Synthesis of Janus structures based on transition metal dichalcogenides by molecular beam epitaxy

Abstract

Two-dimensional materials with Janus structures have garnered immense attention owing to their unique properties and promising prospects in electronics and optoelectronics. This thesis delves into the fabrication and comprehensive characterization of various Janus transition metal dichalcogenides, nitrides, and oxides, using molecular beam epitaxy (MBE) combined with plasma.Janus WSeTe Fabrication and Characterization: Utilizing plasma-assistedMBE, we synthesized large-scale single-crystal Janus WSeTe monolayers (ML) on Au(111) at low temperatures. Remarkably, WSeTe ML can revert to WSe2 ML through Se treatment. Advanced characterization methods including annular darkfield scanning transmission electron microscopy (ADF-STEM), electron diffraction, scanning tunneling microscopy and spectroscopy (STM/S), and x-ray photoelectron spectroscopy (XPS) validated the atomic configuration of Janus WSeTe. Its electronic attributes were examined through ultraviolet photoelectron spectroscopy (UPS), photoluminescence spectroscopy (PL), STS, and corroborated by density functional theory (DFT) calculations. Furthermore, piezoresponse force microscopy (PFM) measurements confirmed the vertical piezoelectricity of the WSeTe ML, deriving a coefficient of approximately 0.2pm/V. Such insights pave the way for harnessing the piezoelectric and potential ferroelectric utilities of Janus transitionmetal dichalcogenides (TMDs) MLs.Synthesis and Analysis of Janus MoSeN: We developed a precise and repeatable synthesis method for monolayer Janus MoSeN using nitrogen plasma under ultra-high vacuum (UHV). The structure was corroborated using comprehensive techniques, including low energy electron diffraction (LEED), STEM, XPS, STM, and auger electron spectroscopy (AES). Time-resolved reflection high energy electron diffraction (RHEED) documented the nuanced dynamic transition from MoSe2 to Janus MoSeN. Subsequently, STS and UPS were employed to elucidate its electronic properties. Moreover, PFM measurements highlighted its vertical piezoelectricity,attributed to out-of-plane dipoles. Exploration of Janus MXOs: Using a straightforward method with oxygen plasma, we sought to fabricate Janus MXOs. The emergence of this novel structure was confirmed by both LEED and RHEED. Subsequent XPS results indicated the oxidation of the O-plasma treated MoSe2. PFM analysis revealed pronounced vertical piezoelectricity, credited to vertical dipoles. Additionally, we examinedthe band structures and potential crystal configurations of Janus MXOs (where M represents V/Mo/W, and X stands for Se/Te) MLs, using DFT calculations.