Abstract
Vanadium dioxide undergoes a metal-insulator transition (MIT) from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g. as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many of its applications. This work presents an atomic-scale investigation of the tetragonal rutile VO$_2$(110)$_\text{T}$ single-crystal surface and reports results obtained with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS), supported by density-functional theory-based (DFT) calculations. The surface reconstructs into an oxygen-rich (2$\times$2) superstructure that coexists with small patches of the underlying, unreconstructed (110)-(1$\times$1) surface. The best structural model for the (2$\times$2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials this reconstruction is more stable than the unreconstructed (110) surface as well as models proposed in the literature.
Highlights
Vanadium(IV) dioxide, VO2, undergoes a first-order metalto-insulator transition (MIT) at a temperature TC of ∼67 ◦C, where the lattice changes from the insulating monoclinic structure M1 to the metallic, tetragonal structure above TC [1]
We present an atomic-scale investigation of the tetragonal rutile VO2(110)T single-crystal surface and report results obtained with scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy, supported by density-functional-theory-based calculations
The single-layer step height of VO2(110)T, ∼320 pm, was used to calibrate the z distances. (This was necessary, as the calibration of the piezo characteristics is usually done at room temperature and can be different at the elevated temperatures used in these scanning tunneling microscopy (STM) measurements)
Summary
Vanadium(IV) dioxide, VO2, undergoes a first-order metalto-insulator transition (MIT) at a temperature TC of ∼67 ◦C, where the lattice changes from the insulating monoclinic structure M1 (distorted rutile) to the metallic, tetragonal (rutile) structure above TC [1]. An intermediate strain- or doping-induced monoclinic phase M2 has been observed during the phase transition, together with a metastable triclinic phase occurring between M1 and M2 [2,3]. VO2 is technologically interesting, as its MIT occurs near room temperature.
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