Growth of Co clusters on thin films Al2O3∕NiAl(100)

Abstract

We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al(2)O(3) thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570 K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6 nm and a height of 0.7 nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3-4-nm-wide crystalline Al(2)O(3) strips where the deposited Co atoms are confined, the same features sustain. Only at 570 K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5 nm and height of 1.4 nm) appears for growth at 450 K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300 K and annealed to 750 K, in which the coalescence is eliminated and the two preferable geometries appear and coexist.

Growth and electronic properties of Au nanoclusters on thin-film Al2O3/NiAl(100) studied by scanning tunnelling microscopy and photoelectron spectroscopy with synchrotron radiation

“…1a and b, with average diameters 0.8 and 3.1 nm and heights 0.25 and 0.9 nm, respectively. Preferential sizes such as for Co clusters [11] are not observed for the Au clusters. The size of Au clusters notably increases with increasing coverage, as plotted in Fig.…”

“…For this effect to be the cause, the surface has to be uniformly defective, because preferential nucleation on particular sites is not seen for both smaller and larger clusters; the clusters of both kinds can be anywhere on the surface. Whereas, evident defects are not anywhere but on the protrusions, indicated by the growth study of Co clusters [10,11]. Another speculation is diffusion of Au to the substrate at elevated sample temperatures.…”

“…Post-oxidation annealing of the oxide thin-films alters length of the stripes and coverage of the crystalline Al 2 O 3 [10]. Most 3D sizable metal nanoclusters grow on the crystalline Al 2 O 3 surface, and the protrusion stripes act as nucleation centres to guide cluster alignment [10,11]. Fig.…”

“…Fig. 1a and b show typical STM images for small (0.02 ML) and large (2.9 ML) coverages of Au deposited on ordered thin-film Al 2 O 3 /NiAl(1 0 0) at 300 K. As the oxide was prepared with a brief post-oxidation annealing (10 min), the amorphous area on the surface was rather limited [10][11][12]. The images show that most of the Au clusters are randomly distributed, independent of the extent of coverage; only a few are aligned into straight lines, as seen in Fig.…”

“…Their size-dependent electronic properties are characterised and discussed. We chose this substrate as it has peculiar surface structures to influence or even to pattern the grown nanoclusters, as already demonstrated for Co and Pt nanoclusters [9][10][11][12].…”