Wolfgang M Heckl

Technische Universität München, München, Bavaria, Germany

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Publications (176)582.05 Total impact

  • Marc-Denis Weitze · Wolfgang M Heckl

    No preview · Article · Oct 2015 · Nanomedicine
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    ABSTRACT: Ullmann coupling of 1,3-diiodobenzene is studied on Cu(111) surfaces in ultra-high vacuum (UHV). In situ Scanning Tunneling Microscopy (STM) at room temperature revealed an unexpected ordered arrangement of highly uniform reaction products adsorbed atop a closed iodine monolayer.
    No preview · Article · Jul 2015 · Chemical Communications
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    ABSTRACT: Monolayer self-assembly of a hexabrominated, three-fold symmetric aromatic molecule is studied at the heptanoic acid – graphite interface. Thermodynamical insights are obtained from an adapted Born-Haber cycle that is utilized to derive the overall enthalpy change including solvent effects. Comparison with theoretical entropy estimates suggests a minor influence of solvation.
    No preview · Article · Sep 2014 · Chemical Communications
  • W Song · N Martsinovich · W M Heckl · M Lackinger
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    ABSTRACT: A direct calorimetric measurement of the overall enthalpy change associated with self-assembly of organic monolayers at the liquid-solid interface is for most systems of interest practically impossible. In previous work we proposed an adapted Born-Haber cycle for an indirect assessment of the overall enthalpy change by using terephthalic acid monolayers at the nonanoic acid-graphite interface as a model system. To this end, the sublimation enthalpy, dissolution enthalpy, the monolayer binding enthalpy in vacuum, and a dewetting enthalpy are combined to yield the total enthalpy change. In the present study the Born-Haber cycle is applied to 4,4'-stilbenedicarboxylic acid monolayers. A detailed comparison of these two aromatic dicarboxylic acids is used to evaluate and quantify the contribution of the organic backbone for stabilization of the monolayer at the nonanoic acid-graphite interface.
    No preview · Article · May 2014 · Physical Chemistry Chemical Physics
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    ABSTRACT: Self-assembly and thermally activated surface chemistry of 1,3,5-benzenetrithiol (BTT) on Cu(111) are studied under ultrahigh vacuum (UHV) conditions by different complementary surface sensitive techniques. Low-energy electron diffraction (LEED) patterns acquired at room temperature and during subsequent heating reveal irreversible phase transitions between in total four different long-range-ordered phases termed α-phase to δ-phase. X-ray photoelectron spectroscopy (XPS) of the different phases facilitates the identification of major chemical changes for the first phase transition from α- to β-phase, whereas in the succeeding phase transitions, no significant chemical shifts are observed anymore. The structural characterization of each phase is carried out by high-resolution scanning tunneling microscopy (STM), and adsorption geometries of the phenyl rings are derived from C 1s near-edge X-ray absorption fine structure (NEXAFS). The combination of the results from this array of experimental techniques leads to a consistent picture of the various phases and underlying processes. Upon room-temperature deposition, BTT fully deprotonates and planar-adsorbed molecules self-assemble into an ordered monolayer. With a temperature onset of 300 K, the carbon–sulfur bonds start dissociating. Sulfur forms a copper sulfide superstructure, whereas the organic remainders form disordered structures. Further heating converts an initial metastable and rarely observed (√3 × √3)R ± 30° copper sulfide superstructure into the more stable and well-known (√7 × √7)R ± 19.1° polymorph.
    No preview · Article · Feb 2014 · The Journal of Physical Chemistry C
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    ABSTRACT: The driving force for self-assembly is the associated gain in free energy with decisive contributions from both enthalpy and entropy differences between final and initial state. For monolayer self-assembly at the liquid-solid interface, solute molecules are initially dissolved in the liquid phase and become then incorporated into an adsorbed monolayer. In this work we present an adapted Born-Haber cycle for obtaining precise enthalpy values for self-assembly at the liquid-solid interface, a key ingredient for a profound thermodynamic understanding of this process. By choosing terephthalic acid as a model system, it is demonstrated that all required enthalpy differences between well-defined reference states can be independently and consistently assessed by both experimental and theoretical methods, giving in the end a reliable value of the overall enthalpy gain for self-assembly of interfacial monolayers. A quantitative comparison of enthalpy gain and entropy cost reveals essential contributions from solvation and dewetting, which lowers the entropic cost and render monolayer self-assembly a thermodynamically favored process.
    No preview · Article · Sep 2013 · Journal of the American Chemical Society
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    Full-text · Dataset · Aug 2013
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    ABSTRACT: Self-assembly of 1,3,5-tris(4'-biphenyl-4''-carbonitrile)benzene monolayers was studied at the liquid-solid interface by scanning tunneling microscopy. Application of different fatty acid homologues as solvents revealed a solvent-induced polymorphism. Yet, tempering triggered irreversible phase transitions of the initially self-assembled monolayers, thereby indicating their metastability. Interestingly, in either case the same, thermodynamically more stable and more densely packed monolayer polymorph is obtained after the thermal treatment, irrespective of the initial structure. Again the same densely packed structure was obtained in complementary solvent-free experiments conducted under ultra-high vacuum conditions. Thus, self-assembly of metastable polymorphs at room temperature is explained by adsorption of partially solvated species under kinetic control. The irreversible phase transitions are induced by thermal desolvation, i.e. desorption of co-adsorbed solvent molecules.
    No preview · Article · Jul 2013 · ACS Nano
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    ABSTRACT: The adsorption geometry of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) on Cu(111) is determined with high precision using two independent methods, experimentally by quantitative low energy electron diffraction (LEED-I(V)) and theoretically by dispersion corrected density functional theory (DFT-vdW). Structural refinement using both methods consistently results in similar adsorption sites and geometries. Thereby a level of confidence is reached that allows deduction of subtle structural details such as molecular deformations or relaxations of copper substrate atoms.
    Full-text · Article · May 2013 · Physical Chemistry Chemical Physics
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    ABSTRACT: The polymerization of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) through covalent aryl-aryl coupling is accomplished using a solution-based approach and investigated by scanning tunneling microscopy. Drop-casting of the TIPB monomer onto Au(111) at room temperature results in poorly ordered non-covalent arrangements of molecules and partial de-halogenation. However, drop-casting on a pre-heated Au(111) substrate yields various topologically distinct covalent aggregates and networks. Interestingly, some of these covalent nanostructures do not adsorb directly on the Au(111) surface, but are loosely bound to a disordered layer of a mixture of chemisorbed iodine and molecules, a conclusion which is drawn from STM data and supported by X-ray photoelectron spectroscopy. We argue that the gold surface becomes covered by a strongly chemisorbed iodine monolayer which eventually inhibits further polymerization.
    Full-text · Article · Mar 2013 · ACS Nano
  • Johanna Eichhorn · Wolfgang M Heckl · Markus Lackinger
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    ABSTRACT: The polymerization of 1,4-diethynylbenzene was studied on a Cu(111) surface using scanning tunneling microscopy (STM) under ultra-high vacuum conditions. Thermal activation yielded disordered covalent networks, where distinct basic structural motifs indicate different coupling reactions.
    No preview · Article · Mar 2013 · Chemical Communications
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    ABSTRACT: Self-assembled monolayers of 1,3,5-tris(4'-biphenyl-4''-carbonitrile)-benzene - a large functional trinitrile molecule - are studied on the (111) surfaces of copper and silver under ultra-high vacuum conditions by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). A densely packed hydrogen bonded polymorph was equally observed on both surfaces. Addi-tionally, deposition onto Cu(111) yielded a well-ordered metal-coordinated porous polymorph that coexisted with the hydrogen bonded structure. The required coordination centers are supplied by the adatom gas of the Cu(111) surface. On Ag(111), however, the well-ordered metal-coordinated network was never observed. Differences in the adatom reactivity between copper and silver and the resulting bond strength of the respective coordination bond are held responsible for this substrate dependence. By utilizing ultra-low deposition rates, we demonstrate that on Cu(111) adatom kinetics plays a decisive role in the expression of intermolecular bonds - and hence for structure selection.
    No preview · Article · Dec 2012 · Journal of the American Chemical Society
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    ABSTRACT: On-surface self-condensation of 1,4-benzenediboronic acid was previously shown to yield extended surface-supported, long-range-ordered two-dimensional covalent organic frameworks (2D COFs). The most important prerequisite for obtaining high structural quality is that the polycondensation (dehydration) reaction is carried out under slightly reversible reaction conditions, i.e., in the presence of water. Only then can the subtle balance between kinetic and thermodynamic control of the polycondensation be favorably influenced, and defects that are unavoidable during growth can be corrected. In the present study we extend the previously developed straightforward preparation protocol to a variety of para-diboronic acid building blocks with the aim to tune lattice parameters and pore sizes of 2D COFs. Scanning tunneling microscopy is employed for structural characterization of the covalent networks and of noncovalently self-assembled structures that form on the surface prior to the thermally activated polycondensation reaction.
    No preview · Article · Jul 2012 · ACS Nano
  • Stefan Schlögl · Wolfgang M. Heckl · Markus Lackinger
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    ABSTRACT: In the present study on-surface polymerisation of aromatic iodinated precursor molecules through radical addition is studied by scanning tunneling microscopy (STM) on Au(111) under ultra-high vacuum (UHV) conditions. Comparison of the two analogous monomers s-triiodobenzene and s-triiodophenylbenzene with similar symmetry and functionalization but differently sized organic backbones aimed to study the influence of monomer size on the morphology of resulting covalent networks. In contrast to previous studies on brominated monomers, here the reaction by-product, i.e., the split-off iodine atoms, was found to adsorb stably on Au(111) in ordered structures. The influence of post-processing by thermal annealing on both the covalent networks and the iodine structures was studied for different temperatures. DFT calculations were applied to evaluate formation energies of commonly observed topological defects and related to their experimental probability of occurrence.
    No preview · Article · Jul 2012 · Surface Science
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    ABSTRACT: Organic-walled microfossils of uncertain origin, classified to an informal group named acritarchs, are most commonly interpreted as the resting cysts of marine eukaryotic phytoplankton. Some acritarchs have recently been interpreted as vegetative cells of chlorococcalean green algae, based on internal bodies that have been interpreted as their asexual reproductive structures (spores). To verify this interpretation, we applied confocal Raman spectroscopy and atomic force microscopy (AFM) to study the ultrastructure and nanostructure of exceptionally preserved acritarchs with internal bodies from the early Silurian cherts (ca 430 Ma-old) of Frankenwald (Germany). Three-dimensional Raman mapping showed the spatial distribution of carbonaceous material and other minerals in the walls of the analysed internal bodies and confirmed that these structures are comparable with spores of chlorococcalean microalgae. Our findings document therefore the oldest thus far known vegetative cells of sporulating green algae. The combination of confocal Raman and AFM techniques yielded detailed information about the nanostructure and fossilisation mode of the mineralised organic walls of both the central vesicles and the enclosed spore-like bodies. Copyright (c) 2011 John Wiley & Sons, Ltd.
    Full-text · Article · Jan 2012 · Journal of Raman Spectroscopy
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    ABSTRACT: Two different straightforward synthetic approaches are presented to fabricate long-range-ordered monolayers of a covalent organic framework (COF) on an inert, catalytically inactive graphite surface. Boronic acid condensation (dehydration) is employed as the polymerization reaction. In the first approach, the monomer is prepolymerized by a mere thermal treatment into nanocrystalline precursor COFs. The precursors are then deposited by drop-casting onto a graphite substrate and characterized by scanning tunneling microscopy (STM). While in the precursors monomers are already covalently interlinked into the final COF structure, the resulting domain size is still rather small. We show that a thermal treatment under reversible reaction conditions facilitates on-surface ripening associated with a striking increase of the domain size. Although this first approach allows studying different stages of the polymerization, the direct polymerization, that is, without the necessity of preceding reaction steps, is desirable. We demonstrate that even for a comparatively small diboronic acid monomer a direct thermally activated polymerization into extended COF monolayers is achievable.
    No preview · Article · Dec 2011 · ACS Nano
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    ABSTRACT: We report on covalent two-dimensional phenylene-boroxine hybrid-networks that were synthesized under ultra-high vacuum conditions from doubly functionalized monomers through thermally activated condensation prior to deposition and successive heterogeneously catalyzed radical addition on Ag(111). Structural properties were characterized in situ by high resolution Scanning-Tunneling-Microscopy (STM).
    No preview · Article · Dec 2011 · Chemical Communications
  • M Lackinger · W M Heckl
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    ABSTRACT: 'Covalent self-assembly', i.e. the on-surface synthesis of covalent organic aggregates and networks, has received considerable attention. This review covers recent scanning tunnelling microscopy (STM) based studies on intermolecular reactions carried out on solid substrates that resulted in surface-confined covalently interlinked organic nanostructures. Experiments showed that their defect density crucially depends on the targeted dimensionality: while zero-dimensional aggregates and one-dimensional chains and ribbons can be synthesized on surfaces with utmost structural perfection, i.e. without any topological defects, realization of long-range ordered two-dimensional (2D) covalently interlinked organic networks has revealed itself as a paramount challenge for on-surface chemists. Different types of reactions, foremost condensation and addition reactions have been proven suitable as polymerization reactions for 2D cross-linked covalent networks. Yet, the emergence of topological defects during the polymerization is difficult to avoid. However, the combined experience and creativity of chemists and surface scientists has yielded encouraging first results which may open up ways for realization of extended, long-range ordered 2D polymers. This review summarizes and compares different approaches, i.e. reaction types, monomers, environments and conditions, for the on-surface synthesis of covalent organic nanostructures. The focus on STM as an analytical tool appears justified, since its unique capabilities render the STM an ideal instrument to study and even control covalent coupling reactions of organic molecules on surfaces.
    No preview · Article · Nov 2011 · Journal of Physics D Applied Physics
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    ABSTRACT: The objective of this work is to study both the dynamics and mechanisms of guest incorporation into the pores of 2D supramolecular host networks at the liquid-solid interface. This was accomplished by adding molecular guests to prefabricated self-assembled porous monolayers and the simultaneous acquisition of scanning tunneling microscopy (STM) topographs. The incorporation of the same guest molecule (coronene) into two different host networks was compared, where the pores of the networks either featured a perfect geometric match with the guest (for trimesic acid host networks) or were substantially larger than the guest species (for benzenetribenzoic acid host networks). Even the moderate temporal resolution of standard STM experiments in combination with a novel injection system was sufficient to reveal clear differences in the incorporation dynamics in the two different host networks. Further experiments were aimed at identifying a possible solvent influence. The interpretation of the results is aided by molecular mechanics (MM) and molecular dynamics (MD) simulations.
    Full-text · Article · Sep 2011 · Langmuir
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    ABSTRACT: Self-assembly of melem on Ag(111) as studied by Scanning-Tunneling-Microscopy (STM) in ultra-high vacuum revealed a great structural variety. In total, five porous and two densely packed monolayer polymorphs were observed. All structures are stabilized by intermolecular hydrogen bonds, where melem–melem arrangements are based on very few basic motifs. Six out of seven polymorphs can be described by a unified concept.
    No preview · Article · Aug 2011 · CrystEngComm

Publication Stats

6k Citations
582.05 Total Impact Points


  • 1985-2015
    • Technische Universität München
      München, Bavaria, Germany
  • 1993-2011
    • Ludwig-Maximilian-University of Munich
      • • Department of Earth and Environmental Sciences
      • • Center for Nanoscience (CeNS)
      • • Department of Biology I
      München, Bavaria, Germany
  • 2006
    • University of Bologna
      Bolonia, Emilia-Romagna, Italy
  • 2005
    • Columbia University
      • Department of Chemistry
      New York, New York, United States
  • 1992-2005
    • University Hospital München
      München, Bavaria, Germany
  • 2002
    • Technische Universität Chemnitz
      • Institute of Physics
      Karl-Marx-Stadt, Saxony, Germany
  • 1990
    • University of Toronto
      • Department of Chemistry
      Toronto, Ontario, Canada