• J. PHYS. CHEM. C: "Simultaneously Understanding the Geometric and Electronic Structure of Anthraceneselenolate on Au(111): A Combined Theoretical and Experimental Study"

    A. M. Track, F. Rissner, G. Heimel, L. Romaner, D. Käfer, A. Bashir, G. M. Rangger, O. T. Hofmann, T. Bucko, G. Witte, E. Zojer
    Journal of Physical Chemistry C 114 (6), 2677-2684 (2010), DOI: 10.1021/jp9102756

    Self-assembled monolayers (SAMs) of π-conjugated organic molecules have attracted significant interest in the field of molecular and organic electronics because of their ability to change electrode work functions combined with a considerable conductivity. Studies simultaneously addressing both their geometrical and morphological structure as well as their electronic properties are, however, scarce. Here, we provide a detailed description of layers consisting of anthracene-2-selenolate on Au(111), which display extraordinarily well and long-range ordered structures. Combining experimental data with the results of slab-type band-structure calculations, we are able to unambiguously determine the alignment of the molecules on the surface. The electronic structure of the SAMs is then determined by ultraviolet photoelectron spectroscopy (UPS) and by density functional theory (DFT) based simulations. For the SAM-induced work-function modification a particularly close agreement between the experimental value of −1.3 eV and the calculated −1.37 eV is found. This supports the notion that the currently available modeling approaches have the potential to quantitatively predict important aspects of the electronic structure of SAMs as long as truly well-ordered monolayers are investigated.

  • Adv. Mater: "Evidence for Band-like Transport in Graphene-based Organic Monolayers"

    D. Käfer, A. Bashir, X. Dou, G. Witte, K. Müllen, C. Wöll
    Advanced Materials 22 (3), 384-388 (2010), DOI: 10.1002/adma.200902123

    Evidence for a band-like, lateral transport of electrons through the cores of HBC-thiolates, forming a highly ordered self-assembled monolayer (SAM) containing a very regular array of HBC-cores, is provided based on a detailed analysis of temperature-dependent scanning tunneling microscopy (STM) data recorded for islands of aromatic SAMs immersed in an insulating matrix.

  • PHYS. CHEM. CHEM. PHYS.: "Influence of OH groups on charge transport across organic-organic interfaces: a systematic approach employing an "ideal" device"

    Z.-H. Wang, D. Käfer, A. Bashir, J. Götzen, A. Birkner, G. Witte, C. Wöll
    Physical Chemistry Chemical Physics 12 (17), 4317-4323 (2010), DOI: 10.1039/B924230A

    The charge transport across a pentacene/SAM interface has been studied by scanning tunnelling spectroscopy (STS) as a function of temperature and film thickness in order to obtain information on the transport mechanisms and in particular on the importance of interfacial OH-groups on n-transport in organic semiconductors. The current–voltage (I–V) characteristics of pentacene thin films deposited on a mercaptoundecanol self-assembled monolayer (SAM) on Au(111) reveal an asymmetric behaviour. At positive sample bias the onset currents shift towards higher voltages for decreasing temperatures, whereas such changes are not seen at negative bias. For lower temperatures, the variation of current onset with layer thickness is absent. These observations are explained by OH-groups at the SAM-surface effectively acting as charge traps. When electrons are caught in these traps at the organic–organic interface, charge transport is severely affected. Imaging of the SAM after loading the traps suggests that the attachment of electrons to the OH-groups exposed at the organic surface is a reversible process.

  • PHYS. REV. B: "Growth and Structure of Pentacene Films on Graphite: Weak Adhesion as a Key for Epitaxial Film Growth"

    J. Götzen, D. Käfer, C. Wöll, G. Witte
    Physical Review B 81 (8), 085440 (2010), DOI: 10.1103/PhysRevB.81.085440

    The microstructure of pentacene films grown on the basal plane of graphite has been investigated. By combining various complementary techniques including scanning tunneling microscopy, atomic force microscopy, x-ray diffraction, thermal desorption spectroscopy, and x-ray absorption spectroscopy the molecular orientation, crystalline structure, and morphology of the films as well as their thermal stability have been characterized in detail as a function of the film thickness. Initial film growth leads to the formation of a commensurate monolayer consisting of flat-lying molecules while upon subsequent deposition epitaxially ordered (022)-oriented pentacene films are formed which adopt the Siegrist phase. The detailed analysis shows that this epitaxial growth of films with an essentially recumbent molecular orientation is brought about by a slight rotation of the molecules in the first layer around their long molecular axis upon deposition of overlying molecular layers. Such a structural modification is unusual and becomes possible by the rather weak adsorption energy on graphite. In contrast, a very different film structure including an upright orientation of molecules even in the first layer is found on nonperfect but rough graphite surfaces leading to the formation of (001)-oriented films which initially reveal the thin-film phase and continue to grow in the Campbell phase of pentacene.

  • CRYS. GROW. DES.: "Rubrene Microcrystals: A Route to Investigate Surface Morphology and Bulk Anisotropies of Organic Semiconductors"

    M. El Helou, O. Medenbach, G. Witte
    Crystal Growth & Design 10 (8), 3496-3501 (2010), DOI: 10.1021/cg1003758

    Rubrene single crystals were grown by deposition near thermal equilibrium conditions which yields distinct microcrystals with sizes up to 100 μm. Such crystallites are rather spherical and exhibit various low index surfaces which enable detailed surface studies as well as directional-dependent bulk measurements. The analysis of the crystal habit reveals characteristic geometries for the confining surfaces which offer an unambiguous identification of their orientation. Atomic force microscopy (AFM) measurements demonstrate that the surfaces of the crystallites are ultraflat with a very low density of defects which is accompanied by a pronounced stability against oxidation at ambient conditions as inferred from laser desorption ionization time-of-flight mass spectroscopy (LDI-TOF MS). To exemplify the ability of such crystals to investigate bulk anisotropies, the refractive index tensor of rubrene was precisely determined at a wavelength of λ=589 nm by employing the Becke line method.