Preparation And Properties Of Two-dimension MoS₂ And Its Lateral Heterostructures

Two-dimensional transition metal dichalcogenides（TMDCs;e.g.,MoS₂,WS₂,WSe₂,WSe₂）have exhibited many novel physical and chemical properties due to their tunable semiconductor electronic structures.Many empirical findings have indicated that TMDCs have a wide range of applications,such as transistors,photodetectors and photovoltaic cells.Among TMDCs,MoS₂has been getting the most extensive attention.As the monolayer MoS₂and other TMDCs were successfully grown on substrates and widely used in thin-film microelectrode devices,heterostructure of TMDCs have been found to have more excellent optoelectronic properties comparing with the single TMDCs,and are also expected to fulfill the continuously increasing demands for the integration and functionality of elect ronic and optoelectronic devices.Realizing the controllable preparation of Mo S ₂ and its heterojunctions is a crucial prerequisite for manufacturing high-performance integrated devices.In recent years,chemical vapor deposition（CVD）has been manifested to be a triumphant approach to prepare various 2D layered materials.However,the study of the growth mechanism of MoS₂ film by CVD is still in its preliminary stage,and the TMDCs based heterojunctions are rarely reported.Therefore,more effects are needed in this regard.In this paper,the controllable preparation of MoS₂ film and its heterojunctions was successfully achieved by the CVD method,and the effects of various growth conditions,the growth process,and the deposition mechanism of them were studied in detail.Besides,the monolayer MoS₂and the heterojunction devices were fabricated and tested,providing a certain theoretical basis and data support for its applications.The details are as follows:1.A high quality,large-area,uniform continuous monolayer MoS₂ film was successfully grown on the SiO₂/Si substrate by the CVD method.In order to improve the repeatability of the experiment,the influences of growth temperature,reaction time,carrier gas flux and other factors on the growth of the monolayer MoS₂ film were investigated.Based on a series of characterization analyses,we presented a possible deposition mechanism of the MoS₂ film by describing its growth process.It has been experimentally proved that the proposed deposition mechanism has a certain guidance significance for realizing the controllable preparation of the continuous monolayer MoS₂ film.2.A monolayer MoS₂ back-gated field effect transistor（1LMoS₂-FET）was constructed based on the conducting channel of the as-grown continuous monolayer MoS₂ film,and an average carrier mobility of～9.8 cm² V^-1s^-1 and a mean ratio of I_on/I_off of～3.1×10⁶ were achieved with an array of 20 1LMoS₂-FETs.The metal-semiconductor contact characteristics between MoS₂ and Ti were analyzed and thoroughly discussed by drawing the contact energy-band diagrams of them under different bias voltages.3.A seamless lateral heterojunction of MoSe₂/MoS₂ was successfully grown on the substrate by using a two-step CVD method.A characterization analysis revealed that the MoSe₂ was laterally and epitaxially grown along the triangle edges of the monolayer MoS₂,eventually leading to the formation of a heterojunction.Our result would benefit the study of the growth mechanism of the lateral heterojunction of MoSe₂/MoS₂ and shed some light on the preparation of other novel heterojunctions and superstructures of TMDCs.4.The energy-band diagrams of the lateral heterojunction of MoSe₂/MoS₂ under the light and the dark state were plotted and analyzed,and the measured volt-ampere characteristics curve of the heterojunctions were analyzed in detail.The band alignment in the MoS₂/MoSe₂ heterojunction was a typical type-Ⅱstaggered gap,which can urge the effective separation of photoinduced electron-hole pairs,leading to a prolonged longevity of the photoexcited charge carrier,and a suppression of the interfacial electron-hole recombination.Efficient p-doping in MoSe₂ and n-doping in MoS₂ nanolayers will be produced handily by light irradiation.