The surface of metal oxide materials is of huge technological relevance in a wide variety of fields. Although the rutile TiO2(110) surface has been extensively studied, the intrinsic difficulties, associated to defects and history of the sample, has lead to a limited knowledge on these materials. This is particularly true for the non-stoichiometric (1x2) reconstruction formed after reducing the surface by annealing up to about 800°C. This reconstruction, although widely studied, presents a large variety of defects, which rends the understanding of its basic properties, still nowadays, unclear. Several models have been proposed to date, but they differ even in the stoichiometry [1,2]. Combining STM, quantitative LEED, and density functional theory, we have determined the atomic surface structure of rutile TiO2(110)-(1x2). STM images show monoatomic steps, wide terraces and no cross-link features. The most suitable model to explain them consists of added Ti2O3 rows along the [001] direction [2]. A more detailed LEED-IV analysis, using a large database of intensities, recorded both normal and off-normal incidences, yields the relaxed surface parameters with a Pendry R-factor Rp = 0.29. Atomic coordinates are in good agreement with the minimum energy structure provided by DFT. Discrepancies can be explained by the large thermal vibration amplitudes found for the Ti2O3 group[2].
Surprisingly, the electronic structure derived from DFT shows a metallic character along the [001] direction. This striking feature is evidenced by parabollic dispersion bands crossing the Fermi level along the [001] direction, whereas in the perpendicular direction bands are rather flat [1]. The long quasi-1D chains display metallic character, do not show any interaction between them, and cannot couple to bulk or surface states in the gap region, forming good one-dimensional atomic wires. STM images show the presence of defect-free atomic chains covering all terraces from one monoatomic step to the next.
[1] M. Blanco-Rey et al. Phys rev. Lett 96, 055502 (2006)
[2] M. Blanco-Rey et al. Phys Rev.B, 75 (2007) 081402(R).
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