Solution of the structure of the high-coverage CO layer on the Ru(0001) surface-A combined study by density functional theory and scanning tunneling microscopy.

Structures formed by dense CO adsorption layers can provide information about the balance between molecule-surface and molecule-molecule interactions. However, in many cases, the structure models are not clear. Using density functional theory (DFT) and scanning tunneling microscopy (STM), we have investigated the high-coverage CO layer on the Ru(0001) surface. Previous investigations by low-energy electron diffraction (LEED) and vibrational spectroscopy led to conflicting results about the structure. In the present study, 88 models with coverages between 0.58 and 0.77 monolayers have been analyzed by DFT. The most stable structures consist of small, compact CO clusters with an internal pseudo 1×1 structure. The CO molecules in the cluster centers occupy on-top sites in an upright position, whereas the molecules farther outside are slightly shifted from these sites and tilted outward. STM data of the CO-saturated surface at low temperatures, corresponding to a coverage of 0.66 monolayers, show a quasi-hexagonal pattern of features with an internal hexagonal fine structure. Simulated images based on the cluster model agree with the experimental data. It is concluded that the high-coverage CO layer consists of the close-packed clusters predicted by DFT as the most stable structure elements. In the experiment, the sizes and shapes of the clusters vary. However, the arrangement is not random but follows defined tiling rules. The structure remains ordered, almost up to room temperature. The LEED data are re-interpreted on the basis of the Fourier transforms of the STM data, solving the long-standing conflict about the structure.

Growing polymers, caught in the act.

Researchers show the polymerization of ethylene at the active centers of a catalyst.

Method for the Manual Analysis of Moiré Structures in STM images.

A method is presented to manually determine the lattice parameters of commensurate hexagonal moiré structures resolved by STM. It solves the problem that lattice parameters of moiré structures usually cannot be determined by inspection of an STM image, so that computer-based analyses are required. The lattice vector of a commensurate moiré structure is a sum of integer multiples both of the two basis vectors of the substrate and of the adsorbed layer. The method extracts the two factors with respect to the adsorbed layer from an analysis of the Fourier transform of an STM image. These two factors are related to the two factors with respect to the substrate layer. Using the cell augmentation method, six possible moiré structures are identified by algebra. When the orientation and lattice constant of the substrate are roughly known, this information is usually sufficient to determine a unique moiré structure and its lattice parameters.

A highly sensitive gas chromatograph for in situ and operando experiments on catalytic reactions.

We describe an automated gas sampling and injection unit for a gas chromatograph (GC). It has specially been designed for low concentrations of products formed in catalytic in situ and operando experiments when slow reactions on single crystal models are investigated. The unit makes use of a buffer volume that is filled with gas samples from the reactor at a reduced pressure. The gas samples are then compressed by He to the injection pressure of 1000 mbar and pushed into two sample loops of the GC, without major intermixing with He. With an additional cryo trap at one of the GC column heads, the design aims at concentrating the gas samples and focusing the peaks. The performance is characterized by experiments on the Fischer-Tropsch synthesis, using H2/CO mixtures (syngas) at 200 and 950 mbar and a Co(0001) single crystal sample as model catalyst. Chromatograms recorded during the reaction display sharp, well separated peaks of saturated and unsaturated C1 to C4 hydrocarbons formed by the reaction, whereas the syngas matrix only gives moderate signals that can be well separated from the product peaks. Detection and quantification limits of 0.4 and 1.3 ppb, respectively, have been achieved and turnover numbers as low as 10-5 s-1 could be measured. The system can be combined with all known analysis techniques used in in situ and operando experiments.

Density fluctuations as door-opener for diffusion on crowded surfaces.

How particles can move on a catalyst surface that, under the conditions of an industrial process, is highly covered by adsorbates and where most adsorption sites are occupied has remained an open question. We have studied the diffusion of O atoms on a fully CO-covered Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling microscopy combined with density functional theory calculations. Atomically resolved trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the clean surface. This finding can be explained by a "door-opening" mechanism in which local density fluctuations in the CO layer intermittently create diffusion pathways on which the O atoms can move with low activation energy.

High-temperature scanning tunneling microscopy study of the ordering transition of an amorphous carbon layer into graphene on ruthenium(0001).

The ordering transition of an amorphous carbon layer into graphene was investigated by high-temperature scanning tunneling microscopy. A disordered C layer was prepared on a Ru(0001) surface by chemical vapor deposition of ethylene molecules at ~660 K. The carbon layer grows in the form of dendritic islands that have almost the same density as graphene. Upon annealing of the fully covered surface, residual hydrogen desorbs and a coherent but still disordered carbon layer forms, with almost the same carbon coverage as in graphene. The ordering of this layer into graphene at 920 to 950 K was monitored as a function of time. A unique mechanism was observed that involves small topographic holes in the carbon layer. The holes are mobile, and on the trajectories of the holes the disordered carbon layer is transformed into graphene. The transport of C atoms across the holes or along the hole edges provides a low-energy pathway for the ordering transition. This mechanism is prohibited in a dense graphene layer, which offers an explanation for the difficulty of removing defects from graphene synthesized by chemical methods.

Surface patterning of silver using an electron- or photon-assisted oxidation reaction.

We have investigated a recently developed method of patterning Ag surfaces. The method uses an electron beam to irradiate Ag surfaces during NO(2) dosing at 300 K and leads to sharp oxide patterns on otherwise metallic surfaces. Investigations were performed on an Ag(111) single crystal and on an Ag foil with LEEM (low-energy electron microscopy), LEED (low-energy electron diffraction), MEM (mirror electron microscopy), and XPEEM (X-ray photo-emission electron microscopy). The oxidation reaction, which is based on the electron-induced desorption of NO molecules, proceeds in steps, from a layer of O atoms adsorbed on the metallic Ag via an intermediate phase to an amorphous Ag(2)O film. Our measurements evidence a high cross section for electron-induced NO desorption with 30-40 eV electrons, indicating that only a few electrons per adsorbing NO(2) molecule are required to initiate the process. The intermediate phase, which forms a partially ordered quadratic structure, contains oxygen species in an oxide-like environment, coexisting with an adsorbate covered metallic Ag(111) surface. While the intermediate phase dissolves within hours under UHV conditions, fully developed oxide patches, consisting of several layers of thick, amorphous Ag(2)O, are kinetically stable. The oxidation method also works with 40 eV (and 700 eV) photons instead of electrons. In preliminary experiments local patterns could also be created with photons, suggesting that mask techniques can be applied for the process.

Microscopic aspects of pattern formation on surfaces.

Recent scanning tunneling microscopy (STM) work gives insight into microscopic processes of surface reactions that play a role for spatio-temporal pattern formation. STM allows to resolve adsorbed particles, follow their surface motion, and monitor reactions with other particles on the atomic scale. The data reveal pronounced deviations from the implicite assumptions of the reaction-diffusion equations traditionally used to model spatio-temporal patterns. In contrast to these descriptions, particles are often not randomly distributed, but cluster in islands because of attractive interactions, and particle hopping can be highly correlated. It is shown that such phenomena can even affect the macroscopic kinetics. The article also discusses a case where the atomic processes inside propagating reaction fronts could be resolved. Here particular strong interaction effects were observed, caused by hydrogen bonds between the reacting species. (c) 2002 American Institute of Physics.