Showing papers by "Torsten Fritz published in 2005"

In situ differential reflectance spectroscopy of thin crystalline films of PTCDA on different substrates

Abstract: We report an investigation of the excitonic properties of thin crystalline films of the archetypal organic semiconductor PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on poly- and single crystalline surfaces. A sensitive setup capable of measuring the optical properties of ultrathin organic molecular crystals via differential reflectance spectroscopy (DRS) is presented. This tool allows to carry out measurements in situ, i.e., during the actual film growth, and over a wide spectral range, even on single crystalline surfaces with high symmetry or metallic surfaces, where widely used techniques like reflection anisotropy spectroscopy (RAS) or fluorescence excitation spectroscopy fail. The spectra obtained by DRS resemble mainly the absorption of the films if transparent substrates are used, which simplifies the analysis. In the case of mono- to multilayer films of PTCDA on single crystalline muscovite mica(0001) and Au(111) substrates, the formation of the solid state absorption from monomer to dimer and further to crystal-like absorption spectra can be monitored.

Line-on-Line Coincidence: A New Type of Epitaxy Found in Organic-Organic Heterolayers

Abstract: We propose a new type of epitaxy, line-on-line coincidence (LOL), which explains the ordering in the organic-organic heterolayer system PTCDA on HBC on graphite. LOL epitaxy is similar to point-on-line coincidence (POL) in the sense that all overlayer molecules lie on parallel, equally spaced lines. The key difference to POL is that these lines are not restricted to primitive lattice lines of the substrate lattice. Potential energy calculations demonstrate that this new type of epitaxy is indeed characterized by a minimum in the overlayer-substrate interaction potential.

Understanding organic-inorganic heteroepitaxial growth of molecules on crystalline substrates : Experiment and theory

Abstract: The physical structure of organic--inorganic heteroepitaxial thin films is usually governed by a fine balance between weak molecule--molecule interactions and a small lateral variation of the molecule--substrate interaction potential. In order to investigate the energetics of such a layer system, one has to consider large molecular domains [Mannsfeld and Fritz, Phys. Rev. B 69, 75416 (2004)]. So far, layer potential calculations for large domains of organic thin films on crystalline substrates were difficult to perform concerning the computational effort which stems from the huge number of atoms which have to be included. Here, we present a technique which enables the calculation of the molecule--substrate interactions for large molecular domains by utilizing potential energy grid files. As an application example, we discuss the growth of titanylphthalocyanine (TiOPc) on Au(111). TiOPc is found to grow homogeneously in extended domains on top of the alternating domains of the Au(111) surface reconstruction, locally exhibiting a point-on-line coincident epitaxy with the substrate lattice. Here, we demonstrate that the optimal structure and orientation of the TiOPc lattice in such extended overlayer domains can be predicted by potential optimization calculations using the technique described herein.