Controlled Growth And Device Applications Of Two-Dimensional ReX₂(X=S,Se) And Their Heterojunctions

In recent years,atomically thin two-dimensional transition metal dichalcogenides(TMDCs)have attracted more and more attention due to their unique structures and tunable electronic and optical properties.Among them,Re-based dichalcogenides ReX2(X=S,Se)with their unique distorted octahedral(1T')crystal structure,interlayer decoupling feature and inherent anisotropy in in-plane optical and electrical properties,have been widely applied in polarization photodetectors,logic inverters and abnormal broadband photodetectors.Deeply exploring the anisotropic growth mechanism of ReX2,and studying the controllable epitaxial growth of ReX2 crystals with large scale and high quality is considered critical to realize their promising applications.In addition,2D transition metal chalcogenides with different compositions,phase structures,and properties offer giant opportunities for building novel 2D heterostructures.However,the studies to date have been mostly limited to homophase TMDCs heterostructures,while the construction of heterophase heterostructures remains a challenge.The present work mainly focuses on the controllable growth of ReX2 crystals,the exploration of anisotropic growth mechanism,the study of ReX2-based field effect transistors and the construction of ReS2-WS2 heterojunction.The thesis is mainly constituted by the following contents:(1)Controlled growth of ReS2 crystals and the application in electronic devicesBy employing angle-resolved Raman spectroscopy and TEM characterization,the domain structure of CVD grown hexagonal ReS2 crystal is systematically studied.By further combining the first-principles calculations,an anisotropic evolution mechanism is proposed to describe the growth process of the domain structure.Based on the understanding of growth mechanism,single-crystal ReS2 with large domain size is obtained,and moreover,large-area and highly crystalline monolayer ReS2 films are successfully synthesized on mica substrates by well controlling the experimental conditions.Furthermore,scalable FET arrays are fabricated based on the large-scale monolayer ReS2,which shows good electrical performances at room temperature.(2)Controlled growth of large-scale ReSe2 monolayer and the application in electrical devicesTwo-dimensional ReSe2 crystals are synthesized on mica substrates by employing the improved space-confined chemical vapor deposition method.By modulating the growth parameters,including the growth temperature,H2 content and the distance between the ReO3 precursor and mica substrates,monolayer ReSe2 with centimeter-scale and highly quality is achieved.Scalable ReSe2 FET arrays are fabricated,and transport measurements are performed to characterize their electrical properties.The results show that the electrodes have good Ohmic contact with the ReSe2 film and the devices have good electrical performance.(3)Controlled growth of 1T'(ReS2)-2H(WS2)van der Waals heterostructuresBased on the two-step CVD growth method,the 1T'-2H ReS2-WS2 lateral heterostructure with high interface quality and vertical heterostructure with nearly 100%overlap are acquired by optimizing the growth conditions,including the growth temperature and time,and the growth mechanism is also analyzed.The morphology,crystal structure and composition of the heterojunctions are studied by means of Raman,AFM and XPS.The physical properties of the heterophase interface,such as band alignments,interfacial charge transfer behaviors,and strain effect in ReS2-WS2 vertical heterojunction,are analyzed by PL spectroscopy.