• ACS APPL. MATER. INTERFACES: "Lattice Matching as the Determining Factor for Molecular Tilt and Multilayer Growth Mode of the Nanographene Hexa-peri-hexabenzocoronene"

    P. Beyer, T. Breuer, S. Ndiaye, A. Zykov, A. Viertel, M. Gensler, J. P. Rabe, S. Hecht, G. Witte, S. Kowarik
    ACS Applied Materials & Interfaces 6 (23), 21484–21493 (2014), DOI: 10.1021/am506465b

    The microstructure, morphology and growth dynamics of hexa-perihexabenzocoronene (HBC, C42H18) thin films deposited on inert substrates of similar surface energies are studied with particular emphasis on the influence of substrate symmetry and substrate–molecule lattice matching on the resulting films of this material. By combining atomic force microscopy (AFM) with x-ray diffraction (XRD), x-ray absorption spectroscopy (NEXAFS) and in-situ x-ray reflectivity (XRR) measurements, it is shown that HBC forms polycrystalline films on SiO2, where molecules are uprightly oriented and adopt the known bulk structure. Remarkably, HBC films deposited on highly oriented pyrolytic graphite (HOPG) exhibit a new, substrate induced polymorph, where all molecules adopt a recumbent orientation with planar π-stacking. Formation of this new phase, however, depends critically on the coherence of the underlying graphite lattice, since HBC grown on defective HOPG reveals the same orientation and phase as on SiO2. These results therefore demonstrate that the resulting film structure and morphology are not solely governed by the adsorption energy, but also by the presence or absence of symmetry- and lattice-matching between substrate and admolecules. Moreover, it highlights that weakly interacting substrates of high quality and coherence can be useful to induce new polymorphs with distinctly different molecular arrangements than the bulk structure.

  • ACS NANO: "Molecular Packing Determines Singlet Exciton Fission in Organic Semiconductors"

    K. Kolata, T. Breuer, G. Witte, S. Chatterjee
    ACS Nano 8 (7), 7377-7383 (2014), DOI: 10.1021/nn502544d

    Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley-Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene (PFP), we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herring bone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet-exciton fission.

  • CRYST. GROW. DES.: "Microstructural Characterization of Organic Heterostructures by (Transmission) Electron Microscopy"

    B. Haas, K. I. Gries, T. Breuer, I. Häusler, G. Witte, K. Volz
    Crystal Growth & Design 14 (6), 3010-3014 (2014), DOI: 10.1021/cg5002896

    Transmission electron microscopy can be a powerful tool to characterize organic heterostructures, if suitable methods are applied. Here, we present with the example of codeposited films of pentacene (PEN) and perfluoropentacene (PFP) different techniques, which can also be applied to the radiation-sensitive organic materials. The structure and morphology of codeposited films of PEN and PFP on a KCl(100) substrate have been investigated by different (transmission) electron microscopy techniques. When prepared by stoichiometrically equivalent coevaporation of both compounds, the films exhibit an intermixed phase that consists of a 1:1 mixture of the two molecules. Unavoidable excess of one of the two molecules was shown to lead to a spread-out film consisting of the respective molecule, which was verified for the case of PEN excess and whose growth was shown in detail. Interestingly, the film of segregated PEN exhibits a distinctly distinguishable growth from the pure PEN on the same substrate material, forming a 4-fold symmetry aligned with the KCl⟨001⟩ directions. The adaption of an automated nanocrystal orientation and phase mapping technique for the transmission electron microscope, commercially available as the so-called ASTAR system, to these beam-sensitive materials will be discussed as this method not only offers additional functionality for the nanoscale characterization of the films but is inherently advantageous for materials that are prone to beam-induced structural changes.

  • J. CHEM. PHYS.: "Analysis of the near-edge X-ray-absorption fine-structure of anthracene: A combined theoretical and experimental study"

    M. Klues, K. Hermann, G. Witte
    Journal of Chemical Physics 140 (1), 014302 (2014), DOI: 10.1021/nn502544d

    The near-edge fine structure of the carbon K-edge absorption spectrum of anthracene was measured and theoretically analyzed by density functional theory calculations implemented in the StoBe code. It is demonstrated that the consideration of electronic relaxation of excited states around localized core holes yields a significant improvement of the calculated excitation energies and reproduces the experimentally observed fine structure well. The detailed analysis of excitation spectra calculated for each symmetry inequivalent excitation center allows in particular to examine the influence of chemical shifts and core hole effects on the excitation energies. Moreover, the visualization of final states explains the large variations in the oscillator strength of various transitions as well as the nature of Rydberg-states that exhibit a notable density of states below the ionization potentials.