| Two-dimensional(2D)transition metal dichalcogenides(TMDs)have attracted considerable interests in recent years owing to their extraordinary physical properties and great potential in electronic,optoelectronic,energy storage and conversion applications.In particular,Mo S2,WS2 among VIB group ultrathin one-layer-thick van der Waals(Vd W)crystals,exhibit nontrivial properties,such as sizeable natural energy bandgap,high charge-carrier mobility,and spin-orbit coupling effects,which is closely related to the layer dependence of band structures.The implementation of precisely tailoring the band structures via growing heterostructures,alloying materials,polymorphism and other approaches are highly demanded.These excellent properties and tunability of band structures provide the powerful platforms for device applications in field-effect transistors(FETs),photodetectors,multifunctional memories and superconductivity.At present,Chemical vapour deposition(CVD)method has become the most widely used for growth of atomically thin 2D materials with high yield,but has yet to be realized entirely controllable and high-quality growth because of the high melting points of the transition metal oxide precursors(MOx)and their low vapour pressure.And little is known about the nucleation and growth mechanisms occurring the CVD process.Besides,a direct epitaxial synthesis technique of lateral heterostructures with a sharp interface is critical to future device applications.It remains a challenge to synthesis of 2D tellurides and their alloys by CVD routes,due to the strong metallicity of Te and the low bonding energy between transition metals and Te atoms.In addition,the possible ways to achieve a phase-selective synthesis still need further development in the controllability.Here,we propose a Na Cl-assisted CVD strategy to grow monolayer Mo S2,WS2and their in-plane heterostructures in one-step.Large-area,doping-free,high-quality 2D materials and heterostructures are obtained during the controlled process.The mechanism of the lowering growth temperatures and the possible reaction pathway during Na Cl-assisted CVD have been confirmed,in which the intermediate Na-containing species could be synthesized as nucleation sites on the substrate,as well as the metal source to be subsequently sulfurized and form 2D materials.On the basis of understanding of the growth mechanism with Na Cl assistance,we design a CVD method to synthesize monolayer WS2(1-x)Te2x alloys with a wide range of Te composition(0<x<1)and achieve the phase-selective synthesis between2H and 1T'phase via modulating the concentration of hydrogen gas.Different atomic distribution of the alloyed S and Te atoms in two phases are studied.Also,the corresponding composition-dependent optical and electrical properties of WS2(1-x)Te2x alloys are investigated in depth.The specific contents of this thesis are as follows:(1)Na Cl-assisted growth of high-quality Mo S2,WS2 monolayers and their in-plane heterostructureA Na Cl-assisted CVD strategy was developed to synthesize large-area?high-quality Mo S2,WS2 monolayers and their in-plane heterostructures in one-step.We systematically investigate the effects of the dosage of Na Cl,growth temperature and gas flow on growth behaviors.As-grown Mo S2 and WS2 are free of intentional doping and evident defects originating from Na Cl,which were demonstrated by Raman,photoluminescence(PL)spectra and high-resolution transmission electron microscope(HRTEM)characterizations.Mo S2-FETs exhibit the electronic characteristic with maximal electron carrier mobility up to 24.5 cm2V-1s-1 and on/off ratios up to 107,which is comparable to the previously reported values from oxide precursors.Owing to assistance with Na Cl,the growth regimes of Mo S2and WS2 are better matched,leading to the formation of Mo S2 at the core prior to WS2 at the peripheral region in a single step.The one-pot grown heterostructures were proved to be of high quality with a clear interface by employing Raman mapping and TEM tests.(2)Analysis of the efforts of Na Cl in CVD processThe addition of Na Cl is found to play vital roles in lowering the reaction temperature with a decrease of 100 and 200°C for growth of large Mo S2 and WS2 monolayers,compared to pure Mo O2/WO3 powder as Mo/W source,respectively.We present the first analysis of the efforts of Na ions in Na Cl assisted-CVD growth in this work by elaborating the transformation rules of precursors with the help of chemical characterizations.Vapored Na Cl could react with MOx into an intermediate ternary Na-containing species e.g.NaxMOycondensed on the substrates.On the one hand,these particles provide the nucleation sites on the substrates.On the other hand,the solid particles as metal precursors could be sulfurized step by step and finally form monolayer Mo S2/WS2.The residual unreacted Na-containing precursors on the substrates could be eliminated by a wet-transfer method.This Na Cl-assisted growth method could be extendable to grow VIB group TMDs selenides e.g.WSe2,tellurides e.g.Mo Te2/WTe2,and is promising for synthesizing other TMDs,heterostructures and their alloys.(3)Controllable synthesis of monolayer 2H/1T'phase WS2(1-x)Te2x alloys with tunable phase structuresMonolayer WS2(1-x)Te2x alloys with a wide range of Te components(x=0-1)were synthesized through the variation of H2 gas flow,and the structural conversion between semiconducting 2H(x<50%)and semimetallic 1T'phase(x>50%)was achieved by altering Te concentration.The optical bandgap energies of the alloys show a trend of the decrease from 1.97 to 1.67 e V with the increase of Te components in semiconducting phase,and a direct fall to 0 e V in semimetallic phase.The Raman spectra of WS2(1-x)Te2x alloys are reported and assigned comprehensively for the first time.Atomically resolved TEM analysis demonstrate that Te atoms are random distributed at chalcogen sites in 2H phase film,while S atoms show the highly selective occupation near W atoms chain in 1T'phase.Finally,the electrical properties of 2H and 1T'phase alloys-based FETs have been studied,which indicates that the substitution of Te atoms can introduce an upshift in the threshold voltage for semiconducting FETs due to a p-type doping effect,and the electrical conduction in semimetallic 1T'phase could be also modulated by alloying of S atoms. |
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