Electronic structure changes during the surface-assisted formation of a graphene nanoribbon

High conductivity and a tunability of the band gap make quasi-one-dimensional graphene nanoribbons (GNRs) highly interesting materials for the use in field effect transistors. Especially bottom-up fabricated GNRs possess well-defined edges which is important for the electronic structure and accordingly the band gap. In this study we investigate the formation of a sub-nanometer wide armchair GNR generated on a Au(111) surface. The on-surface synthesis is thermally activated and involves an intermediate non-aromatic polymer in which the molecular precursor forms polyanthrylene chains. Employing angle-resolved two-photon photoemission in combination with density functional theory calculations we find that the polymer exhibits two dispersing states which we attribute to the valence and the conduction band, respectively. While the band gap of the non-aromatic polymer obtained in this way is relatively large, namely 5.25 ± 0.06 eV, the gap of the corresponding aromatic GNR is strongly reduced which we attribute to the different degree of electron delocalization in the two systems

End states, band gap, and dispersion

Angle-resolved two-photon photoemission and high-resolution electron energy loss spectroscopy are employed to derive the electronic structure of a subnanometer atomically precise quasi-one-dimensional graphene nanoribbon (GNR) on Au(111). We resolved occupied and unoccupied electronic bands including their dispersion and determined the band gap, which possesses an unexpectedly large value of 5.1 eV. Supported by density functional theory calculations for the idealized infinite polymer and finite size oligomers, an unoccupied nondispersive electronic state with an energetic position in the middle of the band gap of the GNR could be identified. This state resides at both ends of the ribbon (end state) and is only found in the finite sized systems, i.e., the oligomers

Tetrahalidocuprates(II)-structure and EPR spectroscopy. Part 1: Tetrabromidocuprates(II)

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Tetrahalidocuprates(II) show a high degree of structural flexibility. We present the results of crystallographic and electron paramagnetic resonance (EPR) spectroscopic analyses of four new tetrabromidocuprate(II) compounds and compare the results with previously reported data. The cations in the new compounds are the sterically demanding benzyltriphenylphosphonium, methyltriphenylphosphonium, tetraphenylphosphonium, and hexadecyltrimethylammonium ions; they were used to achieve a reasonable separation of the paramagnetic Cu(II) ions for EPR spectroscopy. X-Ray crystallography shows that in all four complexes the [CuBr4]2− units have a distorted tetrahedral coordination geometry which is in agreement with DFT calculations. The EPR hyperfine structure was not resolved. This is due to the exchange broadening resulting from still incomplete separation of the paramagnetic Cu(II) centres. Nevertheless, the principal values of the electron Zeemann tensor (gand g⊥) of the complexes could be determined. A correlation of structural (X-ray) parameters with the spin density at the copper centres (DFT) is well reflected in the EPR spectra of the bromidocuprates. This enables the correlation of X-ray and EPR parameters to predict the structure of tetrabromidocuprates in physical states other than the crystalline state. As a result, we provide a method to structurally characterize [CuBr4]2− in, for example, ionic liquids or in solution, which has important implications for e.g. catalysis or materials science

Current concepts in the prevention of pathogen transmission via blood/plasma-derived products for bleeding disorders

The pathogen safety of blood/plasma-derived products has historically been a subject of significant concern to the medical community. Measures such as donor selection and blood screening have contributed to increase the safety of these products, but pathogen transmission does still occur. Reasons for this include lack of sensitivity/specificity of current screening methods, lack of reliable screening tests for some pathogens (e.g. prions) and the fact that many potentially harmful infectious agents are not routinely screened for. Methods for the purification/inactivation of blood/plasma-derived products have been developed in order to further reduce the residual risk, but low concentrations of pathogens do not necessarily imply a low level of risk for the patient and so the overall challenge of minimising risk remains. This review aims to discuss the variable level of pathogenic risk and describes the current screening methods used to prevent/detect the presence of pathogens in blood/plasma-derived products

Eine Fallstudie zum epd-Basisdienst

## Contents 0 Title, contents 0 1 Introduction 1 2 H scattering at Cu(100) [19] 7 2.1 Introduction 7 2.2 Models and potential 11 2.3 Propagation methods 20 2.4 Results and discussion 41 2.5 Conclusions 57 3 Image potential state dynamics o f electrons at Cu(100) 59 3.1 Introduction 59 3.2 Model and eigenstates 65 3.3 Propagation methods 84 3.4 Results and discussion 91 3.5 Summary and conclusions 109 4 Summary and conclusions 111 A Numerical methods 115 A.1 The fast Fourier transform [58] 115 A.2 The split operator technique 118 A.3 The Euler integration method [58] 119 A.4 The Runge Kutta 4th order method [58] 119 Bibliography# Abstract The dynamical interaction of atoms, electrons, or molecules with solid surfaces is not only of practical importance (heterogeneous catalysis, microstructuring of materials), but also of fundamental and methodological interest. In particular when the "adsorbate" is treated quantum mechanically, the problem arises of how to couple it to the practically infinitely many vibrations (phonons) and / or electronic degrees of freedom of the surface. In this thesis the focus will be on two examples where an atom's or an electron's quantum dynamics is influenced by the internal substrate degrees of freedom; in particular, various methods to treat this kind of "system plus bath" problems efficiently within time-dependent quantum mechanics will be described. In the first example the influence of phonons is studied for the scattering of atomic hydrogen at a Cu(100) surface. Three different propagation techniques, namely reduced-dimensionality but ``exact'' wave packet propagation, the Time- Dependent-Self-Consistent-Field (TDSCF) method, and the ``mean-field'' (Ehrenfest) mixed Quantum-Classical-Molecular-Dynamics scheme (QCMD), are tested against each other and compared with classical trajectory results, and with rigid surface calculations. In the second example the dynamics and spectroscopy of electrons at surfaces, created by laser excitation of metal electrons into empty, so-called "image charge" states will be outlined. The latter are coupled to the electron reservoir of the metal surface, which therefore cannot simply be considered as inert, or rigid. Again for the example system Cu(100), recently measured time and energy resolved Two-Photon-Photoemission (2PPE) spectra will be calculated, now with the help of open system density matrix theory.# Zusammenfassung Die Dynamik von Atomen, Elektronen oder Molekülen an Festkörperoberflächen ist nicht nur von praktischer Bedeutung (heterogene Katalyse, Mikrostrukturierung von Materialien), sondern auch von fundamentalem Interesse unter einem theoretischen Blickwinkel. Insbesondere wenn das ,,Adsorbat'' quantenmechanisch behandelt wird, ergibt sich das Problem, wie die praktisch unendlich vielen vibratorischen (Phononen) und elektronischen Freiheitsgrade der Oberfläche in einem geeigneten Modell repräsentiert werden können. In dieser Arbeit werden zwei Beispiele behandelt, in denen die (Quanten-) Dynamik eines Atoms bzw. eines Elektrons durch die inneren Freiheitsgrade der Oberfläche beeinflußt wird. Dabei wird insbesondere eine Reihe von Methoden vorgestellt, um diese Art von ,,System-Bad'' Problemen effektiv innerhalb einer zeitabhängigen quantenmechanischen Beschreibung zu modellieren. Im ersten Beispiel wird der Einfluß von Phononen auf die Streuung von atomarem Wasserstoff an einer Cu(100) Oberfläche behandelt. Drei verschiedene Propagationstechniken werden getestet und untereinander und mit Ergebnissen aus klassischen Trajektorienrechnungen und Rechnungen für eine rigide Oberfläche verglichen. Diese Propagationstechniken sind ,,exakte'' Wellenpaketpropagation in reduzierter Dimensionalität, der Time-Dependent- Self-Consistent-Field (TDSCF) Ansatz und das ``mean-field'' (Ehrenfest) gemischt quanten-klassische Quantum-Classical-Molecular-Dynamics (QCMD) Schema. Im zweiten Beispiel wird die Dynamik und Spektroskopie von Elektronen an Oberflächen beschrieben. Diese Elektronen werden durch einen Laserpuls aus der Metalloberfläche in sogenannte Bildladungszustände angeregt. Die Oberfläche ist hier wiederum Cu(100). Die Bildladungszustände sind an das Elektronenreservoir der Metalloberfläche gekoppelt, welche deshalb nicht als inert oder rigide angesehen werden kann. Es werden kürzlich gemessene, energie- und zeitaufgelöste 2-Photonen-Photo-Emissions-Spektren (2PPE) mit Hilfe der Dichtematrixtheorie offener Quantensysteme berechnet

Photoisomerization Ability of Molecular Switches Adsorbed on Au(111): Comparison between Azobenzene and Stilbene Derivatives

High resolution electron energy loss spectroscopy and two-photon photoemission was employed to derive the adsorption geometry, electronic structure, and the photoisomerization ability of the molecular switch tetra-tert-butyl-stilbene (TBS) on Au(111). The results are compared with the azobenzene analogue, tetra-tert-butyl-azobenzene (TBA), adsorbed on Au(111). TBS was found to adsorb on Au(111) in a planar (trans) configuration similar to TBA. The energetic positions of several TBS-induced electronic states were determined, and in comparison to TBA, the higher occupied molecular states (e.g., the highest occupied molecular orbital, HOMO) are located at similar energetic positions. While surface-bound TBA can be switched with light between its trans and cis configurations, in TBS this switching ability is lost. In TBA on Au(111), the trans → cis isomerization is driven by a substrate-mediated charge transfer process, whereby photogenerated hot holes in the Au d band lead to transient positive ion formation (transfer of the holes to the TBA HOMO level). Even though the energetic positions of the HOMOs in TBA and TBS are almost identical and thus a charge transfer should be feasible, this reaction pathway is obviously not efficient to induce the trans → cis isomerization in TBS on Au(111). Quantum chemical calculations of the potential energy surfaces for the free molecules support this conclusion. They show that cation formation facilitates the isomerization for TBA much more pronounced than for TBS due to the larger gradients at the Franck−Condon point and the much smaller barriers on the potential energy surface in the case of the TBA