition process.Electrical test of the devices made from Bi I3/WSe2 vertical heterojunctions shows obvious p-n diode characteristics,rectification behavior and obvious photovoltaic effect.(3)Scalable and controllable synthesis of van der Waals heterostructure array is a necessary step towards practical integrated circuits of two-dimensional materials.Many of the reported works about the synthesis of vdWHs rely on chance nucleation and growth with rather limited synthetic control.A direct laser cauterization patterning approach to creat periodic arrays of defects on the surface of s-TMDs was developed,and the defects were used as the exclusive nucleation sites for site-specific growth of m-TMDs to obtain m-TMD/s-TMD vdWH arrays.This method is general and not limited to materials with specific chemical composition or lattice structure.A series of vdWH arrays,including 1T-VSe2/WSe2,Ni Te2/WSe2,Co Te2/WSe2,Nb Te2/WSe2,VS2/WSe2,VSe2/MoS2 and VSe2/WS2 have been successfully prepared.Specifically,using highly-oriented continuous monolayer MoS2 thin film,much larger periodic arrays of VSe2/MoS2 2D vdWHs(?12,000 separated VSe2/MoS2 2D-vdWHs)can be achieved with a high yield up to?99%,which unambiguously confirmed the scalability of this approach.Systematic scanning transmission electron microscopy and cross section transmission electron microscopy studies reveal the atomically sharp,nearly ideal vdW interfaces with widely tunable Moirésuperlattices of the synthesized vdWHs.High-performance two-dimensional field-effect transistors with the synthesized vdW contact were fabricated.Based on these atomically clean heterostructures interface,the m-TMDs can function as highly reliable synthetic vdW contacts for the underlying WSe₂ with excellent device performance and yield,delivering a high ON-current density of up to 900?A?m^-1 in a bilayer WSe₂transistors with a 1.8?m long channel.This represents the highest room temperature ON-current ever achieved in monolayer or bilayer TMD semiconductor transistors.