| The 21st century is not only an era of technological revolution,but also an era of two-dimensional(2D)materials.In 2004,Novoselov,Geim and his colleagues successfully exfoliated graphene from graphite using scotch tape,marking the arrival of two-dimensional nano materials.Graphene is a crystalline carbon film with a single atom thick.It has ultrahigh room temperature carrier mobility,quantum Hall effect,bipolar field effect,high Young's modulus,ultrahigh specific surface area,excellent optical transparency,excellent conductivity and thermal conductivities.Under this background,the research boom of new two-dimensional materials has arisen,such as two-dimensional transition metal chalcogenides(TMDs),black phosphorus(BP),hexagonal boron nitride(h-BN),metal oxides and so on.In particular,2D metallic transition metal dichalcogenides(MTMDs)recently have attracted widespread attention in the scientific community due to their rich physical and chemical properties,such as electromagnetism,charge density waves,superconductivity,novel electronic applications and so on.At the same time,2D MTMDs have increasingly become an indispensable metallic-building-blocks for the construction of 2D van der Waals and in-plane heterostructures.Therefore,this paper focuses on the chemical vapor deposition(CVD)synthesis method and thickness-tunable synthesis of high-quality two-dimensional PtTe2,CoTe2,tetragonal and hexagonal CoSe,CoSe-WSe2 metal-semiconductor lateral heterostructures are realized.Moreover,their electrical or electromagnetic properties are also studied.The main research results are as follows:1.High-quality ultrathin PtTe2 nanosheets were firstly controllably synthesized through a chemical vapor deposition method,with thickness of the nanosheets as thin as 1.8 nm.X-ray diffraction(XRD),transmission-electron microscopy(TEM),and electron-diffraction(SAED)studies confirm that the resulting 2D nanosheets are high-quality single crystals.Raman spectroscopic studies show characteristics Eg and A1g vibration modes at?109 and?155 cm?1,with a systematic red shift with increasing nanosheet thickness.Electrical transport studies show the 2D PtTe2 nanosheets display an excellent conductivity up to 2.5×106 S m-1 and show strong thickness-tunable electrical properties,with both the conductivity and its temperature dependence varying considerably with the thickness.Moreover,2D PtTe2 nanosheets show an extraordinary breakdown current density up to 5.7×107 A/cm2,which is the highest breakdown current density achieved in 2D metallic transition-metal dichalcogenides before 2018.2.High-quality ultrathin CoTe2 nanosheets were firstly controllably synthesized by a chemical vapor deposition method,with thickness of the nanosheets as thin as 3.1 nm.X-ray diffraction,transmission electron microscopy,and high-resolution scanning transmission electron microscope(STEM)studies reveal the obtained CoTe2 nano sheets are high-quality single crystals in the hexagonal 1T phase.Electrical transport studies show the 2D CoTe2 nano sheets display excellent electrical conductivities up to 4.0×105 S m-1 and very high breakdown current densities up to 2.1×107 A/cm2,both with strong thickness tunability.3.High-quality ultrathin tetragonal and hexagonal CoSe nanosheets were firstly synthesized controllably through a chemical vapor deposition method,with thickness of nanosheets as thin as 2.3 and 3.7 nm,respectively.X-ray diffraction,transmission electron microscopy,and selected area electron diffraction studies show that both types of nanosheets are high-quality single crystals.Electrical transport studies reveal that both the tetragonal and hexagonal CoSe nanoplates show strong thickness-tunable electrical properties and excellent breakdown current density.The 2D hexagonal CoSe nanoplates display metallic behavior with an excellent conductivity up to 6.6×105 S m-1 and an extraordinary breakdown current density up to 3.9×107 A cm-2,while the square tetragonal nanoplates show considerably lower conductivity up to 8.2×104 S m-1 with angle-dependent magneto resistance and weak antilocalization effect at lower field.4.Tetragonal and hexagonal CoSe-WSe2 lateral heterostructures were synthesized through a two-step vapor phase epitaxy strategy.The 2D CoSe can selectively nucleate at the edge of pre-grown WSe2 nanosheets to form CoSe-WSe2 metal-semiconductor lateral heterostructures.Optical microscopy(OM),scanning electron microscopy(SEM)and atomic force microscopy(AFM)studies reveal distinct contrast across the heterostructure interface.High-resolution transmission electron microscopy(HRTEM)and selected area electron diffraction studies further confirm the micro structure modulation across hetero structure interface.The field-effect transistors(FETs)made of CoSe-WSe2 lateral heterostructures using the in-plane connected CoSe as source and drain electrodes show satisfactory Ohmic contacts and considerably better FET performance over those with deposited Cr/Au contacts. |
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