Robert L. Johnson

Bio: Robert L. Johnson is an academic researcher from University of Hamburg. The author has contributed to research in topics: X-ray photoelectron spectroscopy & Angle-resolved photoemission spectroscopy. The author has an hindex of 46, co-authored 294 publications receiving 8659 citations. Previous affiliations of Robert L. Johnson include Johannes Kepler University of Linz & Humboldt University of Berlin.

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

Abstract: 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.

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

Abstract: 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.

Sharper images by focusing soft X-rays with photon sieves

Abstract: Fresnel zone plates consisting of alternating transmissive and opaque circular rings can be used to focus X-rays1. 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 today2. 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.

Electronic structure of hydrogenated and unhydrogenated amorphous Si N x ( 0 ≤ x ≤ 1 . 6 ) : A photoemission study

Abstract: We present a comprehensive core-level and valence-band photoemission study of hydrogenated and unhydrogenated amorphous silicon nitride ($a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}:\mathrm{H}$ and $a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}$). Position, width, and shape of the Si $2p$ line as a function of $x$ are interpreted in terms of a superposition of five chemically shifted components which correspond to the possible $\mathrm{Si}\ensuremath{-}{\mathrm{Si}}_{4\ensuremath{-}n}{\mathrm{N}}_{n}(n=0,\dots{},4)$ bonding configurations. The chemical shift per Si---N bond is between 0.62 ($x\ensuremath{\le}0.6$) and 0.78 eV ($x\ensuremath{\simeq}1.3$). From the intensities of the chemically shifted Si $2p$ components the number of Si---N bonds is calculated and compared with the total nitrogen concentration. Above $x\ensuremath{\simeq}0.8$ the average number of N---Si bonds per N starts to deviate from three. The addition of hydrogen increases this deviation because N---H bonds are favored over N---Si bonds. A band of N $2p$ lone-pair states is identified at the top of the valence bands in nearly stoichiometric $a$-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$. This band determines the position of the valenceband maximum (VBM) above $x=1.1$. Below $x=1.1$ Si---Si bonding states mark the VBM. The conduction-band minimum (CBM) is determined by Si-Si antibonding states up to $x=1.25$ and its position relative to the core levels is virtually unaffected by the presence of nitrogen or hydrogen. Above $x=1.25$, a transition to Si-N antibonding states occurs which is accompanied by a sharp recession of the CBM. The position of the Fermi energy within the gap is investigated as a function of $x$ and the hydrogen content. Si---H and N---H bonding states are identified at 6.3 and 9.8 eV below the VBM in nearly stoichiometric $a$-${\mathrm{Si}}_{3}$${\mathrm{N}}_{1}$: H. Si---Si bonding defect states lie 0.5 to 1.0 eV above the VBM and the corresponding antibonding states (3.0\ifmmode\pm\else\textpm\fi{}0.3) eV above the VBM. Plasmon energies vary between 17 eV in $a$-Si and 22 eV in $a$-${\mathrm{SiN}}_{1.5}$.

Electronic structure of Ag2O.

Abstract: The electronic structure of ${\mathrm{Ag}}_{2}$O has been investigated by photoelectron, Auger-electron, and bremsstrahlung isochromat spectroscopy. The experimental results are compared with model one-particle band-structure and two-particle cluster calculations. We extract values for the on-site Ag 4d and O 2p energies, the Ag 4d--O 2p transfer integrals, as well as the on-site Coulomb Ag 4d-4d and O 2p-2p interactions. A comparison with ${\mathrm{Cu}}_{2}$O and CuO is made, and the differences are discussed in view of high-${T}_{c}$ superconductivity and the instability of AgO.

Nano‐photocatalytic Materials: Possibilities and Challenges

Abstract: Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

Angle-resolved photoemission studies of the cuprate superconductors

Abstract: The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

Diluted magnetic semiconductors

Abstract: We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...