Self-limiting growth of copper islands on TiO2(110)-(1×1)

We have studied the growth of Cu islands on rutile TiO2(110)-(1×1) using scanning tunneling microscopy for Cu coverages of 0.03–1.25 monolayers (ML). The formation of three-dimensional islands at all the coverages reflects the relatively high mobility of Cu atoms on TiO2 at room temperature (diffusion constant ≥4×10−10 cm2/s) and the weak interaction between Cu and TiO2 compared with CuCu bonding. At low coverages (<0.5 ML), the Cu islands exhibit self-limiting growth: with increasing coverage, the average island size remains almost constant, and the island density increases. At higher coverages (>0.5 ML), the average island size scales with coverage, but this increase in island size is primarily due to an increase in height, not diameter. Although larger islands can be formed by annealing, the average size of the islands is nearly independent of coverage for any given annealing temperature. We propose two general schemes that can simultaneously explain the increase in island density with increasing coverage and the narrow island size distribution. In the first scheme, the rate at which adatoms attach to existing islands drops rapidly as the island size increases. This could be due to the existence of strain fields that accommodate the lattice mismatch at the interface between the Cu islands and the TiO2 substrate. Adatoms rejected by the islands are then available for nucleation of new islands. In the second scheme, the rate at which adatoms reach existing islands drops rapidly as the Cu coverage increases. This could be related to continued nucleation of islands at defects in the TiO2 surface.