Karsten Bromann

Self-organized growth of nanostructure arrays on strain-relief patterns

The physical and chemical properties of low-dimensional structures depend on their size and shape, and can be very different from those of bulk matter. If such structures have at least one dimension small enough that quantum-mechanical effects prevail, their behaviour can be particularly interesting. In this way, for example, magnetic nanostructures can be made from materials that are non-magnetic in bulk, catalytic activity can emerge from traditionally inert elements such as gold, and electronic behaviour useful for device technology can be developed,. The controlled fabrication of ordered metal and semiconductor nanostructures at surfaces remains, however, a difficult challenge. Here we describe the fabrication of highly ordered, two-dimensional nanostructure arrays through nucleation of deposited metal atoms on substrates with periodic patterns defined by dislocations that form to relieve strain. The strain-relief patterns are created spontaneously when a monolayer or two of one material is deposited on a substrate with a different lattice constant. Dislocations often repel adsorbed atoms diffusing over the surface, and so they can serve as templates for the confined nucleation of nanostructures from adatoms. We use this technique to prepare ordered arrays of silver and iron nanostructures on metal substrates.

Controlled Deposition of Size-Selected Silver Nanoclusters

Variable-temperature scanning tunneling microscopy was used to study the effect of kinetic cluster energy and rare-gas buffer layers on the deposition process of size-selected silver nanoclusters on a platinum(111) surface. Clusters with impact energies of ≤1 electron volt per atom could be landed nondestructively on the bare substrate, whereas at higher kinetic energies fragmentation and substrate damage were observed. Clusters with elevated impact energy could be soft-landed via an argon buffer layer on the platinum substrate, which efficiently dissipated the kinetic energy. Nondestructive cluster deposition represents a promising method to produce monodispersed nanostructures at surfaces.

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.

Hard and soft landing of mass selected Ag clusters on Pt(111)

Mass selected Ag/sub n/ clusters (n=1,7,19) from a secondary ion source have been deposited onto a Pt(111) substrate at low temperature. The surface and resulting cluster morphology have subsequently been characterized within the same UHV chamber by variable temperature STM as a function of cluster size, kinetic impact energy, and substrate temperature. The kinetic energy per cluster atom was found to be the decisive parameter for a controlled deposition. Noble gas buffer layers ( approximately=10 ML Ar), which were pre-adsorbed onto the surface at low temperatures, were found to efficiently dissipate the impact energy opening up the possibility of soft landing clusters with elevated kinetic energy.

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

Pseudomorphic growth induced by chemical adatom potential

The transformation from pseudomorphic to dislocated and back to pseudomorphic growth with increasing coverage is reported for molecular beam epitaxy of Ag on Pt(111). Below a critical size of 200 Angstrom two-dimensional Ag islands grow coherently strained, while larger islands relieve strain through the introduction of misfit dislocations. Upon completion of the first monolayer, the dislocations disappear and the Ag film again adopts a pseudomorphic structure. With the help of effective-medium theory calculations, it is shown that this effect is related to the elevated chemical potential of Ag adatoms on top of the first Ag monolayer

The Effect of Strain on Intra- and Interlayer Mass Transport in Metal Epitaxy

Keywords: Nucleation ; Aggregation ; Self-Assembly ; Epitaxial Growth Reference LNS-ARTICLE-1996-006View record in Web of Science Record created on 2009-04-14, modified on 2017-05-12

Surface Diffusion in Metal Epitaxy — Strain Effects

A method is presented to measure both the barriers for intra-and interlayer diffusion for an epitaxial system with great accuracy. It is based upon the application of mean-field nucleation theory to variable temperature STM data. The validity and limits of applying nucleation theory to extract barriers for terrace diffusion are discussed in comparison to alternative methods like Kinetic Monte-Carlo (KMC) simulations. With this approach, a pronounced influence of strain on intra-and interlayer diffusion was established for Ag self diffusion on strained and unstrained Ag(lll) surfaces. The strained surface was the pseudomorphic Ag monolayer on Pt(lll) which is under 4.3% compressive strain. The barrier for terrace diffusion is observed to be substantially lower on the strained, compared to the unstrained Ag/Ag(lll) case, 60+/-10 meV and 97+/-10 meV, respectively. A general method for the quantitative determination of the additional barrier for descending at steps is presented. It is based on the measurement of the nucleation rate on top of previously prepared adlayer islands as a function of island size and temperature. Application of this method reveals a considerable effect of strain also on interlayer diffusion. The additional barrier for interlayer diffusion decreases from 120+/-15 meV for Ag(111)homoepitaxy to only 30+/-5 meV for diffusion from the strained Ag layer down to the Pt(lll) substrate. These examples illustrate the strong influence of strain on the intra-and interlayer mass transport which leads to a new concept of layer-dependent nucleation kinetics for heteroepitaxial systems. Finally, we discuss the relation between corner diffusion and island shapes. Low temperature aggregation on hexagonally close-packed metal surfaces generally is dominated by the microscopic difference between two edge orientations giving rise to anisotropic corner (and edge) diffusion. It is demonstrated how this anisotropy gives rise to dendritic island shapes with trigonal symmetry.

Fractal and Dendritic Growth of Surface Aggregates

Keywords: Nucleation ; Aggregation ; Self-Assembly ; Epitaxial Growth Reference LNS-ARTICLE-1996-005View record in Web of Science Record created on 2009-04-14, modified on 2017-05-12