Large-size niobium disulfide nanoflakes down to bilayers grown by sulfurization

Atomically thin layers of group VB transition metal dichalcogenides (TMDs) provide a unique platform for studying two-dimensional (2D) superconductivity and charge density waves. Thus far, the bottom-up synthesis of these 2D TMDs has often involved precursors that are corrosive or toxic, and their lateral sizes are typically only a few micrometers. In this paper, we report the growth of NbS2 nanoflakes with a thickness down to bilayers and a lateral dimension up to tens of micrometers without using harsh chemical species. NbS2 nanoflakes either standing or lying with respect to the sapphire substrate were obtained by sulfurization of niobium oxide films that were prepared via pulsed laser deposition. Standing nanoflakes are considered to grow epitaxially on the sapphire substrate according to their ordered orientation, while lying nanoflakes with random orientations were grown directly on top of the niobium oxide films. The Raman spectra of the 3R-phase exhibit strong dependence on the layer thickness, where the A(1) mode softens as the layer number decreases. In contrast to the stable bulk NbS2, the ultra-thin nanoflakes were oxidized on their top surfaces after prolonged exposure to air, as revealed by X-ray photoelectron spectroscopy. Our work explores an important route to synthesize large-size NbS2 nanoflakes and studies the oxidation process, which is a critical factor to consider if practical applications should be realized in the future.