Interplay between Steps and Oxygen Vacancies on Curved TiO₂(110)

Abstract

A vicinal rutile TiO2(110) crystal with a smooth variation of atomic steps parallel to the [1 -1 0] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1 -1 1] zigzag faceting to straight [1 -1 0] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (Obr vacs) vanish. The [1 -1 0] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (St) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of Obr vacs plus St protrusions at steps, suggesting that both Obr vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti3+ polaron near steps and at terraces.