Harald Brune

Microscopic view of epitaxial metal growth: nucleation and aggregation

Thin films are often grown away from thermodynamic equilibrium and their morphology becomes determined by kinetics. The final structure of the epitaxial film is decided in the very early stage of submonolayer nucleation and island growth. Recent experiments with scanning tunneling microscopy opened up an unprecedented view of this early stage of epitaxial growth. Variable sample temperatures enabled quantification of the rates of the most important atomic diffusion events and tracing back their interplay yielding the final submonolayer morphology. The present understanding of nucleation and aggregation in light of these new experimental results is reviewed for the case of metals. Examples are given how the growth kinetics can be employed to create well-defined island morphologies and island arrays in a self-organization process.

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.

Interaction of oxygen with Al(111) studied by scanning tunneling microscopy

The interaction of oxygen with Al(111) was studied by scanning tunneling microscopy (STM). Chemisorbed oxygen and surface oxides can be distinguished in STM images, where for moderate tunnel currents and independent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was established by counting O adatoms. The initial sticking coefficient is determined to so=0.005. Upon chemisorption at 300 K the O adlayer is characterized by randomly distributed, immobile, individual O adatoms and, for higher coverages, by small (1×1) O islands which consist of few adatoms only. From the random distribution of the thermalized O adatoms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissociative adsorption. These ‘‘hot adatoms’’ ‘‘fly apart’’ by at least 80 A, before their excess energy is dissipated. A model is derived which explains the unusual island nucleation scheme ...

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.

Radial elasticity of multiwalled carbon nanotubes.

We report an experimental and a theoretical study of the radial elasticity of multiwalled carbon nanotubes as a function of external radius. We use atomic force microscopy and apply small indentation amplitudes in order to stay in the linear elasticity regime. The number of layers for a given tube radius is inferred from transmission electron microscopy, revealing constant ratios of external to internal radii. This enables a comparison with molecular dynamics results, which also shed some light onto the applicability of Hertz theory in this context. Using this theory, we find a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30+/-10 GPa.

Chiral kagome lattice from simple ditopic molecular bricks

Self-assembly techniques allow for the fabrication of highly organized architectures with atomic-level precision. Here, we report on molecular-level scanning tunneling microscopy observations demonstrating the supramolecular engineering of complex, regular, and long-range ordered periodic networks on a surface atomic lattice using simple linear molecular bricks. The length variation of the employed de novo synthesized linear dicarbonitrile polyphenyl molecules translates to distinct changes of the bonding motifs that lead to hierarchic order phenomena and unexpected changes of the surface tessellations. The achieved 2D organic networks range from a close-packed chevron pattern via a rhombic network to a hitherto unobserved supramolecular chiral kagomé lattice.

Reaching the magnetic anisotropy limit of a 3d metal atom

Maximizing atomic magnetic memory A study of the magnetic response of cobalt atoms adsorbed on oxide surfaces may lead to much denser storage of data. In hard drives, data are stored as magnetic bits; the magnetic field pointing up or down corresponds to storing a zero or a one. The smallest bit possible would be a single atom, but the magnetism of a single atom —its spin—has to be stabilized by interactions with heavy elements or surfaces through an effect called spin-orbit coupling. Rau et al. (see the Perspective by Khajetoorians and Wiebe) built a model system in pursuit of single-atom bits—cobalt atoms adsorbed on magnesium oxide. At temperatures approaching absolute zero, the stabilization of the spins magnetic direction reached the maximum that is theoretically possible. Science, this issue p. 988; see also p. 976 A cobalt atom bound to a single oxygen site on magnesia has the maximum magnetic anisotropy allowed for a transition metal [Also see Perspective by Khajetoorians and Wiebe] Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom’s magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.

High-Quality 2D Metal-Organic Coordination Network Providing Giant Cavities within Mesoscale Domains

A surface-supported open metal-organic nanomesh featuring a 24 nm(2) cavity size and extending to mum domains was fabricated by Co-directed assembly of para-hexaphenyl-dicarbonitrile linker molecules in two dimensions. The metallosupramolecular lattice is thermally robust and resides fully commensurate on the employed Ag(111) substrate as directly verified by high-resolution scanning tunneling microscopy observations.

An endohedral single-molecule magnet with long relaxation times: DySc2N@C80.

The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy(III) ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc(2)N@C(80) samples with C(60) increases the zero-field 4f electron relaxation time at 2 K to several hours.

Effect of the TiO2 Reduction State on the Catalytic CO Oxidation on Deposited Size-Selected Pt Clusters

The catalytic activity of deposited Pt(7) clusters has been studied as a function of the reduction state of the TiO(2)(110)-(1 × 1) support for the CO oxidation reaction. While a slightly reduced support gives rise to a high catalytic activity of the adparticles, a strongly reduced one quenches the CO oxidation. This quenching is due to thermally activated diffusion of Ti(3+) interstitials from the bulk to the surface where they deplete the oxygen adsorbed onto the clusters by the formation of TiO(x) (x ≃ 2) structures. This reaction is more rapid than the CO oxidation. The present results are of general relevance to heterogeneous catalysis on TiO(2)-supported metal clusters and for reactions involving oxygen as intermediate.