Electron structure and charge transport properties of thiols and dithiocarbamates in self-assembled monolayers

It is accepted that the potential of molecular electronics for future device applications critically depends on the formation of a stable and defined link between the organic layer and the metallic contacts. Here we present investigations on dithiocarbamates used as an alternative connecting group for metals, from which their promising properties with respect to thermal stability and electrical conductance emerge. The structure and the electronic properties of dithiocarbamate- and comparable thiolate-based self-assembled monolayers is elucidated using photoelectron spectroscopy, scanning tunneling microscopy (STM), contact angle measurements and interlinked nanoparticle arrays. The experimental results are interpreted in the light of density functional theory calculations, that show the principal difference between the two anchor groups. In the first chapter of the thesis, we show that dithiocarbamates improve the electrical coupling to the metal due to the presence of delocalized electronic states at 0.5±0.1 eV below the Fermi level of Au. The significantly increased density of states at the interface, as revealed by photoelectron spectroscopy and density functional theory calculations, proves that these states are related to the hybridization of the metal d band with delocalized orbitals on the dithiocarbamate anchor group. A low charge injection barrier between the monolayer and the metal is the consequence. Finally, the improved stability of dithiocarbamates on gold is shown by thermal desorption experiments. In the second chapter, the overlayer structure of alkanethiol, benzyl-mercaptan and highly conjugated methyl-phenyl-dithiocarbamate self-assembled monolayers on Au(111) is studied by STM and the conductance of those monolayers measured by current-distance spectroscopy. Whereas alkanethiol monolayers exhibit the known c(4 x 2) overlayer structure, benzyl-mercaptan monolayers show a novel reconstruction, consisting of extended, striped phase domains having a commensurate, p(4½√3 x 2) overlayer structure with an oblique unit cell. In contrast, methyl-phenyl-dithiocarbamate monolayers are found to be disordered. The tunnelling decay constant β for the molecular medium, as well as the molecular conductance at the STM tip-monolayer contact point, are determined. A decay constant of β = 1/Å for alkanethiols and β = 0.5/Å for the phenyl ring is found, in line with reported values, whereas the methyl-phenyl-dithiocarbamate is roughly one order of magnitude more conductive than benzyl-mercaptan. In the last chapter, the structure and the electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides and of mixed dithioacetamide/alkanethiol monolayers are characterized. The co-assembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers, and it is found that small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles, using molecular lengths resulting from density functional theory calculations. The difference in the tunneling decay constant β measured for aromatic dithioacetamides (β = 0.74-0.76/Å) and for aliphatic dithioacetamides (β = 0.84-0.91/Å) highlights the influence of the conjugation within the cyclic core on molecular conductance. In conclusion, different alternative approaches have been used to determine the conductance of molecular junctions, and it is shown that molecules coupled to metals via the dithiocarbamate anchor group could overcome some of the fundamental limitations currently encountered in molecular electronics

Structural origins of the cohesive energy in metal-terpyridine oligomer thin-films

FeII-terpyridine based oligomers have attracted considerable interest as key constituents for the realization of highly robust, ultra-thin ordered layers of metal center oligomers (MCOs) for organic electronics applications. By using molecular simulations and nanotribology investigations, we report on the origins of the surprisingly high mechanical and thermal stability in this type of MCO layers, which finds its expression in nanowear resistance values of up to 1.5 μN for the MCO films, as well as in a thermal stability of two-terminal MCO junctions to temperatures up to ∼100 °C under electrical load. A theoretical analysis of the fundamental cohesive forces among the constituents within the context of an electrostatic model reveal that the cohesive energy is essentially based on Coulomb interactions among the ionic constituents of the oligomers, leading to an estimated cohesive energy per molar mass of 0.0132 eV mol g−1 for MCO layers that advantageously compare to the 0.0061 eV mol g−1 reported for pentacene crystals

Molecular Origin of the Charge Carrier Mobility in Small Molecule Organic Semiconductors

Small-molecule organic semiconductors are used in a wide spectrum of applications, ranging from organic light emitting diodes to organic photovoltaics. However, the low carrier mobility severely limits their potential, e.g., for large area devices. A number of factors determine mobility, such as molecular packing, electronic structure, dipole moment, and polarizability. Presently, quantitative ab initio models to assess the influence of these molecule-dependent properties are lacking. Here, a multiscale model is presented, which provides an accurate prediction of experimental data over ten orders of magnitude in mobility, and allows for the decomposition of the carrier mobility into molecule-specific quantities. Molecule-specific quantitative measures are provided how two single molecule properties, the dependence of the orbital energy on conformation, and the dipole-induced polarization determine mobility for hole-transport materials. The availability of first-principles based models to compute key performance characteristics of organic semiconductors may enable in silico screening of numerous chemical compounds for the development of highly efficient optoelectronic devices

Ultrarobust Thin‐Film Devices from Self‐Assembled Metal–Terpyridine Oligomers

Ultrathin molecular layers of Fe(II) -terpyridine oligomers allow the fabrication of large-area crossbar junctions by conventional electrode vapor deposition. The junctions are electrically stable for over 2.5 years and operate over a wide range of temperatures (150-360 K) and voltages (±3 V) due to the high cohesive energy and packing density of the oligomer layer. Electrical measurements reveal ideal Richardson-Shottky emission in surprising agreement with electrochemical, optical, and photoemission data

Low Energy Analyzing Powers in Pion-Proton Elastic Scattering

Analyzing powers of pion-proton elastic scattering have been measured at PSI with the Low Energy Pion Spectrometer LEPS as well as a novel polarized scintillator target. Angular distributions between 40 and 120 deg (c.m.) were taken at 45.2, 51.2, 57.2, 68.5, 77.2, and 87.2 MeV incoming pion kinetic energy for pi+ p scattering, and at 67.3 and 87.2 MeV for pi- p scattering. These new measurements constitute a substantial extension of the polarization data base at low energies. Predictions from phase shift analyses are compared with the experimental results, and deviations are observed at low energies.Comment: 15 pages, 4 figure

Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ resulting in formation of a direct covalent sigma bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated pi-system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the pi-system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls with two to four phenyl units show a hundred-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling as they exhibit an exponential dependence of conductance with oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission with a cross-over to tunneling for the longer oligomers.Comment: Accepted to the Journal of the American Chemical Society as a Communication

pi+- p differential cross sections at low energies

Differential cross sections for pi- p and pi+ p elastic scattering were measured at five energies between 19.9 and 43.3 MeV. The use of the CHAOS magnetic spectrometer at TRIUMF, supplemented by a range telescope for muon background suppression, provided simultaneous coverage of a large part of the full angular range, thus allowing very precise relative cross section measurements. The absolute normalisation was determined with a typical accuracy of 5 %. This was verified in a simultaneous measurement of muon proton elastic scattering. The measured cross sections show some deviations from phase shift analysis predictions, in particular at large angles and low energies. From the new data we determine the real part of the isospin forward scattering amplitude.Comment: 13 pages, 5 figures. To appear in PL

Stalingrad erinnern: Zur Historisierung eines Mythos

In den über 60 Jahren, die seit den Ereignissen bei Stalingrad von 1942/43 vergangen sind, ist das Geschehen in den Erinnerungskulturen zahlreicher Länder immer wieder neu entworfen, umgewandelt und mit unterschiedlichen Deutungsebenen verknüpft worden. Die Muster der Erinnerung waren und sind eng verbunden mit den gesellschaftlichen Verhältnissen und Bedürfnissen zum Zeitpunkt ihrer Verbreitung, und sie standen bzw. stehen noch immer in einem gespannten Verhältnis zu wissenschaftlichen Erkenntnissen über "Stalingrad": So ging der Krieg für die Deutschen bereits im Winter 1941 verloren, und es folgten für Deutschland verlustreichere Kriegsphasen. Dass "Stalingrad" in der deutschen und sowjetischen Erinnerungskultur bis heute eine herausragende Bedeutung einnimmt, steht dazu in offensichtlichem Widerspruch und kann nur gedächtnisgeschichtlich erklärt werden