Formation and coarsening of Ag(110) bilayer islands on NiAl(110): STM analysis and atomistic lattice-gas modeling

Abstract

Scanning tunneling microscopy analysis of the initial stages of film growth during deposition of Ag on NiAl(110) reveals facile formation of bilayer Ag(110) islands at temperatures of 130 K and above. Annealing subsequent to deposition at 130 K induces coarsening of the bilayer island distribution. The thermodynamic driving force for bilayer island formation reflects a lower relative surface energy for films of even layer thicknesses. This feature derives from quantum size effects due to electron confinement in the Ag film. The kinetics of island formation and relaxation is controlled by terrace and edge-diffusion barriers, detachment barriers, interlayer diffusion barriers, and layer-dependent adsorption and interaction energies. These key energies are determined from density-functional theory analysis and incorporated into an atomistic lattice-gas model for homogeneous island formation, where specification of the adatom hop rates is consistent with detailed balance. Model analysis via kinetic Monte Carlo simulation elucidates the role of strongly anisotropic interactions in development during deposition of elongated island growth shapes and also in facilitating upward mass transport needed for bilayer island formation. The model succeeds in recovering island densities at lower temperatures but experimental densities exceed model predictions at higher temperatures plausibly due to heterogeneous nucleation at surface defects. The same model successfully describes postdeposition coarsening of small islands grown at 130 K. Scanning tunneling microscopy analysis of the initial stages of film growth during deposition of Ag on NiAl͑110͒ reveals facile formation of bilayer Ag͑110͒ islands at temperatures of 130 K and above. Annealing subsequent to deposition at 130 K induces coarsening of the bilayer island distribution. The thermodynamic driving force for bilayer island formation reflects a lower relative surface energy for films of even layer thicknesses. This feature derives from quantum size effects due to electron confinement in the Ag film. The kinetics of island formation and relaxation is controlled by terrace and edge-diffusion barriers, detachment barriers, interlayer diffusion barriers, and layer-dependent adsorption and interaction energies. These key energies are determined from density-functional theory analysis and incorporated into an atomistic lattice-gas model for homogeneous island formation, where specification of the adatom hop rates is consistent with detailed balance. Model analysis via kinetic Monte Carlo simulation elucidates the role of strongly anisotropic interactions in development during deposition of elongated island growth shapes and also in facilitating upward mass transport needed for bilayer island formation. The model succeeds in recovering island densities at lower temperatures but experimental densities exceed model predictions at higher temperatures plausibly due to heterogeneous nucleation at surface defects. The same model successfully describes postdeposition coarsenin...

Self-assembly of metal nanostructures on binary alloy surfaces

“…1 and in ref. 20, Ag islands formed at 140-300 K always display rectangular (bilayer) growth shapes, which become progressively more elongated with increasing T in this entire range, achieving aspect ratios of 10 and higher at 300 K. These elongated structures are growth shapes rather than equilibrium shapes (which have smaller aspect ratios). In contrast, for Ag/Ag(110) homoepitaxy, monolayer rectangular islands become less elongated above 220 K, relaxing towards their equilibrium shapes (9).…”

“…STM studies of the deposition of Au at 200 K and 300 K are now described and contrasted with our previous observations for Ag deposition on NiAl(110) (20)(21)(22). First, consider island heights.…”

“…The first approximate strategy is geared to single-component systems with simpler island shapes (20,21). DFT is used to determine barriers for terrace and straight edge configurations.…”

“…3B. ) LG modeling based on a single Ni-br adsorption site using the first approximate strategy described in the Methods section, with DFT input for key energetics, has elucidated the facile kinetics of Ag(110) bilayer island formation even at low T (20,21). A key factor is that weak adatom interactions in one direction facilitate upward transport.…”

“…For homogeneous nucleation on perfect terraces, traditional mean-field rate equations are often used to describe the dependence of island density, N isl , on temperature T or deposition flux F, but they fail to describe the island size distribution (ISD) (9). Alloy surfaces tend to be imperfect, e.g., due to surface segregation which produces exchange defects which can constitute traps for heterogeneous nucleation of islands (20). Then, N isl can reflect the trap density.…”