Research On Controllable Growth And Characteristics Of Two-dimensional Layered Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides（2D-TMDs）are one new type of two-dimensional layered materials which have received intense attention after graphene.They have unique properties such as the layer-number-dependent bandgap,adjustable exciton effect and high carrier mobility.2D-TMDs can be potentially applied in many fields such as photovoltaics,electronic devices,photoelectronic and medical detections,and thus have attracted worldwide research.Although the discovery time of 2D-TMDs is relatively short,the development is quite rapid,including progress and breakthroughs in material preparations,property investigations and practical applications.Obviously,the efficient preparation of 2D-TMDs is the premise of all research and applications.Among different preparation methods,the chemical vapor depositon（CVD）method is regarded as the most promising one.However,like most other methods,the CVD also faces great challenges in controlling the size and the layer number of films,the large-scale fabrication and the convenient-assembly of heterojunctions,which have become an important factor restricting the large-scale industry applications of 2D-TMDs.In addition,the fundamental properties of exciton luminescence,band regulation and interlayer coupling of 2D-TMDs need more studies,which are the bases of their future special applications and have great scientific and practical significance.In view of this,systematic studies on the controllable growth and fundamental properties of 2D-TMDs,particularly the molybdenum disulfide（MoS₂）and its multi-component alloys,are carried out.The main research content and conclusions are as follows.1.A novel confined-space CVD method based on the auxiliary substrate has been proposed,which can be used to fabricate single-layer TMDs efficiently.By constructing a confined growth environment,the contamination on the substrate surface is effectively prevented,and the relative stability of the vapor ratios of the powder sources is maintained in a narrow space.The large area growth of binary（MoS₂,WS₂）and ternary(Mo_1-xW_xS₂,W_1-xMo_xS₂)monolayer films with large size,high quality,low pollution and triangular shape is realized successfully.The results show that the carrier mobility in the field effect transistor（FET）fabricated using the monolayer MoS₂ reaches up to 50-60 cm²V^-1s^-1.The photoluminescence（PL）of the monolayer MoS₂ can be effectively regulated by the back field voltage due to the low initial electron doping level in the film.In addition,the ternary alloy films prepared by the confined-space CVD method have a very uniform element distribution.The developed confined-space CVD method has more advantages in preparing high quality monolayer films.2.A hydrogen（H₂）assisted reverse flow CVD method has been proposed,which can realize the controllable layer-by-layer preparation of various bilayer TMDs films.By introducing the reverse flow into the heating stage after the growth of the first layer,the vertical stacking of nucleation sites on the surface of the first layer is effectively avoided.The surface energy of the substrate and the first layer is effectively reduced by introducing H₂,the density of nucleation sites on the first layer surface is well controlled,and thus the controllable layer-by-layer growth of large area bilayer films of binary（MoS₂ and WS₂）,ternary(Mo_（1-x）W_xS₂)and quaternary(Mo_（1-x）W_xS_2（1-y）Se_2y)alloys are realized successfully.The results show that these bilayer films have large size,uniform thickness and high crystallization quality.The bilayer films can be easily and quickly processed to form the periodical pattern of monolayers and bilayers by laser etching,which shows great potential in fabricating special function devices.The stacking mode of bilayer films is closely related to the temperature difference between the depositon temperatures of the two layers.When the temperature difference is less than 100℃,the AA stacking dominates.When the temperature difference is greater than 100℃,the AB stacking dominates.The FET fabricated using the AA-stacked bilayer MoS₂ film has the higher carrier mobility(up to 74.8 cm²V^-1s^-1),while the FET based on the AB-stacked bilayer MoS₂ film shows the lower carrier mobility(lower than7.8 cm²V^-1s^-1)and obvious rectification characteristics.These differences are caused by different stacking modes and contact conditions between the metal electrode and the film.In addition,the bilayer Mo_（1-x）W_xS_2（1-y）Se_2y quaternary alloy film contains both electron and hole doping components,thus its FET finally shows the bipolar characteristics.3.By using the one-step CVD method,both the conventional MoS₂-WS₂ lateral heterojunction and the MoS₂/WS₂ vertical heterojunction arrays have been fabricated with large range and high yield.Their growth mechanisms and optical properties are studied.The results show that the growth temperature control plays a decisive role in the growth direction of heterojunction.The lateral heterojunction is formed when the growth temperature remains unchanged during the whole process,and the vertical heterojunction is formed when the growth temperature rises in stages.A large number of lateral heterojunctions are uniformly distributed on the whole growth substrate.The edge of the bottom MoS₂ layer in the vertical heterojunction arrays reaches a length up to 600μm.Based on the preparation of simple heterojunctions,the complex heterojunctions such as MoS₂-WS₂/WS₂ and 2L-MoS₂-2L-WS₂are successfully fabricated,and their growth mechanisms and optical properties are also investigated.The results show that these heterojunctions exhibit novel optical characteristics at the stacking or boundary of different materials.The successful fabrication of complex heterojunctions indicates that the directional growth of heterojunctions can be well controlled by controlling the substrate temperature and the introducing time of H₂,which provides an effective experimental method to realize novel heterojunctions with special functions.4.The photodetectors are fabricated using the monolayer MoS₂,the bilayer MoS₂ and the bilayer Mo_（1-x）W_xS_2（1-y）Se_2y quaternary alloy films directly on the original substrates by electron beam lithography,and their photoelectric characteristics such as responsivity and response time are studied.The results show that these photodetectors exhibit good photoelectric properties,including high responsivity and fast response speed.The photodetectors based on the monolayer MoS₂ films prepared by the confined-space CVD have slightly higher responsivity than those based on the monolayer MoS₂ films grown by the conventional CVD,and the photodetectors based on the bilayer Mo_（1-x）W_xS_2（1-y）Se_2y films exhibit the best photoelectric properties such as the highest responsivity and the shortest response time.