2D Vanadium Sulfides: Synthesis, Atomic Structure Engineering, and Charge Density Waves.

Abstract

Two ultimately thin vanadium-rich 2D materials based on VS₂ are created via molecular beam epitaxy and investigated using scanning tunneling microscopy, X-ray photoemission spectroscopy, and density functional theory (DFT) calculations. The controlled synthesis of stoichiometric single-layer VS₂ or either of the two vanadium-rich materials is achieved by varying the sample coverage and sulfur pressure during annealing. Through annealing of small stoichiometric single-layer VS₂ islands without S pressure, S-vacancies spontaneously order in 1D arrays, giving rise to patterned adsorption. Via the comparison of DFT calculations with scanning tunneling microscopy data, the atomic structure of the S-depleted phase, with a stoichiometry of V₄S₇, is determined. By depositing larger amounts of vanadium and sulfur, which are subsequently annealed in a S-rich atmosphere, self-intercalated ultimately thin V₅S₈-derived layers are obtained, which host 2 × 2 V-layers between sheets of VS₂. We provide atomic models for the thinnest V₅S₈-derived structures. Finally, we use scanning tunneling spectroscopy to investigate the charge density wave observed in the 2D V₅S₈-derived islands.