Nucleation and growth of Ag islands on fivefold Al-Pd-Mn quasicrystal surfaces: Dependence of island density on temperature and flux

Abstract

Scanning tunneling microscopy (STM) has been used to investigate the nucleation and growth of Ag islands on the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal. Analysis of the data as a function of deposition temperature, from 127 K to 300 K, reveals that island density is constant, while at higher temperature it decreases. To model this behavior, we first show that the potential energy surface describing bonding of Ag at various locations on the surface is complex, with a few sites acting as traps for clusters of adatoms. We then develop a rate equation model which incorporates enhanced nucleation at trap sites relative to nucleation at regular sites on terraces. It recovers the temperature dependence of the island density, plus previous flux-scaling data. Our model suggests that the critical size for both types of nucleation sites is large-corresponding to stable clusters of at least 6 Ag atoms-and that binding between atoms at trap sites is significantly stronger than at free terrace sites. The data and the model, combined, provide guidance about the conditions of temperature and flux under which saturation of trap sites can be expected. This, in turn, provides a general indicator of the conditions that may favor localized pseudomorphic growth at low coverage, here and in other systems. Scanning tunneling microscopy ͑STM͒ has been used to investigate the nucleation and growth of Ag islands on the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal. Analysis of the data as a function of deposition temperature, from 127 K to 300 K, reveals that island density is constant, while at higher temperature it decreases. To model this behavior, we first show that the potential energy surface describing bonding of Ag at various locations on the surface is complex, with a few sites acting as traps for clusters of adatoms. We then develop a rate equation model which incorporates enhanced nucleation at trap sites relative to nucleation at regular sites on terraces. It recovers the temperature dependence of the island density, plus previous fluxscaling data. Our model suggests that the critical size for both types of nucleation sites is large-corresponding to stable clusters of at least 6 Ag atoms-and that binding between atoms at trap sites is significantly stronger than at free terrace sites. The data and the model, combined, provide guidance about the conditions of temperature and flux under which saturation of trap sites can be expected. This, in turn, provides a general indicator of the conditions that may favor localized pseudomorphic growth at low coverage, here and in other systems.

Direct observation of the basic mechanisms of Pd island nucleation on Au(111)

“…2 shows how the potential nucleation sites, both regular elbows and defects, act like traps which are progressively filled until the island density n (number of islands per unit area) reaches the maximum value n max which represents the density of nucleation traps. This behaviour indicates also that the incident flux is low enough in order not to compromise the ordered growth regime, as shown in different studies regarding dynamics of growth controlled by the presence of traps both by atomistic Monte Carlo simulations [21,22] and by STM of clusters on Au(111) vicinal surfaces [22] and quasicrystals [23]. Fig.…”

Comparison between experimental surface data and bulk structure models for quasicrystalline AlPdMn: Average atomic densities and chemical compositions

“…Cut clusters have been associated with certain local features in STM images of the fivefold surfaces, the so-called dark stars, which are important adsorption sites. 5,56,57,[59][60][61] The two main families differ in the way that the top planes cut bulk clusters in the models. The Pd− family cuts Mackay clusters 0.252 nm above or 0.204 nm below the equator, thereby producing features that are strong candidates for the dark star sites.…”

Templated Quasicrystalline Molecular Ordering

“…For Pb/A-lPb-Mn, 6 Si/Al-Pd-Mn 8 and Bi/Al-Pd-Mn, 5 the network depends on Mn atoms (at the centers of 'white flowers') in the surface layer. For C 60 /Al-Pd-Mn and Ag/Al-Pd-Mn, 28 those molecules in ordered sites decorate the dark stars in the substrate, which occur atop Mn atoms in the subsurface layer. 20 For Al/AlCu-Fe, the network is argued to be analogous to the dark star network in isostructural Al-Pd-Mn, based on subsurface Mn atoms, which leads to the conclusion that, in AlCu-Fe, it is based on Fe atoms.…”