Deposition of metals on binary alloy surfaces offers new possibilities for guiding the formation of functional metal nanostructures. This idea is explored with scanning tunneling microscopy studies and atomistic-level analysis and modeling of nonequilibrium island formation. For Au/NiAl(110), complex monolayer structures are found and compared with the simple fcc(110) bilayer structure recently observed for Ag/NiAl(110). We also consider a more complex codeposition system, ðNi þ AlÞ∕NiAlð110Þ, which offers the opportunity for fundamental studies of self-growth of alloys including deviations for equilibrium ordering. A general multisite lattice-gas model framework enables analysis of structure selection and morphological evolution in these systems.S elf-assembly involves the autonomous organization of components into structures (1). This process requires mobility of aggregating components, and usually occurs on smooth surfaces or in fluids. Some degree of relaxation in the aggregated state is typically also operative. Self-assembly can be manifested in either complex equilibrium structures, e.g., reflecting competing interactions, or in far-from-equilibrium growth structures (1). The latter can be very different and more diverse than the equilibrium forms (2). Significantly, self-assembly provides a practical strategy for creating ensembles of nanostructures with unique size and shape dependent properties, a central goal of nanotechnology.A broad range of systems self-assemble, spanning hard and soft matter, with a wide variety of interactions and component sizes. We consider the formation of metal nanostructures motivated by applications ranging from catalysis to plasmonics (3-5). Our specific focus is vapor deposition of metal atoms on single-crystal metal surfaces under the well controlled conditions of ultrahigh vacuum (UHV). This process leads to the self-assembly of metal nanostructures and growth of epitaxial metal films. Here, the nonaggregated components are rapidly diffusing adsorbed atoms (adatoms) which assemble into islands. Relaxation in the aggregated state can be achieved via diffusion of adatoms along island edges or via detachment-reattachment. Understanding these processes on the atomic scale facilitates guided formation of functional nanostructures with tailored morphologies and (for alloys) compositions. Ideally, control over formation allows tuning of desired properties, e.g., for heterogeneous catalysis (6).Despite the complexity of self-assembly processes, significant advances are being made in the development of predictive models even in soft material systems (7). Our focus on epitaxial growth on perfect single-crystal surfaces (hard materials) under UHV has a special advantage in facilitating extremely detailed and realistic atomistic-level modeling. Localization of adatoms to a periodic array of adsorption sites enables the use of lattice-gas (LG) models for which nonequilibrium evolution can be efficiently analyzed on the appropriate time-and length-scales via kinetic Monte Carlo (KMC)...
Scanning tunneling microscopy studies show that coarsening of arrays of rectangular single-layer Ag islands on Ag(110) at 220 K and below occurs by one-dimensional (1D) decay of narrower islands, which maintain roughly constant width in the 〈001〉 direction. Adatoms mainly detach from the island ends with 〈001〉 step edges. 1D decay derives from the absence of corner rounding diffusion from 〈001〉 to 〈1̅ 10〉 edges and from inhibited nucleation of new layers on 〈1̅ 10〉 edges. In contrast, rounding from 〈1̅ 10〉 to 〈001〉 edges is active. The island decay rate exhibits an unexpectedly low effective Arrhenius energy due to a combination of strong anisotropy in terrace diffusion and a decrease with temperature of typical island end-to-end separations. Behavior is described by atomistic modeling, which accurately captures both the thermodynamics and the edge diffusion kinetics of the system, in contrast to previous treatments. Kinetic Monte Carlo (KMC) simulations assess model behavior and clarify the driving force for coarsening, as well as various detailed features of the 1D decay process. Refined "atom-tracking" KMC simulations for island configurations matching the experiment recover the experimentally observed island decay times and further elucidate spatial aspects of the transfer of adatoms between islands. Scanning tunneling microscopy studies show that coarsening of arrays of rectangular single-layer Ag islands on Ag(110) at 220 K and below occurs by one-dimensional (1D) decay of narrower islands, which maintain roughly constant width in the 001 direction. Adatoms mainly detach from the island ends with 001 step edges. 1D decay derives from the absence of corner rounding diffusion from 001 to 1 10 edges and from inhibited nucleation of new layers on 1 10 edges. In contrast, rounding from 1 10 to 001 edges is active. The island decay rate exhibits an unexpectedly low effective Arrhenius energy due to a combination of strong anisotropy in terrace diffusion and a decrease with temperature of typical island end-to-end separations. Behavior is described by atomistic modeling, which accurately captures both the thermodynamics and the edge diffusion kinetics of the system, in contrast to previous treatments. Kinetic Monte Carlo (KMC) simulations assess model behavior and clarify the driving force for coarsening, as well as various detailed features of the 1D decay process. Refined "atom-tracking" KMC simulations for island configurations matching the experiment recover the experimentally observed island decay times and further elucidate spatial aspects of the transfer of adatoms between islands.
The (111) surface of single-crystal NaAu2 is a model for catalytically active, powdered NaAu2. We prepare and characterize this surface with a broad suite of techniques. Preparation in ultrahigh vacuum consists of the traditional approach of ion bombardment (to remove impurities) and thermal annealing (to restore surface order). Both of these steps, however, cause loss of sodium (Na), and repeated treatments eventually trigger conversion of the surface and near-surface regions to crystalline gold. The bulk has a limited ability to repopulate the surface Na. Under conditions where Na depletion is minimized, electron diffraction patterns are consistent with the bulkterminated structure, and scanning tunneling microscopy reveals mesa-like features with lateral dimensions of a few tens of nanometers. The tops of the mesas do not possess fine structure characteristic of a periodic lattice, suggesting that the surface layer is disordered under the conditions of these experiments. (111) surface of single-crystal NaAu 2 is a model for catalytically active, powdered NaAu 2 . We prepare and characterize this surface with a broad suite of techniques. Preparation in ultrahigh vacuum consists of the traditional approach of ion bombardment (to remove impurities) and thermal annealing (to restore surface order). Both of these steps, however, cause loss of sodium (Na), and repeated treatments eventually trigger conversion of the surface and near-surface regions to crystalline gold. The bulk has a limited ability to repopulate the surface Na. Under conditions where Na depletion is minimized, electron diffraction patterns are consistent with the bulkterminated structure, and scanning tunneling microscopy reveals mesa-like features with lateral dimensions of a few tens of nanometers. The tops of the mesas do not possess fine structure characteristic of a periodic lattice, suggesting that the surface layer is disordered under the conditions of these experiments. Keywords
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