Ulrich Groth

Universität Konstanz, Constance, Baden-Württemberg, Germany

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Publications (11)29.81 Total impact

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    ABSTRACT: Here we report the synthesis, investigation as well as surface deposition of a truly axial symmetry Mn12-diphenylphosphinate (Mn12-phn) single molecule magnet. Out of 16 acetate ligands encapsulating the Mn12O12 core, 12 ligands were exchanged by diphenylphosphinate in this compound. Mn12-phn shows well-defined magnetic hysteresis curves indicating a very high crystal quality. A monolayer of Mn12-phn was chemically grafted on a functionalized Au(111) surface via ligand exchange reaction and studied by means of scanning tunneling microscopy and spectroscopy. Via distance–voltage spectroscopy we determine the real-space height of the Mn12-phn molecules with high accuracy. A large spread in the measured molecular heights obtained from the distance–voltage spectra indicates the absence of preferential orientation of Mn12-phn molecules with respect to the surface which we attribute to the equal anchoring probability of all diphenylphosphinate ligands in Mn12-phn while none of the four acetate ligands are exchanged. These results are compared with the experimental data obtained from a different Mn12 derivative containing 16 thiophenecarboxylate ligands. In general, we show that the substitution of the ligand shell may have a major impact on the surface orientation of the Mn12 clusters deposited on Au, i.e. on the orientation of the easy magnetization axis.
    First publ. in: Polyhedron 28 (2009), 9/10, 1977–1981. 06/2009;
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    ABSTRACT: The possibility to use the Au(100)/Fe(100)/MgO(100) system as a substrate for future spin-polarized transport measurements on Mn12 single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn12 monolayers was studied. The results demonstrate that Mn12 can be deposited on Au(100)/Fe(100)/MgO(100) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies.
    Polyhedron 06/2009; 28(9):1606-1609. · 2.05 Impact Factor
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    ABSTRACT: The stability of single crystals and monolayers of Mn12 single molecule magnets under the influence of X-ray radiation and other possibly disruptive influences has been investigated by means of synchrotron radiation. Clear evidence for radiation induced sample degradation was found for both single crystals and monolayers. The comparison with spectra obtained after damaging the molecules by Ar+ sputtering, metal evaporation or water moistening indicates a possibility to distinguish between radiation damage and other external influences. The results clarify some of the previous conflicting reports on the integrity of Mn12 molecules deposited on surfaces and are linked to the investigations aiming at studies of the electronic and magnetic properties of individual Mn12 clusters.
    Applied Physics A 01/2009; · 1.69 Impact Factor
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    ABSTRACT: Individual Mn12 single-molecule magnets have been investigated by means of scanning tunneling spectroscopy at room temperature. Current-voltage characteristics of a Mn12 derivative are studied in detail and compared with simulations. A few-parameter scalar model for ballistic current flow through a single energy level is sufficient to describe the main features observed in scanning tunneling spectra of individual Mn12 molecules and offers a deeper insight into the electronic transport properties of this class of single-molecule magnets. In addition, distance-voltage spectroscopy performed on individual Mn12 molecules reveals a possibility to identify the orientation of the molecular easy axis. The results indicate a preferential orientation of the easy axis of the molecules nearly perpendicular to the surface.
    First publ. in: Physical Review B 78 (2008), 155403. 10/2008;
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    ABSTRACT: We report on the electronic properties of Mn12 molecules chemically grafted on the functionalized Au(111) surface studied by means of scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from Mn12 molecules showing a large region of low conductance around the Fermi energy. In agreement with the tunneling spectroscopy results the bias voltage variation upon scanning leads to apparent height changes of the Mn12 clusters. We discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn12 molecules.
    Journal of Physics Conference Series 03/2008; 100(5):052070.
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    ABSTRACT: Different approaches to the deposition of Mn(12) single molecule magnets on the Au(111) surface and their characterization by a broad variety of techniques are investigated with respect to their suitability for a profound corroboration of the integrity of the Mn(12) core. In this context, the most recent improvements in the experimental approaches are presented and the latest results on the electronic properties of Mn(12) are linked to each other. The results confirm the high instability of Mn(12) single molecule magnets on surfaces and reveal the need for an amendment of the requirements to define the structural integrity of Mn(12) molecules on surfaces.
    Dalton Transactions 02/2008; · 4.10 Impact Factor
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    ABSTRACT: The authors report on a scanning tunneling microscopy/spectroscopy investigation of the possibility to influence the assembly of monolayers of Mn12 single molecule magnets on a functionalized Au(111) surface by using flexible linker molecules. The results corroborate the general suitability of the deposition via ligand exchange reaction but, on the other hand, reveal the need for a compromise between conductivity and flexibility of the linker molecules. The results are discussed with respect to previous attempts [ A. Naitabdi et al., Adv. Mater. (Weinheim, Ger.) 17, 1612 (2005) ] to deposit ordered monolayers of Mn12 molecules on Au(111).
    Journal of Applied Physics 01/2008; 103(7):07B901-07B901-3. · 2.21 Impact Factor
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    ABSTRACT: A new approach to the deposition of Mn12 single-molecule magnet monolayers on the functionalized Au(111) surface optimized for the investigation by means of scanning tunneling spectroscopy was developed. To demonstrate this method, the new Mn12 complex [Mn12O12(O2CC6H4F)16(EtOH)4].4.4CHCl3 was synthesized and characterized. In MALDI-TOF mass spectra the isotopic distribution of the molecular ion peak of the latter complex was revealed. The complex was grafted to Au(111) surfaces via two different short conducting linker molecules. The Mn12 molecules deposited on the functionalized surface were characterized by means of scanning tunneling microscopy showing homogeneous monolayers of highest quality. Scanning tunneling spectroscopy measurements over a wider energy range compared with previous results could be performed because of the optimized Au(111) surface functionalization. Furthermore, the results substantiate the general suitability of short acidic linker molecules for the preparation of Mn12 monolayers via ligand exchange and represent a crucial step toward addressing the magnetic properties of individual Mn12 single-molecule magnets.
    Journal of the American Chemical Society 12/2007; 129(46):14362-6. · 10.68 Impact Factor
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    ABSTRACT: The authors report on the electronic properties of individual molecules of two Mn12 derivatives chemically grafted on the functionalized Au(111) surface studied by means of ultrahigh vacuum scanning tunneling microscopy/spectroscopy at room temperature. Reproducible current-voltage curves were obtained from both Mn12 molecules, showing a well defined wide band gap. In agreement with the tunneling spectroscopy results, the bias voltage variation upon scanning leads to apparent height changes of the Mn12 clusters. The authors discuss these findings in the light of the recent band structure calculations and electronic transport measurements on single Mn12 molecules.
    Applied Physics Letters 03/2007; 90(13):133104-133104-3. · 3.52 Impact Factor
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    ABSTRACT: We present a detailed study on the electronic properties of monolayers of Mn12 derivatives chemically grafted on clean as well as on functionalized Au(111) surfaces. Scanning tunneling microscopy and x-ray photoelectron spectroscopy were employed to ensure the successful monolayer deposition. Unoccupied and occupied valence band states in the electronic structure of Mn12-clusters were probed by means of x-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES) at the Mn 2p-3d absorption edge, respectively. XAS measurements reveal a significant difference between the Mn oxidation states of Mn12 cores bound to the Au surface compared with the single crystal environment. Direct deposition of Mn12 derivatives onto the Au surface leads to a strong fragmentation of Mn12 cores. An appropriate combination of Mn12 cluster and substrate functionalization leads to formation of Mn12 monolayers with a large fraction of Mn12 cores retaining their structural integrity upon surface deposition. By extracting the Mn 3d partial density of states from the RPES spectra a very good agreement with previously reported LDA+U calculations on Mn12 for U=4 eV [Boukhvalov et al., J. Electron Spectrosc. Relat. Phenom. 137–140, 735 (2004)] was found, showing the importance of electron correlation effects in Mn12 systems.
    Physical review. B, Condensed matter 01/2007; · 3.66 Impact Factor
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    ABSTRACT: The naturally occurring sulfonate N-acetyltaurine was synthesized chemically and its identity was confirmed. Aerobic enrichment cultures for bacteria able to utilize N-acetyltaurine as sole source of fixed nitrogen or as sole source of carbon were successful. One representative isolate, strain NAT, which was identified as a strain of Delftia acidovorans, grew with N-acetyltaurine as carbon source and excreted stoichiometric amounts of sulfate and ammonium. Inducible enzyme activities were measured in crude extracts of this organism to elucidate the degradative pathway. Cleavage of N-acetyltaurine by a highly active amidase yielded acetate and taurine. The latter was oxidatively deaminated by taurine dehydrogenase to ammonium and sulfoacetaldehyde. This key intermediate of sulfonate catabolism was desulfonated by the known reaction of sulfoacetaldehyde acetyltransferase to sulfite and acetyl phosphate, which was further degraded to enter central metabolism. A degradative pathway including transport functions is proposed.
    Archives of Microbiology 08/2006; 186(1):61-7. · 1.91 Impact Factor