apl. Prof. Dr. Frank Joachim Meyer zu Heringdorf

Institute for Experimental Physics
University of Duisburg-Essen

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  • Implementation and operation of a fiber-coupled CMOS detector in a low energy electron Microscope
    Janoschka, D. and Dreher, P. and Rödl, A. and Franz, T. and Schaff, O. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Ultramicroscopy 221 (2021)
    The intrinsically weak signals in ultrafast electron microscopy experiments demand an improvement in the signal-to noise ratio of suitable electron detectors. We provide an experience report describing the installation and operation of a fiber-coupled CMOS based detector in a low energy electron microscope. We compare the detector performance to the traditional multi-channel-plate-based setup. The high dynamic range CMOS detector is capable of imaging spatially localized large intensity variations with low noise. The detector is blooming-free and overexposure appears uncritical. Overall, we find dramatic improvements in the imaging with the fiber-coupled CMOS detector compared to imaging with our previously used multi-channel-plate detector. © 2020 Elsevier B.V.
    view abstract10.1016/j.ultramic.2020.113180
  • Non-conventional bell-shaped diffuse scattering in low-energy electron diffraction from high-quality epitaxial 2D-materials
    Omambac, K. and Kriegel, M. and Brand, C. and Finke, B. and Kremeyer, L. and Hattab, H. and Janoschka, D. and Dreher, P. and Meyer zu Heringdorf, F.-J. and Momeni Pakdehi, D. and Pierz, K. and Schumacher, H.W. and Petrović, M. and van Houselt, A. and Poelsema, B. and Tringides, M.C. and Horn-Von Hoegen, M.
    Applied Physics Letters 118 (2021)
    A broad, bell-shaped intensity component is observed in low-energy electron diffraction from high-quality epitaxial 2D-systems. Three 2D-systems, graphene on Ir(111), graphene on SiC(0001), and hexagonal boron nitride on Ir(111), have been prepared in situ under ultra-high vacuum conditions. In all three systems—independent of substrate material—similar strong diffuse intensity is observed, exhibiting a width as large as 50% of the Brillouin zone and an integrated intensity more than 10 times the intensity of the Bragg spots. The presented experimental results provide evidence for a common origin of such diffuse diffraction intensity in different atomically thin 2D-materials. © 2021 Author(s).
    view abstract10.1063/5.0051220
  • Segregation-Enhanced Epitaxy of Borophene on Ir(111) by Thermal Decomposition of Borazine
    Omambac, K.M. and Petrović, M. and Bampoulis, P. and Brand, C. and Kriegel, M.A. and Dreher, P. and Janoschka, D. and Hagemann, U. and Hartmann, N. and Valerius, P. and Michely, T. and Meyer Zu Heringdorf, F.J. and Horn-Von Hoegen, M.
    ACS Nano 15 (2021)
    Like other 2D materials, the boron-based borophene exhibits interesting structural and electronic properties. While borophene is typically prepared by molecular beam epitaxy, we report here on an alternative way of synthesizing large single-phase borophene domains by segregation-enhanced epitaxy. X-ray photoelectron spectroscopy shows that borazine dosing at 1100 °C onto Ir(111) yields a boron-rich surface without traces of nitrogen. At high temperatures, the borazine thermally decomposes, nitrogen desorbs, and boron diffuses into the substrate. Using time-of-flight secondary ion mass spectrometry, we show that during cooldown the subsurface boron segregates back to the surface where it forms borophene. In this case, electron diffraction reveals a (6 × 2) reconstructed borophene χ6-polymorph, and scanning tunneling spectroscopy suggests a Dirac-like behavior. Studying the kinetics of borophene formation in low energy electron microscopy shows that surface steps are bunched during the borophene formation, resulting in elongated and extended borophene domains with exceptional structural order. ©
    view abstract10.1021/acsnano.1c00819
  • Ultrafast vector imaging of plasmonic skyrmion dynamics with deep subwavelength resolution
    Davis, T.J. and Janoschka, D. and Dreher, P. and Frank, B. and Meyer zu Heringdorf, F.J. and Giessen, H.
    Science 368 (2020)
    Plasmonic skyrmions are an optical manifestation of topological defects in a continuous vector field. Identifying them requires characterization of the vector structure of the electromagnetic near field on thin metal films. Here we introduce time-resolved vector microscopy that creates movies of the electric field vectors of surface plasmons with subfemtosecond time steps and a 10-nanometer spatial scale. We image complete time sequences of propagating surface plasmons as well as plasmonic skyrmions, resolving all vector components of the electric field and their time dynamics, thus demonstrating dynamic spin-momentum coupling as well as the time-varying skyrmion number. The ability to image linear optical effects in the spin and phase structures of light in the single-nanometer range will allow for entirely novel microscopy and metrology applications. © 2020 American Association for the Advancement of Science. All rights reserved.
    view abstract10.1126/science.aba6415
  • Condensation of ground state from a supercooled phase in the Si(111)-(4 × 1) → (8 × 2)-indium atomic wire system
    Hafke, B. and Witte, T. and Janoschka, D. and Dreher, P. and Meyer Zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Structural Dynamics 6 (2019)
    Strong optical irradiation of indium atomic wires on a Si(111) surface causes the nonthermal structural transition from the (8 × 2) reconstructed ground state to an excited (4 × 1) state. The immediate recovery of the system to the ground state is hindered by an energy barrier for the collective motion of the indium atoms along the reaction coordinate from the (4 × 1) to the (8 × 2) state. This metastable, supercooled state can only recover through nucleation of the ground state at defects like adsorbates or step edges. Subsequently, a recovery front propagates with constant velocity across the surface and the (8 × 2) ground state is reinstated. In a combined femtosecond electron diffraction and photoelectron emission microscopy study, we determined - based on the step morphology - a velocity of this recovery front of ∼100 m/s. © 2019 Author(s).
    view abstract10.1063/1.5111636
  • Equilibrium shape of single-layer hexagonal boron nitride islands on iridium
    Petrović, M. and Hoegen, M.H.-V. and Meyer zu Heringdorf, F.-J.
    Scientific Reports 9 (2019)
    Large, high-quality layers of hexagonal boron nitride (hBN) are a prerequisite for further advancement in scientific investigation and technological utilization of this exceptional 2D material. Here we address this demand by investigating chemical vapor deposition synthesis of hBN on an Ir(111) substrate, and focus on the substrate morphology, more specifically mono-atomic steps that are always present on all catalytic surfaces of practical use. From low-energy electron microscopy and atomic force microscopy data, we are able to set up an extended Wulff construction scheme and provide a clear elaboration of different interactions governing the equilibrium shapes of the growing hBN islands that deviate from the idealistic triangular form. Most importantly, intrinsic hBN edge energy and interaction with the iridium step edges are examined separately, revealing in such way the importance of substrate step morphology for the island structure and the overall quality of 2D materials. © 2019, The Author(s).
    view abstract10.1038/s41598-019-56000-1
  • Precision Plasmonics with Monomers and Dimers of Spherical Gold Nanoparticles: Nonequilibrium Dynamics at the Time and Space Limits
    Schumacher, L. and Jose, J. and Janoschka, D. and Dreher, P. and Davis, T.J. and Ligges, M. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X. and Meyer Zu Heringdorf, F. and Sokolowski-Tinten, K. and Schlücker, S.
    Journal of Physical Chemistry C 123 (2019)
    Monomers and dimers of spherical gold nanoparticles (NPs) exhibit highly uniform plasmonic properties at the single-particle level due to their high structural homogeneity (precision plasmonics). Recent investigations in precision plasmonics have largely focused on static properties using conventional techniques such as transmission electron microscopy and optical dark-field microscopy. In this Feature Article, we first highlight the application of femtosecond time-resolved electron diffraction for monitoring the nonequilibrium dynamics of spherical gold NPs after ultrafast optical excitation. The analysis of the transient diffraction patterns allows us to directly obtain quantitative information on the incoherent excitation of the lattice, that is, heating upon electron-lattice equilibration, as well as on the development of strain due to lattice expansion on picosecond time scales. The controlled assembly of two spherical gold NPs into a dimer with a few nanometers gap leads to unique optical properties. Specifically, extremely high electric fields (hot spot) in the gap are generated upon resonant optical excitation. Conventional optical microscopy cannot spatially resolve this unique hot spot due to the optical diffraction limit. We therefore employed nonlinear photoemission electron microscopy to visualize hot spots in single dimers of spherical gold NPs. A quantitative comparison of different single dimers confirms the homogeneity of the hot spots on the single-particle level. Overall, these initial results are highly encouraging because they pave the way to investigate nonequilibrium dynamics in highly uniform plasmonic nanostructures at the time and space limits. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b01007
  • Spatiotemporal Analysis of an Efficient Fresnel Grating Coupler for Focusing Surface Plasmon Polaritons
    Podbiel, D. and Kahl, P. and Frank, B. and Davis, T.J. and Giessen, H. and Horn-von Hoegen, M. and Meyer Zu Heringdorf, F-J.
    ACS Photonics 6 (2019)
    Time-resolved photoemission microscopy is used to investigate the spatiotemporal properties of surface plasmon polaritons launched at a Fresnel-type grating coupler. By milling a multiline structure of segmented grooves with dimensions derived from Fresnel zones into a plasmonic material, efficient focusing of surface plasmon polaritons can be accomplished. We demonstrate the presence of pulse broadening at the focus associated with propagation delays in the device. Moreover, our experimental data implies an enhancement of the plasmonic field energy density at the focus in excess of a factor of 10, with a focal spot size at the Abbe limit for our lens. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsphotonics.8b01565
  • Spatiotemporal Analysis of an Efficient Fresnel Grating Coupler for Focusing Surface Plasmon Polaritons
    Podbiel, D. and Kahl, P. and Frank, B. and Davis, T.J. and Giessen, H. and Hoegen, M.H.-V. and Meyer Zu Heringdorf, F.J.
    ACS Photonics 6 (2019)
    Time-resolved photoemission microscopy is used to investigate the spatiotemporal properties of surface plasmon polaritons launched at a Fresnel-type grating coupler. By milling a multiline structure of segmented grooves with dimensions derived from Fresnel zones into a plasmonic material, efficient focusing of surface plasmon polaritons can be accomplished. We demonstrate the presence of pulse broadening at the focus associated with propagation delays in the device. Moreover, our experimental data implies an enhancement of the plasmonic field energy density at the focus in excess of a factor of 10, with a focal spot size at the Abbe limit for our lens. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsphotonics.8b01565
  • Temperature-Controlled Rotational Epitaxy of Graphene
    Omambac, K.M. and Hattab, H. and Brand, C. and Jnawali, G. and N'Diaye, A.T. and Coraux, J. and van Gastel, R. and Poelsema, B. and Michely, T. and Meyer Zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Nano letters 19 (2019)
    When graphene is placed on a crystalline surface, the periodic structures within the layers superimpose and moiré superlattices form. Small lattice rotations between the two materials in contact strongly modify the moiré lattice parameter, upon which many electronic, vibrational, and chemical properties depend. While precise adjustment of the relative orientation in the degree- and sub-degree-range can be achieved via careful deterministic transfer of graphene, we report on the spontaneous reorientation of graphene on a metallic substrate, Ir(111). We find that selecting a substrate temperature between 1530 and 1000 K during the growth of graphene leads to distinct relative rotational angles of 0°, ± 0.6°, ±1.1°, and ±1.7°. When modeling the moiré superlattices as two-dimensional coincidence networks, we can ascribe the observed rotations to favorable low-strain graphene structures. The dissimilar thermal expansion of the substrate and graphene is regarded as an effective compressive biaxial pressure that is more easily accommodated in graphene by small rotations rather than by compression.
    view abstract10.1021/acs.nanolett.9b01565
  • Lateral heterostructures of hexagonal boron nitride and graphene: BCN alloy formation and microstructuring mechanism
    Petrović, M. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Applied Surface Science 455 (2018)
    Integration of individual two-dimensional materials into heterostructures is a crucial step which enables development of new and technologically interesting functional systems of reduced dimensionality. Here, well-defined lateral heterostructures of hexagonal boron nitride and graphene are synthesized on Ir(1 1 1) by performing sequential chemical vapor deposition from borazine and ethylene in ultra-high vacuum. Low-energy electron microscopy (LEEM) and selected-area electron diffraction (μ-LEED) show that the heterostructures do not consist only of hexagonal boron nitride (an insulator) and graphene (a conductor), but that also a 2D alloy made up of B, C, and N atoms (a semiconductor) is formed. Composition and spatial extension of the alloy can be tuned by controlling the parameters of the synthesis. A new method for in situ fabrication of micro and nanostructures based on decomposition of hexagonal boron nitride is experimentally demonstrated and modeled analytically, which establishes a new route for production of BCN and graphene elements of various shapes. In this way, atomically-thin conducting and semiconducting components can be fabricated, serving as a basis for manufacturing more complex devices. © 2018 Elsevier B.V.
    view abstract10.1016/j.apsusc.2018.06.057
  • Imaging the Nonlinear Plasmoemission Dynamics of Electrons from Strong Plasmonic Fields
    Podbiel, D. and Kahl, P. and Makris, A. and Frank, B. and Sindermann, S. and Davis, T.J. and Giessen, H. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Nano Letters 17 (2017)
    We use subcycle time-resolved photoemission microscopy to unambiguously distinguish optically triggered electron emission (photoemission) from effects caused purely by the plasmonic field (termed "plasmoemission"). We find from time-resolved imaging that nonlinear plasmoemission is dominated by the transverse plasmon field component by utilizing a transient standing wave from two counter-propagating plasmon pulses of opposite transverse spin. From plasmonic foci on flat metal surfaces, we observe highly nonlinear plasmoemission up to the fifth power of intensity and quantized energy transfer, which reflects the quantum-mechanical nature of surface plasmons. Our work constitutes the basis for novel plasmonic devices such as nanometer-confined ultrafast electron sources as well as applications in time-resolved electron microscopy. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.7b02235
  • Microanalysis of single-layer hexagonal boron nitride islands on Ir(111)
    Petrović, M. and Hagemann, U. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    Applied Surface Science 420 (2017)
    Large hexagonal boron nitride (hBN) single-layer islands of high crystalline quality were grown on Ir(111) via chemical vapor deposition (CVD) and have been studied with low-energy electron microscopy (LEEM). Two types of hBN islands have been observed that structurally differ in their shape and orientation with respect to iridium, where the former greatly depends on the iridium step morphology. Photoemission electron microscopy (PEEM) and IV-LEEM spectroscopy revealed that the two island types also exhibit different work functions and bindings to iridium, which provides an explanation for differences in their shape and growth modes. In addition, various temperatures were used for the CVD synthesis of hBN, and it was found that at temperatures higher than ≈950 °C boron atoms, originating either from decomposed borazine molecules or disintegrated hBN islands, can form additional compact reconstructed regions. The presented results are important for advancement in synthesis of high-quality hBN and other boron-based layered materials, and could therefore expedite their technological implementation. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.05.155
  • Revealing the subfemtosecond dynamics of orbital angular momentum in nanoplasmonic vortices
    Spektor, G. and Kilbane, D. and Mahro, A.K. and Frank, B. and Ristok, S. and Gal, L. and Kahl, P. and Podbiel, D. and Mathias, S. and Giessen, H. and Meyer zu Heringdorf, F.-J. and Orenstein, M. and Aeschlimann, M.
    Science 355 (2017)
    The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. The physical properties of light with a helical wavefront can be confined onto two-dimensional surfaces with subwavelength dimensions in the form of plasmonic vortices, opening avenues for thus far unknown light-matter interactions. Because of their extreme rotational velocity, the ultrafast dynamics of such vortices remained unexplored. Here we show the detailed spatiotemporal evolution of nanovortices using time-resolved two-photon photoemission electron microscopy. We observe both long- and short-range plasmonic vortices confined to deep subwavelength dimensions on the scale of 100 nanometers with nanometer spatial resolution and subfemtosecond time-step resolution. Finally, by measuring the angular velocity of the vortex, we directly extract the OAM magnitude of light. © 2017, American Association for the Advancement of Science. All rights reserved.
    view abstract10.1126/science.aaj1699
  • Subfemtosecond and Nanometer Plasmon Dynamics with Photoelectron Microscopy: Theory and Efficient Simulations
    Davis, T.J. and Frank, B. and Podbiel, D. and Kahl, P. and Meyer zu Heringdorf, F.-J. and Giessen, H.
    ACS Photonics 4 (2017)
    We develop a theoretical model of the excitation and interference of surface plasmon polariton (SPP) waves with femtosecond laser pulses and use the model to understand the features in images from subfemtosecond time-resolved two-photon photoelectron microscopy (2PPE-PEEM). The numerically efficient model is based on the optics of SPP modes on multilayer thin films and takes account of the excitation and interference by the incident light, its polarization, the boundary shape on the film where the plasmons are generated, the pulsed form of the excitation and the time integration associated with the PEEM method. The model explains the dominant features observed in the images including the complex patterns formed in experiments involving orbital angular momentum. The model forms the basis of an efficient numerical method for simulating time-resolved 2PPE-PEEM images of SPP wave propagation. The numerics is extremely fast, efficient, and accurate, so that each image can take as little as a few seconds to calculate on a laptop computer, enabling entire PEEM movies to be calculated within minutes. © 2017 American Chemical Society.
    view abstract10.1021/acsphotonics.7b00676
  • Analysis of the contrast in normal-incidence surface plasmon photoemission microscopy in a pump–probe experiment with adjustable polarization
    Podbiel, D. and Kahl, P. and Meyer zu Heringdorf, F.-J.
    Applied Physics B: Lasers and Optics 122 (2016)
    We investigate the fringe contrast in surface plasmon polariton-based two-photon photoemission microscopy in a normal-incidence geometry. In a pump–probe experiment with freely adjustable polarization of the probe pulse, we find a maximum contrast whenever the probe pulse polarization is parallel (or anti-parallel) to the propagation direction of the surface plasmon polariton wave packet. The experimental observation is compared to a wave simulation based on the known TM solution for surface plasmon polaritons. We estimate that at the Au/vacuum interface the in-plane component of the electric field of the surface plasmon polariton inside the metal is about five times larger than its out-of-plane component. We conclude that the locations of maximum plasmon-related nonlinear photoemission yield in a pump–probe experiment are the ones where the in-plane component of the electric field of the surface plasmon polariton is maximal. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-016-6363-6
  • Spatio-temporal imaging of surface plasmon polaritons in two photon photoemission microscopy
    Meyer zu Heringdorf, F.-J. and Podbiel, D. and Raß, N. and Makris, A. and Buckanie, N.M. and Kahl, P.A.
    Proceedings of SPIE - The International Society for Optical Engineering 9921 (2016)
    A two-photon photoemission microscopy experiment with femtosecond time-resolution for imaging of propagating surface plasmon polaritons is discussed. The experimental setup of an actively Pancharatnam's phase stabilized interferometer is described, and a temporal stability in time-resolved two-photon photoemission microscopy of less than 20 attoseconds is demonstrated. The time-resolved setup is applied to investigate the interaction of a surface plasmon polariton wave packet with a plasmonic beam-splitter. Pump-probe data recorded at times before and after the interaction of the surface plasmon polariton wave packet with the beam-splitter indicate transmission and reflection coefficients of T ≈ 0.3 and R ≈ 0.4, respectively. © 2016 SPIE.
    view abstract10.1117/12.2239878
  • Thickness dependent electron-lattice equilibration in thin Bi films studied by time-resolved MeV electron diffraction
    Sokolowski-Tinten, K. and Li, R.K. and Reid, A.H. and Weathersby, S.P. and Quirin, F. and Chase, T. and Coffee, R. and Corbett, J. and Fry, A. and Hartmann, N. and Hast, C. and Hettel, R. and Horn-Von Hoegen, M. and Janoschka, D. and Jermann, M. and Lewandowski, J.R. and Ligges, M. and Meyer zu Heringdorf, F.-J. and Mo, M. and Shen, X. and Vecchione, T. and Witt, C. and Wu, J. and Dürr, H.A. and Wang, X.J.
    Optics InfoBase Conference Papers Part F20-UP 2016 (2016)
    Using time-resolved electron diffraction the electron-lattice equilibration in laserexcited thin Bi-films has been investigated. Our data reveal a pronounced thickness-dependence which is attributed to cross-interfacial coupling of hot electrons in the Bi-film to substrate phonons.
    view abstract10.1364/UP.2016.UTh4A.49
  • Signatures of plasmoemission in two photon photoemission electron microscopy
    Meyer zu Heringdorf, F.-J. and Kahl, P. and Makris, A. and Sindermann, S. and Podbiel, D. and Horn-Von Hoegen, M.
    Proceedings of SPIE - The International Society for Optical Engineering 9361 (2015)
    The imaging of surface plasmon polariton waves in two photon photoemission microscopy has been intensely studied during the past years, with a focus on contrast mechanisms and light-plasmon interaction. The possibility of photoemission from the plasmonic fields alone has so far not been addressed in such experiments. This was justified, since the intensity of the plasmonic fields at the surface was comparatively weak and nonlinear plasmonic effects were not to be expected. Here we discuss the properties of grating couplers for creation of intense and short plasmon polariton pulses for which the emission of electrons purely from the plasmonic field cannot be neglected any more. Two examples for signatures of such nonlinear plasmoemission effects in experimental two photon photoemission microscopy images are discussed. © 2015 SPIE.
    view abstract10.1117/12.2082610
  • Anisotropy of electromigration-induced void and island drift
    Latz, A. and Sindermann, S.P. and Brendel, L. and Dumpich, G. and Meyer zu Heringdorf, F.-J. and Wolf, D.E.
    Journal of Physics Condensed Matter 26 (2014)
    By means of our novel self-learning kinetic Monte Carlo model (Latz et al 2012 J. Phys.: Condens. Matter 24 485005) we study the electromigration-induced drift of monolayer voids and islands on unpassivated surfaces of single crystalline Ag(111) and Ag(001) films at the atomic scale. Regarding the drift velocity, we find a non-monotonic size dependence for small voids. The drift direction is aligned with the electromigration force only along high symmetry directions, while halfway between, the angle enclosed by them is maximal. The magnitude of these directional deviations strongly depends on the system parameter, which are investigated in detail. The simulation results are in accordance with void motion observed in experiments performed on Ag(111). © 2014 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/26/5/055005
  • Europium underneath graphene on Ir(111): Intercalation mechanism, magnetism, and band structure
    Schumacher, S. and Huttmann, F. and Petrović, M. and Witt, C. and Förster, D.F. and Vo-Van, C. and Coraux, J. and Martínez-Galera, A.J. and Sessi, V. and Vergara, I. and Rückamp, R. and Grüninger, M. and Schleheck, N. and Meyer zu Heringdorf, F.-J. and Ohresser, P. and Kralj, M. and Wehling, T.O. and Michely, T.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a (2×2) or a (3×3)R30 superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n doping. The hybridization of the Ir surface state S1 with Gr states is lifted, and the moiré superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially, as found by x-ray magnetic circular dichroism. The (2×2) Eu structure displays plain paramagnetic behavior, whereas for the (3×3)R30 structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.235437
  • Lattice degradation by moving voids during reversible electromigration
    Sindermann, S.P. and Latz, A. and Spoddig, D. and Schoeppner, C. and Wolf, D.E. and Dumpich, G. and Meyer zu Heringdorf, F.-J.
    Journal of Applied Physics 116 (2014)
    Electromigration driven void motion is studied in Ag wires with an initially well-defined single crystal lattice by in situ scanning electron microscopy. Voids are moving in opposite direction to the electron flow. When the electron current is reversed, voids exactly retrace their previous motion path with an increased drift velocity: The microstructure of the Ag wire "remembers" the motion path of the initial voids. To investigate the nature of this memory effect, we analyzed the crystal lattice with electron backscatter diffraction after passing of a void. The results show a permanent lattice degradation caused by the moving void. The implication of this finding for the reversibility of EM will be discussed. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4889816
  • Normal-Incidence Photoemission Electron Microscopy (NI-PEEM) for Imaging Surface Plasmon Polaritons
    Kahl, P. and Wall, S. and Witt, C. and Schneider, C. and Bayer, D. and Fischer, A. and Melchior, P. and Horn-von Hoegen, M. and Aeschlimann, M. and Meyer zu Heringdorf, F.-J.
    Plasmonics 9 (2014)
    We introduce a novel time-resolved photoemission-based near-field illumination method, referred to as femtosecond normal-incidence photoemission microscopy (NI-PEEM). The change from the commonly used grazing-incidence to normal-incidence illumination geometry has a major impact on the achievable contrast and, hence, on the imaging potential of transient local near fields. By imaging surface plasmon polaritons in normal light incidence geometry, the observed fringe spacing directly resembles the wavelength of the plasmon wave. Our novel approach provides a direct descriptive visualization of SPP wave packets propagating across a metal surface. © 2014, Springer Science+Business Media New York.
    view abstract10.1007/s11468-014-9756-6
  • Al-induced faceting of Si(113)
    Klein, C. and Heidmann, I. and Nabbefeld, T. and Speckmann, M. and Schmidt, T. and Meyer zu Heringdorf, F.-J. and Falta, J. and Horn-Von Hoegen, M.
    Surface Science 618 (2013)
    Adsorption of Al on a Si(113) substrate at elevated temperatures causes a faceting transition of the initially flat surface. The (113) surface decomposes into a quasi-periodic sequence of Al terminated (115)- and (112)-facets. The resulting surface morphology is characterized in-situ by reciprocal space maps obtained with in-situ spot profile analyzing low-energy electron diffraction and ex-situ atomic force microscopy. The periodicity length of the faceted surface increases with adsorption temperature from 7 nm at 650 C to 80 nm at 800 C. The stability of the Al terminated Si(112) surface is the driving force for the faceting transition. © 2013 Elsevier B.V.
    view abstract10.1016/j.susc.2013.08.007
  • Interaction of light and surface plasmon polaritons in Ag Islands studied by nonlinear photoemission microscopy
    Buckanie, N.M. and Kirschbaum, P. and Sindermann, S. and Meyer zu Heringdorf, F.-J.
    Ultramicroscopy 130 (2013)
    Two photon photoemission microscopy was used to study the interaction of femtosecond laser pulses with Ag islands prepared using different strategies on Si(111) and SiO2. The femtosecond laser pulses initiate surface plasmon polariton (SPP) waves at the edges of the island. The superposition of the electrical fields of the femtosecond laser pulses with the electrical fields of the SPP results in a moiré pattern that is comparable despite the rather different methods of preparation and that gives access to the wavelength and direction of the SPP waves. If the SPPs reach edges of the Ag islands, they can be converted back into light waves. The incident and refracted light waves result in an interference pattern that can again be described with a moiré pattern, demonstrating that Ag islands can be used as plasmonic beam deflectors for light. © 2013 Elsevier B.V.
    view abstract10.1016/j.ultramic.2013.03.007
  • Lattice dependent motion of voids during electromigration
    Sindermann, S.P. and Latz, A. and Dumpich, G. and Wolf, D.E. and Meyer zu Heringdorf, F.-J.
    Journal of Applied Physics 113 (2013)
    The influence of the crystal lattice configuration to electromigration processes, e.g., void formation and propagation, is investigated in suitable test structures. They are fabricated out of self-assembled, bi-crystalline Ag islands, grown epitaxially on a clean Si(111) surface. The μm-wide and approximately 100 nm thick Ag islands are a composition of a Ag(001) and a Ag(111) part. By focused ion beam milling, they are structured into wires with a single grain boundary, the orientation of which can be chosen arbitrarily. In-situ scanning electron microscopy (SEM) allows to capture an image sequence during electrical stressing and monitors the development of voids and hillocks in time. To visualize the position and motion of voids, we calculate void maps using a threshold algorithm. Most of the information from the SEM image sequence is compressed into one single image. Our present electromigration studies are based on in-situ SEM investigations for three different lattice configurations: Ag(001) (with electron current flow in [110] direction), Ag(111) (with electron current flow in [112] direction), and additionally 90°rotated Ag(111) (with electron current flow in [110] direction). Our experimental results show that not only the formation and shape but also the motion direction of voids strongly depends on the crystal orientation. © 2013 American Institute of Physics.
    view abstract10.1063/1.4798367
  • Ultrafast laser-induced melting and ablation studied by time-resolved diffuse X-ray scattering
    Nicoul, M. and Quirin, F. and Lindenberg, A.M. and Barty, A. and Fritz, D.M. and Zhu, D. and Lemke, H. and Chollet, M. and Reis, D.A. and Chen, J. and Ghimire, S. and Trigo, M. and Fuchs, M. and Gaffney, K.J. and Larsson, J. and Becker, T. and Meyer, S. and Payer, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M. and Jerman, M. and Sokolowski-Tinten, K.
    EPJ Web of Conferences 41 (2013)
    Time-resolved diffuse X-ray scattering with 50 fs, 9.5 keV X-ray pulses from the Linear Coherent Light Source was used to study the structural dynamics in materials undergoing rapid melting and ablation after fs laser excitation. © Owned by the authors, published by EDP Sciences, 2013.
    view abstract10.1051/epjconf/20134104013
  • High-quality epitaxial Bi(111) films on Si(111) by isochronal annealing
    Payer, T. and Klein, C. and Acet, M. and Ney, V. and Kammler, M. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Thin Solid Films 520 (2012)
    Bi(111) films grown on Si(111) at room temperature show a significantly higher roughness compared to Bi films grown on Si(100) utilizing a kinetic pathway based on a low-temperature process. Isochronal annealing steps of 3 min duration each with temperatures up to 200 °C cause a relaxation of the Bi films' lattice parameter toward the Bi bulk value and yield an atomically flat Bi surface. Driving force for the relaxation and surface reordering is the magic mismatch of 11 Bi atoms to 13 Si atoms that emerges at annealing temperatures above 150 °C and reduces the remaining strain to less than 0.2%. © 2012 Elsevier B.V.
    view abstract10.1016/j.tsf.2012.06.004
  • Impact of C 60 Adsorption on Surface Plasmon Polaritons on Self-Assembled Ag(111) Islands on Si(111)
    Kirschbaum, P. and Buckanie, N.M. and Meyer zu Heringdorf, F.-J.
    Plasmonics 7 (2012)
    The influence of C 60 adsorption on the properties of surface plasmon polaritons on small Ag islands is discussed. Under illumination with UV light as well as under illumination with femtosecond laser pulses, a decrease of the photoemission yield with increasing C 60 coverage is observed. With angular resolved measurements, changes of the band structure during deposition are studied. Based on these experiments, an increase of the work function with increasing coverage is measured. In two photon photoemission, the surface plasmons are imaged as a periodic moiré pattern, the wavelength of which changes because of a modified effective surface dielectric function. Our findings imply that the wavelength of the plasmon wave becomes shorter as a result. Finally, a decrease of the intensity of the moiré pattern maxima compared with the intensity of the first maximum with increasing C 60 coverage has been observed. Accordingly, the damping of the plasmon wave becomes stronger. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s11468-011-9298-0
  • Interplay of wrinkles, strain, and lattice parameter in graphene on iridium
    Hattab, H. and N'Diaye, A.T. and Wall, D. and Klein, C. and Jnawali, G. and Coraux, J. and Busse, C. and Van Gastel, R. and Poelsema, B. and Michely, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Nano Letters 12 (2012)
    Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter. Our data reveals a characteristic hysteresis of the graphene lattice parameter that is explained by the interplay of reversible wrinkle formation and film strain. We show that graphene on Ir(111) always exhibits residual compressive strain at room temperature. Our results provide important guidelines for strategies to avoid wrinkling. © 2011 American Chemical Society.
    view abstract10.1021/nl203530t
  • Simulation of electromigration effects on voids in monocrystalline Ag films
    Latz, A. and Sindermann, S. and Brendel, L. and Dumpich, G. and Meyer zu Heringdorf, F.-J. and Wolf, D.E.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    We developed a three-dimensional, atomistic model based on the kinetic Monte Carlo method to investigate how voids penetrating a monocrystalline silver film are affected by electromigration. The simulations show a clear dependency between the nonequilibrium shape of the voids and the crystallographic orientation of the film. The simulation results are in accordance with experimental results on bicrystalline silver wires. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.035449
  • The interplay of topography and energy dissipation in pentacene thin films
    Wall, S. and Thien, D. and Meyer zu Heringdorf, F.-J.
    Journal of Electron Spectroscopy and Related Phenomena 185 (2012)
    Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(0 0 1) and (√3 × √3)R30°-Ag/Si(1 1 1). After an optical excitation of electrons by a λ = 400 nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(0 0 1), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (√3 × √3)R30°-Ag/Si(1 1 1), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay. © 2012 Elsevier B.V.
    view abstract10.1016/j.elspec.2012.09.007
  • Transient anisotropy in the electron diffraction of femtosecond laser-excited bismuth
    Zhou, P. and Streubühr, C. and Ligges, M. and Brazda, T. and Payer, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M. and Von Der Linde, D.
    New Journal of Physics 14 (2012)
    Laser excitation of thin bismuth films leads to a reduction in the diffraction intensity, which exhibits a characteristic angular anisotropy. The anisotropy depends on the polarization of the laser pulse and persists for approximately 150 ps. The effect clearly indicates coherent atomic motion in a preferential direction that we tentatively attribute to a transient shear deformation due to the photoelastic stress induced by the laser pulse. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/14/10/103031
  • Atomically smooth p-doped silicon nanowires catalyzed by aluminum at low temperature
    Moutanabbir, O. and Senz, S. and Scholz, R. and Alexe, M. and Kim, Y. and Pippel, E. and Wang, Y. and Wiethoff, C. and Nabbefeld, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    ACS Nano 5 (2011)
    Silicon nanowires (SiNWs) are powerful nanotechnological building blocks. To date, a variety of metals have been used to synthesize high-density epitaxial SiNWs through metal-catalyzed vapor phase epitaxy. Understanding the impact of the catalyst on the intrinsic properties of SiNWs is critical for precise manipulation of the emerging SiNW-based devices. Here we demonstrate that SiNWs synthesized at low-temperature by ultrahigh vacuum chemical vapor deposition using Al as a catalyst present distinct morphological properties. In particular, these nanowires are atomically smooth in contrast to rough {112}-type sidewalls characteristic of the intensively investigated Au-catalyzed SiNWs. We show that the stabilizing effect of Al plays the key role in the observed nanowire surface morphology. In fact, unlike Au which induces (111) and (113) facets on the nanowire sidewall surface, Al revokes the reconstruction along the [1̄1̄2] direction leading to equivalent adjacent step edges and flat surfaces. Our finding sets the lower limit of the Al surface density on the nanowire sidewalls at ∼2 atom/nm2. Additionally, despite using temperatures of ca. 110-170 K below the eutectic point, we found that the incorporation of Al into the growing nanowires is sufficient to induce an effective p-type doping of SiNWs. These results demonstrate that the catalyst plays a crucial role is shaping the structural and electrical properties of SiNWs. © 2011 American Chemical Society.
    view abstract10.1021/nn1030274
  • Epitaxial Ag wires with a single grain boundary for electromigration
    Sindermann, S. and Witt, C. and Spoddig, D. and Horn-Von Hoegen, M. and Dumpich, G. and Meyer zu Heringdorf, F.-J.
    Review of Scientific Instruments 82 (2011)
    Test structures for electromigration with defined grain boundary configurations can be fabricated using focused ion beam (FIB). We present a novel approach of combining epitaxial growth of Ag islands with FIB milling. Depending on the growth parameters, bi-crystalline Ag islands can be grown on Si(111) surfaces and can be structured into wires by FIB. To avoid doping effects of the used Ga FIB, silicon on insulator (SOI) substrates are used. By cutting through the device layer of the SOI substrate with deep trenches, the Ag wire can be electrically separated from the rest of the substrate. In this way, Ag wires with one isolated grain boundary of arbitrary direction can be assembled. Using scanning electron microscopy we demonstrate the feasibility of our approach. © 2011 American Institute of Physics.
    view abstract10.1063/1.3671802
  • Growth temperature dependent graphene alignment on Ir(111)
    Hattab, H. and N'Diaye, A.T. and Wall, D. and Jnawali, G. and Coraux, J. and Busse, C. and Van Gastel, R. and Poelsema, B. and Michely, T. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Applied Physics Letters 98 (2011)
    The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moiŕ spots reflect graphene in a single orientation epitaxial incommensurate phase. © 2011 American Institute of Physics.
    view abstract10.1063/1.3548546
  • Lost in reciprocal space? Determination of the scattering condition in spot profile analysis low-energy electron diffraction
    Klein, C. and Nabbefeld, T. and Hattab, H. and Meyer, D. and Jnawali, G. and Kammler, M. and Meyer zu Heringdorf, F.-J. and Golla-Franz, A. and Müller, B.H. and Schmidt, T. and Henzler, M. and Horn-Von Hoegen, M.
    Review of Scientific Instruments 82 (2011)
    The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions. © 2011 American Institute of Physics.
    view abstract10.1063/1.3554305
  • Shape, orientation, and crystalline composition of silver islands on Si(111)
    Wall, D. and Tikhonov, S. and Sindermann, S. and Spoddig, D. and Hassel, C. and Horn-von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    IBM Journal of Research and Development 55 (2011)
    Photoemission electron microscopy and spot profile analyzing low-energy electron diffraction have been used to study the temperature-dependent growth of Ag islands on a Si(111) surface. Depending on growth temperature, various island shapes can be formed. At low temperatures, polygonic islands are formed, consisting of both Ag(001) and Ag(111) crystal orientations. At higher temperatures, islands consist mostly of Ag(111) orientation and are predominantly of triangular shape. As the islands grow, it is possible that the crystalline composition of an island changes. We observed that Ag(001)-oriented areas convert into areas of Ag(111) orientation. The rotational orientation of the Ag islands with respect to the substrate is explained by a modified coincidence-site lattice approach. © Copyright 2011 by International Business Machines Corporation.
    view abstract10.1147/JRD.2011.2158761
  • Anisotropy of Ag diffusion on vicinal Si surfaces
    Sindermann, S. and Wall, D. and Roos, K.R. and Horn-Von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    e-Journal of Surface Science and Nanotechnology 8 (2010)
    Photoemission electron microscopy (PEEM) is used to study Ag surface diffusion on vicinal Si surfaces. The diffusion field is represented by Iso-Coverage Zones around Ag islands during desorption. By analyzing the shape and radius of the Iso-Coverage Zone we can determine diffusion parameters. For anisotropic diffusion the zone has an elliptical shape and the aspect ratio gives a measure for the anisotropy. Using this technique, we study the degree of anisotropy of Ag diffusion on vicinal Si(001) and Si(111). With increasing miscut angles, starting from Si(001) as well as from Si(111), we find a gradually increasing anisotropy, caused by the higher step density. On higher index surfaces, like Si(119), Si(115) and Si(113), we find isotropic diffusion for surfaces with comparable dimer and (double) step structure as on Si(001)-4°, where diffusion is strongly anisotropic. © 2010 The Surface Science Society of Japan.
    view abstract10.1380/ejssnt.2010.372
  • Imaging diffusion fields on a surface with multiple reconstructions: Ag/Si(111)
    Wall, D. and Lohmar, I. and Roos, K.R. and Krug, J. and Horn-Von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    New Journal of Physics 12 (2010)
    Photoemission electron microscopy is used for studying the thermal decay of Ag islands grown epitaxially on a Si(111) surface. During the decay, the islands feed adatoms to the surrounding surface. The adatoms diffuse and eventually desorb, resulting in a radial coverage gradient that induces the formation of two concentric reconstructed zones, namely (√3 × √3)-R30 o and (3 × 1), around each island. We have developed a diffusion model to describe this multizone formation and demonstrate how diffusion constants can be determined for different reconstructed phases in a simple experiment. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/12/10/103019
  • Silver induced faceting of Si(112)
    Nabbefeld, T. and Wiethoff, C. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Applied Physics Letters 97 (2010)
    Si nanowires grown in ultrahigh vacuum by metal-catalyzed vapor-liquid-solid epitaxy are known to exhibit sidewalls with {112}-type orientation. For some metals the sidewalls show pronounced faceting. Ag induced faceting on Si(112) surfaces was studied in situ by spot-profile-analyzing low energy electron diffraction and ex situ atomic force microscopy. The (112) surface decomposes into (115)- and (111)- (√3×√3) -facets, both of which are Ag terminated. The width of the facets is kinetically limited and varies between 6 nm at T<550 °C and 30 nm at T=690 °C. © 2010 American Institute of Physics.
    view abstract10.1063/1.3464555
  • electromagnetic wave polarization

  • epitaxial growth

  • low energy electron microscopy

  • nanostructures

  • particle optics

  • photoemission electron microscopy

  • photoemission electron microscopy

  • plasmon resonance

  • quantum theory

  • self assembly

  • surface plasmons

  • surfaces

  • thin films

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