R.L. Johnson

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.

Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

Although an isolated individual molecule clearly has only one ionization potential, multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical pi-conjugated organic compounds on metal substrates combined with first-principles calculations and electrostatic modelling reveal the existence of a surface dipole built into molecular layers. Conceptually different from the surface dipole at metal surfaces, its origin lies in details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. In addition to providing in-depth understanding of this phenomenon, our study offers design guidelines for improved organic-organic heterojunctions, hole- or electron-blocking layers and reduced barriers for charge-carrier injection in organic electronic devices.

Sharper images by focusing soft X-rays with photon sieves.

Fresnel zone plates consisting of alternating transmissive and opaque circular rings can be used to focus X-rays. The spatial resolution that can be achieved with these devices is of the order of the width of the outermost zone and is therefore limited by the smallest structure (20–40 nm) that can be fabricated by lithography today. Here we show that a large number of pinholes distributed appropriately over the Fresnel zones make it possible to focus soft X-rays to spot sizes smaller than the diameter of the smallest pinhole. In addition, higher orders of diffraction and secondary maxima can be suppressed by several orders of magnitude. In combination with the next generation of synchrotron light sources (free-electron lasers) these ‘photon sieves’ offer new opportunities for high-resolution X-ray microscopy and spectroscopy in physical and life sciences.

Tuning the ionization energy of organic semiconductor films: the role of intramolecular polar bonds.

For the prototypical conjugated organic molecules pentacene and perfluoropentacene, we demonstrate that the surface termination of ordered organic thin films with intramolecular polar bonds (e.g., -H versus -F) can be used to tune the ionization energy. The collective electrostatics of these oriented bonds also explains the pronounced orientation dependence of the ionization energy. Furthermore, mixing of differently terminated molecules on a molecular length scale allows continuously tuning the ionization energy of thin organic films between the limiting values of the two pure materials. Our study shows that surface engineering of organic semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable alternative to the surface modification of substrates to control the energetics at organic/(in)organic interfaces.

Structural and electronic properties of pentacene-fullerene heterojunctions

In this study the performance differences of layered and bulk-heterojunction based organic solar cells composed of the prototypical p- and n-type organic semiconductors pentacene (PEN) and fullerene (C60) are correlated with the physical properties of the heterostructures. The electronic structure of layered and codeposited thin PEN and C60 films on the conducting polymer substrate poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated with ultraviolet photoelectron spectroscopy. Layered structures of C60 on PEN precovered PEDOT:PSS exhibited an offset of the highest occupied molecular orbital (HOMO) levels of 1.45 eV. In contrast, codeposited films of PEN and C60 showed a reduced HOMO-level offset of 0.85 eV, which increased to 1.45 eV by precoverage of the substrate with a thin PEN layer. In this case, the PEN-HOMO level was Fermi-level pinned at 0.35 eV binding energy and charge transfer between PEN and PEDOT:PSS decreased the vacuum level by 0.75 eV. In addition, the morpholo...

Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase

A simple and inexpensive approach is used to coat metal oxide surfaces (SBA-15) with thin films of carbon. These carbon films provide improved hydrothermal stability to oxides, such as silica and alumina, which are not otherwise stable at elevated temperatures in the presence of liquid water. Furthermore, the carbon film changes the surface chemistry of the support.

Gold-induced facetting of Si(111)

Abstract We report an STM investigation of the morphology of Au-covered vicinal Si(111) surfaces. High-resolution STM images of the Si(111)(5 × 2)-Au reconstruction are presented and compared to various structural models. Depending on coverage, annealing temperature, and inclination angle of the substrate surface we observed the occurrence of two different high-Miller-index Si facets which are stabilized by Au, namely a (2 × 1) reconstructed (775) facet and a (3 × 1) reconstructed (995) facet. A comparison of high-resolution STM images of the facets with images of the (5 × 2) reconstruction of the (111) terraces indicates that the different structures are closely related.

Controlling energy level offsets in organic/organic heterostructures using intramolecular polar bonds

The impact of intramolecular polar bonds (IPBs) on the energy level alignment in layered systems of rodlike conjugated molecules standing on the substrate was investigated for pentacene (PEN) and perfluoropentacene (PFP) on SiO2 using ultraviolet photoelectron spectroscopy. A remarkably large energy offset of 1.75 eV was found between the highest occupied molecular orbital (HOMO) levels of PEN and PFP caused by IPBs at the surface of standing PFP layers. This large HOMO-level offset results in a narrow intermolecular energy gap of approximately 0.4 eV at the interface between PEN and PFP layers. However, the absence of significant spatial overlap of PEN and PFP electron wave functions across the layers suppresses interlayer optical transitions.

Spectroscopic ellipsometry measurements of AlxGa1−xN in the energy range 3–25 eV

Abstract The dielectric functions of the wide-bandgap semiconductors GaN and Al x Ga 1− x N (hexagonal phase on c-sapphire) are determined directly for the first time in the spectral range 3–25 eV using an ellipsometry set-up operating with synchrotron radiation at the Berlin electron storage ring BESSY I. The compositional dependence of the transition energies of interband-critical points located in the vacuum ultraviolet spectral region, which are not accessible to ellipsometers using conventional light sources, was determined for 0≤ x ≤1. Additional measurements with a visible-UV ellipsometer have been performed to determine precisely the fundamental gap E 0 . A systematic parabolic shift of E 0 towards higher energies was found with increasing Al content while higher interband transitions show a linear shift. Differences in morphological properties of a MBE- and a MOCVD-grown GaN sample and their influence on the measured dielectric function are discussed.

Sulfur induced Cu4 tetramers on Cu(111)

Abstract The structures of the Cu (111) | 4 1 −1 4 |- S and the Cu(111)(√7 × √7)R19.1°-S reconstructions have been studied by surface X-ray diffraction and two new structural models are proposed. Although the unit cells for the two structures are quite different, a high degree of similarity has been revealed. It consists of a Cu4S unit with sulfur adsorbed in the fourfold hollow site on top of a Cu tetramer. The | 4 1 −1 4 | structure contains two Cu4S units, the (√7 × √7)R19.1° structure only one. In both structures additional sulfur atoms are present. The sulfur coverages for the structures are 0.35 and 0.43 ML, respectively. The building block for the structures is similar to the Ni4S unit which we recently found in the Ni(111)(5√3 × 2)-S structure. We propose that the driving force for the reconstruction is the chemical stability of the Cu4S units given by the tendency of sulfur to be fourfold coordinated.