• ACS NANO: "Epitaxial growth of π-stacked perfluoropentacene on graphene-coated quartz"

    I. Salzmann, A. Moser, M. Oehzelt, T. Breuer, X. Feng, Z.-Y. Juang, D. Nabok, R. G. Della Valle, S. Duhm, G. Heimel, A. Brillante, E. Venuti, I. Bilotti, C. Christodoulou, J. Frisch, P. Puschnig, C. Draxl, G. Witte, K. Müllen, N. Koch
    ACS Nano 6 (12), 10874-10883 (2012), DOI: 10.1021/nn3042607

    Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

  • J. PHYS. COND. MATTER: "Structural and optical properties of pentacene films grown on differently oriented ZnO surfaces"

    M. El Helou, E. Lietke, J. Helzel, W. Heimbrodt, G. Witte
    Journal of Physics: Condensed Matter 24 (44), 445012 (2012), DOI: 10.1088/0953

    Pentacene films have been grown on two polar zinc oxide surfaces, i.e., ZnO(0001) and ZnO(000-1), as well as on the mixed-terminated ZnO(10-10) and are characterized by means of atomic force microscopy (AFM), x-ray diffraction (XRD), and thermal desorption spectroscopy (TDS). In all cases, pentacene aggregates in an upright orientation without any evidence for the formation of an interface stabilized wetting layer. Additional films deposited on a highly-defective, oxygen-depleted ZnO(000-1) reveal no altered growth mode. Nearly identical optical absorption spectra have been measured for all films, thus corroborating a weak molecule–substrate interaction. Upon cooling, however, a slightly different relaxation behavior could be resolved for pentacene films on polar ZnO surfaces compared to pentacene on the mixed-terminated ZnO(10-10) surface.

  • J. PHYS. CHEM. C: "Substrate induced thermal decomposition of perfluoro-pentacene thin films on the coinage metals"

    C. Schmidt, T. Breuer, S. Wippermann, W. G. Schmidt, G. Witte
    Journal of Physical Chemistry C 116 (45), 24098-24106 (2012), DOI: 10.1021/jp307316r

    The thermal and chemical stability of perfluoropentacene (PFP) thin films grown by organic molecular beam deposition onto the (111)-oriented surfaces of the coinage metals copper, silver, and gold have been studied by means of temperature dependent X-ray photoelectron spectroscopy (XPS) and Near-Edge X-ray absorption fine structure spectroscopy (NEXAFS). Under vacuum conditions, PFP multilayers are completely desorbed at 425 K while molecules in contact with the Au(111) surface remain intact up to 500 K. By contrast, PFP that is in contact with Cu(111) is distinctly distorted and becomes partially defluorinated already upon thermal desorption of multilayers. A pronounced defluorination of PFP also takes place on Ag(111) at temperatures around 440 K, while further heating causes a complete cracking and defluorination. Additional measurements carried out on a regularly stepped silver surface demonstrate that steps are active sites that promote defluorination already at lower temperatures. van der Waals corrected density-functional (DFT-D) calculations show that PFP, though being weakly adsorbed on all three metal surfaces, exhibits a reduced energy barrier for defluorination, in particular on copper and silver, thus reflecting their catalytic activity. The calculations reveal further that defluorinated molecules are covalently bound to the substrate, leading to a notable bending of the molecular backbone. The present study highlights the importance of also considering chemical reactions when theoretically analyzing molecule/metal interactions and indicates that fluorinated aromatic molecules, though offering interesting electronic properties, actually exhibit a limited stability in contact with some electrode surfaces like silver due to catalytic effects.

  • APPL. SURFACE SCIENCE: "Rapid preparation of highly ordered ultraflat ZnO surfaces"

    J. Götzen, G. Witte
    Applied Surface Science 258 (24), 10144-10147 (2012), DOI: 10.1016/j.apsusc.2012.06.094

    We report a simple procedure for preparing atomically flat and microscopically well ordered ZnO single crystal surfaces of various orientations which consists of an initial cleaning step by ion sputtering, followed by subsequent annealing in air. The resulting atomic ordering and morphology have been validated for both polar basal planes as well as the mixed terminated (10-10) surface by means of low energy electron diffraction (LEED) and atomic force microscopy (AFM) and were further compared with other preparation schemes. It is demonstrated that this procedure allows a very rapid and reliable preparation of unreconstructed surfaces with extended terraces whose size is limited by the precision of the macroscopic sample orientation.

  • J. PHYS. CHEM. C: "Vibrational Davydov-Splittings and Collective Mode Polarizations in Oriented Organic Semiconductor Crystals"

    T. Breuer, M. A. Celik, P. Jakob, R. Tonner, G. Witte
    Journal of Physical Chemistry C 116 (27), 14491-14503 (2012), DOI: 10.1021/jp304080g , Correction to this paper

    Vibrational properties of highly ordered crystalline perfluoropentacene (PFP) films epitaxially grown on KCl(100) and NaF(100) substrates have been studied by means of transmission infrared spectroscopy and density functional theory. The different molecular orientations adopted by PFP on both substrates (standing vs lying) and their epitaxial ordering enable precise polarization-resolved measurements along individual crystallographic directions and thus allow an unambiguous experimental determination of the polarization of the IR modes. Computations of the vibrational spectra beyond the single-molecule approximation were employed at the periodic dispersion-corrected density functional level (PBE-D2PBC) and compared with nonperiodic calculations (PBE-D2/def2-TZVPP). Thereby, a detailed mode assignment based on vibrational energies and polarization information was attained. A microscopic explanation for the experimentally observed Davydov splitting of some modes and the IR inactivity of others was derived based on the mutual coupling of the dynamical dipole moments of the two molecules within the unit cell. Experimentally observed modes not covered by our theoretical analysis have been identified as combination bands of IR-active modes coupled to totally symmetric modes of similar displacement patterns. These findings have important implications for future studies on structure and charge transport in organic semiconductors and the validation of theoretical approaches for the modeling of vibrational spectra.

  • J. APPL. PHYS.: "GaP heteroepitaxy on Si(001): Correlation of Si-surface structure, GaP growth conditions, Si-III/V interface structure"

    A. Beyer, J. Ohlmann, S. Liebich, H. Heim, G. Witte, W. Stolz, K. Volz
    Journal of Applied Physics 111 (8), 083534 (2012), DOI: 10.1063/1.4706573

    GaP-layers on Si(001) can serve as pseudo-substrates for a variety of novel optoelectronic devices. The quality of the GaP nucleation layer is a crucial parameter for the performance of such devices. Especially, anti-phase domains (APDs) evolving at mono-atomic steps on the Si-surface can affect the quality of a layer adversely. The size, shape, and possible charge of the APDs and their boundaries depend on the polarity of the surrounding crystal. The observed polarity of the GaP is caused by the A-type double step configuration of the Si-surface reconstruction prior to GaPgrowth and the prevalent binding of Ga to Si under optimized growth conditions. The polarity of the GaP-layer and hence the atomic configuration at the Si-III/V interface can be changed by altering the growth conditions. With this knowledge, defect-free GaP/Si(001) templates for III/V device integration on Si-substrates can be grown.

  • J. PHYS. CHEM. C: "Temperature Dependent Structural Phase Transition at the Perfluoropentacene/Ag(111) Interface"

    M. Marks, C. Schmidt, C. H. Schwalb, T. Breuer, Gr. Witte, U. Höfer
    Journal of Physical Chemistry C 116 (2), 1904 (2012), DOI: 10.1021/jp2094577

    Monolayers of perfluoropentacene (PFP) on Ag(111) undergo a reversible structural transition from a crystalline (6 × 3) phase at low temperatures to a disordered phase at room temperature. Two-photon photoemission (2PPE) of the low temperature phase revealed a distinct signal of the first image-potential state (n = 1) of Ag(111), thus showing the presence of bare substrate regions between the crystalline PFP islands. At higher temperatures, the entire surface is covered by a disordered phase of diffusing molecules causing a complete suppression of the (n = 1) signal and the electron diffraction (LEED) pattern. X-ray absorption spectroscopy showed that neither a change of the weak chemical interaction nor a change of the molecular tilt angle occurs during this phase-transition. The quantitative analysis of the 2PPE signal and the LEED contrast yields a transition temperature of TC = 145 ± 5 K. The present experiment shows that desorption of excess multilayers leads to an effective coverage of less than a dense monolayer.

  • POLYMER: "Vinyl-functionalized gold nanoparticles as artificial monomers for the copolymerization with methyl methacrylate"

    K. Gries, M. El Helou, G. Witte, S. Agarwal, A. Greiner
    Polymer 53 (8), 1632-1639 (2012), DOI: 10.1016/j.polymer.2012.02.008

    Vinyl-functionalized gold nanoparticles (AuNP) were prepared by surface polymerization of vinyl-functionalized ligands induced by carboxy-functionalized radical initiators followed by vinyl-transformation of the carboxy-group. These AuNP were regarded as artificial molecules as they were used as comonomers for the free radical copolymerization with methyl methacrylate (MMA). Successful copolymerization was proven by gel permeation chromatography (GPC) and by thermogravimetrical analysis (TGA). Further analysis of the novel hybrid material was carried out by transmission electron microscopy (TEM) and by atomic force microscopy (AFM) to proof the presence of AuNP and their arrangement.