Search

The bonding and the temperature-driven metalation of 2H-tetraphenylporphyrin (2H-TPP) on the Cu(111) surface under ultrahigh vacuum conditions were investigated by a combination of x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy with density functional theory calculations. Thin films were prepared by organic molecular beam epitaxy and subsequent annealing. Our systematic study provides an understanding of the changes of the spectroscopic signature during adsorption and metalation. Specifically, we achieved a detailed peak assignment of the 2H-TPP multilayer data of the C1s and the N1s region. After annealing to 420 K both XPS and NEXAFS show the signatures of a metalloporphyrin, which indicates self-metalation at the porphyrin-substrate interface, resulting in Cu-TPP. Furthermore, for 2H-TPP monolayer samples we show how the strong influence of the copper surface is reflected in the spectroscopic signatures. Adsorption results in a strongly deformed macrocycle and a quenching of the first NEXAFS resonance in the nitrogen edge suggesting electron transfer into the LUMO. For Cu-TPP the spectroscopic data indicate a reduced interaction of first-layer molecules with the substrate as demonstrated by the relaxed macrocycle geometry. [ABSTRACT FROM AUTHOR]

The adsorption of CO on the rutile TiO2(110) surface was investigated using He atom scattering (HAS), high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and different types of ab initio electronic structure calculations. The experimental and theoretical results allow to put forward a consistent picture for this rather complicated adsorbate system. At 70 K a (2×1) adlayer with a glide symmetry plane is formed, containing two molecules per unit cell which are tilted in alternate directions by about 20° relative to the surface normal. For this high density phase, the theoretical calculations reveal a substantial repulsion between CO molecules on neighboring lattice sites, in accord with the results of a detailed analysis of the experimental TDS data. The CO binding energy depends strongly on coverage and varies between 0.20 eV for the saturated monolayer and 0.36 eV for isolated molecules. The CO–CO repulsion leads to the desorption of about half of the CO molecules above 70 K and the formation of low density phases. HAS gave no indication of ordered adlayers at these lower coverages. For the internal stretching vibration of the CO molecules a value of 273 meV was determined by HREELS, in very good agreement with the theoretical calculations. [ABSTRACT FROM AUTHOR]

X-ray emission spectroscopy (XES) at the O Kα threshold has been used to investigate the electronic structure of a microporous pure calcined zeolite with the crystal structure of the MFI-type framework (silicalite), a deboronated MFI zeolite (DB-MFI), a pure mesoporous cubic MCM-48 material, a MCM-48 loaded with copper and zinc oxide nanoparticles (Cu/ZnO-MCM-48), and a crystalline layered silicic acid H-RUB-18. For comparison, the XES O Kα spectrum of pure α-quartz has also been recorded. In the nonresonant energy regime the XES O Kα spectra for all these compounds look very similar indicating that the electronic structure of the micro- and mesoporous silica materials is very similar to that of quartz. In the resonant regime, however, the spectra exhibit significant differences. In all the materials under study, the resonant XES O Kα spectra recorded at photon energies close to the positions of the O K edges show Raman-type inelastic peaks with an energy loss of 11 eV, originating from electronic excitations within these insulating materials. The prominent features in the XES O Kα spectra of α-quartz and H-RUB-18 are analyzed by means of quantum chemical ab initio cluster calculations. [ABSTRACT FROM AUTHOR]

A diodelike device has been fabricated by organic molecular beam deposition of pentacene on a surface-modified, single-crystalline Au substrate. Using the substrate as the first and the tip of a scanning tunneling microscope (STM) as the second electrode, transport characteristics of this organic semiconductor (OSC) device were investigated. The probed pentacene islands are single crystalline and defect-free and consist of few molecular layers only. The current-voltage characteristics of this device reveal a pronounced asymmetry. For negative polarity, the current characteristics is almost independent of the layer thickness. For positive polarity, the current onset is shifted significantly to larger voltages with increasing layer thickness. Numerical simulations for a two-dimensional model system allow us to identify the injection properties of the STM tip as reasons for this pronounced asymmetry. For negative substrate bias the creation of holes in the valence band occurs by tunneling of electrons to the tip whereas in the opposite case holes have to be transported through the OSC layer from the substrate. Thus, for low positive voltage the hole current limits the device current. Once the resulting voltage drop between layer and tip becomes larger than the barrier for electron injection, direct tunneling of electrons into the pentacene conduction band becomes possible and n conduction begins to dominate. [ABSTRACT FROM AUTHOR]

The influence of chemical surface modifications of gold electrodes on the morphology and the electrical properties has been studied for pentacene based thin-film transistors with channel lengths of L<=4 μm. Self-assembled monolayers (SAMs) of various aliphatic and aromatic organothiols have been used to selectively modify the metallic source and drain electrodes and are further compared with reference samples with untreated electrodes. For all SAM-treated devices a reduced roughness of the pentacene film is observed which is accompanied by a reduction of the threshold voltage from about VT=2 V for untreated transistors to -0.9 V for transistors with SAM modified electrodes. Using aliphatic SAMs a poor on/off ratio of about 102 was obtained which is attributed to their low conductivity. In contrast, the on/off ratio is enhanced by four orders of magnitude if the surface is modified by an aromatic SAM. In this case a subthreshold swing as low as 0.55 V/decade is achieved which corresponds to a trap density reduced by one order of magnitude compared to the reference sample. [ABSTRACT FROM AUTHOR]

The adsorption of terephthalic acid [C6H4(COOH)2, TPA] on a Pd(111) surface has been investigated by means of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure spectroscopy under ultrahigh vacuum conditions at room temperature. We find the coexistence of one- (1D) and two-dimensional (2D) molecular ordering. Our analysis indicates that the 1D phase consists of intact TPA chains stabilized by a dimerization of the self-complementary carboxyl groups, whereas in the 2D phase, consisting of deprotonated entities, the molecules form lateral ionic hydrogen bonds. The supramolecular growth dynamics and the resulting structures are explained by a self-limiting deprotonation process mediated by the catalytic activity of the Pd surface. Our models for the molecular ordering are supported by molecular mechanics calculations and a simulation of high resolution STM images. [ABSTRACT FROM AUTHOR]

A search for external vibrations of hydrocarbons physisorbed on Cu(100) has been carried out with high-resolution inelastic He-atom scattering. For the saturated hydrocarbons n-hexane, cyclohexane, n-octane, and n-decane, the frustrated translation normal to the surface could be detected with energies of 6.8 meV, 7.3 meV, 7.0 meV, and 7.0 meV, respectively. A very similar value of 7.3 meV for the same vibration is observed for the unsaturated hydrocarbon benzene. No vibrational modes corresponding to frustrated translations parallel to the surface could be detected. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The low energy vibrations of NH3 and CO molecules adsorbed on Fe(110) have been investigated with inelastic He-atom scattering. For ammonia an energy loss of 10 meV was observed in the time-of-flight spectra and is assigned to the frustrated translations. Previously, this mode had only been observed for adsorbed CO molecules. For CO on Fe(110) the energy of the frustrated translations is 4.6 meV and the observed linewidth is smaller than 0.6 meV. [ABSTRACT FROM AUTHOR]

Diffraction of helium atoms from randomly distributed step edges on the otherwise smooth (001) surface of Ni has been observed. Measurements for different azimuths near the <100> direction indicate that the step edges are lined up perpendicular to the <100> direction. The effect of CO adsorption on both the amplitude of the diffraction oscillations and also on specular scattering has been studied. The in-plane diffraction oscillations from the steps and the specular peak intensity show a similar attenuation with CO adsorption. This is interpreted in terms of a simple model for the effect of adsorption on the step diffraction oscillations. With increasing CO adsorption, the step edge diffraction patterns are also found to be slightly shifted suggesting a continuous change in step edge shape with adsorption. [ABSTRACT FROM AUTHOR]

Angular distributions and time-of-flight spectra of a nearly monoenergetic (Δv/v[bar_over_tilde:_approx._equal_to]2% FWHM) He seeded HD beam (ki=6.80 Å-1, Ei=32 meV) have been measured after scattering from a Ni(001) single-crystal surface along the <100> azimuth. The most intense peaks in the angular distributions are attributed to diffraction with rotational excitation while the rotationally elastic diffraction peaks are an order of magnitude smaller. All the time-of-flight distributions reveal intense peaks at ΔE=-11.06 meV corresponding to the HD rotational transition ji=0→jf=1. The rotational energy loss observed at angles between the diffraction peaks is attributed to incoherent inelastic scattering at defects. The other inelastic peaks in the time-of-flight spectra can be assigned to phonons which are created or annihilated in the rotationally inelastic collisions. The measured phonon dispersion curves suggest a predominant interaction with the transverse bulk edge and not with the Rayleigh mode. These results provide the first direct evidence for a coupling of rotational transitions to the phonons of a metal surface. The pronounced rotational inelasticity at step edges does not support the widely held view that nonactivated chemisorption occurs at step edges. [ABSTRACT FROM AUTHOR]

Differential cross sections have been measured for the scattering of helium atoms from isolated CO molecules on a Pt(111) surface. The cross sections reveal an oscillatory structure as a function of scattering angle extending to large momentum transfer on both sides of the specular peak. Using a hard hemisphere model to approximate the interaction potential, the data can be well reproduced by a hard core of radius about 2.5 Å. This result is compared to the charge density profile of the adsorbed molecule and also to the gas phase interaction potential, and in both cases good agreement with the experimentally predicted classical turning points is found. [ABSTRACT FROM AUTHOR]

A new method for studying resonances in the scattering of molecules from metal surfaces is presented. In the experiment the velocity distribution of specularly scattered molecules with an initially broad distribution is analyzed with a time-of-flight technique and reveals sharp dips and peaks with a width of <0.25 meV attributed to Feshbach resonances. The present results for HD–Cu(111) and HD–Au(111) are analyzed by comparison with a close coupling calculation to provide potential well parameters. The method has been used to study the surface temperature dependence of one of the resonances down to 40 K and provides insight into a possible elementary step in physisorption. [ABSTRACT FROM AUTHOR]

Langmuir–Blodgett (LB) films of Cd arachidate and Ca arachidate with thicknesses of 1, 3, 9, and 25 layers were investigated using near edge x-ray absorption fine structure (NEXAFS spectroscopy), small angle x-ray scattering (SAXS), and reflected high energy electron diffraction (RHEED). The results from the x-ray diffraction experiments on layer thickness and interface roughness complement the structural information from NEXAFS on the average tilt angle of the alkyl chains in real space. The RHEED measurements confirm these findings and give additional information on the lateral order in these organic films. We find that the monolayer film shows a substantial degree of disorder, while for three and more LB layers, the molecular orientation is virtually identical to that of the corresponding fatty acid salts, i.e., the hydrocarbon chains are oriented perpendicular to the substrate surface in an all-trans conformation. Furthermore, a simplified and more robust analysis scheme for NEXAFS data obtained for hydrocarbon chains is presented. [ABSTRACT FROM AUTHOR]