Superconducting\n2D NbS₂ Grown Epitaxially\nby Chemical Vapor Deposition

Abstract

Metallic two-dimensional\n(2D) transition metal dichalcogenides\n(TMDCs) are attracting great attention because of their interesting\nlow-temperature properties such as superconductivity, magnetism, and\ncharge density waves (CDW). However, further studies and practical\napplications are being slowed down by difficulties in synthesizing\nhigh-quality materials with a large grain size and well-determined\nthickness. In this work, we demonstrate epitaxial chemical vapor deposition\n(CVD) growth of 2D NbS₂ crystals on a sapphire substrate,\nwith a thickness-dependent structural phase transition. NbS₂ crystals are epitaxially aligned by the underlying c-plane sapphire\nresulting in high-quality growth. The thickness of NbS₂ is well controlled by growth parameters to be between 1.5 and 10\nnm with a large grain size of up to 500 μm. As the thickness\nincreases, we observe in our NbS₂ a transition from a metallic\n3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy,\naberration-corrected scanning transmission electron microscopy (STEM)\nand electrical transport measurements. A Berezinskii–Kosterlitz–Thouless\n(BKT) superconducting transition occurs in the CVD-grown 2H-phase\nNbS₂ below the transition temperature (<i>T</i>_c) of 3 K. Our work demonstrates thickness and phase-controllable\nsynthesis of high-quality superconducting 2D NbS₂, which\nis imperative for its practical applications in next-generation TMDC-based\nelectrical devices.