144 research outputs found
Modeling of Metal(100) Homepitaxial Film Growth at Very Low Temperatures
We model the growth of Ag films deposited on Ag(100) below 140K. Our recent Variable-Temperature Scanning Tunneling Microscopy (VTSTM) studies reveal “smooth growth” from 120-140K, consistent with earlier diffraction studies. However, we also find rougher growth for lower temperatures. This unexpected behavior is modeled by describing the deposition dynamics using a “restricted downward funneling” model, wherein deposited atoms get caught on the sides of steep nanoprotrusions (which are prevalent below 120K), rather than always funneling down to lower four-fold hollow adsorption sites. At OK, where no thermal diffusion processes are operative, this leads to the formation of overhangs and internal defects (or voids). Above 40K, low barrier interlayer diffusion processes become operative, producing the observed smooth growth by 120K. We also discuss how the apparent film morphology mapped out by the STM tip “smears” features of the actual film morphology (which are small at low temperature), and also can lead to underestimation of the roughness
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
Adatom capture by arrays of two-dimensional Ag islands on Ag(100)
We examine the capture of diffusing Ag adatoms by arrays of two-dimensional Ag islands subsequent to deposition on Ag(100) at room temperature. This is achieved by a combination of scanning tunneling microscopy experiments, kinetic Monte Carlo simulations, and diffusion equation analyses. The dependence of the capture rates on Ag-island size is shown to reflect larger island-free regions surrounding the larger islands, i.e., a strong correlation between island sizes and separations. This feature, and the influence of the local environment of the islands on capture, are elucidated by introducing suitable tessellations of the surface into “capture zones” for each island. We show that a Voronoi-type tessellation based on the distance from the island edges accurately reflects adatom capture. However, a tessellation exactly describing adatom capture is only obtained from a solution of the steady-state equation describing adatom deposition, diffusion, and capture by an array of islands distributed as in experiment. The stochastic nature of adatom capture is also quantified by analysis of the dependence on the deposition location of the probability for diffusing adatoms to be captured by a specific island. The experimental island size dependence of adatom capture is found to be entirely consistent with that obtained from a “canonical” model for the irreversible nucleation and growth of square islands
Evolution of far-from-equilibrium nanostructures on Ag(100) surfaces: Protrusions and indentations at extended step edges
Scanning tunneling microscopy is used to monitor the formation and relaxation of nanoprotrusions and nanoindentations at extended step edges following submonolayer deposition of Ag on Ag(100). Deposition of up to about 1/4 ML Ag produces isolated two-dimensional (2D) Ag clusters, which subsequently diffuse, collide, and coalesce with extended step edges, thus forming protrusions. Deposition of larger submonolayer amounts of Ag causes existing step edges to advance across terraces, incorporating 2D islands. The resulting irregular step structure rapidly straightens after terminating deposition, except for a few larger indentations. Relaxation of these far-from-equilibrium step-edge nanoconfigurations is monitored to determine rates for restructuring versus local geometry and feature size. This behavior is analyzed utilizing kinetic Monte Carlo simulations of an atomistic lattice-gas model for relaxation of step-edge nanostructures. In this model, mass transport is mediated by diffusion along the step edge (i.e., “periphery diffusion”). The model consistently fits observed behavior, and allows a detailed characterization of the relaxation process, including assessment of key activation energies
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
Low temperature shape relaxation of 2-d islands by edge diffusion
We present a precise microscopic description of the limiting step for low
temperature shape relaxation of two dimensional islands in which activated
diffusion of particles along the boundary is the only mechanism of transport
allowed. In particular, we are able to explain why the system is driven
irreversibly towards equilibrium. Based on this description, we present a
scheme for calculating the duration of the limiting step at each stage of the
relaxation process. Finally, we calculate numerically the total relaxation time
as predicted by our results and compare it with simulations of the relaxation
process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.
Smoluchowski ripening of Ag islands on Ag(100)
Using scanning tunneling microscopy, we study the post-deposition coarsening of distributions of large, two-dimensional Ag islands on a perfect Ag(100) surface at 295 K. The coarsening process is dominated by diffusion, and subsequent collision and coalescence of these islands. To obtain a comprehensive characterization of the coarsening kinetics, we perform tailored families of experiments, systematically varying the initial value of the average island size by adjusting the amount of Ag deposited (up to 0.25 ML). Results unambiguously indicate a strong decrease in island diffusivity with increasing island size. An estimate of the size scaling exponent follows from a mean-field Smoluchowski rate equation analysis of experimental data. These rate equations also predict a rapid depletion in the initial population of smaller islands. This leads to narrowing of the size distribution scaling function from its initial form, which is determined by the process of island nucleation and growth during deposition. However, for later times, a steady increase in the width of this scaling function is predicted, consistent with observed behavior. Finally, we examine the evolution of Ag adlayers on a strained Ag(100) surface, and find significantly enhanced rates for island diffusion and coarsening
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