ABSTRACT
Dimensional confinement has shown to be an effective strategy to tune competing degrees of freedom in complex oxides. Here, we achieved atomic layered growth of trigonal vanadium sesquioxide (V2O3) by means of oxygen-assisted molecular beam epitaxy. This led to a series of high-quality epitaxial ultrathin V2O3 films down to unit cell thickness, enabling the study of the intrinsic electron correlations upon confinement. By electrical and optical measurements, we demonstrate a dimensional confinement-induced metal-insulator transition in these ultrathin films. We shed light on the Mott-Hubbard nature of this transition, revealing a vanishing quasiparticle weight as demonstrated by photoemission spectroscopy. Furthermore, we prove that dimensional confinement acts as an effective out-of-plane stress. This highlights the structural component of correlated oxides in a confined architecture, while opening an avenue to control both in-plane and out-of-plane lattice components by epitaxial strain and confinement, respectively.
ABSTRACT
Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing. The structural properties, surface morphology, and chemical composition of the films have been systematically investigated by in situ reflection high-energy electron diffraction during the growth and by ex situ techniques, such as atomic force microscopy, scanning tunneling microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). Crystalline monolayer vanadium oxide can be achieved on HOPG by systematically tuning the deposition time of V atoms and by subsequent annealing at 450 °C in controlled atmospheres. Increasing the partial pressure of O2 during the deposition seems to decrease the mobility of V atoms on the graphitic surface of HOPG and promote the formation of a two-dimensional (2D) vanadium oxide. The obtained oxide layers are found to be polycrystalline with an average grain size of 15 nm and to have a mixed-valence state with mainly V5+ and V4+. Moreover, XPS valence band measurements indicate that the vanadium oxide is insulating. These results demonstrate that a 2D insulating vanadium oxide can be grown directly on HOPG and suggest vanadium oxide as a promising candidate for graphene/oxide heterostructures.
