Frank-J. Meyer zu Heringdorf

Growth of graphene on Ir(111)

Catalytic decomposition of hydrocarbons on transition metals attracts a renewed interest as a route toward high-quality graphene prepared in a reproducible manner. Here we employ two growth methods for graphene on Ir(111), namely room temperature adsorption and thermal decomposition at 870–1470 K (temperature programmed growth (TPG)) as well as direct exposure of the hot substrate at 870–1320 K (chemical vapor deposition (CVD)). The temperature- and exposure-dependent growth of graphene is investigated in detail by scanning tunneling microscopy. TPG is found to yield compact graphene islands bounded by C zigzag edges. The island size may be tuned from a few to a couple of tens of nanometers through Smoluchowski ripening. In the CVD growth, the carbon in ethene molecules arriving on the Ir surface is found to convert with probability near unity to graphene. The temperature-dependent nucleation, interaction with steps and coalescence of graphene islands are analyzed and a consistent model for CVD growth is developed.

Electronic acceleration of atomic motions and disordering in bismuth.

The development of X-ray and electron diffraction methods with ultrahigh time resolution has made it possible to map directly, at the atomic level, structural changes in solids induced by laser excitation. This has resulted in unprecedented insights into the lattice dynamics of solids undergoing phase transitions. In aluminium, for example, femtosecond optical excitation hardly affects the potential energy surface of the lattice; instead, melting of the material is governed by the transfer of thermal energy between the excited electrons and the initially cold lattice. In semiconductors, in contrast, exciting ∼10 per cent of the valence electrons results in non-thermal lattice collapse owing to the antibonding character of the conduction band. These different material responses raise the intriguing question of how Peierls-distorted systems such as bismuth will respond to strong excitations. The evolution of the atomic configuration of bismuth upon excitation of its A1g lattice mode, which involves damped oscillations of atoms along the direction of the Peierls distortion of the crystal, has been probed, but the actual melting of the material has not yet been investigated. Here we present a femtosecond electron diffraction study of the structural changes in crystalline bismuth as it undergoes laser-induced melting. We find that the dynamics of the phase transition depend strongly on the excitation intensity, with melting occurring within 190 fs (that is, within half a period of the unperturbed A1g lattice mode) at the highest excitation. We attribute the surprising speed of the melting process to laser-induced changes in the potential energy surface of the lattice, which result in strong acceleration of the atoms along the longitudinal direction of the lattice and efficient coupling of this motion to an unstable transverse vibrational mode. That is, the atomic motions in crystalline bismuth can be electronically accelerated so that the solid-to-liquid phase transition occurs on a sub-vibrational timescale.

In?situ observation of stress relaxation in epitaxial graphene

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations.

Au stabilization and coverage of sawtooth facets on Si nanowires grown by vapor-liquid-solid epitaxy.

Si nanowires grown in UHV by Au-catalyzed vapor-liquid-solid epitaxy are known to exhibit sidewalls with {112}-type orientation that show faceting. To understand the origin of the faceting, Au induced faceting on Si(112) surfaces was studied in situ by spot-profile-analyzing low-energy electron diffraction. With increasing Au coverage at 750 degrees C, the Si(112) surface undergoes various morphological transformations until, at a critical Au coverage of about 3.1 x 10 (14) atoms/cm (2), a phase consisting of large (111) and (113) facets forms, similar in structure to the nanowire sidewalls. This phase is stable at larger Au coverages in equilibrium with Au droplets. We suggest that Si nanowire surfaces exhibit this structure, and we derive the Au coverage on the two types of facets.

Normal-Incidence Photoemission Electron Microscopy (NI-PEEM) for Imaging Surface Plasmon Polaritons

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.

Reciprocal space mapping by spot profile analyzing low energy electron diffraction

We present an experimental approach for the recording of two-dimensional reciprocal space maps using spot profile analyzing low energy electron diffraction (SPA-LEED). A specialized alignment procedure eliminates the shifting of LEED patterns on the screen which is commonly observed upon variation of the electron energy. After the alignment, a set of one-dimensional sections through the diffraction pattern is recorded at different energies. A freely available software tool is used to assemble the sections into a reciprocal space map. The necessary modifications of the Burr-Brown computer interface of the two Leybold and Omicron type SPA-LEED instruments are discussed and step-by-step instructions are given to adapt the SPA 4.1d software to the changed hardware. Au induced faceting of 4° vicinal Si(001) is used as an example to demonstrate the technique.

Impact of C60 Adsorption on Surface Plasmon Polaritons on Self-Assembled Ag(111) Islands on Si(111)

The influence of C60 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 C60 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 C60 coverage has been observed. Accordingly, the damping of the plasmon wave becomes stronger.

Short-range surface plasmonics: Localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface

We demonstrate nanofocusing down to 60 nm with 800-nm light in atomically flat single-crystalline 22-nm-thick gold flakes. We experimentally and theoretically visualize the propagation of short-range surface plasmon polaritons using atomically flat single-crystalline gold platelets on silicon substrates. We study their excitation and subfemtosecond dynamics via normal-incidence two-photon photoemission electron microscopy. By milling a plasmonic disk and grating structure into a single-crystalline gold platelet, we observe nanofocusing of the short-range surface plasmon polariton. Localized two-photon ultrafast electron emission from a spot with a smallest dimension of 60 nm is observed. Our novel approach opens the door toward reproducible plasmonic nanofocusing devices, which do not degrade upon high light intensity or heating due to the atomically flat surface without any tips, protrusions, or holes. Our nanofoci could also be used as local emitters for ultrafast electron bunches in time-resolved electron microscopes.

Three-dimensional size determination of particles with photoelectron emission microscopy

We show that the aspect ratio and the size of particles at surfaces can be estimated with photoelectron emission microscopy when both linear and nonlinear processes are utilized. As the width of the particles is known from regular photoemission microscopy, a complete determination of the particles’ dimensions becomes possible by two-photon photoemission microscopy. Here, the light diffraction pattern of the illuminating light around the particles is emphasized by the nonlinear dependence of the photoemission yield on the electric field components at the surface. This allows the quantitative measurement of the aspect ratio of the particles. The results are in agreement with theory and atomic force microscopy measurements.

Spatio-Temporal Imaging of Surface Plasmon Polaritons in Two Photon Photoemission Microscopy

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.