Main Content

2011

ACS NANO: "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, W. J. Parak
ACS Nano 5 (12), 9870-9876 (2011), DOI: 10.1021/nn2035582

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.
APPL. PHYS. LETT.: "Temperature dependent Optical Properties of Pentacene films on Zinc Oxide"

J. Helzel, S. Jankowski, M. El Helou, G. Witte, W. Heimbrodt
Applied Physics Letters 99 (21), 211102 (2011), DOI: 10.1063/1.3663863

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.
CRYS. GROW. DES.: "Interrelation between substrate roughness and thin-film structure of functionalized acenes on graphite"

T. Breuer, I. Salzmann, J. Götzen, M. Oehzelt, A. Morherr, N. Koch, G. Witte
Crystal Growth & Design 11 (11), 4996-5001 (2011), DOI: 10.1021/cg200894y

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.
J. APPL. PHYS.: "Application of Transmission Electron Microscopy for Microstructural Characterization of Perfluoropentacene Thin Films"

B. Haas, A. Beyer, W. Witte, T. Breuer, G. Witte, K. Volz
Journal of Applied Physics 110 (7), 073514 (2011), DOI: 10.1063/1.3646549

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.
J. PHYS. CHEM.: "Tailoring the Cu(100) work function by substituted benzenethiolate-SAMs"

C. Schmidt, A. Witt, G. Witte
Journal of Physical Chemistry A 115 (25), 7234-7241 (2011), DOI: 10.1021/jp200328r

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.
PHYS. REV. B: "Epitaxial growth of perfluoropentacene films with predefined molecular orientation: A route for single-crystal optical studies"

Tobias Breuer and Gregor Witte
Physical Review B 83 (15), 155428 (2011), DOI: 10.1103/PhysRevB.83.155428

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.
LANGMUIR: "Temporal Evolution of Benzenethiolate-SAMs on Cu(100)"

C. Schmidt, J. Götzen, G. Witte
Langmuir 27 (3), 1025-1032 (2011), DOI: 10.1021/la1022664

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.
LANGMUIR: "Structural Evolution of Perfluoro-Pentacene Films on Ag(111): Transition from 2D to 3D Growth"

J. Götzen, C. H. Schwalb, C. Schmidt, G. Mette, M. Marks, U. Höfer, G. Witte
Langmuir 27 (3), 993-999 (2011), DOI: 10.1021/la1022664

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.
SURF. SCIE.: "Absence of template induced ordering in organic multilayers: the growth of pentacene on a Cu(221) vicinal surface"

J. Götzen, S. Lukas, A. Birkner, G. Witte
Surface Science 605 (5-6), 577-581, (2011), DOI: 10.1016/j.susc.2010.12.022

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.