Research Activities
Molecular self-assembly on surfaces constitutes a promising route for creating novel materials and nano-devices in molecular electronics, opto-electronics and bio-sensing [A. Kühnle Curr. Op. Coll. Interf. Sci. 14 (2009) 157 
]. Self-assembly is governed by forces acting between individual molecules as well as molecules and the substrate. Our research aims at measuring and controlling forces acting during molecular movement on dielectric surfaces to both unravel the mechanisms behind self-assembly and to understand and develop molecular manipulation.
Current Projects
Contrast formation in non-contact atomic force microscopy
In frequency modulation non-contact atomic force microscopy, the change in resonance frequency of an oscillating cantilever is detected while the cantilever is scanned over the surface. Analyzing this frequency shift allows for gaining insights into the physical mechanisms behind contrast formation. We investigate the physical origin of, e.g., contrast inversion in non-contact atomic force microscopy [P. Rahe et al. Phys. Rev. B 77 (2008) 195410 ]. This understanding is a prerequisite for non-contact atomic force microscopy data interpretation. In close collaboration with our theory partners P. Jelínek, C. Gonzáles (Prague) and R. Pérez (Madrid) we have unraveled the constrast mechanisms of "all-inclusive" TiO2(110) imaging [R. Bechstein et al. Nanotechnology 20 (2009) 505703 ].
Atomic resolution on dielectric surfaces
To investigate the interaction of organic molecules with dielectric surfaces, a detailed characterisation of the bare surface structure and surface reactivity at the atomic level is mandatory. We investigated various dielectric surfaces prepared both in ultra-high vacuum (UHV) as well as after cleavage in air. We routinely obtain atomically resolved images of CaF2(111) and the cleavage plane of calcite (CaCO3). Recently, we succeeded in resolving both bare and H-terminated diamond surfaces [M. Nimmrich et al. Phys. Rev. B 81 (2010) 201403(R) ].
Diffusion and self-assembly of organic molecules on dielectric surfaces
Uni-directional molecular structure on calcite [P. Rahe et al. J. Phys. Chem. C 114 (2010) 1547] |
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The interaction of organic molecules with dielectric surfaces is rather weak compared to metallic substrates. This weak interaction generally leads to a high molecular mobility on dielectric surfaces, resulting in clustering at step edges and molecular bulk crystal formation. In order to employ surface templating for molecular self-assembly, strategies need to be developed for increasing the molecule-surface interaction. By exploiting electrostatic interactions of highly polar molecules on ionic crystals we could greatly increase the diffusion barrier of cytosine on CaF2(111) [J. Schütte et al. Phys. Rev. B 80 (2009) 205421 ]. For C60 on CaF2(111) we have unraveled the molecular-scale origin of unusual island morphologies found at room temperature [M. Körner et al. Phys. Rev. Lett. 107 (2011) 016101 ]. For gaining detailed insight into diffusion and self-assembly, we quantitatively determine diffusion parameter such as the diffusion barrier [F. Loske et al. Phys. Rev. B B 82 (2010) 155428 ]. By carefully choosing the molecule/substrate system, we recently succeeded in self-assembling uni-directional wire-like structures [P. Rahe et al. J. Phys. Chem. C. 114 (2010) 1547 ]. We perform our diffusion and self-assembly studies in close collaboration with our chemistry partners, A. Gourdon (Toulouse), I. Stara (Prague) and H. Langhals (Munich). The results of our experiments are interpreted in close collaboration with theoreticians, M. Rohlfing (Osnabrück), P. Maass (Osnabrück), R. Perez (Madrid) and others, performing, e.g., density-functional theory (DFT) calculations and Monte Carlo simulations. Moreover, we collaborate with experimental groups, e.g., the group of Ch. Wöll (Karlsruhe), for supporting our NC-AFM measurements with complementary techniques.
On-surface covalent linking of organic molecules
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Covalent linking of benzoic acid derivatives on calcite [M. Kittelmann et al. ACS Nano (2011)]
On-surface synthesis in ultra-high vacuum provides a promising strategy for creating thermally and chemically stable molecular structures at surfaces. The two-dimensional confinement of the educts, the possibility to work at higher (or lower) temperatures in the absence of solvent and the templating effect of the surface bear the potential of preparing compounds that cannot be obtained in solution. Moreover, covalently linked conjugated molecules allow for efficient electron transport and are, thus, particularly interesting for future molecular electronics applications. When having these applications in mind, electrically insulating substrates are mandatory to provide sufficient decoupling of the molecular structure from the substrate surface. So far, however, on-surface synthesis has only been achieved on metallic substrates. We have demonstrated the covalent linking of organic molecules on a bulk insulator, namely calcite [M. Kittelmann et al. ACS Nano (2011) ]. By varying number and position of the halide substitution, we rationally design the resulting structures, revealing straight lines, zigzag structures as well as dimers, thus providing clear evidence for the covalent linking.
Development of strategies for lateral manipulation of organic molecules with the non-contact atomic force microscope
Tip-induced switching of a PTCDI derivative from one adsorption position into a neighboring row [J. Schütte et al. Nanotechnology 22 (2011) 245701].
Lateral as well as vertical manipulation of molecules has been studied extensively on metallic and semiconducting surfaces using scanning tunneling microscopy, and an impressive degree of experimental control and theoretical understanding has been achieved. Molecular manipulation using the non-contact atomic force microscope is, however, still in its infancy. Besides understanding physical processes during manipulation, an important aspect of NC-AFM manipulation is developing strategies for preparing stable, chemically inert NC-AFM tips. Employing lateral manipulation allows for gaining detailed insight into intermolecular as well as molecule-surface forces, which greatly contributes to the understanding of molecular self-assembly processes. We have investigate the controlled manipulation of both C60 molecules [F. Loske et al. Appl. Phys. Lett. 95 (2009) 043110 ] as well as PTCDI derivatives [J. Schütte et al. Nanotechnology 22 (2011) 245701 ] on TiO2(110) at room temperature.
High-resolution non-contact atomic force microscopy in liquids
Besides working in ultra-high vacuum, we have implemented the high-resolution frequency modulation technique for operation in liquids [S. Rode et al. Rev. Sci. Instrum. 82 (2011) 073703 ]. The adsorption and structure formation of functional molecules in a liquid environment is of utmost interest for a broad range of fields. An important aspect is studying biological molecules in their natural environment such as proteins for surface functionalization. Another important issue is studying functional molecules in an application-relevant environment. Pivotal progress has recently been achieved by T. Fukuma in the group of H. Yamada (Kyoto), demonstrating that frequency modulation non-contact atomic force microscopy is capable of providing atomic resolution in liquids. Prerequisite for a successful implementation of this technique operated in liquids is the optimisation of the signal-to-noise ratio of the instrument. We closely cooperate with H. Yamada, who has hosted our former group member Sebastian Rode for a training period in Japan, where he was able to atomically resolve calcite under liquid conditions [S. Rode et al. Langmuir 25 (2009) 2850 ].
Photocatalytically active, doped titanium dioxide
Titanium dioxide is well-known for its photocatalytic activity and intensively studied in the context of hydrogen synthesis from water. Doping with transition metal atoms has been successfully employed for shifting the absorption into the visible-light range. In a collaboration with H. Onishi (Kobe), we study TiO2(110) surfaces that were doped with chromium and antimony atoms using high-resolution non-contact atomic force microscopy [R. Bechstein et al. J. Phys. Chem. C 113 (2009) 3277 ], [R. Bechstein et al. Nanotechnology 20 (2009) 264003 ] and [R. Bechstein et al. J. Phys. Chem. C 113 (2009) 13199-13203 ].
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