Josef Mysliveček

A versatile fabrication method for cluster superlattices

On the graphene moire on Ir(111) a variety of highly perfect cluster superlattices can be grown as shown for Ir, Pt, W and Re. Even materials that do not form cluster superlattices upon room temperature deposition may be grown into such by low-temperature deposition or the application of cluster seeding through Ir as shown for Au, AuIr and FeIr. Criteria for the suitability of a material to form a superlattice are given and largely confirmed. It is proven that at least Pt and Ir form epitaxial cluster superlattices. The temperature stability of the cluster superlattices is investigated and understood on the basis of positional fluctuations of the clusters around their sites of minimum potential energy. The binding sites of Ir, Pt, W and Re cluster superlattices are determined and the ability to cover samples macroscopically with a variety of superlattices is demonstrated.

Creating single-atom Pt-ceria catalysts by surface step decoration

Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces—monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt2+ ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt2+ and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

Epitaxial Cubic Ce2O3 Films via Ce-CeO2 Interfacial Reaction.

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO2 films. The method yields bulk-truncated layers of cubic c-Ce2O3. Compared to CeO2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce2O3(111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO2(111). In addition, c-Ce2O3 layers represent an experimental realization of a normally unstable polymorph of Ce2O3. During interfacial reaction, c-Ce2O3 nucleates on the interface between CeO2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal-oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.

Anode Material for Hydrogen Polymer Membrane Fuel Cell: Pt – CeO2 RF-Sputtered Thin Films

The interaction of Pt with CeO 2 layers was investigated using photoelectron spectroscopy. Pt-doped CeO 2 layers 30 nm thick were deposited by radio-frequency (rf) magnetron sputtering using a composite CeO 2 -Pt target on a carbon diffusion layer of micropolymer membrane fuel cell covered by double-wall carbon nanotubes. The laboratory X-ray photoelectron spectroscopy and synchrotron radiation soft X-ray photoemission spectra showed the formation of cerium oxide with completely ionized species Pt 2+,4+ embedded in the film. A small amount of Pt 0 is present on the film surface only. Hydrogen/air fuel cell activity measurements normalized to the amount of used Pt revealed specific power up to 30 W/mg(Pt). The activity of this material is explained by high activity of embedded Pt 2+,4+ cations toward H 2 dissociation and formation of protonic hydrogen. Very high specific power and low cost together with planar deposition techniques make the material promising for microfabrication of fuel cells to power mobile systems.

Modification of the conductance of single fullerene molecules by endohedral doping

We use scanning tunneling microscopy to establish controlled contacts to single molecules of endohedrally doped Ce2@C80 fullerenes with C60 as a reference. The stability of the experimental setup allows for the determination of the conductance of Ce2@C80 relative to the conductance of C60. The endohedral doping reduces the conductance of Ce2@C80 by a factor of about five with respect to C60. Ab initio calculations show that the reason for this reduced conductance is the absence of electron orbitals delocalized over the cage of Ce2@C80 in the energy window of the conductance measurement.

Nanoscale charge transport measurements using a double-tip scanning tunneling microscope

We demonstrate the ability of a double-tip scanning tunneling microscope (STM) combined with a scanning electron microscope (SEM) to perform charge transport measurements on the nanoscale. The STM tips serve as electric probes that can be precisely positioned relative to the surface nanostructures using the SEM control and the height reference provided by the tunneling contact. The tips work in contact, noncontact, and tunneling modes. We present vertical transport measurements on nanosized GaAs/AlAs resonant tunneling diodes and lateral transport measurements on the conductive surface of 7×7 reconstructed Si(111). The high stability of the double-tip STM allows nondestructive electrical contacts to surfaces via the tunneling gaps. We performed two-point electrical measurements via tunneling contacts on the Si(111)(7×7) surface and evaluated them using a model for the charge transport on this surface.

Magic islands and barriers to attachment: A S i / S i ( 111 ) 7 × 7 growth model

Institut fu¨r Grenzfl¨achenforschung und Vakuumphysik, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany(February 1, 2008)Surface reconstructions can drastically modify growth kinetics during initial stages of epitaxialgrowth as well as during the process of surface equilibration after termination of growth. Weinvestigate the effect of activation barriers hindering attachment of material to existing islandson the density and size distribution of islands in a model of homoepitaxial growth on Si(111)7×7reconstructed surface. An unusual distribution of island sizes peaked around “magic” sizes anda steep dependence of the island density on the growth rate are observed. “Magic” islands (ofa different shape as compared to those obtained during growth) are observed also during surfaceequilibration.81.15.Aa, 05.70.Ln, 81.15.Hi, 68.35.Bs

One-dimensional ordering of Ge nanoclusters along atomically straight steps of Si(111)

Ge nanostructures grown by molecular beam epitaxy on a vicinal Si(111) surface with atomically well-defined steps are studied by means of scanning tunneling microscopy and spectroscopy. When the substrate temperature during deposition is around 250°C, Ge nanoclusters of diameters less than 2.0nm form a one-dimensional array of the periodicity 2.7nm along each step. This self-organization is due to preferential nucleation of Ge on the unfaulted 7×7 half-unit cells at the upper step edges. Scanning tunneling spectroscopy reveals localized electronic states of the nanoclusters.

Self-assembly of periodic nanoclusters of Si and Ge along atomically straight steps of a vicinal Si(111)

The very initial stage of the molecular beam epitaxy of Si and Ge on Si(111)−7×7 substrates with atomically straight steps has been studied by scanning tunneling microscopy and spectroscopy. The atomically straight steps have been prepared on a miscut Si(111) substrate by annealing at 830 °C with kink-up direct current. The length of the steps can be maximized by selecting a proper annealing time. The steps have a well-defined U(2, 0) step-edge structure. The growth of both Si and Ge at temperatures between 250 and 400 °C starts with formation of a single-adatom-row nanowire (0.67 nm in width) along the lower edge of each U(2, 0) step. Subsequent growth of Si and Ge at temperatures between 250 and 300 °C results in formation of one-dimensional arrays of nanoclusters (less than 2.0 nm in width) in the unfaulted halves of the 7×7 structure along the upper step edges. Scanning tunneling spectroscopy reveals localized electronic states of the nanoclusters. Differences between the growth of Si and Ge nanoclust...

STM study of nucleation of Ag on Si(111)−(7 × 7) surface

Initial stages of Ag on Si(111)−(7 × 7) surface nucleation were studied at submonolayer coverage. Samples were prepared by thermal evaporation of Ag from tungsten wire under UHV conditions (p<2.5 × 10−8 Pa). Various deposition rates (0.002–0.1 ML s−1) were used to prepare Ag island films with coverages (0.002–2) ML (1 ML ≈ 7.58 × 1014 atoms cm−2) at room temperature. We observed preferential growth on faulted half unit cells (F cells). At constant coverage both the island density and ratio of occupied F and U (unfaulted) cells are independent of the deposition rate, which is an evidence for dominant influence of substrate structure. The preference of nucleation in the F cells against U cells decreases with the coverage until the ratio is 1:1 for 1 ML Ag film. We have observed that presence of an Ag island in any type of the half unit cell (F or U) considerably reduces nucleation probability in neighbouring cells. This results in forming of structural patterns observed among randomly grown Ag-islands which is a new feature found for Ag/Si(111)−(7 × 7) system.