  We investigated the inhomogeneous electronic properties at the surface and
interior of VO_{2} thin films that exhibit a strong first-order metal-insulator
transition (MIT). Using the crystal structural change that accompanies a VO_{2}
MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate
the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature
dependence of the p$^{XRD}$ was very closely correlated with the dc
conductivity near the MIT temperature, and fit the percolation theory
predictions quite well: $\sigma$ $\sim$ (p - p_{c})^{t} with t = 2.0$\pm$0.1
and p_{c} = 0.16$\pm$0.01. This agreement demonstrates that in our VO$_{2}$
thin film, the MIT should occur during the percolation process. We also used
surface-sensitive scanning tunneling spectroscopy (STS) to investigate the
microscopic evolution of the MIT near the surface. Similar to the XRD results,
STS maps revealed a systematic decrease in the metallic phase as temperature
decreased. However, this rate of change was much slower than the rate observed
with XRD, indicating that the electronic inhomogeneity near the surface differs
greatly from that inside the film. We investigated several possible origins of
this discrepancy, and postulated that the variety in the strain states near the
surface plays an important role in the broad MIT observed using STS. We also
explored the possible involvement of such strain effects in other correlated
electron oxide systems with strong electron-lattice interactions.
