Enhancing stability by tuning element ratio in 2D transition metal chalcogenides
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onal Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China 2 SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China 3 International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 3050044, Japan § Zhenjia Zhou and Tao Xu contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 7 June 2020 / Revised: 30 July 2020 / Accepted: 4 Augutst 2020
ABSTRACT Two-dimensional (2D) transition metal chalcogenides (TMCs) are known to be susceptible to the atmosphere, which greatly obscures the intrinsic physical and chemical properties. The quantitative origin of the instability on the atomic scale has not been well investigated due to the lack of environmentally stable TMCs sample. Here, we find the stability of the grown TMCs is strongly relevant to their initial element ratios, and thus the stoichiometric bonded TMCs have favorable stability, benefitted from the TMCs with controllable chalcogenisation. In this study, the degree of structural degradation has been quantitatively defined by the reduced element ratio of chalcogen to metal through the time-dependent characterizations, and the non-stoichiometric ratios in TMCs reveal the atomic lattices with point defects like additive bonds or vacancies inside. This study not only provides a potential view to fabricate environmentally stable TMCs based devices, but also will bring an effective feasibility of stacking stable vertical heterostructures.
KEYWORDS transition metal chalcogenides, stability, point defects, stoichiometric ratio, two-step vapor deposition
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Introduction
Two-dimensional (2D) materials and their programmable heterostructures have attracted much more attentions due to their vagaries of physical properties [1–3]. The 2D materials of single component in these heterostructures can be graphene [4], hexagonal boron nitride (h-BN) [5], transition metal chalcogenides (TMCs, including dichalcogenides and monochalcogenides) [1–3], transition metal halides [6], black phosphorus [7–9], and other kinds of materials. Among the reported 2D materials, TMCs and transition metal halides have abundant properties, such as semiconducting [10–13], superconducting [14–17], ferromagnetic [18], ferroelectric [6], electrocatalytic [19], etc. Similar to graphene, most TMCs and transition metal halides can also be prepared through either bottom-up or top-down routes [20–24]. Under the premise of retaining their intrinsic physical properties, micromechanical exfoliation of bulk layered materials, molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) are main methods to prepare TMCs and transition metal halides until now [1, 23, 25, 26]. Although kinds of TMCs, like MoS2, NbSe2, etc., can be obtained by above methods, most of the
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