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Scanning tunneling microscopy shows that large two-dimensional Ag clusters on Ag(100) can diffuse. The value of the diffusion coefficient at room temperature is of order 10-17 cm2s-1 and varies little, if at all, with cluster size in the range studied, 100 to 720 atoms per cluster. This weak variation rules out periphery diffusion as the main mechanism of cluster diffusion, suggesting instead two-dimensional evaporationcondensation. This conclusion is compatible with the energetics of atomic-scale events within the cluster and with the dissolution of small clusters observed at low coverages. Keywords Ames Laboratory, Mathematics Disciplines Mathematics | Physical Chemistry CommentsThis article is from Physical Review Letters 73, no. 19 (1994) .- (a) . (

We describe experimental observations and theoretical analysis of the coarsening of distributions of twodimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu(111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics generally follows the predictions of mesoscale continuum theories. SR kinetics is controlled by the size-dependence of cluster diffusion. However, this size-dependence, together with that of nanostructure shape relaxation upon coalescence, often deviates from mesoscale predictions as a direct consequence of the nanoscale dimension of the clusters. Second, we describe examples for the above systems where trace amounts of a chemical additive lead to dramatic enhancement of coarsening. We focus on the scenario where "facile reaction" of metal and additive atoms leads to the formation of mobile additivemetal complexes which can efficiently transport metal across the surface, i.e., additive-enhanced OR. A suitable reaction-diffusion equation formulation is developed to describe this behavior. ReceiVed: July 19, 2008; ReVised Manuscript ReceiVed: December 25, 2008 We describe experimental observations and theoretical analysis of the coarsening of distributions of twodimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu (111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics generally follows the predictions of mesoscale continuum theories. SR kinetics is controlled by the size-dependence of cluster diffusion. However, this size-dependence, together with that of nanostructure shape relaxation upon coalescence, often deviates from mesoscale predictions as a direct consequence of the nanoscale dimension of the clusters. Second, we describe examples for the above systems where trace amounts of a chemical additive lead to dramatic enhancement of coarsening. We focus on the scenario where "facile reaction" of metal and additive atoms leads to the formation of mobile additivemetal complexes which can efficiently transport metal across the surface, i.e., additive-enhanced OR. A suitable reaction-diffusion equation formulation is developed to describe this behavior.

A remarkable dependence of the friction force on carrier concentration was found on doped silicon substrates. The sample was a nearly intrinsic n-type Si(100) wafer patterned with 2-micrometer-wide stripes of highly B-doped p-type material. The counter surface was the tip of an atomic force microscope coated with conductive titanium nitride. The local carrier concentration was controlled through application of forward or reverse bias voltages between the tip and the sample in the p and the n regions. Charge depletion or accumulation resulted in substantial differences in friction force. The results demonstrate the capability to electronically control friction in semiconductor devices, with potential applications in nanoscale machines containing moving parts.

a b s t r a c tGraphene, a single atomic layer of graphite, has been the focus of recent intensive studies due to its novel electronic and structural properties. Metals grown on graphene also have been of interest because of their potential use as metal contacts in graphene devices, for spintronics applications, and for catalysis. All of these applications require good understanding and control of the metal growth morphology, which in part reflects the strength of the metal-graphene bond. Also of importance is whether the interaction between graphene and metal is sufficiently strong to modify the electronic structure of graphene. In this review, we will discuss recent experimental and computational studies related to deposition of metals on graphene supported on various substrates (SiC, SiO 2 , and hexagonal close-packed metal surfaces). Of specific interest are the metal-graphene interactions (adsorption energies and diffusion barriers of metal adatoms), and the crystal structures and thermal stability of the metal nanoclusters.

We present a different mechanism to explain the occurrence of long-lived oscillations in diffraction spot intensities during epitaxial growth of metal films on fcc (100) substrates at low temperature. Rather than rely on the common picture of cyclical nucleation and growth to produce the oscillations, the model invokes ''downward funneling'' deposition dynamics to fourfold-hollow adsorption sites.

Growth shapes of Ag islands formed on Ag͑111͒ during submonolayer deposition at different temperatures were studied with scanning tunneling microscopy, and analyzed via kinetic Monte Carlo simulation of a suitable atomistic lattice-gas model. Distinct shape transitions can be observed, from dendrites with triangular envelopes at low temperatures ͑below 140 K͒ to more isotropic fat fractal islands at intermediate temperatures, and then to distorted hexagonal shapes with longer B steps and shorter A steps at higher temperatures ͑above 170 K͒. In contrast, the equilibrium island shapes in this system are almost perfect hexagons displaying a near-sixfold symmetry. Modeling reveals that the broken symmetry of growth shapes at low and high temperatures derives from the interplay of diffusion-mediated aggregation with different aspects of a corner diffusion anisotropy. The broken symmetry is less clear at intermediate temperatures, where the near-isotropic fractal shapes reflect in part a kink Ehrlich-Schwoebel effect.