Research On The Preparation And Photoelectric Properties Of Mo-based Transition Metal Compound Alloys And Heterostructures

Mo-based transition metal dichalcogenides(TMDs)are considered as functional materials to realize the application prospect of nanotechnology,and have the potential to play a great role in quantum physics,microelectronics,superconductivity and other fields in the future.Among the currently used preparation methods,chemical vapor deposition(CVD)is considered to be a promising one.In previous studies,solid precursors(such as metal oxides and sulfur powders)are commonly used as the growth materials for CVD synthesis of TMDs.However,the large difference in vapor pressure between different precursors makes it quite difficult to achieve a stable supply of precursors in the CVD reaction,which makes the reaction less controllable and reproducible.On the purpose of promoting the controllable development of the CVD preparation process,three feasible improved CVD methods are proposed in this paper.High-quality MoS₂ crystals,MoS_2(1-x)Se_2x alloys,Mo_1-xSb_xS₂ alloys,MoS₂-WS₂ vertical heterostructures and MoSe₂-WSe₂ vertical heterostructures are successfully synthesized.The element composition,morphology,optical properties and other properities of the crystal thin films are studied via a series of characterization methods.In addition,we construct photodetectors based on the above thin crystal films to further study the related photoelectric properties.The main content of this paper is as follows.1.A CVD method using niobium pentoxide(Nb₂O₅)powder as an oxide inhibitor to assist growth is proposed.High-quality and large-size MoS₂ crystals are successfully prepared on SiO₂/Si substrates.In the preparation,the oxide inhibitor(Nb₂O₅ powder)is completely covered on the molybdenum oxide(Mo O₃)powder.The release of Mo vapor is controlled by adjusting the quality of the Nb₂O₅ powder.When the quality of the oxide inhibitor increases,more Mo vapor is trapped in the Nb₂O₅ powder and can not diffuse,which results less Mo vapor participates in the CVD reaction.With the remaining parameters unchanged,a near-ideal Mo:S atomic ratio environment could be achieved by this method.Moreover,since the selected Nb₂O₅ powder has an ultra-high melting point(?1500?),it doesn't cause elemental contamination at all.The characterization results of microstructure,optics,and elemental composition show the high crystal quality of the prepared MoS₂ crystals.The fastest response time of the constructed monolayer MoS₂ crystal photodetector is 0.8 s,which is significantly better than the previously reported ones.2.MoS_2(1-x)Se_2x and Mo_1-xSb_xS₂ alloy films are prepared by the confined-space CVD method.The confined-space CVD system can effectively reduce the concentration of reactants and provide a stable sample growth space for the synthesis of high-quality alloys.Mo_1-xSb_xS₂ alloys are prepared by adding metal element(Sb)into MoS₂.The characterization results of microstructure,optics,and elemental composition demonstrate the excellent crystal quality and perfect uniformity of prepared alloys.The produced monolayer Mo_1-xSb_xS₂ photodetector has an ultra-fast response time of 40 ms and a responsivity of 19.75 m A/W.MoS_2(1-x)Se_2x alloys with different element contents are prepared by adding Se element into MoS₂.Through the above characterization methods,the high crystal quality and good uniformity of MoS_2(1-x)Se_2x alloys are proved.Monolayer MoS_2(1-x)Se_2x photodetectors with different element contents are also constructed.The study shows that the response time of the alloy photodetector continues to decrease with the increase of Se element.At last,the second-level response time of monolayer MoS₂ device is successfully reduced to millisecond-level,with a fastest response time of 20 ms.3.Low-melting water-soluble sodium tungstate powder(Na₂WO₄)is used instead of high-melting tungsten oxide(WO₃)powder as a tungsten source and molybdenum oxide(Mo O₃)powder is used as a molybdenum source.The MoS₂-WS₂ and MoSe₂-WSe₂ vertical heterostructures are synthesized by a one-step CVD method which combines aqueous solution and solid powder.The method solves the obvious shortcoming of the excessively high melting point of the traditional tungsten source,avoids the problem of element cross-contamination during the preparation process of heterostructures,and ensures the complete periodicity of the crystal domain.Characterization results such as microstructure and optical properties prove good crystal quality of the heterostructures.The produced heterostructure photodetectors all exhibit excellent rectification characteristics,with a maximum rectification ratio of about 10⁴.