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.     

低能电子显微术及其应用

低能电子显微术是新发展起来的一种显微探测技术。它的特点是利用低能（１－３０ｅＶ）电子的弹性背散射使表面实空间实时成像，具有高的横向（１５ｎｍ）和纵向（原子级）分辩率，且易与低能电子衍射及其他电子显微术相结合。近年来它已有效地应用于金属和半导体表面的形貌观测、表面相变、吸附、反应及生长过程的研究。     

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.     

Phase retrieval low energy electron microscopy

We consider the utility of phase-retrieval methods in low energy electron microscopy (LEEM). Computer simulations are presented, demonstrating recovery of the terraced height profile of atomic steps. This recovery uses phase retrieval to decode a single LEEM image, incorporating the effects of defocus, spherical aberration and chromatic aberration. The ability of the method, to obtain temporal sequences of evolving step profiles from a single LEEM movie, is discussed.     

Challenges and strategies of surface modification of electrogalvanized coatings for electron microscopy analysis

Despite wide usage of electrogalvanized coatings in various applications, characterization studies on their micro/crystal structure, and the understanding of how they correspondingly affect the properties, such as corrosion, are rather limited. This is mainly attributed to some difficulties in preparing and examining the zinc coating layers, owing to their intrinsically low corrosion resistance and refined nano-scaled crystallite size. This study aims to examine such challenges systematically and propose some mitigation strategies. Particularly, sample preparation processes, including surface finishing for metallography and sample thinning processes are explored. Furthermore, a range of electron microscopy techniques, including scanning electron microscopy (SEM), electron back scattered diffractometry (EBSD), and transmission electron microscopy (TEM) are investigated in relation to the achievable clarity of microstructural details of electrogalvanized coatings.     

In situ scanning electron microscopy of graphene growth on polycrystalline Ni substrate

Scanning electron microscopy (SEM) is shown to be capable of imaging a monolayer of graphene, and is employed to observe in situ the graphene growth process by segregation of bulk-dissolved carbon on a polycrystalline nickel surface. Because of a wide field of view, SEM could easily track the rapid graphene growth induced by carbon segregation. Monolayer graphene extended on (111)- and (011)-oriented nickel grains, but was excluded from the (001) grains. This is due to the difference in carbon-nickel binding energy among these crystalline faces. This work proves the usefulness of in situ SEM imaging for the investigation of large area graphene growth.     

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.     

Surface imaging with LEEM

In low-energy electron microscopy (LEEM) surfaces are imaged with LEED electrons. The physical and electron-optical principles of the method are described as well as a prototype LEEM microscope. Micrographs of Mo, Si, and Au surfaces illustrate the various types of contrast obtained with LEEM and its application.     

Recent advances in imaging of properties and growth of low dimensional structures for photonics and electronics by XPEEM

Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) is a very powerful and diverse microscopy technique for the investigation of surfaces, interfaces, buried layers and nanoscale objects like nanoparticles and nanowires. The many significant results from photoemission Electron microscopy (PEEM) in recent years are linked with the exploitation of advanced light sources such as synchrotrons and new advanced laser systems. Combined also with low energy electron microscopy (LEEM) it allows a complementary chemical and structural analysis making LEEM/PEEM a versatile multitechnique instrument. To illustrate the extreme diversity, we give a summary of the recent studies with the SPELEEM installed at the soft X-ray beamline I311 at the MAXII synchrotron storage ring and a portable electrostatic PEEM used with ultra-fast XUV laser technology. The examples cover topics such as full-cone 3D band mapping by using the photoelectron diffraction mode of the microscope, growth mechanism and detailed doping profile of III–V nanowires, growth and intercalation of graphene on SiC substrates, droplet dynamics on GaP(1 1 1) surface, surface chemistry and control of nanostructure fabrication. Moreover, the first results of PEEM experiments using extreme ultraviolet attosecond pulse trains are discussed.     

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.     

Buffer layer free large area bi-layer graphene on SiC(0 0 0 1)

The influence of hydrogen exposures on monolayer graphene grown on the silicon terminated SiC(0 0 0 1) surface is investigated using photoelectron spectroscopy (PES), low-energy electron microscopy (LEEM) and micro low-energy electron diffraction (μ-LEED). Exposures to ionized hydrogen are shown to have a pronounced effect on the carbon buffer (interface) layer. Exposures to atomic hydrogen are shown to actually convert/transform the monolayer graphene plus carbon buffer layer to bi-layer graphene, i.e. to produce carbon buffer layer free bi-layer graphene on SiC(0 0 0 1). This process is shown to be reversible, so the initial monolayer graphene plus carbon buffer layer situation is recreated after heating to a temperature of about 950 °C. A tentative model of hydrogen intercalation is suggested to explain this single to bi-layer graphene transformation mechanism. Our findings are of relevance and importance for various potential applications based on graphene-SiC structures and hydrogen storage.     

X-ray and electron microscopy of actinide materials

Actinide materials demonstrate a wide variety of interesting physical properties in both bulk and nanoscale form. To better understand these materials, a broad array of microscopy techniques have been employed, including transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDXS), high-angle annular dark-field imaging (HAADF), scanning electron microscopy (SEM), wavelength dispersive X-ray spectroscopy (WDXS), electron back scattered diffraction (EBSD), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and scanning transmission X-ray microscopy (STXM). Here these techniques will be reviewed, highlighting advances made in the physics, materials science, chemistry, and biology of actinide materials through microscopy. Construction of a spin-polarized TEM will be discussed, considering its potential for examining the nanoscale magnetic structure of actinides as well as broader materials and devices, such as those for computational magnetic memory.     

Probing bacterial interactions: integrated approaches combining atomic force microscopy, electron microscopy and biophysical techniques

Recent developments in the application of Atomic Force Microscopy (AFM) and other biophysical techniques for the study of bacterial interactions and adhesion are discussed in the light of established biological and microscopic approaches. Whereas molecular-biological techniques combined with electron microscopy allow the identification and localization of surface constituents mediating bacterial interactions, with AFM it has become possible to actually measure the forces involved in bacterial interactions. Combined with the flexibility of AFM in probing various types of physical interactions, such as electrostatic interactions, specific ligand-receptor interactions and the elastic forces of deformation and extension of bacterial surface polymers and cell wall, this provides prospects for the elucidation of the biophysical mechanism of bacterial interaction. However, because of the biochemical and a biophysical complexity of the bacterial cell wall, integrated approaches combining AFM with electron microscopy and biophysical techniques are needed to elucidate the mechanism by which a bacterium interacts with a host or material surface. The literature on electron microscopy of the bacterial cell wall is reviewed, with particular emphasis on the staining of specific classes of cell-wall constituents. The application of AFM in the analysis of bacterial surfaces is discussed, including AFM operating modes, sample preparation methods and results obtained on various strains. For various bacterial strains, the integration of EM and AFM data is discussed. Various biophysical aspects of the analysis of bacterial surface structure and interactions are discussed, including the theory of colloidal interactions and Bell's theory of cell-to-cell adhesion. An overview is given of biophysical techniques used in the analysis of the properties of bacterial surfaces and bacterial surface constituents and their integration with AFM. Finally, we discuss recent progress in the understanding of the role of bacterial interactions in medicine within the framework of the techniques and concepts discussed in the paper.     

The application of graphene as a sample support in transmission electron microscopy

Transmission electron microscopy has witnessed rampant development and surging point resolution over the past few years. The improved imaging performance of modern electron microscopes shifts the bottleneck for image contrast and resolution to sample preparation. Hence, it is increasingly being realized that the full potential of electron microscopy will only be realized with the optimization of current sample preparation techniques. Perhaps the most recognized issues are background signal and noise contributed by sample supports, sample charging and instability. Graphene provides supports of single atom thickness, extreme physical stability, periodic structure, and ballistic electrical conductivity. As an increasing number of applications adapting graphene to their benefit emerge, we discuss the unique capabilities afforded by the use of graphene as a sample support for electron microscopy.     

High-resolution electron microscopy for heterogeneous catalysis research

Heterogeneous catalysts are the most important catalysts in industrial reactions. Nanocatalysts, with size ranging from hundreds of nanometers to the atomic scale, possess activities that are closely connected to their structural characteristics such as particle size, surface morphology, and three-dimensional topography. Recently, the development of advanced analytical transmission electron microscopy(TEM) techniques, especially quantitative high-angle annular darkfield(HAADF) imaging and high-energy resolution spectroscopy analysis in scanning transmission electron microscopy(STEM) at the atomic scale, strengthens the power of(S)TEM in analyzing the structural/chemical information of heterogeneous catalysts. Three-dimensional reconstruction from two-dimensional projected images and the real-time recording of structural evolution during catalytic reactions using in-situ(S)TEM methods further broaden the scope of(S)TEM observation. The atomic-scale structural information obtained from high-resolution(S)TEM has proven to be of significance for better understanding and designing of new catalysts with enhanced performance.     

Angle-resolved X-ray photoemission electron microscopy

Synchrotron based photoemission electron microscopy with energy filter combines real space imaging with microprobe diffraction (μ-ARPES), giving access to the local electronic structure of laterally inhomogeneous materials. We present here an overview of the capabilities of this technique, illustrating selected applications of angle resolved photoemission electron microscopy and related microprobe methods. In addition, we report the demonstration of a darkfield XPEEM (df-XPEEM) imaging method for real space mapping of the electronic structure away from Γ at a lateral resolution of few tens of nm. The application of df-XPEEM to the (1 × 12)-O/W(1 1 0) model oxide structure shows the high sensitivity of this technique to the local electronic structure, allowing to image domains with inequivalent adsorption site symmetry. Perspectives of angle-resolved PEEM are discussed.     

Visualization of electron orbitals in scanning tunneling microscopy

Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated.     

Lorentz transmission electron microscopy studies on topological magnetic domains

Lorentz transmission electron microscopy(TEM) is a powerful tool to study the crystal structures and magnetic domain structures in correlation with novel physical properties. Nanometric topological magnetic configurations such as vortices, bubbles, and skyrmions have received enormous attention from the viewpoint of both fundamental science and potential applications in magnetic logic and memory devices, in which understanding the physical properties of magnetic nanodomains is essential. In this review article, several magnetic imaging methods in Lorentz TEM including the Fresnel and Foucault modes, electron holography, and differential phase contrast(DPC) techniques are discussed, where the novel properties of topological magnetic domains are well addressed. In addition, in situ Lorentz TEM demonstrates that the topological domains can be efficiently manipulated by electric currents, magnetic fields, and temperatures, exhibiting novel phenomena under external fields, which advances the development of topological nanodomain-based spintronics.     

Energy-resolved analysis of ferroelectric electron emission from TGS using emission electron microscopy

Ferroelectric electron emission arises when the spontaneous polarization of a ferroelectric is switched due to the application of an electric field. In order to study the origin of emission and the related emission mechanism, space-resolved emission electron microscopy has been employed. The integral energy distribution of the emitted electrons from triglycine-sulfate surfaces has been investigated using a cylindrical sector analyzer and an imaging retarding field analyzer. Space-resolved emission photography and energy distribution measurements were obtained, revealing the effect of ferroelectric switching on the electric field distribution and hence on the emission process. Evidence of secondary electron emission from the metal electrodes has been found.     

Thermally activated stripe reconstruction induced by O on Nb (011)

We report scanning tunneling microscopy and low energy electron microscopy (LEEM) observations for thin films of Nb (011) of stripe-phase behavior by two variants of an O-induced reconstruction. Stripes occur for thin films but not bulk crystals. At low temperatures the less-favored variant is thermally activated as single stripes on surface heterogeneities. Near T0 = 1505 K, where the reconstruction is lifted, the stripes crowd to form a periodic array with a temperature dependent spacing. LEEM permits quantitative insight into stripe behavior and reveals novel details of stripes interacting with topographic features such as steps, facets, and dislocations.