Dipole formation at metal/PTCDA interfaces: Role of the Charge Neutrality Level

The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface barrier is analyzed using weak-chemisorption theory. The electronic structure of the uncoupled PTCDA molecule and of the metal surface is calculated. Then, the induced density of interface states is obtained as a function of these two electronic structures and the interaction between both systems. This induced density of states is found to be large enough (even if the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital. We conclude that the metal/PTCDA interface molecular level alignment is due to the electrostatic dipole created by the charge transfer between the two solids.Comment: 6 page

Point defects on graphene on metals

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in free-standing graphene layers

Augmentation of the mechanical and chemical resistance characteristics of an Al2O3-based refractory by means of high power diode laser surface treatment

Augmentation of the wear rate and wear life characteristics of an Al2O3-based refractory within both normal and corrosive (NaOH and HNO3) environmental conditions was effected by means of high power diode laser (HPDL) surface treatment. Life assessment testing revealed that the HPDL generated glaze increased the wear life of the Al2O3-based refractory by 1.27 to 13.44 times depending upon the environmental conditions. Such improvements are attributed to the fact that after laser treatment, the microstructure of the Al2O3-based refractory was altered from a porous, randomly ordered structure, to a much more dense and consolidated structure that contained fewer cracks and porosities. In a world economy that is increasingly placing more importance on material conservation, a technique of this kind for delaying the unavoidable erosion (wear) and corrosion that materials such as the Al2O3-based refractory must face may provide an economically attractive option for contemporary engineers

A theoretical study of the low-lying excited states of thieno[3,4-b]pyrazine

The low-lying electronic excited states of thieno[3,4-b]pyrazine have been studied using the multiconfigurational second-order perturbation CASPT2 theory with extended atomic natural orbital basis sets. The CASPT2 results allow for a full interpretation of the electronic absorption and emission spectra and provide valuable information for the rationalization of the experimental data. The nature, position, and intensity of the spectral bands have been analyzed in detail. A preliminary comparative study of the ground-state geometry of thieno[3,4-b]pyrazine has been performed at the coupled cluster single and doubles and density functional theory levels using a variety of correlation-consistent basis sets. Thieno[3,4-b]pyrazine exhibits a polyene-like structure in the ground state due to the bond localization in the pyrazine moiety. An aromatization of the pyrazine unit is predicted for the lowest-energy electronic excited [email protected] [email protected] [email protected]

Barrier formation at metal/organic interfaces: dipole formation and the Charge Neutrality Level

The barrier formation for metal/organic semiconductor interfaces is analyzed within the Induced Density of Interface States (IDIS) model. Using weak chemisorption theory, we calculate the induced density of states in the organic energy gap and show that it is high enough to control the barrier formation. We calculate the Charge Neutrality Levels of several organic molecules (PTCDA, PTCBI and CBP) and the interface Fermi level for their contact with a Au(111) surface. We find an excellent agreement with the experimental evidence and conclude that the barrier formation is due to the charge transfer between the metal and the states induced in the organic energy gap.Comment: 7 pages, Proceedings of ICFSI-9, Madrid, Spain (September 2003), special issue of Applied Surface Science (in press

Submodule power losses balancing algorithms for the modular multilevel converter

Tolerance and component aging can cause signif¬icant differences in the capacitance values of the submodules (SMs) in a modular multilevel converter (MMC). Depending on the modulation technique, capacitance mismatches may produce uneven switching transitions of the SMs, hence imbalances in the power losses that can lead to reliability problems. In this paper, a new algorithm that helps to achieve evenly distributed switching and conduction losses within the converter SMs is presented. The proposed algorithm is based on a modification of the common voltage balancing algorithms, balancing a weighted function of voltage and losses. Even distribution of power losses is achieved at the cost of slightly increasing the capacitor voltage ripples. The effectiveness of the strategy has been demonstrated by simulation results of a high-power grid-connected MMC

Tug-of-war between corrugation and binding energy: revealing the formation of multiple moiré patterns on a strongly interacting graphene-metal system

The formation of multidomain epitaxial graphene on Rh(111) under ultra-high vacuum (UHV) conditions has been characterized by scanning tunnelling microscopy (STM) measurements and density functional theory (DFT) calculations. At variance with the accepted view for strongly interacting graphene-metal systems, we clearly demonstrate the formation of different rotational domains leading to multiple moiré structures with a wide distribution of surface periodicities. Experiments reveal a correlation between the STM apparent corrugation and the lattice parameter of the moiré unit cell, with corrugations of just 30-40 pm for the smallest moirés. DFT calculations for a relevant selection of these moiré patterns show much larger height differences and a non-monotonic behaviour with the moiré size. Simulations based on non-equilibrium Green's function (NEGF) methods reproduce quantitatively the experimental trend and provide a detailed understanding of the interplay between electronic and geometric contributions in the STM contrast of graphene systems. Our study sheds light on the subtle energy balance among strain, corrugation and binding that drives the formation of the moiré patterns in all graphene/metal systems and suggests an explanation for the success of an effective model only based on the lattice mismatch. Although low values of the strain energy are a necessary condition, it is the ability of graphene to corrugate in order to maximize the areas of favourable graphene-metal interactions that finally selects the stable configurationsWe acknowledge financial support from Spanish grants MAT2013-41636-P, MAT2011-23627, MAT2011-26534, CSD2010-00024 (MINECO, Spain) and S2009/MAT-1467 (CAM, Spain). A.J.M.G. was supported by a Marie Curie action under the Seventh Framework Programme. P.P. was supported by the Ramón y Cajal Progra

Syntheses, supramolecular architectures and photoluminescence properties of Zn(II) complexes based on 3,5-dihydroxybenzoic and pyridine/pyrazole derived ligands

Five new coordination compounds [Zn(μ‑3,5‑DHB)2(H2O)2]n (1a), [Zn(μ‑3,5‑DHB)(μ‑OH2) (H2O)2]n·(3,5‑DHB)n·(4H2O)n (1b), [Zn(3,5‑DHB)2(Isna)2]·2H2O (2), [Zn(3,5‑DHB)2(4‑Acpy)2]·3H2O (3) and [Zn (3,5‑DHB)2(3‑Mepz)2]·H2O (4) (3,5‑HDHB=3,5‑dihydroxybenzoic, Isna=isonicotinamide, 4‑Acpy=4‑acetylpyridine and 3‑Mepz=3‑methylpyrazole) were synthesized in water or water-methanol as solvents. All these compounds have been characterized by elemental analysis, FTIR-ATR and 1H NMR spectroscopies and Powder X-ray diffraction (PXRD). For compounds 1b-4, X-ray crystal structures have been determined. In these compounds, 3,5‑DHB ligand displays different coordination modes. Complex 1b is a coordination polymer, while the addition of the pyridine/pyrazole ligands in the reaction provokes the formation of monomeric compounds (2-4). Moreover, the crystal packing indicates that these complexes expand into 2D/ 3D network structures mainly by intermolecular hydrogen bond interactions. Finally, the photoluminescent properties of these complexes in solid state have also been investigated. The strong emission observed for 1b indicates that it may be a good candidate for photoluminescent devices