Cryogen-free low-temperature photoemission electron microscopy for high-resolution nondestructive imaging of electronic phases.

An investigation of the stacking fault tetrahedra in ion-implanted and thermally annealed silicon, observed by means of high resolution electron microscopy and computer simulation, shows that an unambiguous distinction can be made between vacancy- and interstitial-type stacking fault tetrahedra by examining the sense of shift of the rows of bright dots in the high resolution images. In this way the observed defects are identified as vacancy-type stacking fault tetrahedra. The methods for estimating the stacking fault energy from the size distribution of the defects are commented on and applied to the present observation.

Due to their small size and complex structure, diagnosing injury of the proximal wrist ligamentous structures can be challenging. The triangular fibrocartilage complex (TFCC) is an example of one such structure, for which lesions may be missed unless high-resolution magnetic resonance imaging (MRI) obtained via a standard matrix with a small field of view or high-resolution imaging matrix (small spatial scale matrix elements/large matrix size) is utilized. While there have been recent advances in increasing MRI spatial resolution, attempts at improved visualization by isolated increase in the spatial resolution will be ineffective if the signal-to-noise ratio (SNR) of the images obtained is low. Additionally, high contrast resolution is important to facilitate a more precise visualization of these structures and their pathology. Thus, a balance of the three important imaging factor qualifications of high spatial resolution, high SNR, and high contrast resolution must be struck for optimized TFCC and wrist imaging. The goal of this article, then, is to elucidate the theory and techniques of effective high-resolution imaging of the proximal ligamentous structures of the wrist, balancing SNR and high contrast resolution constraints, and focusing on imaging of the TFCC as a prototypical example.

The potential of time‐resolved photoemission electron microscopy (PEEM) for imaging ultrafast processes and for aberration correction in full‐field imaging is discussed. In particular, we focus on stroboscopic imaging of precessional magnetic excitations via XMCD‐PEEM exploiting the time structure of synchrotron radiation (magnetic field pulse pump–X‐ray probe). In a special bunch‐compression mode at BESSY, a time resolution of about 15 ps has been obtained. Further, we discuss an all‐optical pump–probe technique using femtosecond laser excitation. A highly promising alternative to stroboscopic imaging is an approach using time‐resolved image detection. As a second application of time‐resolved PEEM we discuss potential ways of aberration correction. These approaches go back to the old ideas of Scherzer in the light of state‐of‐the‐art equipment. The excellent time structure of synchrotron radiation or pulsed lasers along with advanced methods of time‐resolved image detection and fast electronic pulsers opens ways for driving the resolution limit of a PEEM into the range of a few nanometers. Copyright © 2006 John Wiley & Sons, Ltd.

The history and future of transmission electron microscopy (TEM) is discussed as it refers to the eventual development of instruments and techniques applicable to the real time in situ investigation of surface processes with high resolution. To reach this objective, it was necessary to transform conventional high resolution instruments so that an ultrahigh vacuum (UHV) environment at the sample site was created, that access to the sample by various in situ sample modification procedures was provided, and that in situ sample exchanges with other integrated surface analytical systems became possible. Furthermore, high resolution image acquisition systems had to be developed to take advantage of the high speed imaging capabilities of projection imaging microscopes. These changes to conventional electron microscopy and its uses were slowly realized in a few international laboratories over a period of almost 40 years by a relatively small number of researchers crucially interested in advancing the state of the art of electron microscopy and its applications to diverse areas of interest; often concentrating on the nucleation, growth, and properties of thin films on well defined material surfaces. A part of this review is dedicated to the recognition of the major contributions to surface and thin film science by these pioneers. Finally, some of the important current developments in aberration corrected electron optics and eventual adaptations to in situ UHV microscopy are discussed. As a result of all the path breaking developments that have led to today’s highly sophisticated UHV–TEM systems, integrated fundamental studies are now possible that combine many traditional surface science approaches. Combined investigations to date have involved in situ and ex situ surface microscopies such as scanning tunneling microscopy/atomic force microscopy, scanning Auger microscopy, and photoemission electron microscopy, and area-integrating techniques such as x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, temperature programmed desorption, high-resolution electron energy-loss and Fourier-transform infrared spectroscopies, and others. Material systems ranging from atomic layers of metals and semiconductors to biology related depositions are being investigated. In the case of biological materials, however, strict limitations to high-resolution applications are imposed by electron radiation damage considerations.

Low Energy Electron Microscopy (LEEM), and its close cousin Photo Electron Emission Microscopy (PEEM) have evolved from curiosities in the hands of physicists, to powerful tools for dynamic materials analysis. Invented in 1962, and first successfully realized in 1985, there are about 30 combined LEEM/PEEM instruments in the world today, in addition to about an equal number of PEEM-only instruments. Most of these instruments follow a design that is now almost 20 years old. The most advanced instruments are located at synchrotron radiation facilities, but they are relatively few in number, and not readily available to the general user. In the meantime, the field of electron microscopy is undergoing revolutionary advances fueled by dual breakthroughs in electron energy filtering and aberration correction. Similarly, the field of quantum optics is developing ever more powerful light sources in the Vacuum Ultra Violet (VUV) and soft X-ray ranges that do not depend on massive investments in national infrastructure (i.e. synchrotrons), but deliver their photons in a standard laboratory. These advances from two very different fields will combine to set the stage for the development of LEEM and PEEM over the next decade. Aberration correction will improve LEEM resolution from 5–10 nm today, to 1.5–2 nm in the near future, enough to resolve individual unit cells in the famous Si(111)-(7×7) surface, or -probably more important- sufficient to resolve the structure of nanoscale features such as magnetic domain walls. In PEEM the spatial resolution will improve from ∼ 20 nm today to 4–5 nm, while at the same time improving transmission by a factor ∼10. Energy filtering, today mostly used by synchrotron-based instruments, will be ubiquitous, powerful, simple, and relatively inexpensive.

Localized surface plasmon resonance (LSPR) can be supported by metallic nanoparticles and engineered nanostructures. An understanding of the spatially resolved near-field properties and dynamics of LSPR is important, but remains experimentally challenging. We report experimental studies toward this aim using photoemission electron microscopy (PEEM) with high spatial resolution of sub-10 nm. Various engineered gold nanostructure arrays (such as rods, nanodisk-like particles and dimers) are investigated via PEEM using near-infrared (NIR) femtosecond laser pulses as the excitation source. When the LSPR wavelengths overlap the spectrum of the femtosecond pulses, the LSPR is efficiently excited and promotes multiphoton photoemission, which is correlated with the local intensity of the metallic nanoparticles in the near field. Thus, the local field distribution of the LSPR on different Au nanostructures can be directly explored and discussed using the PEEM images. In addition, the dynamics of the LSPR is studied by combining interferometric time-resolved pump-probe technique and PEEM. Detailed information on the oscillation and dephasing of the LSPR field can be obtained. The results identify PEEM as a powerful tool for accessing the near-field mapping and dynamic properties of plasmonic nanostructures. A novel form of microscopy has been used to map the field dynamics of surface plasmon resonances. Hiroaki Misawa and co-workers in Japan used high-resolution (10 nm) multiphoton photoemission electron microscopy (MP-PEEM) to image the field distribution of a variety of metallic nanostructures, such as arrays of gold rods, nanodisks and dimer pairs. MP-PEEM works by recording the electrons emitted from a sample in response to illumination with a series of ultrashort pulses. Illumination with near-infrared pulses caused excited plasmon resonances in the gold nanostructures to show up in the resulting MP-PEEM images as local hot spots, from which the field distribution was determined. The short duration of the pulses made it possible to probe the dynamics of the plasmon resonance field distribution on the femtosecond timescale.

Photoemission electron microscopy (PEEM) is a unique surface-sensitive instrument capable of providing real-time images with high spatial resolution. While similar to the more common electron microscopies, scanning electron microscopy and transmission electron microscopy, the imaging technology relies on the photogeneration of electrons emitted from a sample through light excitation. This imaging technique has found prominence in surface and materials sciences, being well suited for imaging flat surfaces, and changes that occur to that surface as various parameters are changed (e.g. temperature, exposure to reactive gases). Biologically focused PEEM received significant attention in the 1970s, but was not aggressively advanced since that pioneering work. PEEM is capable of providing important insights into biological systems that extend beyond simple imaging. In this article, we identify and establish important issues that affect the acquisition and analysis of biological samples with PEEM. We will briefly review the biological impact and importance of PEEM with respect to our work. The article also concludes with a discussion of some of the current challenges that must be addressed to enable PEEM to achieve its maximum potential with biological samples.

Significance: Visualizing high-resolution hemodynamics in cerebral tissue over a large field of view (FOV), provides important information in studying disease states affecting the brain. Current state-of-the-art optical blood flow imaging techniques either lack spatial resolution or are too slow to provide high temporal resolution reconstruction of flow map over a large FOV.Aim: We present a high spatial resolution computational optical imaging technique based on principles of laser speckle contrast imaging (LSCI) for reconstructing the blood flow maps in complex tissue over a large FOV provided that the three-dimensional (3D) vascular structure is known or assumed.Approach: Our proposed method uses a perturbation Monte Carlo simulation of the high-resolution 3D geometry for both accurately deriving the speckle contrast forward model and calculating the Jacobian matrix used in our reconstruction algorithm to achieve high resolution. Given the convex nature of our highly nonlinear problem, we implemented a mini-batch gradient descent with an adaptive learning rate optimization method to iteratively reconstruct the blood flow map. Specifically, we implemented advanced optimization techniques combined with efficient parallelization and vectorization of the forward and derivative calculations to make reconstruction of the blood flow map feasible with reconstruction times on the order of tens of minutes.Results: We tested our reconstruction algorithm through simulation of both a flow phantom model as well as an anatomically correct murine cerebral tissue and vasculature captured via two-photon microscopy. Additionally, we performed a noise study, examining the robustness of our inverse model in presence of 0.1% and 1% additive noise. In all cases, the blood flow reconstruction error was for most of the vasculature, except for the peripheral vasculature which suffered from insufficient photon sampling. Descending vasculature and deeper structures showed slightly higher sensitivity to noise compared with vasculature with a horizontal orientation at the more superficial layers. Our results show high-resolution reconstruction of the blood flow map in tissue down to and beyond.Conclusions: We have demonstrated a high-resolution computational imaging technique for visualizing blood flow map in complex tissue over a large FOV. Once a high-resolution structural image is captured, our reconstruction algorithm only requires a few LSCI images captured through a camera to reconstruct the blood flow map computationally at a high resolution. We note that the combination of high temporal and spatial resolution of our reconstruction algorithm makes the solution well-suited for applications involving fast monitoring of flow dynamics over a large FOV, such as in functional neural imaging.

Continuous research on small-scale mechanical structures and systems has attracted strong demand for ultrafine deformation and strain measurements. Conventional optical microscope cannot meet such requirements owing to its lower spatial resolution. Therefore, high-resolution scanning electron microscope has become the preferred system for high spatial resolution imaging and measurements. However, scanning electron microscope usually is contaminated by distortion and drift aberrations which cause serious errors to precise imaging and measurements of tiny structures. This paper develops a new method to correct drift and distortion aberrations of scanning electron microscope images, and evaluates the effect of correction by comparing corrected images with scanning electron microscope image of a standard sample. The drift correction is based on the interpolation scheme, where a series of images are captured at one location of the sample and perform image correlation between the first image and the consequent images to interpolate the drift-time relationship of scanning electron microscope images. The distortion correction employs the axial symmetry model of charged particle imaging theory to two images sharing with the same location of one object under different imaging fields of view. The difference apart from rigid displacement between the mentioned two images will give distortion parameters. Three-order precision is considered in the model and experiment shows that one pixel maximum correction is obtained for the employed high-resolution electron microscopic system.

Development of in situ characterization of two-dimensional materials grown on insulator substrates with spectroscopic photoemission and low energy electron microscopy

Photoemission electron microscopy (PEEM) is a powerful and well established tool in surface science. In recent years, PEEM has been increasingly applied to new terrain, such as imaging of complex nano-objects and functional molecular materials, as well as time-resolved experiments. When applying PEEM to such new terrain, information on the mechanisms causing contrast in the PEEM image is particularly valuable. Here, we present a PEEM study on a complex nano-object – an individual multi-walled carbon nanotube (CNT) – to shed light on the origin of PEEM contrast. The presented PEEM images of the nanotube are of unsurpassed resolution and feature intensity variations along the nanotube. Complementary scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements on the same nanotube reveal topography as the dominant cause for the contrast observed along the nanotube. Energy-filtered PEEM measurements demonstrate that the contrast between nanotube and substrate mainly originates from their different electronic structures. The measurements further demonstrate that energy-filtered PEEM has the potential to image electronic structure variations of complex nano-objects and materials on nanometer length scales.

We have been developing photoemission electron microscopy (PEEM) using both soft and hard X-rays at SPring-8 beamline BL 15 XU, in order to realize two-dimensional chemical analysis for commercial materials. In order to clarify the sub-surface of specimens, high kinetic energy photoelectrons are appropriate owing to their longer inelastic mean free path. Low kinetic energy photoelectrons, such as secondary electrons, have a high electron yield so that one can easily adjust the PEEM lens condition in real-time with a high spatial resolution image and obtain the bulk sensitive information. Therefore, detecting both high energy photoelectrons and low energy secondary electrons is highly desirable for the practical PEEM system. Our PEEM can also observe several materials including thick insulators and rugged specimens. Using our PEEM, we realized a wide energy scan, and successfully observed the energy filtered image using photoelectrons emitted from deep inner shell for the first time. In this paper, we present the performance of our XPEEM and a finding of the micro-area X-ray absorption near-edge structure (μ-XANES) of DVD+RW for a practical material.

This document is the final SAND Report for the LDRD Project 105877 - 'Novel Diagnostic for Advanced Measurements of Semiconductor Devices Exposed to Adverse Environments' - funded through the Nanoscience to Microsystems investment area. Along with the continuous decrease in the feature size of semiconductor device structures comes a growing need for inspection tools with high spatial resolution and high sample throughput. Ideally, such tools should be able to characterize both the surface morphology and local conductivity associated with the structures. The imaging capabilities and wide availability of scanning electron microscopes (SEMs) make them an obvious choice for imaging device structures. Dopant contrast from pn junctions using secondary electrons in the SEM was first reported in 1967 and more recently starting in the mid-1990s. However, the serial acquisition process associated with scanning techniques places limits on the sample throughput. Significantly improved throughput is possible with the use of a parallel imaging scheme such as that found in photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM). The application of PEEM and LEEM to device structures relies on contrast mechanisms that distinguish differences in dopant type and concentration. Interestingly, one of the first applications of PEEM was a study ofmore » the doping of semiconductors, which showed that the PEEM contrast was very sensitive to the doping level and that dopant concentrations as low as 10{sup 16} cm{sup -3} could be detected. More recent PEEM investigations of Schottky contacts were reported in the late 1990s by Giesen et al., followed by a series of papers in the early 2000s addressing doping contrast in PEEM by Ballarotto and co-workers and Frank and co-workers. In contrast to PEEM, comparatively little has been done to identify contrast mechanisms and assess the capabilities of LEEM for imaging semiconductor device strictures. The one exception is the work of Mankos et al., who evaluated the impact of high-throughput requirements on the LEEM designs and demonstrated new applications of imaging modes with a tilted electron beam. To assess its potential as a semiconductor device imaging tool and to identify contrast mechanisms, we used LEEM to investigate doped Si test structures. In section 2, Imaging Oxide-Covered Doped Si Structures Using LEEM, we show that the LEEM technique is able to provide reasonably high contrast images across lateral pn junctions. The observed contrast is attributed to a work function difference ({Delta}{phi}) between the p- and n-type regions. However, because the doped regions were buried under a thermal oxide ({approx}3.5 nm thick), e-beam charging during imaging prevented quantitative measurements of {Delta}{phi}. As part of this project, we also investigated a series of similar test structures in which the thermal oxide was removed by a chemical etch. With the oxide removed, we obtained intensity-versus-voltage (I-V) curves through the transition from mirror to LEEM mode and determined the relative positions of the vacuum cutoffs for the differently doped regions. Although the details are not discussed in this report, the relative position in voltage of the vacuum cutoffs are a direct measure of the work function difference ({Delta}{phi}) between the p- and n-doped regions.« less

We describe the layout and the capabilities of a new aberration-corrected low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) facility, which features real- and reciprocal-space spectroscopy. This new setup, named Electronic, Structural, and Chemical Nanoimaging in Real Time (ESCHER), was recently installed at Leiden University. It has three major instrumentation-related goals. First, we aim to reach the ultimate spatial resolution facilitated by aberration correction using an electron mirror, together with advanced electron detection. Second, we want to develop and exploit the spectroscopic possibilities of LEEM and PEEM in a standard laboratory environment. To this end, ESCHER is equipped with an inline energy filter and advanced photon sources. Third, we plan to extend the sample temperature range down to approximately 10 K, which is significantly lower than that achieved to date. Combined, these efforts will broaden the scientific reach of LEEM and PEEM beyond the areas of surface and materials science and into the realms of biosciences and life sciences. Here, we also present images of the first experiments performed with ESCHER focused on the growth of graphene on SiC(0001).

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