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Publications

2015

  • Epitaxial TTF-TCNQ Thin Films on KCl(100): New Preparation Methods and Observation of Interface-Mediated Thin Film Polymorph.
    Alexander Mänz, Tobias Breuer, and Gregor Witte
    Crystal Growth & Design 15, 395–403 (2015), DOI: 10.1021/cg501484p
    Abstract - Full Text

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    Combining organic compounds of complementary ionization potential and electron affinity allows to fabricate charge-transfer complexes which exhibit remarkable properties, resulting e.g. in very high conductivity. Though the bulk properties of the prototypical organic conductor Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ) have been studied in detail, the influence of defects and crystallite size on resulting electronic properties as well as an integration of these materials in organic thin film devices is barely explored. One important requirement for such a comprehension is the precise control over crystallite size and quality. In this study, we report on different strategies to prepare crystalline TTF-TCNQ thin films and compare their structural quality. While conventional organic molecular beam deposition of TTF-TCNQ onto KCl(100) substrates enables the growth of epitaxial thin films with grain dimensions of up to 2 μm, further enhancement of the crystallite dimensions by raising the growth temperature is thermally limited by vanishing sticking and onset of vaporization. Using more sophisticated methods like hot wall evaporation, however, allows to overcome these limitations and yields crystalline islands with extensions enhanced by two orders of magnitude. Furthermore, we identify and provide a full structure solution of a yet unknown interface-mediated thin film polymorph of TTF-TCNQ, which is adopted in films of thicknesses below 1 μm.


2014

  • Lattice Matching as the Determining Factor for Molecular Tilt and Multilayer Growth Mode of the Nanographene Hexa-peri-hexabenzocoronene.
    Paul Beyer, Tobias Breuer, Saliou Ndiaye, Anton Zykov, Andreas Viertel, Manuel Gensler, Jürgen P. Rabe, Stefan Hecht, Gregor Witte, and Stefan Kowarik
    ACS Appl. Mater. Interfaces 6, 21484–21493 (2014), DOI: 10.1021/am506465b
    Abstract - Full Text

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    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.

  • Molecular Packing Determines Singlet Exciton Fission in Organic Semiconductors.
    Kolja Kolata, Tobias Breuer, Gregor Witte, and Sangam Chatterjee
    ACS Nano, 8, 7377-7383 (2014), DOI: 10.1021/nn502544d
    Abstract - Full Text

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    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.

  • Microstructural Characterization of Organic Heterostructures by (Transmission) Electron Microscopy.
    Benedikt Haas, Katharina I. Gries, Tobias Breuer, Ines Häusler, Gregor Witte, and Kerstin Volz
    Crystal Growth & Design, 14, 3010-3014 (2014), DOI: 10.1021/cg5002896
    Abstract - Full Text

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    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.

  • Analysis of the near-edge X-ray-absorption fine-structure of anthracene: A combined theoretical and experimental study.
    Michael Klues, Klaus Hermann and Gregor Witte
    J. Chem. Phys. 140, 14302 (2014), DOI: 10.1021/nn502544d
    Abstract - Full Text

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    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.

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2013

  • Cell-imprinted substrates direct the fate of stem cells.
    Morteza Mahmoudi, Shahin Bonakdar, Mohammad A. Shokrgozar, Haniyeh Aghaverdi, Raimo Hartmann, Andre Pick, Gregor Witte, and Wolfgang Parak
    ACS Nano 7, 8379-8384 (2013), DOI: 10.1021/nn403844q
    Abstract - Full Text - see also Correction to this publication

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Smart nano-environments were obtained by cell-imprinted substrates based on mature and dedifferentiated chondrocytes as templates. Rabbit adipose derived mesenchymal stem cells (ADSCs) seeded on these cell-imprinted substrates were driven to adopt the specific shape (as determined in terms of cell morphology) and molecular characteristics (as determined in terms of gene expression) of the cell types which had been used as template for the cell-imprinting. This method might pave the way for a reliable, efficient, and cheap way of controlling stem cell differentiation. Data also suggest that besides residual cellular fragments, which are presented on the template surface, the imprinted topography of the templates plays a role in the differentiation of the stem cells.

  • Diffusion-controlled growth of molecular hetero-structures: fabrication of 2D, 1D and 0-Dimensional C60-nanostructures on pentacene substrates.
    Tobias Breuer and Gregor Witte
    ACS Applied Materials & Interfaces 5, 9740-9745 (2013), DOI: 10.1021/am402868s
    Abstract - Full Text

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    A variety of low dimensional C60 structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C60 clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS we found that no contiguous pentacene film is formed on the 2D C60 structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic hetero-structures.

  • Thermally activated intermixture in pentacene-perfluoropentacene heterostructures.
    Tobias Breuer and Gregor Witte
    J. Chem. Phys. 138, 114901 (2013), DOI: 10.1063/1.4795004
    Abstract - Full Text

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    Using thermal desorption spectroscopy (TDS) the thermal stability of binary pentacene/perfluoropentacene (PEN/PFP) thin films has been investigated for various preparation protocols. Variation of stoichiometry ratio reveals a significantly enhanced thermal stability in comparison to the single compounds only for films with equimolar stoichiometry. The stabilization also depends on the preparation method and was found for co-deposition as well as for multistacks and subsequently grown PEN/PFP-stacks but not for stacks grown in the reversed order. By systemically varying the substrate temperature during deposition, we prove that the resulting intermixture is caused by a thermally activated diffusion during film growth and not due to post-deposition diffusion induced upon heating during TDS measurements. The different extents of thermal stabilization are discussed in the context of the film morphology studied by means of atomic force microscopy (AFM). For complementary information, optical absorption spectra of the heterostructures are analyzed, where the arisal of new absorption bands and the extinction of excitonic bands existing in the pure compounds are identified as decisive criteria to judge the efficiency of intermixture.

  • Synthesis and solid-state structures of 6,13-Bis(trifluoromethyl)- and 6,13-Dialkoxypentacene.
    Jonas Schwaben, Niels Münster, Tobias Breuer, Michael Klues, Klaus Harms, Gregor Witte, and Ulrich Koert
    European Journal of Organic Chemistry, 1639-1643 (2013), DOI: 10.1002/ejoc.201201714
    Abstract - Full Text

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    Novel 6,13-disubstituted pentacenes were synthesized. Their electrochemical and optical properties and their packing motif in the solid state cold been determined. Treatment of pentacenequinone with TMSCF3 (Ruppert’s reagent) and deprotection led to the 6,13-bis(trifluoromethyl)-pentacene-6,13-diol which was aromatized to 6,13-bis(trifluoromethyl) pentacene using PBr3. 6,13-Dialkoxypentacenes were accessible via alkylation of the corresponding hydroquinopentacenes using dialkyl sulfates. 6,13-Bis(trifluoromethyl) pentacene and 6,13-dimethoxypentacene exhibit slipped face to face π-stacking in the solid state while 6,13-diethoxypentacene forms pairs of π-stacking molecules in the solid state.

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2012

  • Epitaxial growth of pi-stacked perfluoropentacene on graphene-coated quartz.
    Ingo Salzmann, Armin Moser, Martin Oehzelt, Tobias Breuer, Xinliang Feng, Zhen-Yu Juang, Dmitrii Nabok, Raffaele G. Della Valle, Steffen Duhm, Georg Heimel, Aldo Brillante, Elisabetta Venuti, Ivano Bilotti, Christos Christodoulou, Johannes Frisch, Peter Puschnig, Claudia Draxl, Gregor Witte, Klaus Müllen, and Norbert Koch
    ASC Nano, 12, 10874-10883 (2012), DOI: 10.1021/nn3042607
    Abstract - Full Text

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    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.

  • Structural and optical properties of pentacene films grown on differently oriented ZnO surfaces.
    M. El Helou, E. Lietke, J. Helzel, W. Heimbrodt, and Gregor Witte
    Journal of Physics: Condensed Matter, 24, 445012 (2012), DOI: 10.1088/0953-8984/24/44/445012
    Abstract - Full Text

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    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.

  • Substrate induced thermal decomposition of perfluoro-pentacene thin films on the coinage metals.
    Christian Schmidt, Tobias Breuer, Stefan Wippermann, Wolf Gero Schmidt, and Gregor Witte
    Journal of Physical Chemistry C, 116, 24098-24106 (2012), DOI: 10.1021/jp307316r
    Abstract - Full Text

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    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.

  • Rapid preparation of highly ordered ultraflat ZnO surfaces.
    Jan Götzen and Gregor Witte
    Applied Surface Science, 258, 10144-10147 (2012), DOI: 10.1016/j.apsusc.2012.06.094
    Abstract - Full Text

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    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.

  • Vibrational Davydov-Splittings and Collective Mode Polarizations in Oriented Organic Semiconductor Crystals.
    Tobias Breuer, Mehmet Ali Celik, Peter Jakob, Ralf Tonner, and Gregor Witte
    Journal of Physical Chemistry C, 116, 14491-14503 (2012), DOI: 10.1021/jp304080g
    Abstract - Full Text - see also Correction to this publication

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    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.

  • GaP heteroepitaxy on Si(001): Correlation of Si-surface structure, GaP growth conditions, and Si-III/V interface structure.
    A. Beyer, J. Ohlmann, S. Liebich, H. Heim, G.Witte, W. Stolz, and K. Volz
    Journal of Applied Physics 111, 083534 (2012), DOI: 10.1063/1.4706573
    Abstract - Full Text

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    GaP-layers on Si(001) can serve as pseudo-substrates for a variety of novel optoelectronic devices. The quality of the GaPnucleation 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.

  • Temperature Dependent Structural Phase Transition at the Perfluoropentacene/Ag(111) Interface.
    Manuel Marks, Christian Schmidt, Christian H. Schwalb, Tobias Breuer, Gregor Witte and Ulrich Höfer
    Journal of Physical Chemistry C 116, 1904 (2012), DOI: 10.1021/jp2094577
    Abstract - Full Text

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    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.

  • Vinyl-functionalized gold nanoparticles as artificial monomers for the copolymerization with methyl methacrylate.
    Katharina Gries, Mira El Helou, Gregor Witte, Seema Agarwal, Andreas Greiner
    Polymer 53, 1632-1639 (2012), DOI: 10.1016/j.polymer.2012.02.008
    Abstract - Full Text

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    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.

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2011

  • Immobilization of quantum dots via conjugated self-assembled monolayers and their application as a light-controlled sensor for the detection of hydrogen peroxide.
    W. Khalid, M. El Helou, T. Murböck, Z. Yue, J.-M. Montenegro, K. Schubert, G. Göbel, F. Lisdat, G. Witte and W. J. Parak
    ACS Nano 5 (12), 9870 (2011), DOI: 10.1021/nn2035582
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    A light-addressable gold electrode modified with CdS and FePt or with CdS@FePt nanoparticles via an interfacial dithiol linker layer is presented. XPS measurements reveal that trans-stilbenedithiol provides high-quality self-assembled monolayers compared to benzenedithiol and biphenyldithiol, in case they are formed at elevated temperatures. The CdS nanoparticles in good electrical contact with the electrode allow for current generation under illumination and appropriate polarization. FePt nanoparticles serve as catalytic sites for the reduction of hydrogen peroxide to water. Advantageously, both properties can be combined by the use of hybrid nanoparticles fixed on the electrode by means of the optimized stilbenedithiol layer. This allows a light-controlled analysis of different hydrogen peroxide concentrations.

  • Temperature dependent Optical Properties of Pentacene films on Zinc Oxide.
    Jonatan Helzel, Stephanie Jankowski, Mira El Helou, Gregor Witte, Wolfram Heimbrodt
    Applied Physics Letters 99, 211102 (2011), DOI: 10.1063/1.3663863
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    The optical transitions of pentacene filmsdeposited on ZnO have been studied by absorption spectroscopy as a function of temperature in the range of room temperature down to 10 K. The pentacene films were prepared with thicknesses of 10 nm, 20 nm, and 100 nm on the ZnO-O(000-1) surface by molecular beam deposition. A unique temperature dependence has been observed for the two Davydov components of the excitons for different film thicknesses. At room temperature, the energetic positions of the respective absorption bands are the same for all films, whereas the positions differ more than 20 meV at 10 K caused by the very different expansion coefficients of pentacene and ZnO. Although the pentacene is just bonded via van der Waals interaction to the ZnO substrate, the very first pentacene monolayer (adlayer) is forced to keep the initial position on the ZnO surface and suffering, therefore, a substantial tensile strain. For all the subsequent pentacene monolayers, the strain is reduced step by step resulting electronically in a strong potential gradient at the interface.

  • Interrelation between substrate roughness and thin-film structure of functionalized acenes on graphite.
    Tobias Breuer, Ingo Salzmann, Jan Götzen, Martin Oehzelt, Antonia Morherr, Norbert Koch, Gregor Witte
    Crystal Growth & Design 11, 4996 (2011), DOI: 10.1021/cg200894y
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    Using atomic force microscopy (AFM) and X-ray diffraction (XRD), we analyzed the growth of differently modified pentacenes (perfluoropentacene and pentacenetetrone) on graphite and demonstrate that both the resulting morphology and the crystalline structure of the films critically depend on the microroughness of the substrate. On well-ordered highly oriented pyrolytic graphite (HOPG) surfaces prepared by exfoliation, both molecular materials form exceptionally smooth films, which consist of large-area molecularly flat islands yielding an overall low roughness. Interestingly, in these films molecules adopt a recumbent orientation, while on defective substrates, created by brief ion sputtering, the molecules adopt an upright orientation and form nonconnected islands exhibiting a significantly increased film roughness. Our study not only underlines the possibility to prepare very smooth films on a weakly interacting substrate but also emphasizes the importance of a proper substrate preparation and the significance of precise knowledge of substrate-surface properties to control the resulting structure of organic films.

  • Application of Transmission Electron Microscopy for Microstructural Characterization of Perfluoropentacene Thin Films.
    Benedikt Haas, Andreas Beyer, Wiebke Witte, Tobias Breuer, Gregor Witte, Kerstin Volz
    Journal of Applied Physics 110, 073514 (2011), DOI: 10.1063/1.3646549
    Abstract - Full Text

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    The crystalline structure and orientation of perfluoropentacene (C22 F14, PFP) fibers formed upon thin-film deposition onto SiO2 substrates have been studied by means of transmission electron microscopy(TEM),atomic force microscopy(AFM), and x-ray diffraction. The synopsis of TEM micrographs and diffraction patterns enhances the understanding of local crystal orientation on small length scales. The relationship of the PFP fiber morphology with the crystalline arrangement of PFP molecules within single fibers was established using this technique. Radiation damage, which is a critical problem for TEM investigations of organic materials, is described and the sample morphology after TEM investigations is correlated with AFM measurements of samples previously examined by TEM.

  • Tailoring the Cu(100) work function by substituted benzenethiolate-SAMs.
    Christian Schmidt, Alexander Witt, Gregor Witte
    Journal of Physical Chemistry A, 115 (25), 7234-7241 (2011), DOI: 10.1021/jp200328r
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    The structure and electronic interface properties of five differently substituted benzenethiol based self-assembled monolayers (SAMs) on Cu(100) have been studied by means of low energy electron diffraction, thermal desorption spectroscopy, X-ray absorption spectroscopy (NEXAFS), and UV photoelectron spectroscopy. Because highly ordered SAMs are formed of which lateral density had been precisely determined, effective molecular dipole moments were derived from the measured work function shifts. These values are compared with gas phase dipole moments computed by quantum chemical calculations for the individual thiol molecules considering the molecular orientation determined from NEXAFS data. Furthermore, this comparison yields clear evidence for a coverage dependent depolarization effect of the adsorbed molecules within the SAMs.

  • Epitaxial growth of perfluoropentacene films with predefined molecular orientation: A route for single-crystal optical studies.
    Tobias Breuer, Gregor Witte
    Physical Review B 83 (15), 155428 (2011), DOI: 10.1103/PhysRevB.83.155428
    Abstract - Full Text

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    Using atomic-force microscopy and x-ray diffraction we show that perfluoropentacene (C22F14, PFP) forms long-range ordered, epitaxial films on KCl(100) and NaF(100) cleavage planes. On both substrates the films adopt the same crystalline bulk phase, but surprisingly exhibit quite different molecular orientations, being upright oriented on NaF and recumbent oriented on KCl. Accompanied thermal desorption spectroscopy measurements indicate the absence of a stabilized seed layer, like on metals, hence suggesting that in both cases the PFP films are stabilized by an electrostatic point-in-line relationship between the outermost fluorine atoms and the alkali cations of the alkali halide surfaces. Furthermore, the transparency of both substrates was utilized to perform detailed transmission UV/Vis spectroscopy and polarized optical microscopy measurements along well-defined crystallographic directions. From these data the orientation of transition dipole moments of the various optical excitations were experimentally determined and a directional anisotropic exciton coupling was observed, which is attributed to the asymmetric molecular packing motif within the (100) plane of the PFP crystal lattice.

  • Temporal Evolution of Benzenethiolate-SAMs on Cu(100).
    Christian Schmidt, Jan Götzen, Gregor Witte
    Langmuir 27 (3), 1025-1032 (2011), DOI: 10.1021/la103533h
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    The structure and thermal stability of self-assembled monolayers (SAMs) of benzenethiolate (BT) on Cu(100) have been studied by means of thermal desorption spectroscopy (TDS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray adsorption fine structure spectroscopy (NEXAFS). Vapor deposition at room temperature yields a well-ordered, densely packed c(62) saturation structure. At room temperature, this film is, however, metastable and transforms via partial decomposition by cleavage of the S-C bond into a less densely packed layer that reveals a coexisting p(22) phase. Such a transition occurs on a time scale of several days and is accompanied by a reduction of the work function change with respect to the bare Cu(100) surface from Δφ=-0.9 eV for a freshly prepared saturated layer to -0.5 eV for an aged film. TDS experiments exhibit the presence of two distinct desorption channels (dissociative and intact desorption) occurring at different temperatures that reflects a variation of the local Cu-S interaction strength of BT at differently coordinated adsorption sites. Heating to above room temperature causes a rapid degradation and continuous thinning of BT films whereas above 500 K all thiolate species have desorbed or dissociated, leaving a sulfide overlayer behind that is accompanied by a substrate reconstruction. Interestingly, the upright orientation of BT adopted in the saturated monolayer remains almost identical upon heating and demonstrates the absence of downward tilting upon thermally induced thinning of the film.

  • Structural Evolution of Perfluoro-Pentacene Films on Ag(111): Transition from 2D to 3D Growth.
    Jan Götzen, Christian H. Schwalb, Christian Schmidt, Gerson Mette, Manuel Marks, Ulrich Höfer, Gregor Witte
    Langmuir 27 (3), 993-999 (2011), DOI: 10.1021/la1022664
    Abstract - Full Text

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    The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the ⟨110⟩ azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along ⟨110⟩ azimuth directions.

  • Absence of template induced ordering in organic multilayers: the growth of pentacene on a Cu(221) vicinal surface.
    Jan Götzen, Simon Lukas, Alexander Birkner, Gregor Witte
    Surface Science 605, 577-581, (2011), DOI: 10.1016/j.susc.2010.12.022
    Abstract - Full Text

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    The structure and morphology of pentacene films grown on a Cu(221) vicinal surface have been studied by means of scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). Well ordered saturated monolayer films were prepared by first depositing few nanometers pentacene at room temperature followed by brief heating above 400 K in order to sublimate the multilayer excess. The chemisorbed monolayer adopts a (1 × 7) superstructure with a uniform alignment of the molecules along the step edges while upon further deposition this template induced ordering is not transferred to multilayer films. Instead the molecules continue to grow in an upright orientation and form pyramidal islands without any substrate related azimuthal orientation.

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2010

  • Simultaneously Understanding the Geometric and Electronic Structure of Anthraceneselenolate on Au(111): A Combined Theoretical and Experimental Study.
    Anna M. Track, Ferdinand Rissner, Georg Heimel, Lorenz Romaner, Daniel Käfer, Asif Bashir, Gerold M. Rangger, Oliver T. Hofmann, Tomaš Bucko, Gregor Witte, and Egbert Zojer
    Journal of Physical Chemistry C 114 (6), 2677 (2010), DOI: 10.1021/jp9102756
    Abstract - Full Text

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    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.

  • Evidence for Band-like Transport in Graphene-based Organic Monolayers.
    D. Käfer, A. Bashir, X. Dou, G. Witte, K. Müllen and Ch. Wöll
    Adv. Mat 22 (3), 384 (2010), DOI: 10.1002/adma.200902123
    Abstract - Full Text

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    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.

  • Influence of OH groups on charge transport across organic-organic interfaces: a systematic approach employing an "ideal" device.
    Zhi-Hong Wang, Daniel Käfer, Asif Bashir, Jan Götzen, Alexander Birkner, Gregor Witte and Christof Wöll
    Physical Chemistry Chemical Physics 12, 4317 (2010), DOI: 10.1039/B924230A
    Abstract - Full Text

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    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.

  • Growth and Structure of Pentacene Films on Graphite: Weak Adhesion as a Key for Epitaxial Film Growth.
    Jan Götzen, Daniel Käfer, Christof Wöll and Gregor Witte
    Physical Review B. 81, 085440 (2010), DOI: 10.1103/PhysRevB.81.085440
    Abstract - Full Text

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    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.

  • Rubrene Microcrystals: A Route to Investigate Surface Morphology and Bulk Anisotropies of Organic Semiconductors.
    Mira El Helou, Olaf Medenbach and Gregor Witte
    Crystal Growth & Design, 10, 3496 (2010), DOI: 10.1021/cg1003758
    Abstract - Full Text

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    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.

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2009

  • Thermally activated dewetting of organic thin films: the case of pentacene on SiO2 and gold.
    D. Käfer, C. Wöll, G. Witte
    Appl. Phys. A 95, 273 (2009)
    Abstract - Full Text

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    The morphology of pentacene organic thin films deposited on SiO2 and Au(111) surfaces using organic molecular beam deposition (OMBD) has been characterized by a multi-technique approach. Among the techniques applied were X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and thermal desorption spectroscopy (TDS). Our rather detailed studies reveal that on both substrates the growth is strongly influenced by dewetting and islanding phenomena, yielding very rough surfaces. Surprisingly, substantial changes in the morphology were observed also after deposition on room-temperature samples on a time scale of several hours. The rather extensive set of in situ XPS data was analyzed in the framework of a simple model, which allows us to derive rather detailed information on the roughness parameters.

  • Molecular mechanisms of electron-induced cross-linking in aromatic SAMs.
    A. Turchanin, D. Käfer, M. El-Desawy, Ch. Wöll, G. Witte, A. Gölzhäuser
    Langmuir 25 (13), 7342-7352 (2009)
    Abstract - Full Text

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    When aromatic self-assembled monolayers (SAMs) are electron-irradiated, intermolecular cross-links are formed and the SAMs transform into carbon nanosheets with molecular thickness. These nanosheets have a very high mechanical stability and can withstand temperatures above 1000 K. In this report, we investigate the electron induced cross-linking of 1,1′-biphenyl-4-thiol (BPT) SAMs on gold by combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UV photoelectron spectroscopy (UPS). The experimental data were acquired as a function of electron dose and temperature and compared with quantum chemical calculations. Details of the intermolecular cross-linking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. Our analysis shows that room-temperature electron irradiation causes a lateral cross-linking via the formation of C−C linked phenyl species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and saturates at ∼50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete cross-linking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked molecules. At temperatures above 800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.

  • A microwave discharge source operating at pressures of several atmospheres.
    V.M. Akimov, G. Witte, L.I. Kolesnikova, L.Y. Rusin, J.P. Toennies
    Instrum. and Exp. Techn. 52 (3), 394 (2009)
    Abstract - Full Text

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    The design of a microwave source in which a discharge is initiated by an electromagnetic surface wave at 2.45 GHz is described. A stable discharge was supported at a gas pressure p0 exceeding the atmospheric pressure in He, N2, and in H2-Ar, H2-He, and O2-He mixtures in a 2-mm inner diameter quartz tube with a 0.15-mm diameter nozzle at a 50- to 115-W microwave power. A degree of dissociation of up to 80% was reached for pure H2 at p0 = 6 Torr and a 6% mixture of H2 and He at p0 = 50 Torr. When p0 increases to 19 Torr for H2 and to 300 Torr for the mixture, the hydrogen-atom beam intensity, in spite of a decrease in the degree of dissociation, increases due to narrowing of the beam particle velocity distribution.

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2008

  • Resolving the Depth Coordinate in Photoelectron Spectroscopy -  Comparison of Excitation Energy Variation vs. Angular Resolved XPS for the Analysis of a Self-assembled Monolayer Model System.
    S.V. Merzlikin, N.N. Tolkachev,  Th. Strunskus, G. Witte, Th. Glogowski, Ch.Wöll, and W. Grünert
    Surf. Sci. 602, 755 (2008).
    Abstract
    - Full Text
  • Influence of contact metals on the performance and morphology of pentacene bottom-contact field-effect transistors.
    C. Bock, D.V. Pham, U. Kunze, D. Käfer, G. Witte, Ch. Wöll
    Physica E  40, 2107 (2008)
    Abstract - Full Text

  • Deposition and characterization of thin organic films on metals for applications in organic electronics.
    G. Witte and Ch. Wöll
    phys. stat. sol. (a) 205, 497 (2008)
    Abstract - Full Text

  • Work function Changes Induced by Charged Adsorbates:  Origin of the Polarity Asymmetry.
    P. Bagus, D. Käfer, G. Witte, Ch. Wöll
    Phys. Rev. Lett. 100, 126101 (2008)
    Abstract - Full Text

  • Selenium as a Key Element for Highly Ordered Aromatic Self-Assembled Monolayers.
    Asif Bashir, Daniel Käfer, Jan Müller, Christof Wöll, Andreas Terfort, and Gregor Witte*
    Angewandte Chemie Int. Ed. 47, 5250 (2008)
    Abstract - Full Text

  • Selen als Schlüsselkomponente für hochgeordnete aromatische selbstorganisierte Monoschichten.
    Asif Bashir, Daniel Käfer, Jan Müller, Christof Wöll, Andreas Terfort, and Gregor Witte*
    Angewandte Chemie 120, 5328 (2008)
    Abstract - Full Text

  • Packing of planar organic molecules: interplay of van-der-Waals and electrostatic interaction
    Daniel Käfer, Mira El Helou, Christian Gemel, and Gregor Witte*
    Crystal Growth & Design, 8, 3053 (2008)
    Abstract - Full Text

  • Reply to a comment of J. Zemek, Prague, regarding the paper ‘‘resolving the depth coordinate in  photoelectron spectroscopy – comparison of excitation energy variation vs. angular-resolved XPS for the analysis of a self-assembled monolayer model system”.
    S.V. Merzlikin, N.N. Tolkachev, T. Strunskus, G. Witte, T. Glogowski, C. Wöll, W. Grünert
    Surf. Sci. 602, 23, 3634 (2008)
    Abstract - Full Text

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2007

  • Growth of Pentacene on Clean and Modified Gold Surfaces.
    D. Käfer, L. Ruppel, and G. Witte*
    Phys. Rev. B  75, 085309 (2007)
    Abstract - Full Text

  • Energy-dispersive X-ray reflectivity and GID for Real-time Growth Studies of Pentacene Films.
    S. Kowarika, A. Gerlach, W. Leitenberger, J. Hua, G. Witte, C. Wöll, U. Pietsch, F. Schreiber
    Thin Solid Films, 515 (14), 5606 (2007)
    Abstract
    - Full Text

  • Reduced Sheet Resistance In Pentacene Field-Effect Transistors Using Thiol-Modified Electrodes.
    D.V. Pham, Y. Gravenstein, C. Bock, U. Kunze, D. Käfer, G. Witte, and Ch. Wöll
    AIP Conference Proceedings, Vol. 893 Issue 1, 363 (2007)
    Abstract - Full Text

  • Interplay of Anchoring and Ordering in aromatic Self-Assembled.
    D. Käfer, A. Bashir and G. Witte*
    J. Phys. Chem. C  111 (28), 10546 (2007)
    Abstract - Full Text

  • Evolution of Pentacene Films on Ag(111): growth beyond the first monolayer.
    D. Käfer and G. Witte*
    Chem. Phys. Lett.  442 (4-6), 376 (2007)
    Abstract - Full Text

  • Molecules coining patterns into a metal: the hard core of soft matter.
    G. Witte*, K. Hänel, C. Busse, A. Birkner, and  Ch. Wöll
    Chem. Mater.  19, 4228 (2007)
    Abstract - Full Text

  • Influence of anthracene-2-thiol treatment on the device parameters of pentacene bottom-contact transistors.
    C. Bock, D.V. Pham, U. Kunze, D. Käfer, G. Witte, A. Terfort
    Appl. Phys. Lett.    91, 052110 (2007)
    Abstract - Full Text

  • Defect-free pentacene diode with a tunnelling contact: n-conduction for positively biased substrate.
    L. Ruppel, A. Birkner, G. Witte, C. Busse, T. Lindner,  G. Paasch, Ch. Wöll
    J. Appl. Phys. 102, 033708 (2007)
    Abstract - Full Text

  • Structure-Property Relationships in Self-Assembled Monolayers of Thiols: A Study on Phase Transition.
    P. Cyganik, M. Buck, Th. Strunskus, A. Shaporenko, G. Witte, M.N. Zharnikov, Ch. Wöll
    J. Phys. Chem. C  111, 16909 (2007)
    Abstract - Full Text

     

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2006

  • Growth and morphology of thin films of aromatic molecules on metals: the case of perylene.
    G. Witte*, K. Hänel, S. Söhnchen, Ch. Wöll  (invited Paper)
    Appl. Phys. A 82, 447 (2006)
    Abstract
    - Full Text
  • Self-Assembled Monolayers of Anthracenethiol on Gold: Preparation, Structure and Stability.
    D. Käfer, G. Witte*, P. Cyganik, A. Terfort, and Ch.Wöll
    J. Am. Chem. Soc. 128, 1723  (2006)
    Abstract - Full Text

  • Improved morphology and charge carrier injection in pentacene field-effect transistors with thiol treated electrodes.
    C. Bock, D.V. Pham, U. Kunze, D. Käfer, G. Witte*, and  Ch. Wöll
    J. Appl. Phys. 100, 114517 (2006)
    Abstract - Full Text

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2005

  • organic Chemical Vapor Deposition (MOCVD) of Palladium: Spectroscopic Study of Cyclopentadienyl-allyl-palladium Deposition on A Palladium Substrate
    A. Niklewski, T. Strunskus, G. Witte and Ch. Wöll,
    Chem. Mater. 17, 861 (2005).
    Abstract

  • Growth of crystalline Rubrene films with enhanced stability
    D. Käfer and G. Witte*,
    Phys. Chem. Chem. Phys. 7(15), 2850 (2005).
    Abstract

  • Deposition of palladium from cyclopentadienyl-allyl-palladium precurser on differently pretreated Sibased substrates: The role of surface Si-OH and Si-H species studied by x-ray photoelectron spectroscopy
    Q.-H. Wu, M. Gunia, T. Strunskus, G. Witte, M. Muhler, Ch. Wöll,
    Chem. Vap. Deposition 11(8-9), 355 (2005).
    Abstract

  • Molecular beam deposition of perylene on copper: formation of ordered phases
    S. Söhnchen, K. Hänel, A. Birkner, G. Witte* and Ch. Wöll,
    Chem. Mater. 17(21), 5297 (2005).
    Abstract

  • The role of molecular conformations in thin film growth: Why soft matter can be special
    D. Käfer, L. Ruppel, G. Witte and Ch. Wöll,
    Phys. Rev. Lett. 95, 166602 (2005).
    Abstract

  • Vacuum level alignment at organic/metal junctions: The “cushion” effect and the interface dipole
    G. Witte, S. Lukas, P. Bagus, and Ch. Wöll,
    Appl. Phys. Lett. 87, 263502 (2005). 
    Abstract

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2004


  • Structural and electronic properties of pentacene molecule and molecular pentacene solid
    R.G.Endres, C.Y. Fong, L.H. Yang, G. Witte, and Ch. Wöll,
    Computational Materials Science,  29, 362 (2004).
    Abstract

  • Epitaxial growth of Pentacene films on metal surfaces
    S. Lukas, S. Söhnchen, G. Witte, and Ch. Wöll,
    ChemPhysChem 5, 266 (2004).
    Abstract

  • Epitaxial growth of Pentacene films on Cu(110)
    S. Söhnchen, S. Lukas and G. Witte*,
    J. Chem. Phys. 121, 525  (2004).
    Abstract

  • Organic molecular beam epitaxy of perylene on Cu(110): Results from NEXAFS, XPS and AFM.
    K. Hänel, S. Söhnchen, S. Lukas, G. Beernink, A. Birkner, T. Strunskus, G. Witte and Ch. Wöll,
    J. Mater. Res. 19, 2049 (2004).
    Abstract

  • Growth of aromatic molecules on solid substrates for applications in organic electronics
    G. Witte and Ch. Wöll (Review),
    J. Mater. Res. 19, 1889 (2004).
    Abstract

  • Probing metallization transitions by metastable He-atom scattering
    G. Witte,
    J. Phys: Condens. Matter  16, S2937 (2004).
    Abstract

  • The importance of dewetting in organic molecular beam deposition: pentacene on gold
    G. Beernink, T. Strunskus, G. Witte* and Ch. Wöll,
    Appl. Phys. Lett. 85, 398 (2004).
    Abstract

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2003


  • Organic Surface Science: Creating order and complexity using self-assembly
    G. Witte* and Ch. Wöll,
    Phase Transitions, 76(4), 291-305 (2003).
    Abstract

  • Self-assembled monolayers of biphenyl-substituted alkanethiols: odd-even differences in the molecular arrangement and packing density.
    W. Azzam, P. Cyganik, G. Witte, M. Buck and Ch. Wöll,
    Langmuir , 19(20),  8262  (2003).
    Abstract

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2002


  • Novel mechanism for molecular self-assembly on metal substrates: Unidirectional rows of pentacene on Cu(110) produced by a substrate-mediated repulsion
    S. Lukas, G. Witte, and Ch. Wöll,
    Phys. Rev. Lett. 88, 28301 (2002).
    Abstract

  • Low frequency vibrational modes of adsorbates
    Gregor Witte,
    Surf. Sci. 502/503, 405 (2002).
    Abstract

  • Microstructure of the Al2O3(11-20) surface .
    T. Becker, A. Birkner, G. Witte* and Ch. Wöll,
    Phys. Rev. B 65, 115401 (2002).
    Abstract

  • Adsorption of dodecanethiol on Cu(110): structural ordering upon thiolate formation
    A. Kühnle, S. Vollmer, T.R. Linderoth, G. Witte*, Ch. Wöll and F. Besenbacher,
    Langmuir  18,  5558  (2002).
    Abstract

  • Structural, chemical, and magnetic properties of Fe films grown on InAs(100)
    L. Ruppel, G. Witte*, Ch. Wöll, T. Last, S.F. Fischer and U. Kunze,
    Phys. Rev. B 66, 245307  (2002).
    Abstract

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2001


  • Growth and dynamics of ultrathin Barium films on Cu(100),
    S. Bartholmei, P. Fouquet and G. Witte*,
    Surf. Sci. 473, 227 (2001).
    Abstract

  • Chemisorption of Acenes on flat and vicinal Cu(111) surfaces: step induced alignment,
    S .Lukas, S. Vollmer, G. Witte* and Ch. Wöll,
    J. Chem. Phys. 114, 10123 (2001).
    Abstract

  • Structural characterization of organothiolate adlayers on gold: the case
    of rigid, aromatic backbones,

    Claus Fuxen, Waleed Azzam, Ralf Arnold, Gregor Witte, Andreas Terfort, Christof Wöll,
    Langmuir 17, 3689 (2001).
    Abstract

  • Low Energy Dynamics of NO and CO chemisorbed on Rh(111),
    Gregor Witte,
    J. Chem. Phys. 115, 2757 (2001).
    Abstract

  • Metallization and demetallization of clean and oxygen covered ultrathin alkali metal films on GaAs(100) 
    P. Fouquet and G. Witte,
    Appl. Surf. Sci. 180, 286 (2001).
    Abstract

  • Structural analysis of saturated alkanethiolate monolayers on Cu(100): coexistence of thiolate and sulfide species
    Stefan Vollmer, Gregor Witte and Christof Wöll,
    Langmuir 17, 7560 (2001).
    Abstract

  • Determination of site specific adsorption energies of CO on copper
    Stefan Vollmer, Gregor Witte* and Christof Wöll,
    Catal. Lett. 77 (1-3), 97 (2001).
    Abstract

2000


  • Monitoring of Electronic Poisoning of Ultrathin Alkali Metal Films by Oxygen and Carbon Monoxide Adsorption,
    P. Fouquet and G. Witte,
    Surf. Sci. 454-456, 256 (2000).
    Abstract

  • A Helium Atom Scattering Study of the Adsorption of NO on Pt(111),
    P. Fouquet, A.P. Graham and G.Witte
    J. Chem. Phys. 112, 7600 (2000).
    Abstract

  • Nanopatterning of Copper (111) Vicinal Surfaces by Oxygen induced Mesoscopic Faceting,
    S. Vollmer, A. Birkner, S. Lukas, G. Witte* and Ch. Wöll,
    Appl. Phys. Lett. 76, 2686 (2000).
    Abstract

  • Variable temperature and ex-situ SPLEEM,
    E.D. Tober, G. Witte and H. Poppa,
    J. Vac. Sci. Tec. A  18, 1845 (2000).
    Abstract

  • Low Energy Vibrational Dynamics of Alkanethiols Adsorbed on Copper Surfaces,
    S.Vollmer, P.Fouquet, G.Witte*, Ch. Boas, M.Kunat, U.Burghaus and Ch. Wöll,
    Surf. Sci. 462, 135 (2000).
    Abstract

  • Nanostrukturierung von Kupferoberflächen durch Sauerstoff- induzierte Rekonstruktionen,
    S. Vollmer, A. Birkner, S. Lukas, G. Witte and Ch. Wöll,
    Zeitschrift für Materialwissenschaft und Werkstofftechnik 31, 845 (2000).
    Abstract

  • Phonons of a Quasi-2D Metal: a Cesium Monolayer on Cu(100),
    G. Witte*and J.P. Toennies,
    Phys. Rev. B 62, 7771 (2000).
    Abstract

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1999


  • Low Frequency Vibrations of Alkali Atoms on Cu(001),
    P. Senet, J.P. Toennies and G.Witte*,
    Chem. Phys. Lett. 299, 389 (1999).
    Abstract

  • Metastable (23S) Helium Atom Scattering from Molecular Adsorbates on Pd and Cu Surfaces,
    P. Fouquet and G. Witte*,
    Surf. Rev. Lett. 6, 103 (1999).
    Abstract

  • Low-Frequency Adsorbate Vibrational Relaxation and Sliding Friction,
    B.N.J. Perrson, E. Tosatti, D. Fuhrmann, G. Witte and Ch. Wöll,
    Phys. Rev. B 59, 11777 (1999).
    Abstract

  • Adsorption of Heptanethiol on Cu(110),
    G. Loepp, S. Vollmer, G. Witte and Ch. Wöll,
    Langmuir 15, 3767 (1999).
    Abstract

  • Observation of Metallization Transition of 2D Alkali Metal Films,
    P. Fouquet and G. Witte,
    Phys. Rev. Lett. 83, 360 (1999).
    Abstract

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1998


  • Damping of molecular motion on a solid substrate: evidence for electron-hole pair creation,
    G. Witte, K. Weiss, P. Jakob, J. Braun, K.L. Kostov and Ch. Wöll,
    Phys. Rev. Lett. 80, 121 (1998).
    Abstract

  • Metastable (23S) Helium Atom Scattering from NiO(100) and Cu(100) Surfaces,
    P.K. Day, P. Fouquet and G. Witte*,
    Surf. Sci. 400, 140 (1998).
    Abstract

  • Second-harmonic generation from sodium covered Si(111)7x7 surfaces
    J. Boness, G. Marowsky, J. Braun, G. Witte and H.G. Rubahn,
    Surf. Sci. 402-404, 513 (1998).
    Abstract

  • Self-assembled lateral multilayers from thin film alloys of immiscible metals,
    E.D. Tober, R.C.F. Farrow, R.F. Marks, G. Witte, K. Kalki and D.D. Chambliss,
    Phys. Rev. Lett. 81, 1897 (1998).
    Abstract

  • Oxygen induced Reconstructions on Cu(211),
    G. Witte*, J. Braun, D. Nowack, L. Bartels, B. Neu and G. Meyer,
    Phys. Rev. B 58, 13224 (1998).
    Abstract

  • A Helium-Atom-Scattering Study of the Structure and Phonons of the NiO(100) and CoO(100) Surfaces,
    G. Witte*, P. Senet and J.P. Toennies,
    Phys. Rev. B 58, 13264 (1998).
    Abstract

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1997


  • Lattice Dynamics of the Ni(977) Surface,
    G. Witte* and J.P. Toennies,
    Phys. Rev. B 55, 1395 (1997).
    Abstract

  • Surface Phonon Dispersion Curves for a Hexagonally Close Packed Metal Surface: Ru(0001),
    J. Braun, K.L. Kostov, G. Witte, L. Surnev, J.G. Skofronik, S. Safron and Ch. Wöll,
    Surf. Sci. 372, 132 (1997).
    Abstract

  • Low Energy Molecular Vibrations investigated by Inelastic Scattering of Thermal Energy He Atoms,
    G. Witte, D. Fuhrmann and Ch. Wöll,
    Chem. Phys, Lett. 265, 347 (1997).
    Abstract

  • CO overlayers on Ru(0001) studied by helium atom scattering: Structure, dynamics, and the influence of coadsorbed H and O.
    J. Braun, K.L. Kostov, G. Witte and Ch. Wöll,
    J. Chem. Phys. 106, 8262 (1997).
    Abstract

  • Micrometer-Sized Nozzles and Skimmers for Atomic-Beam Production of Supersonic He Atom Beams,
    J. Braun, P.K. Day, J.P. Toennies, G. Witte* and E. Neher,
    Rev. Sci. Instrum. 68, 3001 (1997).
    Abstract

  • A Helium Atom Scattering Study of Low Energy Vibrations of CO on Ni(110),
    M. Bertino and G. Witte*,
    Surf. Sci. Lett. 385, L984 (1997).
    Abstract

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1996


  • The low-energy thermal excitation spectrum of nitrogen molecules adsorbed on Ni(110): Implications for molecular adsorption sites,
    M. Bertino, W. Steinhögl, H. Range, F. Hofmann, E. Hulpke, G. Witte and Ch. Wöll,
    Appl. Phys. A 62, 95 (1996).
    Abstract

  • A Helium Atom Scattering Study of the Frustrated Translational Mode of CO Chemisorbed on Defects on Copper Surfaces,
    J. Braun, A.P. Graham, F. Hofmann, W. Silvestri, J.P. Toennies and G. Witte,
    J. Chem. Phys. 105, 3258 (1996).
    Abstract

  • Surface-scattering study of the interaction potential of He atoms with the step edges of the Cu(211) and Cu(511) vicinal surfaces,
    S. Miret-Artes, J.P. Toennies and G. Witte,
    Phys. Rev. B 54, 5881 (1996).
    Abstract

1995


  • Comparison of Surface Phonon Dispersion Curves of the Clean and Hydrogen Covered Rh(111) Surface,
    G. Witte, J.P. Toennies and Ch. Wöll,
    Surf. Sci. 323, 228 (1995).
    Abstract

  • A Helium Atom Scattering Study of the Frustrated Translation Mode of CO adsorbed on the Cu(001) Surface,
    J. Ellis, J.P. Toennies and G. Witte,
    J. Chem. Phys. 102, 5059 (1995).
    Abstract

  • Helium Atom Scattering Study of the Dispersion Curves of Step Localized Phonons on Cu(211) and Cu(511),
    G. Witte, J. Braun, A. Lock and J.P. Toennies,
    Phys. Rev.  B 52, 2165 (1995).
    Abstract

  • External vibrations of hydrocarbons on Cu(001),
    G. Witte and Ch. Wöll,
    J. Chem. Phys. (Letters to the Editor) 103, 5860 (1995).
    Abstract

  • A SPALEED structural study of cesium adsorption on stepped copper surfaces Cu(211) and Cu(511),
    J. Braun, J.P. Toennies and G. Witte,
    Surf. Sci. 340, 265 (1995).
    Abstract

  • Untersuchung der Dynamik und Struktur von sauberen und adsorbatbedeckten Oxid- und Metalloberflächen (Investigation of the dynamics and structure of clean and adsorbate-covered oxide and metal surfaces).
    G. Witte,
    Dissertation Georg-August Universität Göttingen (1995).
    Max-Planck-Institut für Strömungsforschung, Bericht 8/1995, ISSN 0436-1199
    Abstract

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1994

  • Surface Phonon Dispersion Curves and the Longitudinal Resonance in Ag(001) observed by Helium Atom Scattering,
    N. Bunjes, N.S. Luo, P. Ruggerone, J.P. Toennies and G. Witte,
    Phys. Rev. B 50, 8897 (1994).
    Abstract


1993


  • Observation of the Low-Energy External Vibrations of Benzene on Rh(111),
    G. Witte, H. Range, J.P. Toennies and Ch. Wöll,
    Phys. Rev. Lett. 71, 1063 (1993).
    Abstract

  • High Resolution He-Atom Scattering on NiO(100),
    J.P. Toennies, G. Witte, A.M. Shikin and K.H. Rieder,
    J. Electron Spectrosc. Rel. Phenom. 64/65, 677 (1993).
    Abstract

  • External Vibrations of Molecules on Surfaces: The case of CO and Benzene,
    G. Witte, H. Range, J.P. Toennies and Ch. Wöll,
    J. Electron Spectrosc. Rel. Phenom. 64/65, 715 (1993).
    Abstract

  • The observation of well defined two and three phonon bands in time of flight spectra of helium atoms scattered from the (2x2) structure of CO on Cu(001),
    J. Ellis. A. Reichmuth, J.P. Toennies and G. Witte,
    J. Electron Spectrosc. Rel. Phenom. 64/65, 725 (1993).
    Abstract

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1991


  • Application of Computer Algebra to Surface Lattice Dynamics,
    R. Berndt, A. Lock, G. Witte and Ch. Wöll,
    Proceedings of the International Symposium on Symbolic and Algebraic Computation,
    p.433ff, S.M. Watt (ed.), acm Press (1991).
    Abstract

  • ORFEUS Alignment Concept,
    R. Graue, D. Kampf, H. Rippel and G. Witte,
    Space Astronomical Telescope and Instruments, SPIE  1494, 377 (1991).
    Abstract

1990


  • Photoemission and He Atom Scattering from Xe Adsorbed on Al(332),
    B.J. Hinch, A. Lock, H.H. Madden, J.P. Toennies and G. Witte,
    J. Electron Spectrosc. Rel. Phenom. 51, 673 (1990).
    Abstract

  • A Helium Atom Scattering Investigation of Facetting of the Stepped Al(332) Surface,
    B.J. Hinch, A. Lock, H.H. Madden, J.P. Toennies and G. Witte,
    Phys. Rev. B 42, 1547 (1990).
    Abstract

  • Surface Phonons of Stepped Metal Surface,
    A. Lock, J.P. Toennies and G. Witte,
    J. Electron Spectrosc. Rel. Phenom. 54/55, 309 (1990).
    Abstract

  • Lattice Dynamical Calculations for stepped Surfaces,
    A. Lock, J.P. Toennies and G. Witte,
    Proceedings of the Third International Conference on Phonon Physics and Sixth International Conference on Condensed Matter, Heidelberg, 1989, p.901ff (World Scientific, Singapore, 1990).

 

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Zuletzt aktualisiert: 29.01.2015 · Gabriele Nau

 
 
 
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