| AVS 56th International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS1-TuM |
| Session: | Oxide Surfaces: Reactions and Structure |
| Presenter: | W. Yin, University of Virginia |
| Authors: | W. Yin, University of Virginia P. Reinke, University of Virginia S. Wolf, University of Virginia C. Ko, Harvard University S. Ramanathan, Harvard University |
| Correspondent: | Click to Email |
Vanadium dioxide (VO2) is one of the most interesting thermochromic materials exhibiting a metal-insulator transition (MIT) close to room temperature (TMI ≈ 340 K). This study presents an observation of the progression of the MIT with scanning tunneling microscopy and spectroscopy as a function of temperature with high spatial resolution. The high-quality VO2 thin films used in this study were deposited on (0001) Al2O3 substrates using a sputtering technique. Electron tunneling current (conductivity) maps showed that the material surface was semiconducting before heating, and highly metallic above TMI. The spatial distribution of bandgaps, semiconducting and metallic regions, is compared to the sample topography, and a model describing the progression of the MIT across the surface will be presented. In particular, the role of the chemical modification and reduction of the surface during heating will be discussed, and compared to the stability of the bulk phase. The surface was still partially metallic after cooling down but tended to recover its semiconducting nature over a long period of time. We attribute such irreversible surface electrical changes to the loss of oxygen and the strain relaxation in the VO2 lattice. The spatial distribution of bandgaps indicates that the phase transition probably started from grain boundaries and the VO2 surface exhibited an inhomogeneous behavior both above TMI and after heat treatment.