Achim Schöll

Energetics of excited states in the conjugated polymer poly(3-hexylthiophene)

There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimisations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene) (P3HT), we demonstrate the general applicability of our novel approach. From photoemission spectroscopy of occupied and unoccupied states we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.

Determination of transport levels of organic semiconductors by UPS and IPS

In this paper, we present a comprehensive discussion on the determination of transport levels of organic semiconductors using a combination of UV and inverse photoemission spectroscopies (UPS and IPS, respectively). We discuss the key question of how the spectra should be evaluated to obtain the correct values for the transport levels. Our evaluation results in much smaller exciton binding energies than generally assumed, ranging from 0.1 to 0.4 eV for CuPc, PTCDA and Alq3. Moreover, we find that polarization effects, vibrations, experimental resolution and inhomogeneous broadening contribute very little to the position and width of the broad photoelectron spectroscopy (PES) and IPS signals. Thus, we suggest a model that explains much of the broadening as a result of dynamic charge delocalization. We discuss our data in analogy to those obtained for inorganic semiconductors (IOSCs), which are presented in a separate paper.

Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver

We demonstrate the application of orbital k-space tomography for the analysis of the bonding occurring at metal-organic interfaces. Using angle-resolved photoelectron spectroscopy, we probe the spatial structure of the highest occupied molecular orbital and the former lowest unoccupied molecular orbital (LUMO) of one monolayer 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(110) and (111) surfaces and, in particular, the influence of the hybridization between the orbitals and the electronic states of the substrate. We are able to quantify and localize the substrate contribution to the LUMO and thus prove the metal-molecule hybrid character of this complex state.

Deagglomeration and surface modification of thermally annealed nanoscale diamond

Thermally annealed nanodiamond has been functionalized by C-C coupling of the partially graphitized diamond surface using aryl diazonium salts. Depending on the terminal functional groups, the modified bucky diamond nanoparticles show good solubility (up to 0.63mgmL(-1)) in different solvents. The agglomerate size of the originally strongly bound detonation diamond (>0.5μm) is substantially reduced to ∼20-50nm by this chemical procedure and without using mechanical techniques such as strong ultrasound or milling. Arylation with functionalized aryl diazonium salts carrying COOH, SO(3)H, NO(2) or bromoethyl groups opens the way for further covalent grafting of organic structures. Arylation with Ar-COOH or Ar-SO(3)H leads to the formation of stable colloidal solutions in water and physiological media (i.e. PBS buffer), an important prerequisite for biomedical applications.

Energy calibration and intensity normalization in high-resolution NEXAFS spectroscopy

Abstract Using high-brilliance synchrotron radiation and an ultrahigh-resolution monochromator a wealth of new fine structures can be observed in near-edge X-ray absorption spectra. The potential information gain, however, requires an accurate calibration of the energy scale and a perfect intensity normalization in order to avoid erroneous results, e.g., the occurrence of spurious peaks. By means of the most problematic C 1s edge it is shown how large these effects can be and how appropriate energy calibration and intensity normalization can be achieved.

Substrate-mediated band-dispersion of adsorbate molecular states

Charge carrier mobilities in molecular condensates are usually small, as the coherent transport, which is highly effective in conventional semiconductors, is impeded by disorder and the small intermolecular coupling. A significant band dispersion can usually only be observed in exceptional cases such as for π-stacking of aromatic molecules in organic single crystals. Here based on angular resolved photoemission, we demonstrate on the example of planar π-conjugated molecules that the hybridization with a metal substrate can substantially increase the delocalization of the molecular states in selective directions along the surface. Supported by ab initio calculations we show how this mechanism couples the individual molecules within the organic layer resulting in an enhancement of the in-plane charge carrier mobility.

Disordering of an Organic Overlayer on a Metal Surface Upon Cooling

Coolly Disordered Generally speaking, at higher temperature phases materials are more disordered—for example, when solids melt to form liquids. In some cases, however—for example, under high pressure—a more disordered phase can emerge upon cooling. Schöll et al. (p. 303) now show that an organic molecule, a naphthalene derivative, adsorbed on a silver surface disorders upon cooling below room temperature. This process is driven by the surface bond becoming stronger upon cooling and preventing weaker interactions that allow ordering between molecules in the plane. Strengthening of the bond of an organic molecule to a silver surface upon cooling disrupts intermolecular ordering. Inverse melting or disordering, in which the disordered phase forms upon cooling, is known for a few cases in bulk systems under high pressure. We show that inverse disordering also occurs in two dimensions: For a monolayer of 1,4,5,8-naphthalene-tetracarboxylic dianhydride on Ag(111), a completely reversible order-disorder transition appears upon cooling. The transition is driven by strongly anisotropic interactions within the layer versus with the metal substrate. Spectroscopic data reveal changes in the electronic structure of the system corresponding to a strengthening of the interface bonding at low temperatures. We demonstrate that the delicate, temperature-dependent balance between the vertical and lateral forces is the key to understanding this unconventional phase transition.

Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot

Graphenes peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.

Complete determination of molecular orbitals by measurement of phase symmetry and electron density

Several experimental methods allow measuring the spatial probability density of electrons in atoms, molecules and solids, that is, the absolute square of the respective single-particle wave function. But it is an intrinsic problem of the measurement process that the information about the phase is generally lost during the experiment. The symmetry of this phase, however, is a crucial parameter for the knowledge of the full orbital information in real space. Here, we report on a key experiment that demonstrates that the phase symmetry can be derived from a strictly experimental approach from the circular dichroism in the angular distribution of photoelectrons. In combination with the electron density derived from the same experiment, the full quantum mechanical wave function can thus be determined experimentally.

Electronic structure at the perylene-tetracarboxylic acid dianhydride/Ag(111) interface studied with two-photon photoelectron spectroscopy

The electronic structure of the prototype metal/organic contact 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on a Ag(111)-surface has been investigated using time- and angle-resolved two-photon photoelectron spectroscopy (2PPE). Our analysis addresses particularly the nature of the interface state (IS) emerging at the interface due to the substrate-adsorbate interaction [C. H. Schwalb, S. Sachs, M. Marks et al., Phys. Rev. Lett. 101, 146801 (2008)]. Its free-electron-like dispersion and a possible backfolding at the surface Brillouin zone boundaries are discussed. Time-resolved pump-probe experiments reveal the inelastic electron lifetime along the dispersion parabola and show its decrease for increasing parallel momentum. The temperature dependence of the peak linewidth indicates a coupling of the IS to molecular vibrations. Moreover, additional aspects are addressed, such as the determination of the electron attenuation length of photoelectrons for low kinetic energy originating from the IS and the work function change of the sample upon PTCDA adsorption with very high energy resolution.