The metal-insulator transition in V2O3(0001) thin films:: surface termination effects

Epitaxially grown V2O3(0001) thin films have been prepared with different surface terminations, as evidenced by atomically resolved scanning tunnelling microscopy and high-resolution electron energy loss spectroscopy (HREELS) phonon spectra. The spectral changes observed in valence band photoemission spectra and HREELS on cooling the V2O3 samples from 300 to 100 K have been associated with the metal-insulator transition (MIT) in the bulk of the V2O3 film. The reconstructed surface regions per se do not display the MIT, but affect the MIT signature observed with surface sensitive techniques, depending on the thickness of the reconstructions. Whereas the thermodynamically stable (I x 1) vanadyl V=O surface termination allows the observation in photoemission and HREELS of a clear signature of the MIT, the latter is screened on a (root 3 x root 3)R30 degrees surface formed by V=O defect structures. Doping of the (root 3 x root 3)R30 degrees surface with small amounts of adsorbed water restores reversibly the MIT spectral fingerprints. These observations are discussed in terms of the different geometrical and electronic structures of the different surface terminations.