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

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(1), catalytic activity can emerge from traditionally inert elements such as gold(2), and electronic behaviour useful for device technology can be developed(3,4). 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

Isolation and purification of ent-pimara-8(14),15-diene from engineered Aspergillus nidulans by accelerated solvent extraction combined with HPLC

Peer reviewe

Self-organized growth of cluster arrays

We present a novel method for the fabrication of well-ordered, two-dimensional nanocluster arrays. The method is based on the confined nucleation of adatoms within the superstructure cells of periodic surface dislocation networks, which form in many heteroepitaxial systems. Sire show how quantitative understanding of adatom diffusion and heterogeneous nucleation on such surfaces can be obtained through kinetic Monte-Carlo simulations and discuss the potential of this approach

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. (C) 1997 Elsevier Science B.V

Strain mediated two-dimensional growth kinetics in metal heteroepitaxy: Ag/Pt(1111)

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. (C) 1997 Elsevier Science B.V

Interlayer Mass-Transport in Homoepitaxial and Heteroepitaxial Metal Growth

We describe a general method for the quantitative determination of the interlayer mass transport in epitaxial growth. Through measurement of the nucleation rate on top of islands as a function of island size and temperature, the additional barrier for an adatom to descend the step edge Delta E(s) can be determined with high accuracy. This approach is applied to the growth of Ag on the (111) surfaces of Ag and Pt. In the homoepitaxial system, the barrier is found to be Delta E(s) = 120 +/- 15 meV, whereas in the heteroepitaxial case it is substantially lowered, Delta E(s) = 30 +/- 5 meV

Stress relief via island formation of an isotropically strained bimetallic surface layer: The mesoscopic morphology of the Ag/Pt (111) surface alloy

Upon annealing above 620 K, submonolayers of Ag deposited on Pt(111) are known to mix into the first surface layer. Thereby the individual Ag atoms reduce the strain fields caused by the lattice mismatch. As observed by scanning tunneling microscopy, careful annealing of the intermixed surface leads to the formation of small round islands with a preferential diameter of 150-300 Angstrom on terraces wider than a critical width of about 400 Angstrom. This indicates that the microscopically intermixed surface is still under considerable stress, which is released by a mesoscopic change of the morphology. An effective release of elastic stress at the island and step edges is thought to compensate for the formation energy of steps and the repulsive elastic step-step interaction