Holger Röder

Anisotropic corner diffusion as origin for dendritic growth on hexagonal substrates

Ag aggregation on Ag(111), Pt(111), and 1 ML Ag pseudomorphically grown on Pt(111), has been studied with variable temperature STM. These systems all have in common that dendritic patterns with trigonal symmetry rather than randomly ramified aggregates, which would be expected for a simple hit and stick mechanism, form. Dendrites are characterized by preferential growth in the [ 2]-directions, i.e., perpendicular to A-steps. The key process for their formation has been found to be diffusion of one-fold comer atoms towards neighboring steps. Calculations with the effective medium theory show that this relaxation is highly asymmetric with respect to the two different kinds of close-packed steps. It leads to dendritic growth as verified by kinetic Monte-Carlo simulations which agree well with experiment.

Kinetic processes in metal epitaxy studied with variable temperature STM: Ag/Pt(111)

Variable-temperature scanning tunneling microscopy has been applied to study kinetic processes involved in epitaxial growth. This paper concentrates on nucleation and aggregation of submonolayer Ag films on a Pt(111) surface. From island density versus temperature data, the activation barrier for Ag adatom diffusion as well as the stability of adsorbed Ag dimers are determined. From the adsorbed aggregate shapes conclusions on Ag perimeter diffusion can be drawn. An anisotropy in edge diffusion leads to dendritic aggregates with the trigonal substrate symmetry. A crossover to randomly ramified fractals is observed upon lowering of the deposition flux.

Strain mediated two-dimensional growth kinetics in metal heteroepitaxy: Ag/Pt(111)

We have investigated the influence of strain on the morphology in metal heteroepitaxy at temperatures where growth is dominated by kinetics. Whereas Ag(111) homoepitaxy is three dimensional below 400 K, the growth of Ag On Pt(111) proceeds two dimensionally up to a critical film thickness after which a transition to 3D growth is observed. This critical thickness increases from 1 ML at 130 K to 6-9 ML at 300 K. It is demonstrated that the 2D growth in the heteroepitaxial system is due to the particular growth kinetics induced by the compressive strain of the Ag films. The strained Ag layers are found to have substantially lower activation barriers for interlayer mass transport compared to strain free Ag(111). Further, strain and its relief in dislocations also lead to layer-dependent nucleation densities. Both these effects strongly promote layer-by-layer growth. The transition to 3D growth is triggered by the structural transition from strained Ag layers to a perfect Ag(111) termination. It is generally expected that compressive strain promotes 2D growth

Structure and complete chemical passivation of Pt(997)

Abstract We report on scanning tunneling microscopy and infrared reflection absorption spectroscopy investigations of the stepped Pt(997) surface. The surface shows a regular terrace-step-structure with only moderate variations of the terrace widths with respect to the nominal structure. The adsorption of CO as well as the coadsorption of hydrogen and CO is studied. It is shown that covering the surface with a full monolayer of hydrogen at 165 K provides its complete passivation against adsorption from the residual gas atmosphere, without affecting the terrace-step-configuration. Thus the surface can be kept clean in UHV over months, a necessary precondition to serve as diffractive element in atom beam optics.

Aggregation of Fractal and Dendritic Ag Clusters on a Pt(111) Surface

We have used variable-temperature scanning tunneling microscopy to study the aggregation of two-dimensional Ag clusters on Pt(111). A transition from randomly ramified to dendritic fractal growth is observed in the diffusion-limited regime. Atomic-scale observations have identified the anisotropy of edge diffusion as microscopic origin of this crossover. Dependent on the deposition flux, this anisotropy is either amplified to the macroscopic-cluster shape and trigonal dendrites result, or it is degenerated and randomly ramified fractals occur. Our study elucidates the close relation between fractal and dendritic pattern formation in diffusion-limited aggregation on a two-dimensional lattice.