A class of nanostructured surface phases on SrTiO 3 (001) is reported and characterized through atomic-resolution scanning tunneling microscopy and Auger electron spectroscopy. These surface phases are created via argon ion sputtering and UHV annealing and form close-packed domains of highly ordered nanostructures. Depending on the type of nanostructures present, the domain ordering exhibit either (6 × 2), (9 × 2), (12 × 2), (6 × 8), or (7 × 4) surface patterning. The nanostructures are composed of TiO 2 -derived complexes surrounded by a TiO 2 surface termination. Such surface ordering phenomena introduce another level of complexity in the chemistry of perovskite oxide surfaces and provide a basis from which potential photocatalytic and molecularordering applications may be developed.

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Scanning tunneling microscopy ͑STM͒ was used to produce atomic resolution images of the Nb-doped SrTiO 3 ͑001͒-͑ ͱ 5 ϫ ͱ 5͒-R26.6°reconstruction and its precursor surfaces. Low-energy electron diffraction was used to identify the surface structure in combination with the STM imaging. The ͑ ͱ 5 ϫ ͱ 5͒-R26.6°reconstruction was observed growing as an adlayer on top of the underlying terraces. The formation of this structure could routinely be detected by an increase in the crystal's electrical conductivity during ultrahigh vacuum ͑UHV͒ annealing at 1300°C. This rise in conductivity occurs due to significant reduction of the crystal. Scanning electron microscopy ͑SEM͒ on these samples showed small islands with a rectangular base separated by flat regions. Auger electron spectroscopy showed titanium enrichment, and strontium and oxygen depletion of the surface region. The islands seen in the SEM images are TiO crystals, as reported by S. B. Lee et al. ͓Ultramicroscopy 104, 30 ͑2005͔͒. The flat regions between the islands are composed of a Sr adlayer on the surface that is consistent with the ͑ ͱ 5 ϫ ͱ 5͒-R26.6°adatom model of Kubo and Nazoye ͓Phys. Rev. Lett. 86, 1801 ͑2001͔͒. We propose that the TiO islands and the ͑ ͱ 5 ϫ ͱ 5͒-R26.6°Sr adlayer reconstruction form due to phase separation in the surface region of the crystal. This phase separation is driven by oxygen depletion which occurs during high-temperature UHV annealing of the sample.

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