Electron optics for high throughput low energy electron microscopy

Novel electron-optical components and concepts aiming at improving the throughput and extending the applications of a low energy electron microscope (LEEM) have been developed. An immersion magnetic objective lens can substantially reduce e-e interactions and the associated blur, as electrons do not form a sharp crossover in the back-focal plane. The resulting limited field of view of the immersion objective lens in mirror mode can be eliminated by immersing the cathode of the electron gun in a magnetic field. A dual illumination beam approach is used to mitigate the charging effects when the LEEM is used to image insulating surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of the secondary beam with an electron yield exceeding 1. On substrates illuminated with a tilted beam near glancing incidence, large shadows are formed on even the smallest topographic features, easing their detection. On magnetic substrates, the magnetic flux leaking above the surface can be detected with tilted illumination and used to image domain walls with high contrast.     

Surface plasmon microscopy using an energy-filtered low energy electron microscope

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.     

Phase retrieval in low-coherence interferometric microscopy

We present compensating methods that address inherent errors in quantitative phase reporting for low-coherence interferometric techniques. A brief theoretical treatment of the problem and experimental validation using spectral domain phase microscopy demonstrate mitigation of the degrading effects of phase leakage on accurate measurement of optical path length in the vicinity of closely spaced reflectors. This result has direct implications for phase-sensitive interferometry techniques, such as Doppler imaging, as well as amplitude-based quantitative reporting. Corrected phase retrieval is demonstrated for conversion of interferometric phase to optical path length in cell surface deflections of beating cardiomyocytes.     

Low energy electron microscopy and photoemission electron microscopy investigation of graphene

Low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) are two powerful techniques for the investigation of surfaces, thin films and surface supported nanostructures. In this review, we examine the contributions of these microscopy techniques to our understanding of graphene in recent years. These contributions have been made in studies of graphene on various metal and SiC surfaces and free-standing graphene. We discuss how the real-time imaging capability of LEEM facilitates a deeper understanding of the mechanisms of dynamic processes, such as growth and intercalation. Numerous examples also demonstrate how imaging and the various available complementary measurement capabilities, such as selected area or micro low energy electron diffraction (μLEED) and micro angle resolved photoelectron spectroscopy (μARPES), allow the investigation of local properties in spatially inhomogeneous graphene samples.     

解说低能量/光电子显微镜（LEEM/PEEM）

文章介绍近年来倍受关注的低能量/光电子显微镜(LEEM/PEEM)的基本原理和应用.LEEM/PEEM拥有成像、光电子能谱和衍射功能,可对样品进行综合全面的分析.通过一系列的应用实例,特别是和同步辐射软X射线结合的成果,展示该实验手段在表面科学和纳米技术方面的应用.     

Low energy electron microscopy of surfaces

In low energy electron microscopy (LEEM) surfaces are imaged with LEED electrons. Either the (00) beam (bright field mode) or one of the other diffracted beams (dark field mode) can be used for producing a true (non scanning) image of the surface. One can also obtain the LEED pattern of the illuminated area (typically 5–10 μm diameter) on the final screen.     

Low energy electron point source microscopy

The setup of the LEEPS (low energy electron point source) microscope is described and images of carbon fibres and nanotubes are shown and compared to simulated images. We used a Kirchhoff-Helmholtz type transform to reconstruct the (wave front at the) object. We also showed that this transform can be used to reconstruct optical in-line holograms.     

The EIGER detector for low‐energy electron microscopy and photoemission electron microscopy

EIGER is a single‐photon‐counting hybrid pixel detector developed at the Paul Scherrer Institut, Switzerland. It is designed for applications at synchrotron light sources with photon energies above 5 keV. Features of EIGER include a small pixel size (75 µm × 75 µm), a high frame rate (up to 23 kHz), a small dead‐time between frames (down to 3 µs) and a dynamic range up to 32‐bit. In this article, the use of EIGER as a detector for electrons in low‐energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) is reported. It is demonstrated that, with only a minimal modification to the sensitive part of the detector, EIGER is able to detect electrons emitted or reflected by the sample and accelerated to 8–20 keV. The imaging capabilities are shown to be superior to the standard microchannel plate detector for these types of applications. This is due to the much higher signal‐to‐noise ratio, better homogeneity and improved dynamic range. In addition, the operation of the EIGER detector is not affected by radiation damage from electrons in the present energy range and guarantees more stable performance over time. To benchmark the detector capabilities, LEEM experiments are performed on selected surfaces and the magnetic and electronic properties of individual iron nanoparticles with sizes ranging from 8 to 22 nm are detected using the PEEM endstation at the Surface/Interface Microscopy (SIM) beamline of the Swiss Light Source.     

Probing of energy gap distribution in superconducting tunnel junctions by low temperature scanning electron microscopy

A two-dimensional voltage image of the energy gap distribution of a superconducting tunnel junction was obtained by scanning the current biased junction with an electron beam and detecting the voltage change δV. The value of the energy gap at the point of irradiation was determined quantitatively from the δV σ(V) curves, where σ(V) is the electric conductance of the junction. Further the quasiparticle diffusion length was found by measuring the length of the transition between a high- and low-gap region generated by a double tunnel junction configuration. The theoretical predictions could be verified by investigating a double tunnel junction configuration, where the energy gap could be changed deliberately by quasiparticle injection.     

Missing spots in low energy electron diffraction

Recent attempts to justify the use, after appropriate data averaging, of kinematical theory in surface structure determination by LEED, make it important to be able to distinguish truly kinematical features from those which can also be predicted from entirely general symmetry arguments. Such arguments are therefore developed formally into a set of rules determining the consequences of symmetry for LEED patterns. In particular the conditions for missing spots due to the presence of glide planes are given. It is pointed out that in some circumstances the isotropic scatterer model leads to spurious selection rules, that will not be satisfied in general.     

Phase retrieval

The problem of phase retrieval arises in experimental uses of diffraction to determine intrinsic structure because the modulus of a Fourier transform is all that can usually be measured after diffraction occurs. For finite distributions, the phase retrieval problem can be solved by methods of factorization in suitable rings of polynomials; for continuous distributions with compact support, the methods of complex analysis are needed to solve the phase retrieval problem. These methods are discussed and examples are given for illustration.Partially supported by NSF Grant MCS 8218800     

pH-dependent effect of heat-induced antigen retrieval of epoxy section for electron microscopy

The aim was to examine how the pH in the antigen retrieval medium (citrate) affects the yield of immunogold labeling of epoxy sections. Renal swine tissue with glomerular immune complex deposits with reactivity against IgG was embedded in epoxy resin. Prior to immunogold labeling with anti-IgG, ultrathin sections from these blocks were exposed to antigen retrieval by heating in citrate solution (pH 6, 9 or 12) at 95 degrees C in a PCR-machine or at 121 or 135 degrees C min in an autoclave. The level of immunogold labeling was significantly higher for pH 12 than for pH 6 when heated at 95 degrees C (50% more intense), but at the cost of the ultrastructural preservation of the tissue. At pH 12 and temperature 135 degrees C the epoxy sections were completely destroyed. The sections which had been heated at 135 degrees C, pH 6 appeared significantly better both with respect to intensity of immunogold labeling (85% more intense) and to ultrastructural preservation than those which were heated at 95 degrees C, pH 12. Therefore, our results indicate that relatively low pH (pH 6) and high temperature is the method of choice, but low temperature and high pH can be used when an autoclave is not available.     

Atomic-resolution electron energy loss spectroscopy imaging in aberration corrected scanning transmission electron microscopy

The "delocalization" of inelastic scattering is an important issue for the ultimate spatial resolution of innershell spectroscopy in the electron microscope. It is demonstrated in a nonlocal model for electron energy loss spectroscopy (EELS) that delocalization of scanning transmission electron microscopy (STEM) images for single, isolated atoms is primarily determined by the width of the probe, even for light atoms. We present experimental data and theoretical simulations for Ti L-shell EELS in a [100] SrTiO3 crystal showing that, in this case, delocalization is not significantly increased by dynamical propagation. Issues relating to the use of aberration correctors in the STEM geometry are discussed.     

Elementary pinning forces measured using low temperature scanning electron microscopy

Magnetic flux penetrating Josephson tunnel junctions parallel and transverse to the tunnel barrier has been imaged using low temperature scanning electron microscopy. Employing this technique we have studied the behavior of magnetic flux quanta under the influence of external forces. The strength of pinning centers acting on vortices existing within the barrier has been determined by measuring the turning angle of these flux quanta in rotating magnetic fields. Trapped transverse flux quanta could be depinned by applying a sufficient Lorentz force or electron beam irradiation. The strength of the pinning centers has been determined.     

Characterization of superconducting YBaCuO-films by low temperature scanning electron microscopy

Low-temperature scanning electron microscopy has been performed for imaging the spatial distribution of the critical current densityj _c(x,y) and of the critical temperatureT _c(x,y) in polycrystalline superconducting YBaCuO films. Strongly inhomogeneous behavior has been observed, and the spatial resolution limit has been found to be 1–2 m. The local temperature increment in the specimen film caused by the electron beam scanning has been demonstrated experimentally as the underlying mechanism of the imaging principle, and the beam-induced thermal perturbation of the high-T _c film/substrate configuration is discussed in detail. The radiation hardness of the sample films against the electron beam irradiation in our imaging experiments has been evaluated. No radiation damage could be detected up to the maximum applied dose of well above 10²⁰ electrons/cm² for a typical beam energy of 26 keV.