a route towards defined surface functionalization

We investigate the surface-catalyzed dissociation of the archetypal molecular switch azobenzene on the Cu(111) surface. Based on X-ray photoelectron spectroscopy, normal incidence X-ray standing waves and density functional theory calculations a detailed picture of the coverage-induced formation of phenyl nitrene from azobenzene is presented. Furthermore, a comparison to the azobenzene/Ag(111) interface provides insight into the driving force behind the dissociation on Cu(111). The quantitative decay of azobenzene paves the way for the creation of a defect free, covalently bonded monolayer. Our work suggests a route of surface functionalization via suitable azobenzene derivatives and the on surface synthesis concept, allowing for the creation of complex immobilized molecular systems

Does face inversion change spatial frequency tuning?

International audienceThe authors examined spatial frequency (SF) tuning of upright and inverted face identification using an SF variant of the Bubbles technique (F. Gosselin & P. G. Schyns, 2001). In Experiment 1, they validated the SF Bubbles technique in a plaid detection task. In Experiments 2a-c, the SFs used for identifying upright and inverted inner facial features were investigated. Although a clear inversion effect was present (mean accuracy was 24% higher and response times 455 ms shorter for upright faces), SF tunings were remarkably similar in both orientation conditions (mean r = .98; an SF band of 1.9 octaves centered at 9.8 cycles per face width for faces of about 6 degrees ). In Experiments 3a and b, the authors demonstrated that their technique is sensitive to both subtle bottom-up and top-down induced changes in SF tuning, suggesting that the null results of Experiments 2a-c are real. The most parsimonious explanation of the findings is provided by the quantitative account of the face inversion effect: The same information is used for identifying upright and inverted inner facial features, but processing has greater sensitivity with the former

Interacting Interactions: A Study on the Interplay of Molecule-Molecule and Molecule-Substrate Interactions at Metal-Organic Interfaces

In this work a surface science study on metal-organic interfaces is presented to resolve their geometric and electronic properties and study the interplay of molecule molecule and molecule-substrate interactions. The organic molecules benzene, azobenzene, 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA), and terephthalicacid (TPA) are deposited on low index Ag and Cu surfaces to form monolayer andsub-monolayer structures which are investigated by normal incidence X-ray standing waves and angle resolved photoemission spectroscopy, which leads to several surprising findings. Investigating the adsorption of benzene, we find it physisorbed in a flat geometry for benzene on Ag(111). Enhancing the molecule-substrate interaction by exchanging Ag(111) with the stronger interacting Cu(111) is expected to simply lower the adsorption height. However, we find flat molecules at an elevated adsorption height forbenzene/Cu(111), which seem to be stabilized via intermolecular interactions due to the coexistence with upright standing benzene molecules. The interplay of molecule-molecule and molecule-substrate interactions is further explored on a metal-organic network formed by codeposition of TPA and Fe atoms on Cu(100). The coordination of TPA molecules by the Fe atoms reduces the TPA substrate interaction. An additional sitespecific adsorption of oxygen again alters this balance. In case of PTCDA a comprehensive study for its adsorption on low index Ag surfacesis presented. From linking the geometric and electronic stucture properties, it is understood that the electron density spill-out of the surface and its uptake by the adsorbing molecule is a decisive molecule-substrate interaction channel. This explains the finding that the resulting binding energies of the lowest unoccupied molecular orbital (LUMO) as well as the adsorption height of PTCDA on Ag are determined by the work function. Moving to the archetypal molecular switch azobenzene, which is studied on Cu(111), three different azobenzene monolayer phases which are formed along with a coverage dependent dissociation of the molecule are revealed. The higher the density of molecules get, the stronger molecule-molecule interactions become and force the molecule to bend. However, its strong molecule-substrate bond prevents a conformational change and the resulting stress ultimately leads to a dissociation. The surprising results of this work show that the understanding of interactions at metal-organic interfaces is still only rudimentary and stress the importance of further fundamental research

Overview and key findings from the global economic model comparison component of the Agricultural Intercomparison and Improvement Project (AgMIP)

Recent studies assessing plausible futures for agricultural markets and global food security have had contradictory outcomes. Ten global economic models that produce long-term scenarios were asked to compare a reference scenario with alternate socio-economic, climate change and bioenergy scenarios using a common set of key drivers. Results suggest that, once general assumptions are harmonized, the variability in general trends across models declines, and that several common conclusions are possible. Nonetheless, differences in basic model parameters, sometimes hidden in the way market behavior is modeled, result in significant differences in the details. This holds for both the common reference scenario and for responses to the various shocks applied. We conclude that agro-economic modelers aiming to inform the agricultural and development policy debate require better data and analysis on both economic behavior and biophysical drivers. More interdisciplinary modeling efforts are required to cross-fertilize analyses at different scales

Overview and key findings from the global economic model comparison component of the Agricultural Intercomparison and Improvement Project (AgMIP)

Recent studies assessing plausible futures for agricultural markets and global food security have had contradictory outcomes. Ten global economic models that produce long-term scenarios were asked to compare a reference scenario with alternate socio-economic, climate change and bioenergy scenarios using a common set of key drivers. Results suggest that, once general assumptions are harmonized, the variability in general trends across models declines, and that several common conclusions are possible. Nonetheless, differences in basic model parameters, sometimes hidden in the way market behavior is modeled, result in significant differences in the details. This holds for both the common reference scenario and for responses to the various shocks applied. We conclude that agro-economic modelers aiming to inform the agricultural and development policy debate require better data and analysis on both economic behavior and biophysical drivers. More interdisciplinary modeling efforts are required to cross-fertilize analyses at different scales

Single Component and Compound Monolayers of CuPc and PTCDA on a Ag(110) Surface

This work is motivated by the recent international effort to create an experimentally self-sustained dynamo. The dynamo effect, whose existence was proposed by Larmor at the beginning of the 20th century, is believed to be the explanation for the magnetic field of Earth and other celestial bodies due to the flow of a conducting fluid. In order to numerically study the von Kármán flow, which models the configuration of the dynamo experiment implemented at Cadarache, we have developed a new numerical approach for solving the magnetohydrodynamic equations in potential formulation in a finite cylindrical geometry. The poloidal-toroidal decomposition has been used to ensure the solenoidal character of the velocity and magnetic fields. We use the influence matrix technique to impose the boundary conditions for the velocity and the continuity between the internal and external magnetic fields. The computational power of the code, which is the result of the MPI-based parallelization, enabled us to investigate two problems concerning turbulence in cylindrical geometry: axisymmetric turbulence and a bifurcation between turbulent flows.Ce travail est motivé par l'effort international actuel de créer expérimentalement une dynamo fluide auto-entretenue. L'effet dynamo, dont l'existence a été prevu par Larmor au début du XXème siècle, est considéré comme étant responsable de la production du champ matnétique terrestre et d'autres objets célestes par l'intermédiaire de l'écoulement d'un fluide conducteur. Afin d'étudier numériquement l'écoulement de von Kármán, qui modélise la configuration d'une expérience dynamo mise en place à Cadarache, nous avons développé une approche numérique originale permettant la résolution des équations magnétohydrodynamiques dans une géométrie dylindrique en formulation potentielle. La décomposition en potentiels poloïdal et toroïdal a été utilisée pour garantir la nature solénoïdale des champs de vitesse et magnétique. Nous utilisons la technique de la matrice d'influence pour satisfaire aux conditions aux limites et aux conditions de continuité du champ magnétique à la paroi du cylindre. La grande puissance de calcul, résultant de la parallélisation MPI du code, a presmis de l'appliquer deux problèmes concernant la turbulence dans la géométrie cylindrique : la turbulence axisymétrique et une bifurcation entre états turbulents

Modification of the PTCDA-Ag bond by forming a heteromolecular bilayer film

The understanding of the fundamental physical properties of metal-organic and organic-organic interfaces is crucial for improving the performance of organic electronic devices. This is particularly true for (multilayer) systems containing several molecular species due to their relevance for donor-acceptor systems. A prototypical heteromolecular bilayer system is copper-II-phthalocyanine (CuPc) on 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on Ag(111). In an earlier work we have reported a commensurate registry between both organic layers and an enhanced charge transfer from the Ag substrate into the organic bilayer film [Phys. Rev. Lett. 108, 106103 (2012)], which both indicate an unexpectedly strong intermolecular interaction across the organic-organic interface. Here we present new details regarding electronic and geometric structure for the same system. In particular, we provide evidence that the enhanced charge transfer from the substrate into the organic bilayer does not involve CuPc electronic states, hence, there is no significant charge transfer into the second organic layer. Furthermore, we report vertical bonding distances revealing a shortening of the PTCDA-Ag(111) distance upon CuPc adsorption. Thus, electronic and geometric properties (charge transfer and bonding distance, respectively) both indicate a strengthening of the PTCDA-Ag(111) bond upon CuPc adsorption. We explain these findings—in particular the correlation between CuPc adsorption and increased charge transfer into PTCDA—in a model involving an intermolecular screening mechanism