Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina

We explore theoretically the scanning tunneling microscopy of single molecules on substrates using a framework of two local probes. This framework is appropriate for studying electron flow in tip/molecule/substrate systems where a thin insulating layer between the molecule and a conducting substrate transmits electrons non-uniformly and thus confines electron transmission between the molecule and substrate laterally to a nanoscale region significantly smaller in size than the molecule. The tip-molecule coupling and molecule-substrate coupling are treated on the same footing, as local probes to the molecule, with electron flow modelled using the Lippmann-Schwinger Green function scattering technique. STM images are simulated for various positions of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We find that these images have a strong dependence on the substrate probe position, indicating that electron flow can depend strongly on both tip position and the location of the dominant molecule-substrate coupling. Differences in the STM images are explained in terms of the molecular orbitals that mediate electron flow in each case. Recent experimental results, showing STM topographs of Zn(II)-etioporphyrin I on alumina/NiAl(110) to be strongly dependent on which individual molecule on the substrate is being probed, are explained using this model. A further experimental test of the model is also proposed.Comment: Physical Review B, in pres

Theory of a Scanning Tunneling Microscope with a Two-Protrusion Tip

We consider a scanning tunneling microscope (STM) such that tunneling occurs through two atomically sharp protrusions on its tip. When the two protrusions are separated by at least several atomic spacings, the differential conductance of this STM depends on the electronic transport in the sample between the protrusions. Furthermore two-protrusion tips commonly occur during STM tip preparation. We explore possible applications to probing dynamical impurity potentials on a metallic surface and local transport in an anisotropic superconductor.Comment: revtex, 11 pages, 6 figures upon reques

Role of water in metal catalyst performance for ketone hydrogenation: a joint experimental and theoretical study on levulinic acid conversion into gamma-valerolactone

While Ru is a poor hydrogenation catalyst compared to Pt or Pd in the gas phase, it is efficient under aqueous phase conditions in the hydrogenation of ketones such as the conversion of levulinic acid into gamma-valerolactone. Combining DFT calculations and experiments, we demonstrate that water is responsible for the enhanced reactivity of Ru under those conditions

Ru catalysts for levulinic acid hydrogenation with formic acid as a hydrogen source

International audienceThe catalytic hydrogenation of levulinic acid (LA) with formic acid (FA) as a hydrogen source into [gamma]-valerolactone (GVL) is considered as one of the crucial sustainable processes in today's biorefinery schemes. In the current work, we investigated the modification of Ru/C as efficient catalysts for both formic acid decomposition and levulinic acid hydrogenation in comparison with Pd and Pt catalysts. In order to better understand what features are responsible for high catalytic performance, we combined experimental tests, DFT calculations together with extensive material characterization. In LA hydrogenation with FA as a hydrogen source, the intermediate surface formate inhibits at least partially the LA hydrogenation. In addition, the FA decomposition is highly sensitive to the kind of the preparation method of the Ru/C catalyst: (i) the process looks structure sensitive favored on larger particles and (ii) residual chlorine decreases significantly the FA decomposition rate

Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water

International audienceThe kinetic stability of any material in water relies on the presence of surface weak spots responsible for chemical weathering by hydrolysis. Being able to identify the atomistic nature of these sites and the first steps of transformation is therefore critical to master the decomposition processes. This is the challenge that we tackle here: combining experimental and modeling studies we investigate the stability of alumina in water. Exploring the reactivity of shape-controlled crystals, we identify experimentally a specific facet as the location of the weak spots. Using biased ab initio molecular dynamics, we recognize this weak spot as a surface exposed tetra-coordinated Al atom and further provide a detailed mechanism of the first steps of hydrolysis. This understanding is of great importance to heterogeneous catalysis where alumina is a major support. Furthermore, it paves the way to atomistic understanding of interfacial reactions, at the crossroad of a variety of fields of research

Biology and dynamics of potential malaria vectors in Southern France

BACKGROUND: Malaria is a former endemic problem in the Camargue, South East France, an area from where very few recent data concerning Anopheles are available. A study was undertaken in 2005 to establish potential malaria vector biology and dynamics and evaluate the risk of malaria re-emergence. METHODS: Mosquitoes were collected in two study areas, from March to October 2005, one week every two weeks, using light traps+CO(2), horse bait traps, human bait catch, and by collecting females in resting sites. RESULTS: Anopheles hyrcanus was the most abundant Anopheles species. Anopheles melanoon was less abundant, and Anopheles atroparvus and Anopheles algeriensis were rare. Anopheles hyrcanus and An. melanoon were present in summer, whereas An. atroparvus was present in autumn and winter. A large number of An. hyrcanus females was collected on humans, whereas almost exclusively animals attracted An. melanoon. Based on an enzyme-linked immunosorbent assay, almost 90% of An. melanoon blood meals analysed had been taken on horse or bovine. Anopheles hyrcanus and An. melanoon parity rates showed huge variations according to the date and the trapping method. CONCLUSION: Anopheles hyrcanus seems to be the only Culicidae likely to play a role in malaria transmission in the Camargue, as it is abundant and anthropophilic

Microscopic origin of the conducting channels in metallic atomic-size contacts

We present a theoretical approach which allows to determine the number and orbital character of the conducting channels in metallic atomic contacts. We show how the conducting channels arise from the atomic orbitals having a significant contribution to the bands around the Fermi level. Our theory predicts that the number of conducting channels with non negligible transmission is 3 for Al and 5 for Nb one-atom contacts, in agreement with recent experiments. These results are shown to be robust with respect to disorder. The experimental values of the channels transmissions lie within the calculated distributions.Comment: 11 pages, 4 ps-figures. Submitted to Phys. Rev. Let

Coherent electron-phonon coupling and polaron-like transport in molecular wires

We present a technique to calculate the transport properties through one-dimensional models of molecular wires. The calculations include inelastic electron scattering due to electron-lattice interaction. The coupling between the electron and the lattice is crucial to determine the transport properties in one-dimensional systems subject to Peierls transition since it drives the transition itself. The electron-phonon coupling is treated as a quantum coherent process, in the sense that no random dephasing due to electron-phonon interactions is introduced in the scattering wave functions. We show that charge carrier injection, even in the tunneling regime, induces lattice distortions localized around the tunneling electron. The transport in the molecular wire is due to polaron-like propagation. We show typical examples of the lattice distortions induced by charge injection into the wire. In the tunneling regime, the electron transmission is strongly enhanced in comparison with the case of elastic scattering through the undistorted molecular wire. We also show that although lattice fluctuations modify the electron transmission through the wire, the modifications are qualitatively different from those obtained by the quantum electron-phonon inelastic scattering technique. Our results should hold in principle for other one-dimensional atomic-scale wires subject to Peierls transitions.Comment: 21 pages, 8 figures, accepted for publication in Phys. Rev. B (to appear march 2001