Origin of Discrepancies in Inelastic Electron Tunneling Spectra of Molecular Junctions

We report inelastic electron tunneling spectroscopy (IETS) of multilayer molecular junctions with and without incorporated metal nano-particles. The incorporation of metal nanoparticles into our devices leads to enhanced IET intensity and a modified line-shape for some vibrational modes. The enhancement and line-shape modification are both the result of a low lying hybrid metal nanoparticle-molecule electronic level. These observations explain the apparent discrepancy between earlier IETS measurements of alkane thiolate junctions by Kushmerick \emph{et al.} [Nano Lett. \textbf{4}, 639 (2004)] and Wang \emph{et al.} [Nano Lett. \textbf{4}, 643 (2004)].Comment: 4 pages, 4 figures accepted for publication in Physical Review Letter

Comparison of XFEM and Voxelbased FEM for the Approximation of Discontinuous Stress and Strain at Material Interfaces

When analyzing bimaterial problems, often stress concentrations appear close to material interfaces and cause nonlinear effects like damage or plastic yielding. Therefore, a precise approximation of stresses at interfaces is desirable. However, jumps in material parameters lead to non-smooth solutions (i.e. kinks in the displacements and jumps in the stresses and strains), which reduce the accuracy of standard discretization methods including the finite element method (FEM). The popular extended finite element method (XFEM) belongs to the class of methods, where additional unknowns are introduced to approximate jumps of the unknowns on meshes which are not aligned to the interface or boundaries. The field of application is the stress analysis of microstructures of two-phase materials with high contrast between the phases including nonlinear effects. The focus of this paper is to considers the XFEM approximation quality of strains and displacements at material interfaces. XFEM and standard FEM solutions are compared for varying mesh sizes. Therefore an analytical solution for a spherical inclusion in a finite domain under specific loading conditions is considered. This analytical solution is then compared in detail to different finite element discretizations with standard FEM and XFEM

Multi-Scale Simulation of Viscoelastic Fiber-Reinforced Composites

This paper presents an effective algorithm to simulate the anisotropic viscoelastic behavior of a fiber-reinforced composite including the influence of the local geometric properties, like fiber-orientation and volume fraction. The considered composites consist of a viscoelastic matrix which is reinforced by elastic fibers. The viscoelastic composite behavior results anisotropic due to the local anisotropic fiber-orientations. The influence of the local time-dependent viscoelastic properties are captured within two elastic microscopic calculations for each fiberorientation in the composite part. These calculations can be performed within a preprocessing step, and thus no expensive, time-dependent viscoelastic multi-scale simulation has to be carried out to incorporate the local properties. The advantage of the presented approach is that the locally varying microscopic properties can be captured in a one-scale simulation within a commercial finite element tool like ABAQUS

First-principles calculation on the transport properties of molecular wires between Au clusters under equilibrium

Based on the matrix Green's function method combined with hybrid tight-binding / density functional theory, we calculate the conductances of a series of gold-dithiol molecule-gold junctions including benzenedithiol (BDT), benzenedimethanethiol (BDMT), hexanedithiol (HDT), octanedithiol (ODT) and decanedithiol (DDT). An atomically-contacted extended molecule model is used in our calculation. As an important procedure, we determine the position of the Fermi level by the energy reference according to the results from ultraviolet photoelectron spectroscopy (UPS) experiments. After considering the experimental uncertainty in UPS measurement, the calculated results of molecular conductances near the Fermi level qualitatively agree with the experimental values measured by Tao et. al. [{\it Science} 301, 1221 (2003); {\it J. Am. Chem. Soc.} 125, 16164 (2003); {\it Nano. Lett.} 4, 267 (2004).]Comment: 12 pages,8 figure

Zur Simulation von Klebeverbindungen für Scheibenbauteile mit Level-Set-Funktionen und erweiterter Finite-Elemente-Methode

Das Kleben ist noch eine relativ neue Art der Verbindung von Betonbauteilen. Bei der iterativen Optimierung der Fugengestalt wird eine Folge von unterschiedlichen Fugenverläufen analysiert. Um eine Neuvernetzung für jede einzelne Fugengestalt zu vermeiden und gleichzeitig die Verzerrungen und Spannungen an der Grenzfläche zwischen HPC-Platte und Klebefuge effizient und genau zu berechnen, wird in dieser Arbeit eine Variante der erweiterten Finite-Elemente-Methode (XFEM) als Strukturanalyseverfahren vorgeschlagen. Es wird gezeigt, dass die Methode sehr gut zur Strukturanalyse bei der Optimierung der Fugengestalt, die implizit über eine Level-Set-Funktion beschrieben wird, geeignet ist. Die Ergebnisse der Gestaltoptimierung werden diskutiert

Zur Simulation von Klebeverbindungen für Scheibenbauteile mit Level-Set-Funktionen und erweiterter Finite-Elemente-Methode

Das Kleben ist noch eine relativ neue Art der Verbindung von Betonbauteilen. Bei der iterativen Optimierung der Fugengestalt wird eine Folge von unterschiedlichen Fugenverläufen analysiert. Um eine Neuvernetzung für jede einzelne Fugengestalt zu vermeiden und gleichzeitig die Verzerrungen und Spannungen an der Grenzfläche zwischen HPC-Platte und Klebefuge effizient und genau zu berechnen, wird in dieser Arbeit eine Variante der erweiterten Finite-Elemente-Methode (XFEM) als Strukturanalyseverfahren vorgeschlagen. Es wird gezeigt, dass die Methode sehr gut zur Strukturanalyse bei der Optimierung der Fugengestalt, die implizit über eine Level-Set-Funktion beschrieben wird, geeignet ist. Die Ergebnisse der Gestaltoptimierung werden diskutiert

Giant Thermoelectric Effect from Transmission Supernodes

We predict an enormous order-dependent quantum enhancement of thermoelectric effects in the vicinity of a higher-order `supernode' in the transmission spectrum of a nanoscale junction. Single-molecule junctions based on 3,3'-biphenyl and polyphenyl ether (PPE) are investigated in detail. The nonequilibrium thermodynamic efficiency and power output of a thermoelectric heat engine based on a 1,3-benzene junction are calculated using many-body theory, and compared to the predictions of the figure-of-merit ZT.Comment: 5 pages, 6 figure

Crystalline silicate dust around evolved stars I. The sample stars

This is the first paper in a series of three where we present the first comprehensive inventory of solid state emission bands observed in a sample of 17 oxygen-rich circumstellar dust shells surrounding evolved stars. The data were taken with the Short and Long Wavelength Spectrographs on board of the Infrared Space Observatory (ISO) and cover the 2.4 to 195 micron wavelength range. The spectra show the presence of broad 10 and 18 micron bands that can be attributed to amorphous silicates. In addition, at least 49 narrow bands are found whose position and width indicate they can be attributed to crystalline silicates. Almost all of these bands were not known before ISO. We have measured the peak positions, widths and strengths of the individual, continuum subtracted bands. Based on these measurements, we were able to order the spectra in sequence of decreasing crystalline silicate band strength. We found that the strength of the emission bands correlates with the geometry of the circumstellar shell, as derived from direct imaging or inferred from the shape of the spectral energy distribution. This naturally divides the sample into objects that show a disk-like geometry (strong crystalline silicate bands), and objects whose dust shell is characteristic of an outflow (weak crystalline silicate bands). All stars with the 33.6 micron forsterite band stronger than 20 percent over continuum are disk sources. We define spectral regions (called complexes) where a concentration of emission bands is evident, at 10, 18, 23, 28, 33, 40 and 60 micron. We derive average shapes for these complexes and compare these to the individual band shapes of the programme stars.Comment: 41 pages, 20 figures, accepted by A&A. Tables 4 to 20 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A

Rapid and Complete Surface Modification with Strain‐Promoted Oxidation‐Controlled Cyclooctyne‐1,2‐Quinone Cycloaddition (SPOCQ)

Strain‐promoted oxidation‐controlled cyclooctyne‐1,2‐quinone cycloaddition (SPOCQ) between functionalized bicyclo[6.1.0]non‐4‐yne (BCN) and surface‐bound quinones revealed an unprecedented 100 % conjugation efficiency. In addition, monitoring by direct analysis in real time mass spectrometry (DART‐MS) revealed the underlying kinetics and activation parameters of this immobilization process in dependence on its microenvironment