Heterostructures of tellurium on NbSe2 from sub-monolayer to few-layer films

Abstract

As a single-elemental system, tellurium can exist stably in the form of layers with an intriguing multivalence character, which constructs a new member of the 2D family. However, the growth and electronic structure of tellurium films are still far from known at present. Here, combined with molecular beam epitaxy, scanning tunneling microscopy/spectroscopy measurements and density functional theory calculations, we report the geometric and electronic structures of tellurium grown on NbSe₂ from sub-monolayer to few-layer films. At the sub-monolayer coverage, we obtain two types of adatom-induced ordered superstructures that are strongly coupled with NbSe₂. With the increase in coverage, the few-layer tellurium films adopt the α-phase form, showing internal strain-induced ripple patterns in the few-layers and bulk-like in thick layers with distinct edge geometries. The band gap of α-tellurium films decreases with the increase in thickness, which is associated with notable in-gap states. These observations, corroborated with DFT calculations, emphasize the important role of the NbSe₂ substrate in modulating the structural and electronic properties of tellurium films. Moreover, the interaction between tellurium adatoms and tellurium films leads to √2 × √2 surface reconstruction prior to a new monolayer, conforming to our theoretical calculations. Our work clarifies the kinetic growth of tellurium films on NbSe₂ and reveals the tunability of electronic properties via substrate modulation or surface decoration.