Advances and Applications of Atomic-Resolution Scanning Transmission Electron Microscopy
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Microscopy and Microanalysis (2021), 27, 943–995 doi:10.1017/S1431927621012125 Review Article Advances and Applications of Atomic-Resolution Scanning Transmission Electron Microscopy Jingyue (Jimmy) Liu Department of Physics, Arizona State University, Tempe, AZ 85287, USA Abstract Although scanning transmission electron microscopy (STEM) images of individual heavy atoms were reported 50 years ago, the applications of atomic-resolution STEM imaging became wide spread only after the practical realization of aberration correctors on field-emission STEM/ TEM instruments to form sub-Ångstrom electron probes. The innovative designs and advances of electron optical systems, the fundamental understanding of electron–specimen interaction processes, and the advances in detector technology all played a major role in achieving the goal of atomic-resolution STEM imaging of practical materials. It is clear that tremendous advances in computer technology and electronics, image acquisition and processing algorithms, image simulations, and precision machining synergistically made atomic-resolution STEM imaging routinely accessible. It is anticipated that further hardware/software development is needed to achieve three-dimensional atomic- resolution STEM imaging with single-atom chemical sensitivity, even for electron-beam-sensitive materials. Artificial intelligence, machine learning, and big-data science are expected to significantly enhance the impact of STEM and associated techniques on many research fields such as materials science and engineering, quantum and nanoscale science, physics and chemistry, and biology and medicine. This review focuses on advances of STEM imaging from the invention of the field-emission electron gun to the realization of aberration-corrected and monochromated atomic-resolution STEM and its broad applications. Key words: atomic resolution, electron microscopy, imaging, materials science, nanostructure (Received 1 January 2021; revised 5 June 2021; accepted 8 June 2021) Introduction The invention of both the telescope and the microscope has unlocked countless mysteries of Nature and enabled numerous Human beings have long been, and will continue to be, fascinated by discoveries that have positively impacted our daily life. Nature and how it works. Curiosity and a thirst for understanding In this review, the goal is to account for the major advances in Nature have fueled scientific and technological breakthroughs one particular type of microscope that is now broadly used for which have been continuously improving our quality of life. We analyzing matter at the atomic scale: the scanning transmission develop our perception of Nature by sensing of which visualization electron microscope (STEM). Although STEM was used to plays a critical role. Since human eyes have a very limited angular image single metal atoms as early as 1970 (Crewe et al., 1970), it resolution (∼1 arcminute or approximately 0.0003 radians), our took a long time for the broader research communities to effec- naked eyes cannot directly visualize either tiny features/creatures, tively utilize this powerful characterization method. The practical such as individual living bacteria, or gigantic individual stars within realization of the correction of lens aberrations to routinely achieve constellations that are far away. The early practice of making sub-Ångstrom image resolution with picometer precision and high spectacles and magnifiers with the goal of “seeing” small features chemical sensitivity greatly enhanced the impact of STEM and more clearly led to the development of two important scientific associated techniques on many research frontiers. Such an accom- tools: The telescope for clearly observing things far away and the plishment has revolutionized how we understand matter at the microscope for examining small features/creatures that our naked atomic level and will have a tremendous impact on how we under- eyes could not do. Both the early telescope and microscope relied stand Nature. The significance of this accomplishment is evidenced on the unique properties of glass lenses that possess the power to by the recent award of the 2020 Kavli Prize in Nanoscience to manipulate light rays. The continuous improvement in telescopes Harald Rose, Maximilian Haider, Knut Urban, and Ondrej has vastly expanded our knowledge of the Universe, while the devel- L. Krivanek “for sub-Ångstrom resolution imaging and chemical opment of various types of microscopes has enabled us to directly analysis using electron beams” (Rose et al., 2020). observe bacteria, viruses, molecules, and even individual atoms. Since many excellent reviews have been published on the topic of aberration-correction (e.g., Rose, 2008, 2009; Hawkes, 2009a, Email: [email protected] 2009b, 2015; Septier, 2017; Hawkes & Krivanek, 2019) we will Cite this article: Liu J (2021) Advances and Applications of Atomic-Resolution Scanning Transmission Electron Microscopy. Microsc Microanal 27, 943–995. focus, in this review, on the advances of forming high-resolution doi:10.1017/S1431927621012125 images by scanning a finely focused electron probe and how to © The Author(s), 2021. Published by Cambridge University Press on behalf of the Microscopy Society of America. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.219, on 28 Sep 2021 at 20:16:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1431927621012125 944 Jingyue (Jimmy) Liu correctly interpret such images. For the fundamental principles In 1957, Marvin Minsky filed a patent describing new approaches and applications of aberration-corrected STEM (ac-STEM), the to constructing a confocal scanning microscope (Minsky, 1957, interested reader is strongly encouraged to read the excellent 1988). The subsequent incorporation of a laser beam into reviews/books on these topics (Rose, 2009; Pennycook & Nellist, Minsky’s design (Davidovits & Egger, 1969, 1971), and the use of 2011; Erni, 2014; Tanaka, 2014; Hawkes & Krivanek, 2019; fluorescent markers for three-dimensional (3D) detection of biolog- Nellist, 2019). ical systems (Cremer & Cremer, 1978), significantly improved the image resolution and made it possible to obtain optical sectioning of 3D objects (Wilson, 2011). By using innovative approaches to A Brief History of the Light Microscope and the Birth of the generating and detecting fluorescent signals, the recent development Electron Microscope of super-resolution microscopes, tremendously aided by the devel- The Dutch eyeglass makers Hans Martens/Zacharias Janssen are opment of faster computers, high-quality lasers, fluorophores, and frequently credited for inventing the compound microscope algorithms for image acquisition and processing, proved extremely (Orchard & Nation, 2014), while Hans Lippershey, another successful in imaging biological objects with an image resolution Dutch eyeglass maker, is considered to be the first person to patent of <20 nm (which can be improved to ∼1 nm), enabling a telescope by assembling a concave eyepiece aligned with a convex observations of dynamic movements of subcellular systems under objective lens (Van Helden, 1977). The early practical applications physiologically relevant environment (Moerner & Kador, 1989; of these lens-based telescopes and microscopes were clearly Betzig & Trautman, 1992; Betzig & Chichester, 1993;Hell& reflected in their respective nicknames as spyglasses and flea Wichmann, 1994; Betzig, 1995; Dickson et al., 1997;Klaretal., glasses. Galileo Galilei, in 1609, constructed his own telescope 2000; Hell, 2003; Betzig et al., 2006;Rustetal.,2006; Huang et al., and discovered that the moon was not a perfect sphere; and with 2009; Pujals et al., 2019). finely polished lenses, he subsequently conducted scientific obser- Abbe’s theory predicts that the use of much shorter wavelengths vations of celestial objects and discovered many distant systems should provide improved image resolution. The de Broglie’s that had not been previously known (Drake, 2003). Even though hypothesis of matter waves (de Broglie, 1923, 1924), the subsequent the modern James Webb Space Telescope (expected to be launched experiments by Davisson and Germer to unambiguously verify the in late 2021) may cost more than $10 billion, our understanding wave nature of electrons (Davisson & Germer, 1927), and Busch’s of the Universe started by stacking two glass lenses to form a theoretical prediction that a cylindrical magnetic lens could be used rudimentary telescope. to focus electrons (Busch, 1926), analogous to the way light is Although the early development and applications of the scien- refracted by a glass lens, led to the emergence of new microscopes tific microscope are difficult to trace exactly, Robert Hooke’s publi- based on electrons instead of light. Ruska and Knoll audaciously cation of Micrographia in 1665 (Hooke, 1665) clearly demonstrated took the adventure of constructing a new type of microscope by the power of using compound microscopes to observe tiny features using high-energy electrons emitted from a metal tip, which pos- that naked human eyes had not been able to do. Hooke also coined sess a wavelength much shorter than that