MODEL FOR THE GROWTH AND REACTIVITY OF METAL-FILMS ON OXIDE SURFACES - CU ON ZNO(0001)-O

In the many studies of vapor deposition of late transition metals on single-crystal oxide surfaces, never has a low-energy electron diffraction superstructure been observed in the first layer. Although layer-by-layer behavior is often reported at less-than-or-equal-to 300 K, annealing the films generally leads to irreversible clustering into thick islands that cover a tiny part of the oxide. We discuss here a growth model that explains these general observations and is also consistent with the expected energetics for metal atom diffusion over oxide and metal terraces and up and down over the edges of metal islands. We briefly review our recent studies which led to this model, in which the structural, electronic, and chemical properties of Cu films on the oxygen-terminated ZnO(0001BAR)-O surface were examined. At coverages beyond a few percent, vapor-deposited Cu adatoms become nearly neutral. Their chemical behavior indicates that they form two-dimensional (2D) islands with metal-like Cu-Cu distances, much shorter than the substrate lattice parameter. Further deposition of Cu leads to spreading of these 2D islands without forming thicker layers, until about 55% of the surface is covered. Thereafter, these Cu islands grow thicker without filling the voids between islands, except at a rate much slower than the rate at which Cu vapor strikes in these voids. Our new model shows that a large fraction of an oxide surface will first be covered by 2D islands during metal deposition at low temperatures due to a kinetic effect. This occurs even when the metal's adsorption energy on itself significantly exceeds its adsorption energy on the oxide (i.e., when 3D clustering is thermodynamically favored), provided the difference in these energies does not exceed the energy of 2D evaporation from island edges onto terraces. As soon as islands nucleate in the second layer, 2D spreading ceases and energetic pathways open that allow metal atoms to migrate up from the oxide terrace into this new layer. This can lead to thick, flat-topped islands. Atom-thin (2D) Cu islands on ZnO(0001BAR)-O react with CO, O2, H2O, HCOOH, and CH3OH very much like Cu(110), although there are some differences: The transiently adsorbed H and CO2 products of HCOOH dissociation migrate off or under the Cu islands and interact strongly with ZnO sites, and O(a) is not as good a Bronsted base as on Cu(110). Annealed 3D islands behave much like Cu(111). We present here new results for methyl iodide adsorption and methyl dissociation on these Cu islands which further support this general picture.