apl. Prof. Dr. Heinz Hövel

Experimental Physics/DELTA
TU Dortmund University


  • Silver clusters shape determination from in-situ XANES data
    Timoshenko, J. and Roese, S. and Hövel, H. and Frenkel, A.I.
    Radiation Physics and Chemistry 175 (2020)
    Knowledge of nanoparticle size, shape and morphology and of their in-situ transformations is crucial for establishing structure-properties relationship in nanosized materials that find applications, e.g., in plasmonic devices and heterogenous catalysis. Here we demonstrate that this information can be extracted reliably from in-situ X-ray absorption near edge structure (XANES) data, by combining ab-initio XANES simulations and machine learning (artificial neural network (NN)) approaches. Here we use NN-XANES method to extract information about the size, shape and interatomic distances in silver clusters, and to monitor their changes during the temperature-controlled particle aggregation. © 2019 Elsevier Ltd
    view abstract10.1016/j.radphyschem.2018.11.003
  • Cluster Assemblies Produced by Aggregation of Preformed Ag Clusters in Ionic Liquids
    Roese, S. and Kononov, A. and Timoshenko, J. and Frenkel, A.I. and Hövel, H.
    Langmuir 34 (2018)
    Room-temperature ionic liquids (RTILs) can be used as electrosterical stabilizers for nanoparticles without adding stabilizing agents. However, the nanoparticle stability and its mechanisms are still in discussion. We deposited preformed 2 nm ±0.6 nm silver clusters into the ionic liquid C4MIM PF6 using in situ UV/vis absorption to monitor the deposition process. The time- and temperature-dependent cluster aggregation process was studied with ex situ UV/vis absorption spectroscopy analyzed with electrodynamic calculations using generalized Mie theory. On an atomistic level, the sample structure was investigated using EXAFS and a neural network based analysis of XANES. The combination of all methods shows that an aggregation of the original 2 nm clusters without coalescence takes place, which can be controlled or stopped by choosing an appropriate sample temperature. This approach allows the controlled production of chainlike cluster aggregates in RTIL, promising for a number of applications. © 2018 American Chemical Society.
    view abstract10.1021/acs.langmuir.7b03984
  • Comparison of technologies for nano device prototyping with a special focus on ion beams: A review
    Bruchhaus, L. and Mazarov, P. and Bischoff, L. and Gierak, J. and Wieck, A.D. and Hövel, H.
    Applied Physics Reviews 4 (2017)
    Nano device prototyping (NDP) is essential for realizing and assessing ideas as well as theories in the form of nano devices, before they can be made available in or as commercial products. In this review, application results patterned similarly to those in the semiconductor industry (for cell phone, computer processors, or memory) will be presented. For NDP, some requirements are different: thus, other technologies are employed. Currently, in NDP, for many applications direct write Gaussian vector scan electron beam lithography (EBL) is used to define the required features in organic resists on this scale. We will take a look at many application results carried out by EBL, self-organized 3D epitaxy, atomic probe microscopy (scanning tunneling microscope/atomic force microscope), and in more detail ion beam techniques. For ion beam techniques, there is a special focus on those based upon liquid metal (alloy) ion sources, as recent developments have significantly increased their applicability for NDP. © 2017 Author(s).
    view abstract10.1063/1.4972262
  • Long-range p-d exchange interaction in a ferromagnet-semiconductor hybrid structure
    Korenev, V. L. and Salewski, M. and Akimov, I. A. and Sapega, V. F. and Langer, L. and Kalitukha, I. V. and Debus, J. and Dzhioev, R. I. and Yakovlev, D. R. and Muller, D. and Schroder, C. and Hövel, H. and Karczewski, G. and Wiater, M. and Wojtowicz, T. and Kusrayev, Y. G. and Bayer, M.
    Nature Physics 12 (2016)
    Hybrid structures synthesized from different materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg) Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an effective p-d exchange that is mediated by elliptically polarized phonons.
    view abstract10.1038/NPHYS3497
  • PDMS embedded Ag clusters: Coalescence and cluster-matrix interaction
    Roese, S. and Engemann, D. and Hoffmann, S. and Latussek, K. and Sternemann, C. and Hövel, H.
    Journal of Physics: Conference Series 712 (2016)
    Polydimethylsiloxane (PDMS) has proven to be a suitable embedding medium for silver clusters to prevent aggregation. In order to investigate the influence of the PDMS on the electronic and local atomic structure of the clusters the measurement of x-ray absorption near edge structure (XANES) spectra for different coverages of silver clusters in PDMS and calculations of corresponding XANES spectra have been performed. The coalescence process and the cluster-PDMS interaction were investigated with XANES.
    view abstract10.1088/1742-6596/712/1/012068
  • Preformed 2 nm Ag Clusters Deposited into Ionic Liquids: Stabilization by Cation-Cluster Interaction
    Engemann, D.C. and Roese, S. and Hövel, H.
    Journal of Physical Chemistry C 120 (2016)
    Recently, the formation of nanoparticles by sputter deposition of metal atoms onto the surface of room-temperature ionic liquids (RTIL) was reported; however, the growth and stabilization mechanism within the ionic liquid are still in discussion. Here, we present another approach by depositing Ag clusters with a diameter of 2 nm preformed in a supersonic nozzle expansion into an ionic liquid. Thus, the properties and size distribution of the clusters are well-known before deposition. The mixture of the clusters with the ionic liquid is investigated in situ and ex situ with UV/vis measurements and X-ray absorption near-edge structure (XANES) spectroscopy at the Ag L2 edge. The plasmon resonances of the Ag clusters show that up to 10 μg/mL, the clusters stay separated in the RTIL and suggest an interaction process between the cations in the liquid and the surfaces of the clusters, which is confirmed by a shift of the absorption edge in the XANES measurements. For higher cluster concentration and on a longer time scale, the stabilization ability of ionic liquids can be investigated. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b12120
  • Decay kinetics of cluster-beam-deposited metal particles
    Grönhagen, N. and Järvi, T.T. and Miroslawski, N. and Hövel, H. and Moseler, M.
    Journal of Physical Chemistry C 116 (2012)
    High-precision experiments and atomistic simulations are used to determine the flattening kinetics of mass-selected 55-147 atom Ag clusters deposited on Au(111). The clusters are shown to align epitaxially and decay through an exchange pathway with a range of rate-limiting barriers, from ca. 0.25 to 0.4 eV, depending on the shape of the particle. It is also shown that nonlocal effects at the Au-Ag interface lead to a dramatic reduction in the barrier of the dominant transition pathway, requiring ab initio methods for correct modeling. As a result, quantitative correspondence between experimental and simulated island heights is obtained. © 2012 American Chemical Society.
    view abstract10.1021/jp305089d
  • High resolution and high density ion beam lithography employing HSQ resist
    Bruchhaus, L. and Bauerdick, S. and Peto, L. and Barth, U. and Rudzinski, A. and Mussmann, J. and Klingfus, J. and Gierak, J. and Hövel, H.
    Microelectronic Engineering 97 (2012)
    In the early stages of focused ion beam (FIB) development, ion beam lithography (IBL) employing organic resists showed potential advantages over electron beam lithography (EBL) (most notably less proximity effects and higher sensitivity [1,2]). However, EBL has always been more popular for various reasons (e.g., instrument capabilities, well known process technology, higher resolution, absence of potential shot noise effects, no unintended doping of the irradiated substrate area [3]). We will present and detail our results obtained with a gallium (Ga) liquid metal ion source (LMIS) nanofabrication system studying the resolution limits related to this kind of IBL. We will show that minimum feature sizes below 10 nm and periodicity of 30 nm are possible in a 6 nm hydrogen silsesquioxane (HSQ) layer. Our experiments have revealed that the minimum reachable feature sizes are limited by a phenomenon called "discontinued lines" [2,4]. Our results indicate that statistical dose fluctuations do not dominate this effect. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2012.04.033
  • III-V photovoltaics - Recent developments and prospects
    Sosa, N.E. and Van Kessel, T.G. and Martin, Y. and Hovel, H.J.
    ECS Transactions 33 (2010)
    III-V materials are the unchallenged front runner for high-efficiency photovoltaics. This high-efficiency arises from the tunability of the materials system. In particular, III-Vs can be epitaxially grown and, by varying the stoichiometry of binary (ternary or quarternary) systems, the lattice parameter can be controlled to yield devices with very low crystal defect density and simultaneously result in different band gaps. Thus, one p-n junction can be grown epitaxially on top of the next, each absorbing a different portion of the solar spectrum - ultimately yielding multiple p-n junctions in tandem with the ability to harness the solar spectrum more efficiently than a single junction or a typical silicon-based solar cell. Furthermore, III-V photovoltaics have the dual advantage of being applicable in thin-film form and to operate under high concentration. There lies the beauty of III-V photovoltaics. This dual ability is lacking in Si- and CIGS-based PV. ©The Electrochemical Society.
    view abstract10.1149/1.3487623
  • Penetration of thin C 60 films by metal nanoparticles
    Duffe, S. and Grönhagen, N. and Patryarcha, L. and Sieben, B. and Yin, C. and Von Issendorff, B. and Moseler, M. and Hövel, H.
    Nature Nanotechnology 5 (2010)
    Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances but desirable in others, and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C 60 supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C 60 supported on gold, or on one monolayer of C 60 supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer. © 2010 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nnano.2010.45
  • Ultraviolet photoelectron spectroscopy of supported mass selected silver clusters
    Wortmann, B. and Mende, K. and Duffe, S. and nhagen, N.G. and von Issendorff, B. and Hövel, H.
    Physica Status Solidi (B) Basic Research 247 (2010)
    Ultraviolet photoelectron spectroscopy (UPS) was used to investigate size selected Ag923±9 and Ag55 clusters which were softlanded on a clean graphite substrate (HOPG) at 100 and 50 K, respectively. With increasing cluster coverage closer to the centre of the deposition spot a continuous change of the d-band signal is observed. Differences in the fine structure of the d-band and comparison to UPS spectra of clusters grown at nanopits on HOPG show that the clusters in the centre of the deposition spot coalesced. However, Ag55 spectra measured at the rim of the deposition spot indicate that the clusters stay separated in regions of lower coverage for a deposition temperature of 50 K. This is corroborated by scanning tunnelling microscopy (STM) images measured at 5K using 1 monolayer (ML) Xe to fix the Ag55 clusters to the substrate, thus making them observable with STM. By comparison to UPS data taken on different sample positions in a 1×1mm2 grid it was determined that at the rim of the deposition spot the coverage of 30 clusters per 100×100 nm2 was low enough for an UPS measurement of single separated Ag55 clusters. Differences in the spectra for the largest coverage of Ag55 and Ag923 clusters in the deposition spot centre indicate that the resulting Ag film has a partial (111) orientation for the deposition of Ag55 at 50K whereas it is mostly polycrystalline for Ag923 deposited at 100 K. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200945586
  • low-temperature scanning

  • photoemission

  • x-ray absorption

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