Dorin Geiger

Technische Universität Dresden, Dresden, Saxony, Germany

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Publications (31)107.55 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Pt x Pd y ‐, Pt‐ und Pd‐Aerogele mit großen Oberflächen und hohen Porositäten wurde nach einer einfachen und umweltfreundlichen Strategie kontrolliert hergestellt und als hochaktive Katalysatorsysteme für die Sauerstoffreduktion in PEFC‐Kathoden eingesetzt. Diese Edelmetallaerogele haben das Potenzial, hohe Stabilität und große spezifische Oberflächen zu vereinen.
    Angewandte Chemie 09/2013; 125(37):10033–10037. DOI:10.1002/ange.201303109
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    ABSTRACT: Best of both worlds: Ptx Pdy , Pt, and Pd aerogels with high surface area and porosity can be synthesized in a controlled fashion by a straightforward and environmentally benign strategy. These materials, which are highly active and stable catalysts for the oxygen reduction reaction in PEFC cathodes, combine the high stability of extended surfaces with the high surface area of nanoparticles.
    Angewandte Chemie International Edition 09/2013; 52(37). DOI:10.1002/anie.201303109 · 11.26 Impact Factor
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    ABSTRACT: Hochaktive Leichtgewichte: Mit α‐, β‐ oder γ‐Cyclodextrin modifizierte Aerogele wurden durch Selbstorganisation von Pd‐Nanopartikeln erhalten. Die Pd‐Aerogele zeigen eine ausgezeichnete katalytische Aktivität in der Elektrooxidation von Ethanol, die auf die trägerfreie nanoskalige Struktur des Aerogelnetzwerks sowie auf die Wechselwirkungen des Ethanols mit den Kavitäten der Cyclodextrinmoleküle zurückgeführt werden kann.
    Angewandte Chemie 06/2012; 124(23). DOI:10.1002/ange.201108575
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    ABSTRACT: Nanostructures as catalysts: Pd aerogels modified with α-, β-, and γ-cyclodextrins can be obtained by the spontaneous self-assembly of in situ generated Pd nanoparticles. The Pd aerogels show excellent electrocatalytic activity for the oxidation of ethanol. The catalytic activity is believed to arise from the nonsupported nanometer-scale structure of the aerogel network and the interactions of ethanol with the cyclodextrin.
    Angewandte Chemie International Edition 06/2012; 51(23):5743-7. DOI:10.1002/anie.201108575 · 11.26 Impact Factor
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    ABSTRACT: Ordered mesoporous boron carbide materials with high sp. surface areas up to 778 m2/g and hexagonal pore arrangement symmetries were obtained for the first time using a nanocasting strategy and the mol. bisdecaboranyl-hexane precursor for the infiltration into an ordered mesoporous silica template (SBA-15). Different prepn. conditions were investigated, and it was found that by adjusting the precursor loading either nanorod structured replicas or tubular structured (CMK-5-like) materials were obtained. Changing the impregnation techniques, the solvents, and/or the pyrolysis temps. allows tailoring of the properties of the hexagonal ordered mesoporous boron carbide replicas. The pore arrangement was altered to cubic ordered mesoporous boron carbide using KIT-6 silica matrix for nanocasting. [on SciFinder(R)]
    Chemistry of Materials 08/2010; 22(16-16):4660-4668. DOI:10.1021/cm100837r · 8.54 Impact Factor
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    ABSTRACT: Electron holography has been shown to allow a posteriori aberration correction. Therefore, an aberration corrector in the transmission electron microscope does not seem to be needed with electron holography to achieve atomic lateral resolution. However, to reach a signal resolution sufficient for detecting single light atoms and very small interatomic fields, the aberration corrector has turned out to be very helpful. The basic reason is the optimized use of the limited number of “coherent” electrons that are provided by the electron source, as described by the brightness. Finally, quantitative interpretation of atomic structures benefits from the holographic facilities of fine-tuning of the aberration coefficients a posteriori and from evaluating both amplitude and phase.
    Microscopy and Microanalysis 07/2010; 16(04):434 - 440. DOI:10.1017/S1431927610093633 · 1.76 Impact Factor
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    ABSTRACT: The defect structure of Ge(111) epilayers grown by molecular beam epitaxy on cubic Pr <sub>2</sub> O <sub>3</sub>(111)/ Si (111) support systems was investigated by means of transmission electron microscopy and laboratory-based x-ray diffraction techniques. Three main types of defects were identified, namely, rotation twins, microtwins, and stacking faults, and studied as a function of Ge film thickness and after annealing at 825 ° C in ultrahigh vacuum. Rotation twins were found to be localized at the Ge(111)/cubic Pr <sub>2</sub> O <sub>3</sub>(111) interface and their amount could be lowered by the thermal treatment. Microtwins across {11 1 } were detected only in closed Ge films, after Ge island coalescence. The fraction of Ge film volume affected by microtwinning is constant within the thickness range of ∼20–260 nm . Beyond 260 nm, the density of microtwins is clearly reduced, resulting in thick layers with a top part of higher crystalline quality. Microtwins resulted insensitive to the postdeposition annealing. Instead, the density of stacking faults across {11 1 } planes decreases with the thermal treatment. In conclusion, the defect density was proved to diminish with increasing Ge thickness and after annealing. Moreover, it is noteworthy that the annealing generates a tetragonal distortion in the Ge films, which get in-plane tensely strained, probably due to thermal mismatch between Ge and Si.
    Journal of Applied Physics 11/2009; 106(7-106):073502 - 073502-8. DOI:10.1063/1.3224947 · 2.19 Impact Factor
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    Hannes Lichte · Dorin Geiger · Martin Linck
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    ABSTRACT: Electron holography allows the reconstruction of the complete electron wave, and hence offers the possibility of correcting aberrations. In fact, this was shown by means of the uncorrected CM30 Special Tübingen transmission electron microscope (TEM), revealing, after numerical aberration correction, a resolution of approximately 0.1 nm, both in amplitude and phase. However, it turned out that the results suffer from a comparably poor signal-to-noise ratio. The reason is that the limited coherent electron current, given by gun brightness, has to illuminate a width of at least four times the point-spread function given by the aberrations. As, using the hardware corrector, the point-spread function shrinks considerably, the current density increases and the signal-to-noise ratio improves correspondingly. Furthermore, the phase shift at the atomic dimensions found in the image plane also increases because the collection efficiency of the optics increases with resolution. In total, the signals of atomically fine structures are better defined for quantitative evaluation. In fact, the results achieved by electron holography in a Tecnai F20 Cs-corr TEM confirm this.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 10/2009; 367(1903):3773-93. DOI:10.1098/rsta.2009.0126 · 2.86 Impact Factor
  • B Freitag · S Kujawa · M Linck · D Geiger · T Niermann · M Lehmann · H Lichte
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009
    Microscopy and Microanalysis 06/2009; 15:1098 - 1099. DOI:10.1017/S1431927609094252 · 1.76 Impact Factor
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009
    Microscopy and Microanalysis 06/2009; 15:1460 - 1461. DOI:10.1017/S1431927609096160 · 1.76 Impact Factor
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    ABSTRACT: New ordered mesoporous carbide derived carbon materials with extraordinary high specific surface areas up to 2800 m2 g−1 were synthesized by selective extraction of silicon from ordered mesoporous silicon carbide. Although the degree of mesostructure ordering is lower than that of the CMK-type materials they exhibit higher specific surface areas and high protein adsorption capacities. We show that they can be effectively functionalized with sulfonic groups and become excellent solid acid catalysts for processing large organic molecules.
    The Journal of Physical Chemistry C 04/2009; 113(18). DOI:10.1021/jp808470s · 4.77 Impact Factor
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    ABSTRACT: Iron nanoparticles embedded in MgO crystals were synthesized by Fe <sup>+</sup> ion implantation at an energy of 100 keV and varying fluences from 3×10<sup>16</sup> to 3×10<sup>17</sup> cm <sup>-2</sup> . Investigations of structural and magnetic properties of Fe nanoparticles have been performed using magnetometry, x-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy, as well as by theoretical Preisach modeling of bistable magnetic systems. It has been found that α - and γ -Fe nanoparticles are formed for all fluences. The content of the α -Fe phase increases at higher fluences and after annealing. The influence of postimplantation annealing at 800 ° C in vacuum and under enhanced hydrostatic pressure on the formation of nanoparticles has been analyzed.
    Journal of Applied Physics 04/2009; 105(6-105):064906 - 064906-7. DOI:10.1063/1.3086265 · 2.19 Impact Factor
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    ABSTRACT: A new synthesis scheme for the formation of porous CeO2/Pt-polycarbosilane composites using inverse microemulsions is presented. Aq. hexachloroplatinic acid was used as a hydrosilylation catalyst causing crosslinking of allyl groups in a liq. polycarbosilane (PCS) (SMP-10). The resulting polymers are temp. stable and highly porous. The Pt catalyst content and post-treatment of the polymer can be used to adjust the porosity. For the first time hydrophobic polymers with sp. surface areas up to 896 m2/g were obtained by catalytic crosslinking of polycarbosilanes. Ceria nanoparticles 2-3 nm in diam. are well dispersed in the PCS matrix, as proven using high resoln. electron microscopy. Porosity of the hydrophobic materials could be increased up to 992 m2/g by adding divinylbenzene in the oil phase. Pyrolysis at 1200-1500° and post-oxidative treatment at various temps. produce porous ceramic structures with surface areas up to 423 m2/g. X-Ray diffration investigations show that the crystallinity of
    Journal of Materials Chemistry 03/2009; 19(11):1543-1553. DOI:10.1039/b813669f · 7.44 Impact Factor
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    ABSTRACT: The integration of alternative semiconductor layers on the Si material platform via oxide heterostructures is of interest to increase the performance and/or functionality of future Si-based integrated circuits. The single crystalline quality of epitaxial (epi) semiconductor–insulator–Si heterostructures is however limited by too high defect densities, mainly due to a lack of knowledge about the fundamental physics of the heteroepitaxy mechanisms at work. To shed light on the physics of stacking twin formation as one of the major defect mechanisms in (111)-oriented fcc-related heterostructures on Si(111), we report a detailed experimental and theoretical study on the structure and defect properties of epi-Si(111)/Y 2 O 3 /Pr 2 O 3 /Si(111) heterostructures. Synchrotron radiation-grazing incidence x-ray diffraction (SR-GIXRD) proves that the engineered Y 2 O 3 /Pr 2 O 3 buffer dielectric heterostructure on Si(111) allows control of the stacking sequence of the overgrowing single crystalline epi-Si(111) layers. The epitaxy relationship of the epi-Si(111)/insulator/Si(111) heterostructure is characterized by a type A/B/A stacking configuration. Theoretical ab initio calculations show that this stacking sequence control of the heterostructure is mainly achieved by electrostatic 4 Author to whom any correspondence should be addressed. 2 interaction effects across the ionic oxide/covalent Si interface (IF). Transmission electron microscopy (TEM) studies detect only a small population of misaligned type B epi-Si(111) stacking twins whose location is limited to the oxide/epi-Si IF region. Engineering the oxide/semiconductor IF physics by using tailored oxide systems opens thus a promising approach to grow heterostructures with well-controlled properties.
    New Journal of Physics 11/2008; 10(10). DOI:10.1088/1367-2630/10/11/113004 · 3.67 Impact Factor
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    ABSTRACT: The synthesis of cerium dioxide nanoparticles using an inverse microemulsion technique and precipitation method was investigated. Cerium hydroxide nanoparticles were synthesized by adding diluted ammonia to n-heptane–surfactant–cerium nitrate system. The micelle and particle size in the range of 5–12 nm were controlled by varying the molar water to surfactant ratio and analyzed by dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM). Cerium hydroxide nanoparticles were isolated and subsequently treated at 100–600 °C to obtain nanoscale ceria. Crystallite sizes of cerium dioxide in the range of 6–16 nm were estimated by Scherrer analysis by X-ray diffraction (XRD) and HRTEM. The catalytic activity of particles annealed at 400 and 600 °C in soot combustion reactions was characterized by temperature-programmed oxidation (TPO) indicating a size-dependant activity. Crystallite sizes and catalytic stability of elevated ceria systems were tested in second combustion cycles.
    Journal of Solid State Chemistry 07/2008; 181(7):1614–1620. DOI:10.1016/j.jssc.2008.04.036 · 2.20 Impact Factor
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    ABSTRACT: Nanocrystalline ternary V–Mo nitrides were prepared via nitridation of amine intercalated oxide foams or bulk ternary oxides. Specific surface areas were in the range between 40 and 198m2g−1 and strongly depended on the preparation method (foam or bulk oxide). Foamed precursors were favorable for vanadium rich materials, while for molybdenum rich samples bulk ternary oxides resulted in higher specific surface areas. The materials were characterized via nitrogen physisorption at 77K, X-ray diffraction patterns, electron microscopy, and elemental analysis.
    Journal of Solid State Chemistry 04/2008; 181(4):935-942. DOI:10.1016/j.jssc.2008.01.028 · 2.20 Impact Factor
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    ABSTRACT: Cs correctors have revolutionized transmission electron microscopy (TEM) in that they substantially improve point resolution and information limit. The object information is found sharply localized within 0.1 nm, and the intensity image can therefore be interpreted reliably on an atomic scale. However, for a conventional intensity image, the object exit wave can still not be detected completely in that the phase, and hence indispensable object information is missing. Therefore, for example, atomic electric-field distributions or magnetic domain structures cannot be accessed. Off-axis electron holography offers unique possibilities to recover completely the aberration-corrected object wave with uncorrected microscopes and hence we would not need a Cs-corrected microscope for improved lateral resolution. However, the performance of holography is affected by aberrations of the recording TEM in that the signal/noise properties ("phase detection limit") of the reconstructed wave are degraded. Therefore, we have realized off-axis electron holography with a Cs-corrected TEM. The phase detection limit improves by a factor of four. A further advantage is the possibility of fine-tuning the residual aberrations by a posteriori correction. Therefore, a combination of both methods, that is, Cs correction and off-axis electron holography, opens new perspectives for complete TEM analysis on an atomic scale.
    Microscopy and Microanalysis 03/2008; 14(1):68-81. DOI:10.1017/S143192760808001X · 1.76 Impact Factor
  • D. Geiger · S. Thiel · J. Mannhart · H. Lichte
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    ABSTRACT: Intrinsic functionalities of complex oxides like ferroelectricity, magnetism, superconductivity or multiferroic behaviour can be combined in electronic devices based on epitaxially grown heterostructures. Searching for new materials with special properties in nanoscience implies also the control of interfaces down to the atomic level.
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    ABSTRACT: An ordered mesoporous silica (SBA) was used to assist space-confined formation of FePt nanoparticles. ordered mesoporous oxides were found to be excellent templates for the preparation of well-defined nanoparticles, nanowires, and three-dimensional nanostructures. It was found that sintering agglomeration can be minimized and transformation into ferromagnetic tetragonal FePt is achieved, without a significant increase of the particle diameter, due to the high temperature stability of the surrounding oxide matrix. It was also found that the growth of the particles was controlled by the pore size of the oxide matrix. Two different strategies were developed for the integration of FePt nanoparticles and in both cases FePt/SBA15 composites were prepared by incipient impregnation of ordered mesoporous silica.
    Advanced Materials 09/2007; 19(19):3021 - 3026. DOI:10.1002/adma.200601367 · 17.49 Impact Factor
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    ABSTRACT: The stoichiometry, structure, and defects of self-assembled heteroepitaxial Ge nanodots on twin-free type B oriented cubic Pr <sub>2</sub> O <sub>3</sub>(111) layers on Si(111) substrates are studied to shed light on the fundamental physics of nanocrystal based nonvolatile memory effects. X-ray photoelectron spectroscopy studies prove the high stoichiometric purity of the Ge nanodots on the cubic Pr <sub>2</sub> O <sub>3</sub>(111)/ Si (111) support system. Synchrotron based x-ray diffraction, including anomalous scattering techniques, was applied to determine the epitaxial relationship, showing that the heteroepitaxial Ge(111) nanodots crystallize in the cubic diamond structure with an exclusive type A stacking configuration with respect to Si(111). Grazing incidence small angle x-ray scattering was used in addition to analyze the average shape, size, and distance parameters of the single crystalline Ge nanocrystal ensemble. Furthermore, transmission electron micrographs report that partial dislocations are the prevailing extended defect structure in the Ge nanodots, mainly induced by surface roughness on the atomic scale of the cubic Pr <sub>2</sub> O <sub>3</sub>(111) support.
    Journal of Applied Physics 09/2007; 102(3-102):034107 - 034107-10. DOI:10.1063/1.2767374 · 2.19 Impact Factor

Publication Stats

438 Citations
107.55 Total Impact Points

Institutions

  • 2004–2013
    • Technische Universität Dresden
      • • Fakultät Mathematik und Naturwissenschaften
      • • Inorganic Chemistry
      Dresden, Saxony, Germany
  • 1999
    • University of Tuebingen
      • Institute of Inorganic Chemistry
      Tübingen, Baden-Württemberg, Germany