The Synthesis Of Two-dimensional Transition Metal Dichalcogenides,and Its Lateral Heterostructures By Reverse Flow Method

Two-dimensional atomic crystals have attracted great interest from the scientific and engineering communities due to its abundant and tunable electronic,optical,chemical,mechanical,magnetic and thermal properties recently.Two-dimensional(2D)transition metal dichalcogenides(TMDs)have attracted considerable attention for their distinct physical properties,such as atomically thin geometry,extraordinary mechanical flexibility,layer-number dependent electronic and optoelectronic properties,and tunable spin and valley polarization,and their potential applications in2D electronics,optoelectronics,and spintronics.However,the studies to date have mainly been limited to the exfoliated flakes with a limited control of the lateral sizes and thickness,which is difficult to scale for functional electronic and photonic devices.Therefore,the development of a large-area single-layer TMDs preparation strategy is essential for practical applications.At the same time,rational design and synthesis of 2D lateral heterostructures,as well as more complex multi-heterojunctions and superlattice structures,are essential for exploring new scientific phenomena within the thickness of several atomic layers,and developing new types of atomic thin devices.To this end,considerable efforts have been devoted to developing synthetic strategies to obtain high-quality 2D lateral heterojunctions,the synthesis of 2D lateral heterojunctions is still challenging,and 2D superlattices have not been successfully synthesized so far.The monolayer 2D crystals are usually too delicate to survive multiple sequential growth steps necessary for the formation of2D heterostructures or superlattices.In view of this,this article has developed a new chemical vapor deposition method,successfully realized the rapid preparation and layer thickness control of WS₂,WSe₂,and that a wide range of 2D heterostructures and multi-heterostructures,and superlattices were readily prepared.The main research results are as follows:1.We report a modified CVD approach with a controllable reverse flow for the rapid growth of large domain single crystals of monolayer TMDs.With the reverse flow to precisely control the chemical vapour supply in thermal CVD process,we can effectively prevent the undesired nucleation before reaching optimum growth temperature and enable rapid nucleation and growth of monolayer TMDs single crystals at a high temperature that is difficult to access in a typical thermal CVD process.We show millimetre-scale monolayer WS₂and WSe₂single crystals can be prepared in<30 s,with a highest lateral growth rate up to 45?m/s.Electronic characterizations show the resulting monolayer material exhibit excellent electronic properties with the carrier mobility up to 90 cm²V^-1s^-1,comparable to that of the best exfoliated monolayers.Our study provides a robust pathway for rapid growth of high-quality TMDs single-crystals.2.We report the robust growth of high-quality WSe₂single crystal domains with a selectively controlled thickness in a reverse-flow chemical vapor deposition system with solid precursor.By introducing a pre-annealing step to tune elemental distribution and volatilization rate of the solid precursor,we stabilize the vapor supply to achieve a highly uniform nucleation and growth,and thus ensure a precise control of the layer number for highly selective growth of monolayer or bilayer WSe₂single crystals(>500?m).The transmission electron microscopy and optical characterizations of the resulting WSe₂single crystals demonstrate excellent crystalline quality with systematically tunable optical properties.Electrical transport studies further show the WSe₂field-effect transistors exhibit p-type semiconductor characteristics with effective hole carrier mobility up to 92 cm²V^-1s^-1in monolayer,145 cm²V^-1s^-1in bilayer materials at room temperature.This simple approach opens up a new avenue to highly controlled synthesis of WSe₂atomic layers for both the fundamental studies and technological applications.3.We report a general synthetic strategy for highly robust growth of diverse heterostructures from two-dimensional(2D)atomic crystals.A reverse flow during the temperature swing stage in sequential vapor deposition growth process allowed us to cool the existing 2D crystals to prevent undesired thermal degradation and uncontrolled homogeneous nucleation,thus enabling highly robust block-by-block epitaxial growth.Raman and photoluminescence mapping studies showed that a wide range of 2D heterostructures were readily prepared with precisely controlled spatial-modulation.Transmission electron microscope studies show clear chemical modulation with atomically sharp interfaces.Electrical transport studies of WSe₂-WS₂lateral junctions show well-defined p-n diode characteristics with a rectification ratio up to 10⁵.4.Furthermore,two-dimensional multi-heterostructures(such as WS₂-MoS₂-WS₂,WS₂-WSe₂-MoS₂,WS₂-MoSe₂-WSe₂)and two-dimensional superlattices(such as WS₂-WSe₂-WS₂-WSe₂)were readily prepared with precisely controlled spatial-modulation.Transmission electron microscope studies show clear chemical modulation with atomically sharp interfaces.By precisely control the width of the epitaxial heterojunction,we have obtained a narrow nano-ribbon with 10 nm,which will help the development of quantum well devices.The successful preparation of these structures provides a reliable material platform for 2D electronics and optoelectronics.