The effect of oxygen on the growth and relaxation of Ag thin films and associated nanostructures deposited on Ag(100)

We report the effects of oxygen on the nucleation, growth and relaxation of Ag nanostructures on Ag(100). Comparisons with previous observations on the clean, oxygen-free Ag(100) surface, provide an indirect method to determine how oxygen affects the atomic-scale diffusional processes on the surface. Experiments were performed in UHV using High-Resolution Low Energy Electron Diffraction and Variable Temperature Scanning Tunneling Microscopy. Oxygen exposures were performed prior-to, during, and after the deposition of Ag at temperatures usually ≤250 K.;Experimental data show that the mechanism for submonolayer-island coarsening changes from island diffusion/coalescence to Ostwald ripening after exposure to oxygen. The change in mechanism is manifest in an increase in the rate for island coarsening. Exposure to oxygen also enhances the rate for smoothing of mounded multilayer films. Decay analysis of multilayer island stacks reveals that oxygen adatoms (Oad) are mobile on the Ag terraces, and are able to freely attach/detach from terrace steps. It is speculated that O ad aids in the detachment of AgnO (n = 1,2) from the surface steps, driving the coarsening process. After extended island evolution, Ag islands undergo a change in size and shape, with island and terrace steps adopting the less favorable (open) [100] orientation. The change in surface step geometry is a result of oxygen-induced formation and stabilization of kinks at the surface steps.;We also show that the presence of Oad prior to the deposition of Ag results in the decrease in the initial island density of surface islands. This implies that mobile Oad and/or AgnO interferes with Ag deposition. It is also possible that incorporation of oxygen destabilizes small Ag clusters, causing them to break apart, or makes the entire cluster itself mobile. Further analysis through multiple-step deposition experiments verifies the existence of mobile O-species. The change in island density demonstrates that the pre-exposure of the Ag surface to oxygen provides an interesting means to control the formation of Ag nanostructures during the deposition process

Additive-enhanced coarsening and smoothening of metal films: Complex mass-flow dynamics underlying nanostructure evolution

Exposure of Ag/Ag(100) thin films to molecular oxygen (O2) at 220–250 K is shown to activate low-temperature coarsening of submonolayer island distributions, and a smoothing of multilayer films with “mounded” morphologies. Dissociation of O2 at kink sites populates step edges with atomic oxygen (O), modifying the step-edge energetics, and facilitating Ostwald ripening of film nanostructures. We propose that ripening occurs by “easy” detachment and terrace diffusion of an AgnO species. Cluster diffusion does not play a significant role, contrasting with the O-free system

Development and ordering of mounds during metal(100) homoepitaxy

Scanning-tunneling microscopy studies combined with atomistic modeling for Ag/Ag(100) homoepitaxy reveal complex growth behavior at 300 K: initial smooth growth up to ∼25 ML, where three-dimensional (3D) mounds develop from 2D islands; then an extended regime of mound steepening for ∼1000 ML producing unexpected rough growth; and finally an asymptotic regime with cooperative mound ordering and coalescence dynamics quite distinct from that in systems with up-down symmetry. The steepening regime is compressed upon lowering temperature, so while initial growth is rougher, asymptotic growth is actually smoother

Morphology of multilayer Ag/Ag(100) films versus deposition temperature: STM analysis and atomistic lattice-gas modeling

Scanning tunneling microscopy is used to analyze the nanoscale morphology of 25 ML films of Ag deposited on Ag(100) at temperatures (T) between 55 and 300 K. A transition from self-affine growth to “mound formation” occurs as T increases above about 140 K. The roughness decreases with increasing T up until 140 K in the self-affine growth regime, and then increases until about 210 K before decreasing again in the mounding regime. We analyze mounding behavior via a lattice-gas model incorporating: downward funneling of depositing atoms from step edges to lower fourfold hollow adsorption sites; terrace diffusion of adatoms with a barrier of 0.40 eV leading to irreversible island formation in each layer; efficient transport of adatoms along island edges to kink sites; and downward thermal transport of adatoms inhibited by a step-edge barrier of 0.06–0.07 eV along close-packed step edges (but with no barrier along kinked or open steps). This model reasonably recovers the T-dependence of not just the roughness, but also of the mound slopes and lateral dimensions above 190 K. To accurately describe lateral dimensions, an appropriate treatment of the intralayer merging of growing islands is shown to be critical. To describe behavior below 190 K, one must account for inhibited rounding of kinks by adatoms at island edges, as this controls island shapes, and thus the extent of open steps and of easy downward transport. Elsewhere, we describe the low-T regime of self-affine growth (with no terrace diffusion) accounting for a breakdown of the simple downward funneling picture