The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
R M Tromp - One of the best experts on this subject based on the ideXlab platform.
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nonuniversal transverse Electron mean free path through few layer graphene
Physical Review Letters, 2019Co-Authors: Daniel Geelen, Johannes Jobst, E E Krasovskii, S J Van Der Molen, R M TrompAbstract:In contrast to the in-plane transport Electron mean-free path in graphene, the transverse mean-free path has received little attention and is often assumed to follow the "universal" mean-free path (MFP) curve broadly adopted in surface and interface science. Here we directly measure transverse Electron scattering through graphene from 0 to 25 eV above the vacuum level both in reflection using low energy Electron Microscopy and in transmission using Electronvolt transmission Electron Microscopy. From these data, we obtain quantitative MFPs for both elastic and inelastic scattering. Even at the lowest energies, the total MFP is just a few graphene layers and the elastic MFP oscillates with graphene layer number, both refuting the universal curve. A full theoretical calculation taking the graphene band structure into consideration agrees well with experiment, while the key experimental results are reproduced even by a simple optical toy model.
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nanoscale measurements of unoccupied band dispersion in few layer graphene
Nature Communications, 2015Co-Authors: Johannes Jobst, Daniel Geelen, R M Tromp, Jaap Kautz, Sense Jan Van Der MolenAbstract:The properties of any material are fundamentally determined by its Electronic band structure. Each band represents a series of allowed states inside a material, relating Electron energy and momentum. The occupied bands, that is, the filled Electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on Low-Energy Electron Microscopy. It relies on the dependence of the Electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only.
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low energy Electron potentiometry contactless imaging of charge transport on the nanoscale
Scientific Reports, 2015Co-Authors: Jaap Kautz, Johannes Jobst, R M Tromp, Christian Sorger, Heiko B Weber, S J Van Der MolenAbstract:Charge transport measurements form an essential tool in condensed matter physics. The usual approach is to contact a sample by two or four probes, measure the resistance and derive the resistivity, assuming homogeneity within the sample. A more thorough understanding, however, requires knowledge of local resistivity variations. Spatially resolved information is particularly important when studying novel materials like topological insulators, where the current is localized at the edges, or quasi-two-dimensional (2D) systems, where small-scale variations can determine global properties. Here, we demonstrate a new method to determine spatially-resolved voltage maps of current-carrying samples. This technique is based on Low-Energy Electron Microscopy (LEEM) and is therefore quick and non-invasive. It makes use of resonance-induced contrast, which strongly depends on the local potential. We demonstrate our method using single to triple layer graphene. However, it is straightforwardly extendable to other quasi-2D systems, most prominently to the upcoming class of layered van der Waals materials.
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large area graphene single crystals grown by low pressure chemical vapor deposition of methane on copper
Journal of the American Chemical Society, 2011Co-Authors: Carl W Magnuson, R M Tromp, Archana Venugopal, James B Hannon, Eric M Vogel, Luigi Colombo, Rodney S RuoffAbstract:Graphene single crystals with dimensions of up to 0.5 mm on a side were grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor. Low-Energy Electron Microscopy analysis showed that the large graphene domains had a single crystallographic orientation, with an occasional domain having two orientations. Raman spectroscopy revealed the graphene single crystals to be uniform monolayers with a low D-band intensity. The Electron mobility of graphene films extracted from field-effect transistor measurements was found to be higher than 4000 cm2 V−1 s−1 at room temperature.
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thermodynamics and kinetics of graphene growth on sic 0001
Physical Review Letters, 2009Co-Authors: R M Tromp, James B HannonAbstract:The formation of surface phases on the Si-terminated SiC(0001) surface, from the Si-rich (3x3) structure, through the intermediate (1x1) and (sqrt[3]xsqrt[3])-R30 degrees structures, to the C-rich (6sqrt[3]x6sqrt[3]) phase, and finally epitaxial graphene, has been well documented. But the thermodynamics and kinetics of these phase formations are poorly understood. Using in situ low energy Electron Microscopy, we show how the phase transformation temperatures can be shifted over several hundred degrees Celsius, and the phase transformation time scales reduced by several orders of magnitude, by balancing the rate of Si evaporation with an external flux of Si. Detailed insight in the thermodynamics allows us to dramatically improve the morphology of the final C-rich surface phases, including epitaxial graphene.
Bene Poelsema - One of the best experts on this subject based on the ideXlab platform.
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size fluctuations of near critical and gibbs free energy for nucleation of bda on cu 001
Physical Review Letters, 2012Co-Authors: Daniel Schwarz, Henricus J W Zandvliet, Raoul Van Gastel, Bene PoelsemaAbstract:We present a Low-Energy Electron Microscopy study of nucleation and growth of BDA on Cu(001) at low supersaturation. At sufficiently high coverage, a dilute BDA phase coexists with c(8×8) crystallites. The real-time microscopic information allows a direct visualization of near-critical nuclei, determination of the supersaturation and the line tension of the crystallites, and, thus, derivation of the Gibbs free energy for nucleation. The resulting critical nucleus size nicely agrees with the measured value. Nuclei up to 4-6 times larger still decay with finite probability, urging reconsideration of the classic perception of a critical nucleus.
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quantum size effect driven structure modifications of bi films on ni 111
Physical Review Letters, 2011Co-Authors: Tjeerd Rogier Johannes Bollmann, Henricus J W Zandvliet, Raoul Van Gastel, Bene PoelsemaAbstract:The quantum-size effect (QSE) driven growth of Bi film structures on Ni(111) was studied in situ using low energy Electron Microscopy and selective area low energy Electron diffraction (μLEED). Domains with a (3×3), [3/1 -1/2], and (7×7) film structure are found with a height of 3, 5, and 7 atomic layers, respectively. A comparison of I/V-μLEED curves with tensor LEED calculations shows perfectly accommodated Fermi wavelengths, indicative that not only the quantized height, but also the film structure is driven by QSE.
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anomalous decay of Electronically stabilized lead mesas on ni 111
Physical Review Letters, 2011Co-Authors: Tjeerd Rogier Johannes Bollmann, Henricus J W Zandvliet, Raoul Van Gastel, Bene PoelsemaAbstract:With their low surface free energy, lead films tend to wet surfaces. However, quantum size effects (QSE) often lead to islands with distinct preferred heights. We study thin lead films on Ni(111) using low energy Electron Microscopy and selected area low energy Electron diffraction. Indeed, the grown lead mesas show distinct evidence for QSE’s. At about 526 K metastable mesas reshape into hemispheres within milliseconds, driven by a huge reduction in interfacial free energy. The underlying diffusion rate is many orders of magnitude faster than expected for lead on bulk lead
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2 1 1 1 phase transition on ge 001 dimer breakup and surface roughening
Physical Review Letters, 2003Co-Authors: Esther Van Vroonhoven, Henricus J W Zandvliet, Bene PoelsemaAbstract:Using low energy Electron Microscopy, we have investigated the (2×1)-(1×1) phase transition occurring above 925 K on Ge(001). Dimer breakup has been identified as the physical origin of this transition. A quantitative description of the dimer concentration during the transition involves configuration entropy of random monomers within the dimer matrix. The dimer formation energy amounts to 1.2±0.3 eV. Dimer breakup promotes reversible surface disorder by step proliferation and irreversible surface roughening above 1130 K.
U Starke - One of the best experts on this subject based on the ideXlab platform.
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formation and structural analysis of twisted bilayer graphene on ni 111 thin films
Surface Science, 2014Co-Authors: Alexei Zakharov, Takayuki Iwasaki, T Eelbo, Marta Waśniowska, R Wiesendanger, J H Smet, U StarkeAbstract:We synthesized twisted bilayer graphene on single crystalline Ni(111) thin films to analyze the statistical twist angle distribution on a large scale. The twisted bilayer graphene was formed by combining two growth methods, namely the catalytic surface reaction of hydrocarbons and carbon segregation from Ni. Low energy Electron diffraction (LEED) investigations directly revealed dominant twist angles of 13 degrees, 22 degrees, 38 degrees, and 47 degrees. We show that the angle distribution is closely related to the sizes of Moire superlattices which form at commensurate rotation angles. In addition to the commensurate angles, quasi-periodic Moire structures were also formed in the vicinity of the dominant angles, confirmed by microscopic observations with low energy Electron Microscopy and scanning tunneling Microscopy (STM). The quasi-periodic Moire patterns are presumably caused by insufficient mobility of carbon atoms during the segregation growth while cooling. Micro-LEED studies reveal that the size of single twisted domains is below 400 nm. Atomic-scale characterization by STM indicates that the twisted layer grown by segregation is located underneath the layer grown by surface reaction, i.e. between the Ni surface and the top single-crystal graphene layer. (C) 2014 Elsevier B.V. All rights reserved.
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ambipolar doping in quasifree epitaxial graphene on sic 0001 controlled by ge intercalation
Physical Review B, 2011Co-Authors: Konstantin V. Emtsev, Stiven Forti, Camilla Coletti, Alexei Zakharov, U StarkeAbstract:The Electronic structure of decoupled graphene on SiC(0001) can be tailored by introducing atomically thin layers of germanium at the interface. The Electronically inactive (6 root 3 x 6 root 3)R30 degrees reconstructed buffer layer on SiC(0001) is converted into quasi-free-standing monolayer graphene after Ge intercalation and shows the characteristic graphene pi bands as displayed by angle-resolved photoElectron spectroscopy. Low-Energy Electron Microscopy (LEEM) studies reveal an unusual mechanism of the intercalation in which the initial buffer layer is first ruptured into nanoscopic domains to allow the local in-diffusion of germanium to the interface. Upon further annealing, a continuous and homogeneous quasifree graphene film develops. Two symmetrically doped (n- and p-type) phases are obtained that are characterized by different Ge coverages. They can be prepared individually by annealing a Ge film at different temperatures. In an intermediate-temperature regime, a coexistence of the two phases can be achieved. In this transition regime, n-doped islands start to grow on a 100-nm scale within p-doped graphene terraces as revealed by LEEM. Subsequently, the n islands coalesce but still adjacent terraces may display different doping. Hence, lateral p-n junctions can be generated on epitaxial graphene with their size tailored on a mesoscopic scale. (Less)
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precise in situ thickness analysis of epitaxial graphene layers on sic 0001 using low energy Electron diffraction and angle resolved ultraviolet photoElectron spectroscopy
Applied Physics Letters, 2008Co-Authors: Christian Riedl, Alexei Zakharov, U StarkeAbstract:We demonstrate an easy and practical method for the thickness analysis of epitaxial graphene on SiC(0001) that can be applied continuously during the preparation procedure. Fingerprints in the spot intensity spectra in low energy Electron diffraction (LEED) allow for the exact determination of the number of layers for the first three graphene layers. The LEED data have been correlated with the Electronic bandstructure around the K¯-point of the graphene Brillouin zone as investigated by laboratory based angle resolved ultraviolet photoElectron spectroscopy using He II excitation. The morphology and homogeneity of the graphene layers can be analyzed by low energy Electron Microscopy.
E. Bauer - One of the best experts on this subject based on the ideXlab platform.
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quantum size effect in exchange asymmetry of ultrathin ferromagnetic films studied with spin polarized low energy Electron Microscopy
Applied Surface Science, 2019Co-Authors: R Zdyb, E. BauerAbstract:Abstract The magnetic properties of ultrathin ferromagnetic films are studied by means of Spin Polarized Low Energy Electron Microscopy. Measurements of the onset of ferromagnetic order, distribution and shape of magnetic domains, magnetization direction and their change are now well established standards for this technique. Here, the asymmetry parameter has been determined as a function of film coverage and energy of the incident Electron beam. It reveals oscillatory behavior which is usually described as due to the quantum size effect (QSE). We explore the origin of the characteristic features observed in the asymmetry curves and distinguish between the QSE oscillations and other phenomena influencing the shape of the asymmetry curves. As an example we discuss the asymmetry of ultrathin iron films grown on the W(1 1 0) surface.
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surface Microscopy with low energy Electrons
2014Co-Authors: E. BauerAbstract:Chapter 1. Introduction Abstract 1.1. The early years 1.2. The postwar revival References Chapter 2. Basic Interactions Abstract 2.1 Fundamental theories of Electron emission 2.2 Photoemission 2.2.1 General considerations 2.2.2 The free Electron gas approximation 2.2.3 Band structure UV photoemission 2.2.4 Spin effects in UV photoemission 2.2.5 Surface plasmon photoemission 2.2.6 X-ray photoemission 2.2.6.1 PhotoElectron emission 2.2.6.2 Secondary Electron emission 2.3 Electron reflection 2.3.1 General considerations 2.3.2 Elastic scattering 2.3.3 Inelastic scattering 2.3.4 Surface effects 2.3.5 VLEED, LEETS, TCS 2.3.6 Quantum well effects 2.3.7 Other aspects References Chapter 3. Instrumentation Abstract 3.1 Instruments: from simple to complex3.1.1 PEEM 3.1.2 LEEM 3.1.3 Aberration-corrected instruments 3.1.4 Spectroscopic imaging instruments 3.1.5 Spin-resolved imaging instruments 3.2 Components 3.2.1 Objective lens and other axial-symmetric lenses 3.2.2 Magnetic deflectors 3.2.3 Electron mirrors 3.2.4 Aberration correctors 3.2.5 Energy filters 3.2.6 Wien filters 3.2.7 Photon sources 3.2.8 Electron sources 3.2.9 Other components (image detectors vacuum system including airlock and specimen preparation chamber, Electronics) References Chapter 4. Theory of image formation Abstract 4.1 Introduct ion 4.2 Wave propagation: The contrast transfer function 4.2.1 Low energy Electron Microscopy. The wave amplitude| 4.2.2 The image intensity 4.2.3 Mirror Electron Microscopy 4.2.4 Emission Electron Microscopy 4.3 Through-focus series image improvement 4.4 Information transfer in the image acquisition system 4.5 Summary and outlook References Chapter 5. Applications in surface science Abstract 5.1 Surface microstructure 5.1.1 Metals 5.1.2 Semiconductors 5.1.2.1 Si(111) 5.1.2.2 Si(100) 5.1.2.3 Other Si surfaces 5.1.3 Other inorganic semiconductor surfaces 5.1.4 Other inorganic compound surfaces 5.2 Adsorption 5.2.1 Adsorption on metals 5.2.1.1 Nonmetallic adsorbates 5.2.1.2 Coadsorption and reaction: catalysis 5.2.1.3 Metallic adsorbates 5.2.2 Adsorption on Semiconductors 5.2.2.1 Metallic adsorbates 5.2.2.2 Nonmetallic adsorbates 5.3 Film growth and structure 5.3.1 Films on semiconductors 5.3.1.1 Metal films 5.3.1.2 Ge on Si 5.3.1.3 Other films on semiconductors 5.3.1.4 Nanostructures and droplets on semiconductors5.3.2 Films on metals 5.3.2.1 Metal films 5.3.2.2 Inorganic compound films 5.3.3 Organic films References Chapter 6. Applications in other fields Abstract 6.1 Graphene 6.1.1 Introduction 6.1.2 Graphene on SiC 6.1.2.1 Growth and microstructure 6.1.2.2 Intercalation 6.1.3 Graphene on metals 6.1.3.1 Introduction 6.1.3.2 Growth and microstructure 6.1.3.3 Intercalation 6.2 Plasmons 6.2.1 Introduction 6.2.2 Linear structures 6.2.3 Nanostructures 6.2.4 Complex wave fields 6.2.5 Limitations 6.3 Technological applications 6.3.1 General materials applications 6.3.2 Electronics 6.4 Biology 6.5 A multimethod case study References Chapter 7. Magnetic imaging Abstract7.1 Introduction 7.2 Ferromagnetic films 7.2.1 Single layers 7.2.2 Quantum well effects 7.2.3 Bilayers 7.2.4 Trilayers 7.2.5 Multilayers 7.2.6 Compound layers 7.3 Bulk magnetic materials 7.3.1 Ferromagnetic materials 7.3.2 Antiferromagnetic materials 7.4 Ferromagnetic-antiferromagnetic interfaces 7.5 Nanostructures 7.5.1 Introduction 7.5.2 Static domain structure 7.5.3 Field and current influence 7.5.4 Nanodots and nanostructure arrays 7.6. Ferroelectrics / Multiferroics 7.6.1 Ferroelectries 7.6.2 Multiferroics Chapter 8. Other surface imaging methods with Electrons Abstract 8.1 Scanning Low Energy Electron Microscopy 8.2 Scanning Low Energy Electron Diffraction Microscopy 8.3 Reflection Electron Microscopy 8.4 Secondary and Auger Electron Microscopy 8.5 Scanning Electron Microscopy with Spin Analysis 8.6 Scanning PhotoElectron Emission Microscopy 8.7 Concluding remarks
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nanomagnetism studies with spin polarized low energy Electron Microscopy and x ray magnetic circular dichroism photoemission Electron Microscopy
Surface and Interface Analysis, 2005Co-Authors: Andrea Locatelli, R Zdyb, S Heun, S Cherifi, R Belkhou, E. BauerAbstract:The spin reorientation transition (SRT) of ultrathin FeCo alloy films on an Au(111) surface and the magnetic/structural phase transition of epitaxial MnAs films on a GaAs(100) surface are studied by two Electron microscopic techniques with very high surface sensitivity, i.e. spin-polarized Low-Energy Electron Microscopy (SPLEEM) and x-ray magnetic circular dichroism photoemission Electron Microscopy (XMCDPEEM), combined with Low-Energy Electron Microscopy (LEEM) and Low-Energy Electron diffraction (LEED), respectively. A new SRT mechanism is found and the interrelation between ferromagnetism and structure is elucidated. Copyright © 2005 John Wiley & Sons, Ltd.
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high lateral resolution spectroscopic imaging of surfaces the undulator beamline nanospectroscopy at elettra
Journal De Physique Iv, 2003Co-Authors: A. Locatelli, S Heun, S Cherifi, A Bianco, D Cocco, M Marsi, M Pasqualetto, E. BauerAbstract:High lateral resolution direct imaging of surfaces with chemical sensitivity is of increasing importance for basic and applied research in the field of surface and materials science. A novel and versatile beamline, to be employed for the spectromicroscopic study of surfaces in the submicron range, is now available at Elettra. The beamline, named «Nanospectroscopy», serves an end-station equipped with a Spectroscopic Photo-Emission and Low Energy Electron Microscope (SPELEEM). This microscope combines the ability to perform XPEEM (X-ray Photo-Emission Electron Microscopy), small spot XPS (X-ray PhotoElectron Spectroscopy), XPD (X-ray PhotoElectron Diffraction), LEEM and LEED (Low Energy Electron Microscopy and Diffraction, respectively).
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leem observation of formation of cu nano islands on si 111 surface by hydrogen termination
Surface Science, 2001Co-Authors: Tsuneo Yasue, M Jalochowski, Takanori Koshikawa, E. BauerAbstract:Abstract The growth of Cu on Si(1 1 1) surface with and without hydrogen termination was studied with low energy Electron Microscopy. On the clean surface the two-dimensional “5×5” incommensurate layer is first formed followed by the formation of three-dimensional (3D) islands. On the hydrogen-terminated surface the formation of the “5×5” structure is suppressed and nano-scale 3D islands decorate the steps and domain boundaries of the δ7×7 structure. Many LEED spots from the nano-islands move with Electron energy, which indicates that the islands are faceted. From the analysis of the LEED pattern it is suggested that the nano-islands are the (1 1 1)-oriented β-phase Cu–Si alloy and are terminated by (1 1 1), {5 5 4} and {15 16 13} faces.
Rodney S Ruoff - One of the best experts on this subject based on the ideXlab platform.
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carbon assisted chemical vapor deposition of hexagonal boron nitride
2D Materials, 2017Co-Authors: Ariel Ismach, Harry Chou, Patrick C Mende, Andrei Dolocan, Rafik Addou, Shaul Aloni, Robert M Wallace, R M Feenstra, Rodney S RuoffAbstract:We show that in a low-pressure chemical vapor deposition (CVD) system, the residual oxygen and/or air play a crucial role in the mechanism of the growth of hexagonal boron nitride (h-BN) films on Ni foil 'enclosures'. Hexagonal-BN films grow on the Ni foil surface via the formation of an intermediate boric-oxide (BOx) phase followed by a thermal reduction of the BOx by a carbon source (either amorphous carbon powder or methane), leading to the formation of single- and bi-layer h-BN. Low energy Electron Microscopy (LEEM) and diffraction (LEED) were used to map the number of layers over large areas; Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), x-ray photoElectron spectroscopy (XPS) and scanning tunneling Microscopy (STM) were used to characterize the structure and physical quality of the ultra-thin h-BN film. The growth procedure reported here leads to a better understanding and control of the synthesis of ultra-thin h-BN films.
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large area graphene single crystals grown by low pressure chemical vapor deposition of methane on copper
Journal of the American Chemical Society, 2011Co-Authors: Carl W Magnuson, R M Tromp, Archana Venugopal, James B Hannon, Eric M Vogel, Luigi Colombo, Rodney S RuoffAbstract:Graphene single crystals with dimensions of up to 0.5 mm on a side were grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor. Low-Energy Electron Microscopy analysis showed that the large graphene domains had a single crystallographic orientation, with an occasional domain having two orientations. Raman spectroscopy revealed the graphene single crystals to be uniform monolayers with a low D-band intensity. The Electron mobility of graphene films extracted from field-effect transistor measurements was found to be higher than 4000 cm2 V−1 s−1 at room temperature.
