Temperature-dependent orientation of self-organized nanopatterns on ion-irradiated TiO_{2}(110)

Temperature-dependent self-organized formation of nanoripples on TiO2(110) surfaces irradiated by low-energy Ar+ beams has been investigated by scanning tunnelling microscopy in UHV conditions. Under the experimental conditions employed (2 keV Ar+ and oblique incidence, 75∘ off-normal, T = 120, 300, 620, and 720 K) on the irradiated surface the ripple structure of periodicity ∼10 nm has developed. Interestingly, the orientation of the nanopatterns switches reversibly by 90∘ with the systematic change of the substrate temperature during irradiation. We have demonstrated that formation of the surface nanomorphology is determined by the interplay between the erosion of the monatomic step edges at grazing incidence, anisotropic surface diffusion along the favoured crystallographic orientation and, at elevated temperatures, the diffusion into the bulk of the excess Ti ions. As indicated by density functional theory (DFT) calculations used for modelling the diffusion processes on the ion irradiated TiO2(110), the significant surface mass transport required to form the nanoripples is dominated by the highly mobile Ti atoms diffusing in assistance of O adatoms.