Browsing by Author "Petersen, Jörg"

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Open Access
Epitaxy and scanning tunneling microscopy image contrast of copper-phthalocyanine on graphite and MoS2
(1994) Ludwig, Christoph; Strohmaier, Rainer; Petersen, Jörg; Gompf, Bruno; Eisenmenger, Wolfgang
Monolayers of copper–phthalocyanine (Cu–Pc) on highly oriented pyrolytic graphite (HOPG) and MoS2 prepared by organic molecular beam epitaxy have been investigated by scanning tunneling microscopy. On both substrates there exist well defined preparation conditions leading to ordered two-dimensional arrays of flat lying molecules. On HOPG they form a close-packed structure with a nearly quadratic unit cell, whereas on MoS2 we found two phases, one close-packed and one rowlike phase. This rowlike phase can be explained by a long range interaction due to an adsorbate induced superstructure of the substrate, which also can be seen in the scanning tunneling microscopy images. In images with submolecular resolution, the molecules appear different on the two substrates. On MoS2 they look like a four-leaved clover, on graphite they show a more detailed inner structure.
Open Access
STM investigations of C6Br6 on HOPG and MoS2
(1994) Strohmaier, Rainer; Ludwig, Christoph; Petersen, Jörg; Gompf, Bruno; Eisenmenger, Wolfgang
Monolayers of the organic molecule hexabromobenzene on HOPG and MoS2 have been imaged with scanning tunneling microscopy. The flat lying molecules form an ordered close-packed array with unit cell parameters corresponding to their van der Waals radii. In both cases the overlayers are incommensurable to the substrate. In the images with submolecular resolution, the molecules appear different on the two substrates. On HOPG the observed STM image contrast shows a detailed submolecular pattern depending on the tunneling voltage, whereas on MoS2 the molecules appear with a less pronounced inner structure. Neither on HOPG nor on MoS2 the observed submolecular contrast depends on the adsorption site.
Open Access
STM investigations of PTCDA and PTCDI on graphite and MoS2 : a systematic study of epitaxy and STM image contrast
(1994) Ludwig, Christoph; Gompf, Bruno; Petersen, Jörg; Strohmaier, Rainer; Eisenmenger, Wolfgang
Monolayers of the organic molecules perylene-3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and diimide (PTCDI) on graphite and MoS₂ have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations.
Open Access
Thermal motion of one-dimensional domain walls in monolayers of a polar polymer observed by Video-STM
(1993) Ludwig, Christoph; Eberle, Gernot; Gompf, Bruno; Petersen, Jörg; Eisenmenger, Wolfgang
Scanning tunneling microscopy (STM) has been used to investigate monolayers of the ferroelectric copolymer polyvinylidenefluoride/trifluoroethylene P(VDF/TrFE) showing images of ordered polymer monolayers. By scanning with video frame rate, direct observation of the motion of onedimensional domain walls was also possible for the first time. The images clearly show domain walls normal to the polymer chains. From measurements of the temperature dependence of the domain wall velocities the activation energy for the thermally generated kink motion was estimated. These results are compared with theoretical models describing domain wall motion in ferroelectric PVDF.
Open Access
Video-STM, LEED and X-ray diffraction investigations of PTCDA on graphite
(1992) Ludwig, Christoph; Gompf, Bruno; Glatz, Wolfgang; Petersen, Jörg; Eisenmenger, Wolfgang; Möbus, Manfred; Zimmermann, Uwe; Karl, Norbert
Thin films of the organic molecule perylene-3,4,9,10-tetracarboxylic-dianhydride ("PTCDA") on graphite (0001) have been investigated from the mono- to the multilayer regime with low energy electron diffraction (LEED), X-ray-diffraction in Bragg-Brentano geometry, and high resolution scanning tunneling microscopy (STM). These different methods proved epitaxial growth in a coincident superstructure and yielded congruent results concerning details of the crystallographic structure of the epilayer. In addition it was possible to resolve submolecular structures in high resolution STM images; a comparison of the 10 resolved maxima of the tunneling current with the molecular structure leads us to question the conventional model description of tunneling.