Epitaxial Growth of Single‐Layer Niobium Selenides with Controlled Stoichiometric Phases

Abstract

AbstractThe phase diagram of 2D monolayer material often differs from the bulk crystal due to electronic instability and interactions with the substrate. At the monolayer level, stoichiometric phases can transform into substoichiometric crystalline phases due to changes in chemical potential, however conditions leading to this are still relatively unexplored. Here, the growth of two distinct phases of niobium selenide by tuning the chemical potential during molecular beam epitaxy (MBE) is demonstrated. NbSe2 monolayers are grown on Au(111) under Se‐rich (low Nb:Se flux ratio) condition, whereas a new phase of niobium selenide (NbSex) monolayers is formed under Nb‐rich (high Nb:Se flux ratio) condition. Scanning tunnelling microscopy and spectroscopy are employed to investigate the geometric and electronic structures of the as‐grown NbSe2 and NbSex phases. The NbSex phase is identified as Nb9Se8 through experimental and theoretical investigation; its distorted single‐layer structure indicates a dimensionality induced reconstruction compared with its bulk counterpart (Nb3Se4). This work shows that MBE affords a high degree of control in growing complex chemical phases, which should be applicable to other metallic transition metal dichalcogenides system.