Two-dimensional metallic NbS2: growth, optical identification and transport properties

Progress on researches of two-dimensional (2D) metals strongly relies on development of the growth technique. Studies on preparation of 2D metals have so far been limited, and this is in stark contrast to the situation of 2D semiconductors, where various layered semiconductors, including MoS2, WS2, MoSe2, WSe2, have been isolated in its monolayer form. In this work, we have developed a facile method to prepare 2D metallic transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD) method, where direct growth of few-layered NbS2 (3R phase) on atomically flat hexagonal boron nitride (hBN) has been demonstrated. Structural characterization of the so-grown NbS2 was performed with atomic force microscopy, optical microscopy, electron microscopy and optical spectroscopy, revealing that the utilization of hBN as growth substrates is a key factor for the first successful CVD growth of 2D metallic TMDCs with large single-domain size (several mu m). Electrical transport measurements have clearly shown that NbS2 atomic layers down to few-layer-thickness are metal. The current study opens up a new synthetic route for controllable growth of 2D layered metallic materials, which is of great importance in study of rich physics in 2D metals, as well as in search for novel 2D superconductors.