The effect of long-range order on intermolecular interactions in organic semiconductors: zinc octaethyl porphyrin molecular thin film model systems.

In order to optimize the performance of devices based on porphyrin thin films it is of great importance to gain a physical understanding of the various factors which affect their charge transport and light-harvesting properties. In this work, we have employed a multi-technique approach to study vacuum deposited zinc octaethyl porphyrin (ZnOEP) thin films with different degrees of long-range order as model systems. An asymmetrical stretching of the skeletal carbon atoms of the porphyrin conformer has been observed and attributed to ordered molecular stacking and intermolecular interactions. For ordered films, a detailed fitting analysis of the X-ray absorption near edge structure (XANES) using the MXAN code establishes a symmetry reduction in the molecular conformer involving the skeletal carbon atoms of the porphyrin ring; this highlights the consequences of increased π-π stacking of ZnOEP molecules adopting the triclinic structure. The observed asymmetrical stretching of the π conjugation network of the porphyrin structure can have significant implications for charge transport and light harvesting, significantly influencing the performance of porphyrin based devices.

Local environment of metal ions in phthalocyanines: K-edge X-ray absorption spectra.

We report a detailed study of the K-edge X-ray absorption spectra of four transition metal phthalocyanines (MPc, M = Fe, Co, Cu and Zn). We identify the important single and multiple scattering contributions to the spectra in the extended energy range and provide a robust treatment of thermal damping; thus, a generally applicable model for the interpretation of X-ray absorption fine structure spectra is proposed. Consistent variations of bond lengths and Debye Waller factors are found as a function of atomic number of the metal ion, indicating a variation of the metal-ligand bond strength which correlates with the spatial arrangement and occupation of molecular orbitals. We also provide an interpretation of the near edge spectral features in the framework of a full potential real space multiple scattering approach and provide a connection to the local electronic structure.

Solid state effects on the electronic structure of H2OEP.

We present the results of a joint experimental and theoretical investigation concerning the effect of crystal packing on the electronic properties of the H2OEP molecule. Thin films, deposited in ultra high vacuum on metal surfaces, are investigated by combining valence band photoemission, inverse photoemission, and X-ray absorption spectroscopy. The spectra of the films are compared, when possible, with those measured in the gas phase. Once many-body effects are included in the calculations through the GW method, the electronic structure of H2OEP in the film and gas phase are accurately reproduced for both valence and conduction states. Upon going from an isolated molecule to the film phase, the electronic gap shrinks significantly and the lowest unoccupied molecular orbital (LUMO) and LUMO + 1 degeneracy is removed. The calculations show that the reduction of the transport gap in the film is entirely addressable to the enhancement of the electronic screening.

Surface octahedral distortions and atomic design of perovskite interfaces.

Atomic engineering of perovskite films and interfaces is significantly improved by in situ optimization of reflection high-energy electron diffraction (RHEED) features resulting from surface BO6 octahedral rotations seen during molecular-beam epitaxy growth. This approach yields Sr-doped manganite films across the phase diagram with magnetotransport properties that are, for the first time, identical to bulk single crystals. Careful structural analysis of manganite/titanate interfaces shows that cation intermixing and unit cell dilations are eliminated, while BO6 rotations and Jahn-Teller-type elongations are nearly completely suppressed at the interface.

Interaction of atomic hydrogen with the beta-SiC(100) 3 x 2 surface and subsurface.

We investigate clean and atomic hydrogen exposed beta-SiC(100) 3 x 2 surfaces by synchrotron radiation-based Si 2p core-level photoemission spectroscopy. The clean 3 x 2 surface reconstruction exhibits three surface and subsurface components. Upon hydrogen exposures, those surface and subsurface components are shifted to lower binding energies by large values, indicating significant charge transfer to the surface and subsurface regions, in excellent agreement with the recently discovered H-induced beta-SiC(100) 3 x 2 surface metallization. In addition, the interaction of hydrogen results in a large reactive component at Si 2p supporting an asymmetric charge transfer in the third plane below the surface, in agreement with previous experimental investigations. However, the results are inconsistent with recent ab initio theoretical "frozen" calculations predicting H atom to be in a bridge-bond position.

Comparative evaluation of corneal epithelial permeability after the use of diclofenac 0.1% and flurbiprofen 0.03% after phacoemulsification.

PURPOSE: To assess the corneal epithelial function after prolonged topical administration of diclofenac 0.1% and flurbiprofen 0.03% single-dose eyedrops. SETTING: University Eye Clinic of Trieste, Trieste, Italy. METHODS: This randomized prospective study comprised 24 patients scheduled for phacoemulsification. The patients were randomly assigned to receive diclofenac or flurbiprofen eyedrops for 2 months after surgery. Corneal epithelial permeability was determined by fluorophotometry 7, 37, and 67 days after surgery. RESULTS: An increase in corneal epithelial permeability was observed in the diclofenac group 37 and 67 days after surgery. No epithelial function alterations occurred in the flurbiprofen group. CONCLUSIONS: Subclinical impairment of the epithelial function was observed during topical treatment with diclofenac 0.1% single-dose eyedrops after phacoemulsification. The mechanism responsible for this effect remains unknown.

NEXAFS multiple scattering calculations of KO2.

Since many years the oxidation of alkali metals has being attracted much interest due to the catalytic properties of metal promoters and the simple electronic structure of alkali atoms. The alkali-oxides phase diagram indicates that the interaction of oxygen with alkali metals can lead to the formation of different atomic O2- ions and molecular O2(-) and O(2)2- ions. Potassium superoxide has been prepared in situ and high resolution O k-edge absorption NEXAFS spectra have been measured at the VUV beam-line at ELETTRA facility. The experimental data have been analyzed by multiple scattering approach deriving many geometrical and electronic details. In particular, we have found that the growth material structure is of the KO2 type with an O-O distance of about 1.35A and that the transition involving single pi molecular empty state of the superoxide O2(-) anion has a fine structure. Multiple Scattering self consistent calculation indicates that the bond between oxygen anion and K atom is totally ionic and that the fine structure is essentially due to solid state effects.

Study of C60/Au(110)-p(6x5) reconstruction from In-plane X-Ray diffraction data

Fullerene molecules absorbed on the highly anisotropic Au(110)-p(1x2) surface induce an ordered p(6x5) superstructure that has been solved by applying the 2D "direct methods" difference sum function to the surface x-ray diffraction data set. We found that the C (60)-gold interface is structurally much more complex than the one previously suggested by scanning tunneling microscopy data [J. K. Gimzewski, S. Modesti, and R. R. Schlittler, Phys. Rev. Lett. 72, 1036 (1994)]. Indeed a large fraction of Au surface atoms are displaced from their original positions producing microscopic pits that may accommodate the fullerene molecules.