Superconducting 2D NbS Grown Epitaxially by Chemical Vapor Deposition.

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS below the transition temperature () of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS, which is imperative for its practical applications in next-generation TMDC-based electrical devices.