Preparation And Characterization Of Graphenetransition Metal Dichalcogenides Vertical Heterostructures By Electrochemical Deposition

Two-dimensional(2D)vertical heterostructures have attracted considerable attentions in electronics and opto-electronics.CVD method has been widely used to synthesize 2D vertical heterostructures.However,due to the nature of the chemical reaction process,the growth window of vertical heterostructures is usually narrower than that of the lateral heterostructures.Moreover,the dimension size of 2D vertical heterostructures is limited,which strongly impedes the device application and integration.A controllable approach to synthesize high-quality and large-area 2D vertical heterostructures is highly desirable.In this work,our strategy using controllable electrochemical deposition(ECD)has been extended to obtain large-area WS₂/graphene heterostructures for high-performance photodetectors with a photoresponsivity up to 2.62 A/W,a fast response time of 49 ms and a noise equivalent power(NEP)of 3.3×10-⁷ WHz-^1/2 due to the decoupled WS₂films on graphene.Both the Schottky barrier height(?_SB=310 m V)and the built-in potential(?_Bi=270m V),which help the separation and transport of the excited carriers,have been determined.Firstly,the conventional single crystalline silicon wafer was used as the substrate in the aqueous solution of gold acid chloride trihydrate(HAu Cl₄·3H₂O,0.1 m M in DI water).The gold film deposited by the electrochemical deposition followed the lattice orientation of the single crystal silicon substrate,which was used as the cathode.A quartz crystal microbalance was used to quantitatively obtain the time dependence of the film thickness,which could be controlled by the applied voltage in a fixed concentration of 0.1 m M.A thin oxide(Si Ox)layer was prepared by the silicon oxidation under the light illumination.Then,polydimethylsiloxane(PDMS)thin layer was prepared and attached to the sample as the supporting material,and the thin oxide layer was etched away by the diluted hydrofluoric acid(HF).The gold film was transferred to the target substrate via the self-built transfer platform,which served as the device electrode.Then,in order to optimize the substrates for electrochemical deposition,both analyses of ITO conductive glass and CVD graphene were conducted.The surface morphology of the two substrates and the deposited samples were analyzed by AFM,and it was found that the surface roughness of ITO was higher than that of graphene.The surface of the sample deposited by ITO was rough,which seriously deviated from the goal of obtaining a flat and dense film.Compared with traditional 3D electrodes/substrates,conducting 2D graphene sheet is an appealing substrate as both the inert electrode in the chemical bath and the template in the following annealing process,because:it has an atomically smooth surface with relatively free of dangling bonds;it is relatively chemically inert due to conjugation;and it has high thermal stability in vacuum or inert atmosphere.Finally,by adopting controllable electrochemical deposition of WS₂ on the large-area CVD graphene film,followed by an optimized annealing process,high-quality 2D heterostructures based on WS₂/graphene have been fabricated.High-performance photodetectors based on the 20-nm-thick WS₂/graphene heterostructure with a photoresponsivity up to 2.62 A/W(@1.19?W,532 nm),a fast response time of 49 ms,and a room temperature noise equivalent power(NEP)of 3.3×10-⁷W·Hz-^1/2 have been demonstrated.Moreover,the underlying electronic properties,including the Schottky barrier height(?_SB=310 m V)and the built-in potential(?_Bi=270 m V),which help the separation and transport of the excited carriers,have been quantitatively determined by the study of KPFM and I–V characteristics.Our work paves a way for a simple strategy to fabricate high-quality WS₂/graphene heterostructures in a large area and presents the essential electrical parameters for future optoelectronic applications.