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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.

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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.

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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.

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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).

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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.

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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.

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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.

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We report on in situ electromigration and potentiometry measurements on single-crystalline Ag nanowires under ultra-high vacuum (UHV) conditions, using a four-probe scanning tunnelling microscope (STM). The Ag nanowires are grown in place by self-organization on a 4° vicinal Si(001) surface. Two of the four available STM tips are used to contact the nanowire. The positioning of the tips is controlled by a scanning electron microscope (SEM). Potentiometry measurements on an Ag nanowire were carried out using a third tip to determine the resistance per length. During electromigration measurements current densities of up to 1 × 10(8) A cm(-2) could be achieved. We use artificially created notches in the wire to initiate electromigration and to control the location of the electromigration process. At the position of the notch, electromigration sets in and is observed quasi-continuously by the SEM.

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Linear and nonlinear photoemission microscopy is used to study the origin of space charge effects that are frequently observed if amplified femtosecond lasers are used for generation of photoelectrons. Space charge effects are apparent in the width of the photoemission spectra, but also create image blur. The onset threshold for space charge effects is determined by recording the width of photoemission spectra and by finding the conditions under which spectral broadening is just less than the energy resolution of the microscope. The principal findings are independent if harmonics of the fundamental of the fs laser pulses are used, but the space charge effects are found to be more dominant at lower repetition rates. By inserting apertures into the electron path, the place at which space charge effects occur can be localized.

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Photoemission electron microscopy is used to study the growth of single-crystalline silver nanowires on flat and vicinal Si(001) substrates. The growth experiments were performed at various temperatures and showed a temperature dependence of nanowire formation. The nanowires on Si(001) are evenly distributed in the [110] and [Formula: see text] directions on the surface, whereas on a 4° vicinal surface the wires grow only along the steps, in the [Formula: see text] direction. This change in wire distribution is attributed to the increasing diffusion anisotropy as the vicinality of the substrate increases.

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Photoemission electron microscopy is used to study the thermal decay of Ag islands grown epitaxially on Si(001) surfaces. (2 x 3) Ag reconstructed zones, due to migrating Ag atoms supplied to the surface by the decaying islands, surround each of the islands. The shape of these reconstructed zones depends on the degree of diffusion isotropy in the system. We demonstrate that the imaging of these reconstructed "isocoverage zones" constitutes a unique experimental method for directly observing diffusion fields in epitaxial systems. We describe the dynamics of the thermal decay of the islands and the isozones in the context of a continuum diffusion model.

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The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

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We have investigated the adsorption of cesium on the Si(100) surface with photoelectron emission microscopy using linearly polarized green laser light. We observe a polarization dependent contrast between the (2 x 1) or (1 x 2) reconstructed terraces. Density-functional calculations reveal the geometric and electronic structure of the Cs/Si(100) surface. The contrast between the (2 x 1) or (1 x 2) reconstructed domains is explained on the basis of dipole selection rules for the photoemission matrix elements.

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Characterization and controlled manipulation of surfaces is a crucial factor in modern processing of the technologically relevant Si(100) surface. Using spot profile analyzing low energy electron diffraction, the morphological changes from a single stepped vicinal Si(100) surface to a single-domain (2x1) reconstructed surface have been investigated in situ during Si deposition. The temperature range for formation of this kinetically-stabilized single-domain surface was found to be 400-500 degrees C. This single-domain surface could be preserved for further characterization and experiments after quenching to room temperature.

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The recent demonstration of single-crystal organic optoelectronic devices has received widespread attention. But practical applications of such devices require the use of inexpensive organic films deposited on a wide variety of substrates. Unfortunately, the physical properties of these organic thin films do not compare favourably to those of single-crystal materials. Moreover, the basic physical principles governing organic thin-film growth and crystallization are not well understood. Here we report an in situ study of the evolution of pentacene thin films, utilizing the real-time imaging capabilities of photoelectron emission microscopy. By a combination of careful substrate preparation and surface energy control, we succeed in growing thin films with single-crystal grain sizes approaching 0.1 millimetre (a factor of 20-100 larger than previously achieved), which are large enough to fully contain a complete device. We find that organic thin-film growth closely mimics epitaxial growth of inorganic materials, and we expect that strategies and concepts developed for these inorganic systems will provide guidance for the further development and optimization of molecular thin-film devices.

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Au induced faceting of a 4 degrees vicinal Si(001) surface was studied with chemical resolution using soft x-ray photoemission electron microscopy. For the first time a direct and quantitative determination of the local Au coverage in situ and during deposition was possible. Au atoms, necessary for the expansion of (001) terraces, are accummulated from a lattice gas, resulting in a phase separation between Au enriched terraces and Au depleted step bunches. During a second stage Au also adsorbs on the step bunches and transforms them into (119) facets. A simple Monte Carlo simulation shows that the initial coverage difference between terraces and bunches determines the regularity of the formed mesoscopic grating.