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The imaging of dynamical processes at interfaces and on the nanoscale is of great importance throughout science and technology. While light-optical imaging techniques often cannot provide the necessary spatial resolution, electron-optical techniques damage the specimen and cause dose-induced artifacts. Here, optical near-field electron microscopy (ONEM) is proposed, an imaging technique that combines noninvasive probing with light, with a high-spatial-resolution readout via electron optics. Close to the specimen, the optical near fields are converted into a spatially varying electron flux using a planar photocathode. The electron flux is imaged using low-energy electron microscopy, enabling label-free nanometric resolution without the need to scan a probe across the sample. The specimen is never exposed to damaging electrons.

Transmission electron microscopy at very low energy is a promising way to avoid damaging delicate biological samples with the incident electrons, a known problem in conventional transmission electron microscopy. For imaging in the 0-30 eV range, we added a second electron source to a low energy electron microscopy (LEEM) setup, enabling imaging and spectroscopy in both transmission and reflection mode at nanometer (nm) resolution. The latter is experimentally demonstrated for free-standing graphene. Exemplary eV-TEM micrographs of gold nanoparticles suspended on graphene and of DNA origami rectangles on graphene oxide further establish the capabilities of the technique. The long and short axes of the DNA origami rectangles are discernable even after an hour of illumination with low energy electrons. In combination with recent developments in 2D membranes, allowing for versatile sample preparation, eV-TEM is paving the way to damage-free imaging of biological samples at nm resolution.

The effects of space charge, aberrations and relativity on temporal compression are investigated for a compact spherical electrostatic capacitor (α-SDA). By employing the three-dimensional (3D) field simulation and the 3D space charge model based on numerical General Particle Tracer and SIMION, we map the compression efficiency for a wide range of initial beam size and single-pulse electron number and determine the optimum conditions of electron pulses for the most effective compression. The results demonstrate that both space charge effects and aberrations prevent the compression of electron pulses into the sub-ps region if the electron number and the beam size are not properly optimized. Our results suggest that α-SDA is an effective compression approach for electron pulses under the optimum conditions. It may serve as a potential key component in designing future time-resolved electron sources for electron diffraction and spectroscopy experiments.

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 electron-Volt Transmission Electron Microscopy. From this 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.

Photo electron emission microscopy (PEEM), going back to the earliest days of electron microscopy, and low-energy electron microscopy (LEEM), successfully deployed since the late 1980s, are examples of cathode lens microscopy in which the sample itself is an integral part of the image forming system. While applications have naturally gravitated towards the acquisition of images to elucidate structure and structural evolution, recent years have also seen a rapidly expanding range of spectroscopic capabilities. These address, for example, the occupied and unoccupied electronic band structures of materials, electrical transport in 2-D systems, crystal growth and 2-D strain, inelastic electron energy loss mechanisms, as well as radiation damage in organic materials during low-energy electron irradiation. In this chapter, we discuss applications of these new spectroscopic methods, as well as recent instrumental developments that further expand the potential uses of cathode lens microscopy.

Measurement of chromatic aberration in a Low Energy Electron Microscope (LEEM) or Photo Electron Emission Microscope (PEEM) is necessary for quantitative image interpretation, and for accurate correction of chromatic aberration in an aberration-corrected instrument. While methods have been developed for measuring the spherical aberration coefficient, C3, measuring the chromatic aberration coefficient, Cc, remains a more difficult task. Here a novel method is introduced to simplify such measurements. The viability and accuracy is demonstrated using detailed electron-optical ray-tracing calculations. Experimental results show that the method is easily reduced to practice.

BackgroundPostoperative ileus is a common complication of abdominal surgery, leading to patient discomfort, morbidity and prolonged postoperative length of hospital stay (LOS). Previous studies suggested that chewing gum stimulates bowel function after abdominal surgery, but were underpowered to evaluate its effect on LOS and did not include enhanced recovery after surgery (ERAS)-based perioperative care. This study evaluated whether chewing gum after elective abdominal surgery reduces LOS and time to bowel recovery in the setting of ERAS-based perioperative care.MethodsA multicentre RCT was performed of patients over 18 years of age undergoing abdominal surgery in 12 hospitals. Standard postoperative care (control group) was compared with chewing gum three times a day for 30 min in addition to standard postoperative care. Randomization was computer-generated; allocation was concealed. The primary outcome was postoperative LOS. Secondary outcomes were time to bowel recovery and 30-day complications.ResultsBetween 2011 to 2015, 1000 patients were assigned to chewing gum and 1000 to the control arm. Median LOS did not differ: 7 days in both arms (P = 0·364). Neither was any difference found in time to flatus (24 h in control group versus 23 h with chewing gum; P = 0·873) or time to defaecation (60 versus 52 h respectively; P = 0·562). The rate of 30-day complications was not significantly different either.ConclusionThe addition of chewing gum to an ERAS postoperative care pathway after elective abdominal surgery does not reduce the LOS, time to bowel recovery or the rate of postoperative complications. Registration number: NTR2594 (Netherlands Trial Register).

We study the differential reactivity of alkanethiols with submonolayers of Au on Si(111), where the surface is a mixture of the Au-rich 3 × 3 structure and the Au-poor 5 × 2 structure. We expose these binary 2D alloy samples to alkanethiols, while monitoring the induced spatial and structural transformations in real time using a low energy electron microscope (LEEM). Energy-dependent diffraction measurements show that, surprisingly, there is no alkanethiol growth on the Au submonolayers. Instead, S–Si interactions induce a disproportionation of the 5 × 2 structure into the unexpectedly inert 3 × 3 phase and a Au-depleted phase. We show how this process can be used to reduce the dimensionality of a 2D electron system to 1D.

Graphene on silicon carbide (SiC) bears great potential for future graphene electronic applications because it is available on the wafer scale and its properties can be custom tailored by inserting various atoms into the graphene/SiC interface. It remains unclear, however, how atoms can cross the impermeable graphene layer during this widely used intercalation process. Here we demonstrate that in contrast to the current consensus, graphene layers grown in argon atmosphere on SiC are not homogeneous, but instead are composed of domains of different crystallographic stacking as they have been observed in other systems. We show that these domains are intrinsically formed during growth and that dislocations between domains dominate the (de)intercalation dynamics. Tailoring these dislocation networks, e.g., through substrate engineering, will increase the control over the intercalation process and could open a playground for topological and correlated electron phenomena in two-dimensional superstructures.

High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the ‘chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of., Heterostructures of graphene and hexagonal boron nitride have great potential for high-mobility electronics, yet little is known about the electronic interaction between these two atomically thin materials. Here, the authors perform angle-resolved reflected-electron spectroscopy to unveil their interplay.

We describe a new design for an aberration-corrected low energy electron microscope (LEEM) and photo electron emission microscope (PEEM), equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control over the chromatic and spherical aberration coefficients C(c) and C(3), as well as the mirror focal length, to match and correct the aberrations of the objective lens. For LEEM (PEEM) the theoretical resolution is calculated to be approximately 1.5 nm (approximately 4 nm). Unlike previous designs, this instrument makes use of two magnetic prism arrays to guide the electron beam from the sample to the electron mirror, removing chromatic dispersion in front of the mirror by symmetry. The aberration correction optics was retrofitted to an uncorrected instrument with a base resolution of 4.1 nm in LEEM. Initial results in LEEM show an improvement in resolution to approximately 2 nm.

We measure the role of structural defects, including grain boundaries and step edges, in determining the electrical transport characteristics of polycrystalline graphene monolayers synthesized on C-face SiC(0 0 0 ) by thermal decomposition. A combination of multi-probe scanning tunneling microscopy/potentiometry and low-energy electron microscopy allows the transport properties of individual grain boundaries to be correlated with their misorientation and atomic-level structure, without any device fabrication. We find that different types of grain boundary show dramatically different transport properties, and that boundaries can change structure and resistivity along their length. Boundary regions made up of dislocation superlattices separated by continuous graphene exhibit relatively low resistivity which is comparable to the resistivity of the graphene sheet itself. Other grain boundaries display trench structures with a resistivity 1–2 orders of magnitude greater and sufficient to dominate transport through the polycrystalline sheet. We also measure the transport properties of step edges and monolayer-bilayer boundaries on C-face graphene and compare them to Si-face graphene. Such measurements offer a guideline for optimizing graphene growth on SiC to improve its electronic properties.

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the p-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the p-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure.

The problem of separating single-wall carbon nanotubes (CNTs) by diameter and/or chirality is one of the greatest impediments toward the widespread application of these promising materials in nanoelectronics. In this paper, we describe a novel physical-chemical method for diameter-selective CNT separation that is both simple and effective and that allows up-scaling to large volumes at modest cost. Separation is based on size-selective noncovalent matching of an appropriate anchor molecule to the wall of the CNT, enabling suspension of the CNTs in solvents in which they would otherwise not be soluble. We demonstrate size-selective separation in the 1-2 nm diameter range using easily synthesized oligo-acene adducts as a diameter-selective molecular anchor. CNT field effect transistors fabricated from diameter-selected CNTs show markedly improved electrical properties as compared to nonselected CNTs.

Background: The risk of dengue transmitted by travellers is known. Methods to estimate the transmission by transfusion (TT) risk from blood donors travelling to risk areas are available, for instance, the European Up-Front Risk Assessment Tool (EUFRAT). This study aimed to validate the estimated risk from travelling donors obtained from EUFRAT. Methods: Surveillance data on notified dengue cases in Suriname and the Dutch Caribbean islands (Aruba, Curaçao, St. Maarten, Bonaire, St. Eustatius and Saba) in 2001-2011 was used to calculate local incidence rates. Information on travel and donation behaviour of Dutch donors was collected. With the EUFRAT model, the TT risks from Dutch travelling donors were calculated. Model estimates were compared with the number of infections in Dutch travellers found by laboratory tests in the Netherlands. Results: The expected cumulative number of donors becoming infected during travels to Suriname and the Dutch Caribbean from 2001 to 2011 was estimated at 5 (95% CI, 2-11) and 86 (45-179), respectively. The infection risk inferred from the laboratory-based study was 19 (9-61) and 28 (14-92). Given the independence of the data sources, these estimates are remarkably close. The model estimated that 0·02 (0·001-0·06) and 0·40 (0·01-1·4) recipients would have been infected by these travelling donors. Conclusions: The EUFRAT model provided an estimate close to actual observed number of dengue infections. The dengue TT risk among Dutch travelling donors can be estimated using basic transmission, travel and donation information. The TT risk from Dutch donors travelling to Suriname and the Dutch Caribbean is small.

The nucleation and growth of pentacene thin films are controlled largely by the energies associated with the interfaces. We have used low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) to investigate the nucleation and growth of pentacene thin films on Si(001) surfaces modified with two different molecular monolayers. Clean Si(001)-(2 × 1) surfaces were modified with either 1,5-cyclooctadiene or 1-dodecene prior to pentacene growth to study the effects of exposed π bonds at the interface, orientation of those π bonds relative to each other, and rigidity of the molecular layer on pentacene nucleation, growth, and crystalline orientation. Both molecular monolayers weaken the substrate−pentacene interaction sufficiently to allow for low pentacene nucleation density and good pentacene diffusion, leading to the growth of pentacene grains as large as 100 μm. Pentacene grows epitaxially on both functionalized surfaces, adopting an orthorhombic unit cell that follows the orientatio...

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., The electronic properties of a material depend on both the filled and the unoccupied electron states. Here, the authors present a technique based on low-energy electron microscopy that is able to directly probe the unoccupied bands of few-layer graphene, as well as other materials.

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.

Purpose The goal of this article was to review the key clinical trials that resulted in the recent recommendation from the Advisory Committee on Immunization Practices (ACIP) to vaccinate all adults aged ≥65 years with the 13-valent pneumococcal polysaccharide conjugate vaccine (PCV13) in addition to the previously recommended 23-valent pneumococcal polysaccharide vaccine (PPSV23). Methods Pertinent articles were identified through searches of EMBASE and MEDLINE by using the terms pneumococcal polysaccharide conjugate vaccine , pneumococcal vaccine , and PCV13 . Searches were limited to articles published between January 1, 2013, and January 31, 2015, and were limited to clinical trials. Resources from the Centers for Disease Control and Prevention's ACIP recommendations and cited references were also reviewed. Findings Recent clinical trials have focused on the order of administration of PPSV23 and PCV13, comparisons in immunogenicity of PPSV23 and PCV13, and efficacy of PCV13 in adults aged ≥65 years. Immunogenicity trials have shown that PCV13 elicits an equal or greater immune response than PPSV23 for most of the serotypes that both vaccines share. The evidence suggests that PCV13 should be administered before PPSV23 when possible. Most recently, clinical data demonstrated the efficacy of PCV13 in adults aged ≥65 years. Implications Recent randomized clinical trials and disease trends have prompted the ACIP to recommend that all adults aged ≥65 years receive a single dose of PCV13. This is in addition to the previous recommended single dose of PPSV23 in the same population. The ACIP and the Centers for Disease Control and Prevention plan to monitor disease trends and clinical data to determine if this recommendation will need to be changed in the future.

▪ Abstract The use of low-energy electron microscopy (LEEM) to study reversible surface phase transitions is reviewed. Representative experiments are described that highlight the key advantages of LEEM: the ability to image surfaces in situ, at elevated temperature, with good spatial and temporal resolution. With these capabilities, the evolution of individual surface features—domains, facets, islands, steps, etc.—can be measured. Real-time and real-space imaging make LEEM a powerful tool for characterizing the thermodynamics and kinetics that govern surface phase transformations.

We have used low-energy electron microscopy to investigate the thermal fluctuation of phase boundaries separating 7×7 and 1×1 regions of the Si(111) surface near the phase transition temperature of 1135 K. We find that the fluctuations are described by a phase boundary stiffness of 180±30 meV/nm. Analysis of the meandering of the phase boundary is used to characterize the elastic repulsion between phase boundaries. Our estimate of the surface stress difference between the phases, 0.06±0.01 eV/A2, is consistent with the measurement of Twesten and Gibson [R. D. Twesten and J. M. Gibson, Phys. Rev. B 50, 17628 (1994)].

The equilibrium thermal roughening of thin Ge layers (one and two monolayers) deposited on Si(001) has been investigated with low-energy electron microscopy. A Ge-coverage-dependent roughening is observed. For two monolayers, the temperature at which imaging contrast is lost due to surface roughness is $900\ifmmode\pm\else\textpm\fi{}25\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, between the roughening temperatures of Ge(001) and Si(001). Lower Ge coverages move this temperature closer to that of Si(001). The roughening is confined to the Ge overlayers. It is believed that this phenomenon represents a new type of surface roughening transition that should be generally applicable for heteroepitaxial films.

The rate of misfit strain relaxation in semiconductor heterostructures is controlled by the kinetics of misfit dislocation nucleation, propagation and interaction. Although there have been a number of detailed theoretical studies of the dislocation–dislocation interaction process in these systems, there exist limited experimental data regarding the regime of epilayer thickness and composition where these interactions affect strain relaxation. Because of its high spatial and temporal resolution, in-situ transmission electron microscopy is an ideal experimental tool with which to observe dislocation interactions. In this work, the unique capabilities of an ultrahigh vacuum transmission electron microscope have been exploited to permit direct, real-time observation of dislocation interactions. This microscope is equipped with in-situ chemical vapour deposition capabilities which allow imaging of dislocation motion during both growth and annealing of SiGe/Si (001) heterostructures. This has permitted...

▪ Abstract In situ Ultra-High Vacuum (UHV) electron microscopy, including Transmission Electron Microscopy (TEM) at 300 keV electron energy and Low-Energy Electron Microscopy (LEEM) at 0-30 eV electron energy, has advanced enormously over the last decade. Growth of thin films such as epitaxial Si1−xGex alloy thin films on Si substrates has become routine, allowing high-resolution video-rate studies of processes such as misfit dislocation injection and interaction, surface roughening and faceting, self-assembly of quantum dots, and shape transitions in such quantum dots. We review results obtained in the SiGe/Si system in the last five years. In addition we discuss new directions in in situ electron microscopy as they apply to thin film formation in a range of materials and environments. [ Erratum ]

For surface science, the 1980s were the decade in which the microscopes arrived. The scanning tunneling microscope (STM) was invented in 1982. Ultrahigh vacuum transmission electron microscopy (UHVTEM) played a key role in resolving the structure of the elusive Si(111)-7 × 7 surface. Scanning electron microscopy (SEM) as well as reflection electron microscopy (REM) were applied to the study of growth and islanding. And low-energy electron microscopy (LEEM), invented some 20 years earlier, made its appearance with the work of Telieps and Bauer.LEEM and TEM have many things in common. Unlike STM and SEM, they are direct imaging techniques, using magnifying lenses. Both use an aperture to select a particular diffracted beam, which determines the nature of the contrast. If the direct beam is selected (no parallel momentum transfer), a bright field image is formed, and contrast arises primarily from differences in the scattering factor. A dark field image is formed with any other beam in the diffraction pattern, allowing contrast due to differences in symmetry. In LEEM, phase contrast is the third important mechanism by which surface and interface features such as atomic steps and dislocations may be imaged. One major difference between TEM and LEEM is the electron energy: 100 keV and above in TEM, 100 eV and below in LEEM. In LEEM, the imaging electrons are reflected from the sample surface, unlike TEM where the electrons zip right through the sample, encountering top surface, bulk, and bottom surface. STM and TEM are capable of ~2 Å resolution, while LEEM and SEM can observe surface features (including atomic steps) with -100 Å resolution.

Low energy electron microscopy (LEEM) and photo-emission electron microscopy (PEEM) traditionally use microchannel plates (MCPs), a phosphor screen and a CCD-camera to record images and diffraction patterns. In recent years, however, MCPs have become a limiting factor for these types of microscopy. Here, we report on a successful test series using a solid state hybrid pixel detector, Medipix 2, in LEEM and PEEM. Medipix 2 is a background-free detector with an infinite dynamic range, making it very promising for both real-space imaging and spectroscopy. We demonstrate a significant enhancement of both image contrast and resolution, as compared to MCPs. Since aging of the Medipix 2 detector is negligible for the electron energies used in LEEM/PEEM, we expect Medipix to become the detector of choice for a new generation of systems.

We have developed an electron lithography method, Hot Electron Emission Lithography (HEEL), which is capable of printing integrated circuits with an exposure time of only a few seconds. The basic design of the mask, manufactured by standard MOS technology, will be discussed. Patterns printed into e-beam resist by a 1:1 projection system show the applicability of the mask for lithography purposes. The minimum feature size projected so far is 160 nm in a system capable of 90 nm resolution. Further improvements in resolution to 50 nm are possible

Low energy electron microscopy (LEEM) has developed into one of the premier techniques for in situ studies of surface dynamical processes, such as epitaxial growth, phase transitions, chemisorption and strain relaxation phenomena. Over the last three years we have designed and constructed a new LEEM instrument, aimed at improved resolution, improved diffraction capabilities and greater ease of operation compared to present instruments.

One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologies and their temperature dependence can be explained on the basis of the thermodynamics of the surface and the role of surface diffusion and evaporation.One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologie...

We have used low energy electron microscopy to study the time dependence of atomic height islands and holes at the extrema of a fabricated biperiodic grating on Si(001). From the pairwise disappearance of these islands and holes as the grating thermally smooths we conclude that steps on Si(001) are remarkably permeable, i.e., adatoms can cross steps without becoming incorporated into them. Comparison of the time dependence of the island areas with model calculations shows that the adatom incorporation probability is actually much smaller than the crossing probability.

Two new optical imaging methods with submonolayer surface sensitivity are applied together to investigate pattern formation of adsorbate concentrations on surfaces during heterogeneously catalysed reactions. Ellipso-microscopy for surface imaging (EMSI) is based on an ellipsometric effect, reflection anisotropy microscopy (RAM) on the polarising properties of surface anisotropies. For the CO oxidation reaction on a Pt(110) surface features like front propagation, target patterns and spiral waves are observed, similar as in prior investigations by photoemission electron microscopy (PEEM). By applying EMSI and RAM simultaneously and imaging the same spot of the surface, supplementary information is gained on the properties of the reaction diffusion fronts. When observing a reaction front by EMSI and RAM at temperatures between 330 and 420 K the image of the front in RAM trails behind its image in EMSI. EMSI seems to display changes in the coverage of adsorbates while RAM relates to variations in surface structure.

Two surface-sensitive optical imaging methods, Ellipso-Microscopy for surface Imaging (EMSI) and Reflection Anisotropy Microscopy (RAM) are introduced. They allow imaging of pattern formation on surfaces, e.g., due to submonolayer coverages of adsorbates, at any arbitrary pressure.

This paper gives a brief review of low-energy electron microscopy (LEEM) as used for in situ studies of surface dynamical processes. The capabilities of LEEM are illustrated with two examples. One is a kinetic instability observed during growth of the first layer of CaF 2 on Si (111). The second concerns the nucleation of misfit dislocations during the growth of thicker, epitaxial CaF 2 films on Si (111), as the critical thickness is exceeded. Both examples highlight the importance of real time, in situ observations of surface dynamical processes.

In Part I we described a new design for an aberration-corrected Low Energy Electron Microscope (LEEM) and Photo Electron Emission Microscope (PEEM) equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control of the chromatic and spherical aberration coefficients Cc and C3, as well as the mirror focal length. In this Part II we discuss details of microscope operation, how the microscope is set up in a systematic fashion, and we present typical results.

Aberration-corrected microscopes with sub-atomic resolution will impact broad areas of science and technology. However, the experimentally observed lifetime of the corrected state is just a few minutes. Here we show that the corrected state is intrinsically unstable; the higher its quality, the more unstable it is. Analyzing the Contrast Transfer Function near optimum correction, we define an 'instability budget' which allows a rational trade-off between resolution and stability. Unless control systems are developed to overcome these challenges, intrinsic instability poses a fundamental limit to the resolution practically achievable in the electron microscope., Comment: accepted, Physical Review Letters

In this paper we discuss a topic that was close to Prof. Gertrude Rempfer s interests for many years. On this occasion of her 100th birthday, we remember and honor Gertrude for her many outstanding contributions, and for the inspiring example that she set. We derive theoretical expressions for the aberration coefficients of the uniform electrostatic field up to 5th order and compare these with raytracing calculations for the cathode lens used in Low Energy Electron Microscopy and Photo Electron Emission Microscopy experiments. These higher order aberration coefficients are of interest for aberration corrected experiments in which chromatic (Cc) and spherical (C3) aberrations of the microscope are set to zero. The theoretical predictions are in good agreement with the results of raytracing. Calculations of image resolution using the Contrast Transfer Function method show that sub-nanometer resolution is achievable in an aberration corrected LEEM system.

The high carrier mobility of graphene is key to its applications, and understanding the factors that limit mobility is essential for future devices. Yet, despite significant progress, mobilities in excess of the 2×10(5) cm(2) V(-1) s(-1) demonstrated in free-standing graphene films have not been duplicated in conventional graphene devices fabricated on substrates. Understanding the origins of this degradation is perhaps the main challenge facing graphene device research. Experiments that probe carrier scattering in devices are often indirect, relying on the predictions of a specific model for scattering, such as random charged impurities in the substrate. Here, we describe model-independent, atomic-scale transport measurements that show that scattering at two key defects--surface steps and changes in layer thickness--seriously degrades transport in epitaxial graphene films on SiC. These measurements demonstrate the strong impact of atomic-scale substrate features on graphene performance.

We have determined the growth mode of graphene on SiC(0001) and $\mathrm{SiC}(000\overline{1})$ using ultrathin, isotopically labeled $\mathrm{Si}^{13}\mathrm{C}$ ``marker layers'' grown epitaxially on the $\mathrm{Si}^{12}\mathrm{C}$ surfaces. Few-layer graphene overlayers were formed via thermal decomposition at elevated temperature. For both surface terminations (Si face and C face), we find that the $^{13}\mathrm{C}$ is located mainly in the outermost graphene layers, indicating that, during decomposition, new graphene layers form underneath existing ones.

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 cm(2) V(-1) s(-1) at room temperature.

Depth profiling of graphene with high-resolution ion beam analysis is a practical method for analysis of monolayer thicknesses of graphene. Not only is the energy resolution sufficient to resolve graphene from underlying SiC, but by use of isotope labeling it is possible to tag graphene generated from reacted ethylene. Furthermore, we are able to analyze graphene supported by oxidized Si(100) substrates, allowing the study of graphene films grown by chemical vapor deposition on metal and transfered to silicon. This introduces a powerful method to explore the fundamentals of graphene formation.

In this Letter we show that a coherent low energy electron beam (100 eV) can be used to obtain real space images of structures and defects buried deep below the surface of the sample. The elastic strain fields of such buried structures, extending to the free surface, are found to give rise to localized phase shifts in the reflected electron waves, resulting in excellent image contrast under slight objective lens defocus conditions. We can now image the formation and evolution of buried interfaces and defects in situ, and in real time. Because of the very low electron energies used, this imaging method is nondestructive.

Two different structural models have been recently proposed to explain the ordering observed in Si x Ge 1-x alloys grown at low temperatures on Si(001). We show that, through dark field imaging of the different domains of the ordered phase, it is possible to differentiate between the two structures unequivocally. In this way, we determine that the ordered phase corresponds to ordering along a single set of {111} planes, where Ge-rich double layers alternate with Si-rich double layers. We also show that this conclusion is consistent with all other experimental data reported so far

Graphene, a two-dimensional carbon form with the highest intrinsic carrier mobility and many desirable physical properties at room temperature, is considered a promising material for ultrahigh speed and low power devices with the possibility of strong scaling potential due to the ultra-thin body. (Fig. 1) [1–3] Here IBM reports progress in graphene nanoelectronics, synthesizing wafer-scale monolayer-controlled graphene and fabricating high-speed single atomic layer graphene FETs (GFET) with the highest value reported cut-off frequency (f T ) 50 GHz, exceeding that of the same gate length Si FETs. It is achieved by improving gate oxide deposition and reducing series resistance. Systematic characterization and small-signal models enable further engineering and optimization for even higher performance. The high I on /I off ratios from bi-layer graphene suggest potential not only for analog but also for logic applications.

When epitaxial graphene layers are formed on SiC(0001), the first carbon layer (known as the ``buffer layer''), while relatively easy to synthesize, does not have the desirable electrical properties of graphene. The conductivity is poor due to a disruption of the graphene $\ensuremath{\pi}$ bands by covalent bonding to the SiC substrate. Here we show that it is possible to restore the graphene $\ensuremath{\pi}$ bands by inserting a thin oxide layer between the buffer layer and SiC substrate using a low temperature, complementary metal-oxide semiconductor-compatible process that does not damage the graphene layer.

We have determined the atomic structure of the Si(111)-(\ensuremath{\surd}7\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}7)Co surface using scanning tunneling microscopy and ion scattering. The unit cell contains one surface substitutional cobalt atom centered under a six silicon adatom cluster. High-temperature annealing produces a low density lattice gas of these ringlike clusters, giving rise to an ``impurity stabilized 1\ifmmode\times\else\texttimes\fi{}1'' structure. They occur for several other transition metals, suggesting a stable, universal, silicide related structure.

The application of electron microscopy, scanning tunneling microscopy, and medium-energy ion scattering to microelectronics is reviewed. These analysis techniques are playing an important role in advancing the technology. Their use in the study of relevant phenomena regarding surfaces, interfaces, and defects is discussed. Recent developments and applications are illustrated using results obtained at the IBM Thomas J. Watson Research Center. Potential advances in the techniques are also discussed.

Although ultraviolet photoelectron emission spectroscopy and microscopy have served as powerful tools for electronic structure analysis in both reciprocal and real space, the integration of spectroscopy into microscopy still remains a challenge due to the technical difficulty of achieving both high spatial and energy resolution with slow electrons. We demonstrate photoelectron emission spectroscopy and microscopy covering the real, reciprocal, and energy spaces efficiently, enabled by a recently developed energy-filtered microscope. Observation of photoemission from micron-sized silver islands using $\text{He}\text{ }\text{I}$ radiation reveals the importance of the unreduced final-state wave vectors, which determine the photoelectron angular distribution.

We have studied the formation of graphene on the Si face of SiC(0001)-6H and -4H using in situ electron microscopy. By imaging the nucleation and growth of the $6\sqrt{3}$ ``buffer layer'' during annealing in vacuum we identify key factors responsible for the appearance of deep pits during graphene formation. Pits form because domains of the buffer layer pin decomposing surface steps. Graphene is observed to nucleate in the pits, where the step density is high.

We present low energy electron microscope (LEEM) spectromicroscopy studies of surface plasmons, localized on micro- and nanoscale epitaxial Ag islands. Excellent agreement is found in a direct comparison of wave vector dependent plasmon intensity with theory, demonstrating that high quality quantitative data can be obtained with a large improvement in both spatial and temporal resolution over traditional electron scattering experiments. The plasmon signal from Ag islands is successfully imaged with a spatial resolution of less than 35 nm. LEEM based plasmon spectromicroscopy promises to be a powerful tool for furthering our understanding of nanoplasmonics.