State-of-the-Art Electron Microscopy For Physical Sciences Research

The ability to provide high-resolution information about a variety of materials has made electron microscopy a valuable tool for the physical sciences research community. In recent years, many advanced electron microscopy techniques have been developed to provide access to new classes of materials or provide new types of information about materials. This Methods Collection focuses on a selection of these state-of-the-art techniques, including those that provide access to information about materials in liquids and gases and those that provide new or enhanced information about more conventional solid materials.

The first article, written by Moon et al., describes coupled focused ion beam (FIB) and scanning electron microscopy (SEM) techniques performed at cryogenic temperatures, which enable liquid-solid interfaces to be characterized in an intact state at the nanoscale¹. The authors provide a workflow for performing cryo-SEM and energy dispersive X-ray (EDX) spectroscopy on FIB-milled samples, including instrument setup, preservation of liquid-solid interfaces, milling of samples by FIB, and imaging and EDX analysis of the resulting cross-sectional surfaces. In addition to the workflow, the authors provide a set of representative results for lithium metal-liquid electrolyte interfaces and a discussion about how to avoid artifacts during different steps in the process. The techniques outlined in this article provide unprecedented high-resolution access to intact liquid-solid interfaces and are invaluable for studying electrochemical energy storage and conversion devices, for example.

In the next article, Ohtsuka and Muto introduce a method known as high-angular-resolution electron-channeling X-ray spectroscopy (HARECEXS) that utilizes electron channeling paired with EDX and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to study site occupancies and site-dependent chemical information about impurities/dopants in materials². The authors present the experimental procedures necessary to perform these experiments, including sample preprocessing, instrument setup/operation, and data analysis. In addition to representative results for BeTiO₃ and Ca_1.8Eu_0.2Y_0.2Sn_0.8O₄, the authors also provide a discussion that includes, among other topics, guidelines for users of instruments not equipped with certain features. The HARECEXS technique described here is a useful tool for tracking atomic-scale chemical information in a wide range of industrial products.

In the following article, authors Miao et al. outline methods for tracking the positions of atomic columns in crystalline materials with picometer precision using a STEM³. The authors discuss techniques for acquisition of high-quality atomic-resolution STEM images, pre-position tracking data processing, atomic column position quantification and the corresponding measurement of lattice strain, and visualization of the results. A MATLAB graphical user interface (GUI) that aids in these processes is also presented, as well as representative results for perovskite structures and a discussion about various available analysis packages, among other topics. These techniques allow strain fields, for example, to be tracked in a range of materials with high precision and high spatial resolution.

Next, Unocic et al. present a method for performing high-resolution in situ measurements of materials undergoing reactions in gaseous environments in a STEM, known as an in situ closed-cell gas reaction (CCGR)⁴. The experimental protocol for performing CCGR experiments is presented, including sample preparation, STEM holder and experimental setup preparation, and how to run the experiment and perform the residual gas analysis (RGA). The authors provide representative results for a Pt nanoparticle heterogeneous catalyst as well as an additional discussion on sample preparation and applications of the technique. Using the CCGR-STEM technique, localized dynamic reactions can be identified, which is very challenging using other methods but is important for studying catalysts and structural materials, for example.

In the final article in the collection, Zheng et al. demonstrate methods for mapping nanoscale magnetic fields by off-axis electron holography in a transmission electron microscope (TEM)⁵. The authors include a description of the steps involved in performing the experiment as well as in analyzing the data and interpreting the results. The authors also provide representative results for magnetic skyrmions and a discussion about how to optimize experiments. The holographic TEM technique described in the article allows magnetic fields to be mapped in two dimensions with nanoscale resolution and, in the future, will enable measurements to be performed in three dimensions when combined with tomographic techniques, thus providing the opportunity to study three-dimensional magnetic nanostructures.

Over time, electron microscopy is encompassing an ever more versatile range of techniques, which are expanding its reach from obtaining basic structural and elemental information about solid materials to providing more detailed and high-resolution information about these properties, measuring entirely new properties like electric/magnetic field distributions, and characterizing samples in contact with volatile materials such as liquids and gases. This Methods Collection covers a wide range of these techniques, represents a valuable resource for researchers beginning to work in the various fields in which these state-of-the-art electron microscopy techniques are invaluable, and provides a solid background for those that will further develop these techniques in the future.

The authors declare no conflicts of interest.

This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

This work was supported by the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.