INTRINSIC DEFECTS ON A TIO2(110)(1X1) SURFACE AND THEIR REACTION WITHOXYGEN - A SCANNING-TUNNELING-MICROSCOPY STUDY

We report a scanning tunneling microscopy (STM) study of the rutile Ti O2(110) surface. The surface was prepared by sputtering and annealing in an ultrahigh vacuum (UHV), After annealing to 1100 K in UHV, a (1 x 1) surface with a terrace width of similar to 100 Angstrom is obtaine d. The terraces are separated by monoatomic step edges running predomi nantly parallel to [001] and [1 (1) over bar 1] type directions. Appro ximately half of the [001]-type steps have a kinked appearance that is attributed to a (4 x 1)-reconstructed step edge. Atomic models for au tocompensated step edges are presented. Oxygen vacancies (point defect s) in the bridging oxygen rows are created by the high-temperature ann eal in UHV. In STM images, these oxygen vacancies appear as bright fea tures centered on dark rows. Their density is 7 +/- 3% per surface uni t cell and is reduced upon exposure to molecular oxygen at room temper ature. Dark features on bright rows are also seen; these are not affec ted by molecular oxygen and are tentatively assigned to subsurface def ects. Hydroxyl groups from spurious water in the oxygen gas stream are observed to adsorb dissociatively at step edges and on the in-plane T i rows on the terraces. The appearance of the surface oxygen vacancies depends on the state of the STM tip; asymmetric tips skew the appeara nce of the point defects and may even render images where they are inv isible. Tip changes occur frequently, especially when the surface has been exposed to oxygen, and may lead to images that are hard to interp ret. The ''normal'' tip state where the vacancies appear as bright spo ts connecting bright rows can be regained reproducibly by scanning wit h a high (up to +10V) tip voltage; the tip is then possibly covered wi th substrate material. The oxygen vacancies show strong interactions w ith the STM tip, i.e. tip-induced oxidation and mobility. These intera ctions depend strongly on the state of the tip, and are enhanced by th e presence of oxygen in the ambient. To explain these effects a model for the tip-induced oxidation is presented where oxygen atoms hop betw een the tip and sample. (C) 1998 Elsevier Science B.V. All rights rese rved.