Shape and size of cobalt nano-islands formed spontaneously on cobalt terraces during Fischer-Tropsch synthesis

Cobalt-based catalysts undergo a massive and spontaneous reconstruction to form uniform triangular nanoislands under Fischer-Tropsch (FT) conditions. This reconstruction is driven by the unusual and synergistic adsorption of square-planar carbon and CO at the 4-fold edge sites of the nanoislands, driving the formation of triangular islands. The size of the nanoislands is determined by the balance between energy gain from creating C/CO-covered edges and energy penalty to create C/CO-covered corners. For carbon chemical potentials corresponding to FT conditions, triangular Co islands with 45 Co atoms (about 2 nm) are the most stable surface structure. Decreasing the carbon chemical potential and hence the stability of square planar carbon favors the formation of larger islands, until reconstruction becomes unfavorable and CO-covered terraces are thermodynamically the most stable. The predicted structure of the islands is consistent with in situ scanning tunneling microscopy images obtained for the first time under realistic FT reaction conditions on a Co(0001) surface

The ring structure and organization of light harvesting 2 complexes in a reconstituted lipid bilayer, resolved by atomic force microscopy

Biological and Soft Matter PhysicsQuantum Matter and Optic

Combined (1x2)-->(1x1) transition and atomic roughening of Ge(001)studied with surface x-ray diffraction.

FWN – Publicaties zonder aanstelling Universiteit Leide

Growth mode and interface structure of ag on the HF-treated Si(111):H surface

A growth mode and interface structure analysis has been performed for Ag deposited at a high temperature of 300 degrees C on the HF-treated Si(111):H surface by means of medium-energy ion scattering and elastic recoil detection analysis of hydrogen. The measurements show that Ag grows in the Volmer-Weber mode and that the Ag islands on the surface are epitaxial with respect to the substrate. The preferential azimuthal orientation is A-type only when Ag is deposited slowly. The interface does not reconstruct to the root 3x root 3-Ag structure, which is normally observed for Ag deposition above 200 degrees C on the Si(111)7 x 7 surface, but retains its bulk-like structure. The presence of hydrogen at the interface is demonstrated after deposition of thick (1100 Angstrom) Ag films. However, the amount of hydrogen at the interface is not a full monolayer. This partial desorption of hydrogen from the interface explains why the Schottky barrier heights of Ag/Si(111):H diodes are close to those of Ag/Si(111)7 x 7 and Ag/Si(111)2 x 1

X-ray-intensity oscillations occuring during growth of Ge on Ge(111)-a comparison with RHEED

The electron paramagnetic resonance (EPR) of ternary oxides of Cu(II) has been studied between 4.2 and 300 K. The systems include those with 180 degrees Cu-O-Cu interactions (such as Ln2CuO4, Sr2CuO2Cl2, Sr2CuO3 and Ca2CuO3) or 90 degrees Cu-O-Cu interactions (such as Y2Cu2O5 or BaCuO2) as well as those in which the Cu2+ ions are isolated (such as Y2BaCuO5, La1.8Ba1.2Cu0.9O4.8 and Bi2CuO4). The change in the EPR susceptibility as a function of temperature is compared with that of the DC magnetic susceptibility. Compounds with extended 180 degrees Cu-O-Cu interactions which have a low susceptibility also do not give EPR signals below room temperature. For compounds such as Ca2CuO3 with one-dimensional 180 degrees Cu-O-Cu interactions a weak EPR signal is found the temperature dependence of which is very different from that of the DC susceptibility. For Y2BaCuO5 as well as for La1.8Ba1.2Cu0.9O4.8 the EPR susceptibility as well as its temperature variation are comparable with those of the static susceptibility near room temperature but very different at low temperatures. Bi2CuO4 also shows a similar behaviour. In contrast, for Y2Cu2O5, in which the copper ions have a very distorted nonsquare-planar configuration, the EPR and the static susceptibility show very similar temperature dependences. In general, compounds in which the copper ions have a square-planar geometry give no EPR signal in the ground state (0 K) while those with a distortion from square-planar geometry do give a signal. The results are analysed in the light of recent MS Xalpha calculations on CuO46- square-planar clusters with various Cu-O distances as well as distortions. It is suggested that in square-planar geometry the ground state has an unpaired electron in anionic orbitals which is EPR inactive. Competing interactions from other cations, an increase in Cu-O distance or distortions from square-planar geometry stabilise another state which has considerably more Cu 3d character. These states are EPR active. Both these states, however, are magnetic. For isolated CuO46- clusters the magnetic interactions seem to involve only the states which have mainly anionic character