  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.
