Controlling island size distributions: a comparison of nickel and copper growth on TiO2ð110Þ

Scanning tunneling microscopy studies of Ni islands deposited on TiO2(1 1 0) show that the islands grow threedimensionally at room temperature with an average height of approximately 6.5 A and diameter of 30 A. The broadness of the island size distributions can be controlled by the diffusion (D) to deposition flux (F ) ratio, with the narrowest distribution corresponding to the smallest D=F ratio. Decreasing the flux or increasing the diffusion rate by increasing the deposition temperature results in a lower island density, larger islands and a broader size distribution. Although the Ni–TiO2 interfacial energy should be greater than that of Cu–TiO2, the Ni islands do not grow flatter on the surface compared to Cu. The higher surface free energy for Ni relative to Cu apparently dominates over the greater interfacial energy for Ni, resulting in Cu and Ni islands with the same aspect ratios. The island densities, average sizes and spatial distributions are roughly the same for both Cu and Ni growth, and we find no evidence that Ni diffusion is slower on TiO2 compared to Cu diffusion. A major difference between Ni and Cu growth is in island sintering at elevated temperatures. Large Cu islands (67 A diameter, 22 A height) are produced after deposition at room temperature and annealing to 800 K, whereas the Ni islands undergo much slower growth under the same conditions (34 A diameter, 6.6 A height). This effect is explained by the stronger metal–metal bond strength for Ni compared to Cu since adatom detachment is the rate limiting step in island sintering. 2003 Elsevier Science B.V. All rights reserved.