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 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.     

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.     

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.     

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.     

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.     

Real-space study of the growth of magnesium on ruthenium

The growth of magnesium on ruthenium has been studied by low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). In LEEM, a layer-by-layer growth is observed except in the first monolayer, where the completion of the first layer in inferred by a clear peak in electron reflectivity. Desorption from the films is readily observable at 400 K. Real-space STM and low-energy electron diffraction confirm that sub-monolayer coverage presents a moiré pattern with a 12 Å periodicity, which evolves with further Mg deposition by compressing the Mg layer to a 22 Å periodicity. Layer-by-layer growth is followed in LEEM up to 10 ML. On films several ML thick a substantial density of stacking faults are observed by dark-field imaging on large terraces of the substrate, while screw dislocations appear in the stepped areas. The latter are suggested to result from the mismatch in heights of the Mg and Ru steps. Quantum size effect oscillations in the reflected LEEM intensity are observed as a function of thickness, indicating an abrupt Mg/Ru interface.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.     

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.     

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.     

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.     

Quantum size effect in low energy electron diffraction of thin films

Low energy electron microscopy (LEEM) is used to study the quantum size effect (QSE) in electron reflectivity from thin films. Strong QSE interference peaks are seen below 20 eV for Cu and Ag films on the W(1 1 0) surface and Sb films on the Mo(0 0 1) surface. Simple inspection of QSE interference peaks reveals that all three metals grow atomic layer-by-atomic layer. Layer-specific I(V) spectra obtained with LEEM permit structural analysis by full dynamical multiple scattering LEED calculations for a layer-by-layer view of thin film structure.     

W(110)面二维氧化的低能电子显微镜研究

应用低能电子显微术对W（110）面二维氧化结构进行了初步研究．随着氧暴露量的增加，低能电子衍射图样由清洁表面（1×1）结构转变成p（2×1），再变为带有复杂衍射卫星点的p（2×2）结构．利用低能电子显微术的暗场像模式，对（00）束附近的分数衍射斑点进行了放大成像，发现表面由两种对比度相差很大的区域组成，它们就是具有不同方位取向的氧超结构畴区．两种畴区的分布与衬底表面缺陷特别是表面台阶有一定的关系，而且温度对这种钨表面的二维氧化起着重要作用 关键词：     

Enhancement of SEM to scanning LEEM

By introducing a cathode lens below or inside the objective lens of a scanning electron microscope, many experiments similar to those typical of the LEEM method can be performed. The conditions for the diffraction of slow electrons are modified by the convergence of the primary beam, and challenges include the necessity of managing the signal species propagating along the optical axis in a direction opposite to that of the primary beam. However, even a simple implementation, providing the integral dark-field signal only, has not only delivered plenty of results in the very low energy range below 50 eV, but the performance in the range of hundreds of eV and units of keV has also been substantially improved. The scanning LEEM method is illustrated using experimental results acquired by additionally employing multichannel detection and detection of transmitted electrons.     

The Si(111)−(5 × 1) Au structure

By combining recent results from STM, LEEM, LEED and X-ray diffraction a structure model is developed for the (5 × 1) structure observed in the Au/Si(111) system at low coverages.     

Epitaxial structures of self-organized, standing-up pentacene thin films studied by LEEM and STM

Growth of pentacene (Pn) thin films has been studied in situ by means of low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). A very low nucleation density of Pn grains has been observed on Bi(0 0 0 1)/Si(1 1 1) template, resulting in formation of large, monolayer-high Pn grains with diameter exceeding several hundreds of micrometers. We determined that formation of self-organized, standing-up Pn epitaxial layers was stabilized by a weak interaction between the substrate and Pn molecules and by the presence of the commensurate structure between the oblique Pn lattice and trigonal substrate surface lattice. The ‘point-on-line’ commensurability has been found along a-axis of Pn and one of the primitive vectors of substrate surface lattice. Strong ‘point-on-line’ commensurability in Pn/Bi(0 0 0 1)/Si(1 1 1) system resulted in a bulk-like epitaxial thin film growth, starting from the first layer. The presence of twins, often having a mirror line parallel to the direction of the ‘point-on-line’ matching, has been also detected using an asymmetric dark-field imaging mode in LEEM experiments, which, we believe, is the first LEEM demonstration of molecular tilt imaging.     

III–V on silicon: Observation of gallium phosphide anti-phase disorder by low-energy electron microscopy

The formation of anti-phase disorder is a major obstacle in the heteroepitaxy of III–V semiconductors on silicon. For an investigation of the anti-phase domain (APD) structure of GaP/Si(100) samples on mesoscopic length scales, we applied dark-field imaging in a low-energy electron microscope (LEEM) to thin GaP films grown on Si(100) substrates by metal organic vapor phase epitaxy (MOVPE). A contamination-free transfer of the samples from the MOVPE ambient to the ultra-high vacuum chamber of the microscope ensured that the atomically well-ordered, P-rich (2 × 2)/c(4 × 2) reconstruction of the surface was preserved. Mutually perpendicular oriented domains of the characteristic GaP(100) reconstruction identify the APDs in the GaP film at the surface and enabled us to achieve high contrast LEEM images. Striped patterns of APDs reflect the regular terrasse structure of the two-domain Si(100)(2 × 1) substrate far away from defects. APDs in the proximity of the defects have larger lateral extensions and are arranged in target pattern-like structures around the defects. In contrast to transmission electron microscopy, which was also applied in a specific dark-field mode for comparison, the characterization of anti-phase disorder by LEEM is non-destructive, does not require elaborate sample preparation, and addresses extended length scales.     

A brief history of PEEM

The evolution of cathode lens-based photo emission electron microscopy (PEEM) from the simple beginnings in the early 1930s to its sophisticated present state is discussed. In addition to conventional ultraviolet light-excited PEEM (UV-PEEM), laser excited PEEM and the various modes of synchrotron X-ray-excited PEEM (XPEEM) particular emphasis is placed on the complementary combination of these methods with low energy electron microscopy (LEEM).     

Spontaneous domain switching during phase separation of Pb on Ge(111)

Using low-energy electron microscopy (LEEM), we have discovered a novel phase separation mechanism for Pb on Ge(111). When the low Pb coverage (1 x 1) phase coexists with the high coverage beta phase, the surface consists of approximately 100 nm sized domains that spontaneously switch from one phase to the other. We argue this striking mechanism occurs because nanometer-scale domains can have density fluctuations comparable to the density difference between the two phases.