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Litao Sun Tao Xu Ze Zhang Editors In-Situ Transmission Electron Microscopy In-Situ Transmission Electron Microscopy · · Litao Sun Tao Xu Ze Zhang Editors In-Situ Transmission Electron Microscopy Editors Litao Sun Tao Xu School of Electronic Science School of Electronic Science and Engineering and Engineering Southeast University Southeast University Nanjing, Jiangsu, China Nanjing, Jiangsu, China Ze Zhang School of Materials Science and Engineering Zhejiang University Hangzhou, Zhejiang, China ISBN 978-981-19-6844-0 ISBN 978-981-19-6845-7 (eBook) https://doi.org/10.1007/978-981-19-6845-7 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore Foreword When I started my career in electron microscopy, it was with in situ experiments under direct observation in the transmission electron microscope. The 1970s and the 1980s were the time of the “dinosaurs” in electron microscopy. To the great disappointment not only of electron opticians but also of materials scientists, it had not been possible to correct the optical aberrations of the electromagnetic lenses by then, despite intensive efforts, so that the then generation of electron microscopes with 100 kV accelerating voltage, could not be led to a higher resolution in this way. On the other hand, advances in electronics and accelerator technology had made it possible to go to higher accelerating voltages, with the result that in the late sixties and early seventies electron microscopes were constructed first at 200 kV, then at 650 kV, 1000 kV, and even up to 3000 kV. The latter three, the so-called high-voltage electron microscopes, were “gigantic”, not only in terms of acquisition costs but also in terms of their dimensions. They occupied halls for installation, two stories in height and with the lateral dimensions of a large experimental laboratory. There were three reasons that led to the rapid growth in the number of high-voltage electron microscope installations worldwide. The first had its origin in Scherzer’s equation for the maximum resolution (smallest object distance that can still be resolved) under phase contrast conditions, which goes with λ3/4 (λ is the electron wavelength) and thus improves with decreasing electron wavelength, i.e. increasing acceleration voltage. The second reason originated from the Bethe–Bloch relation for inelastic electron scattering, whose effective cross section decreases with the square of the electron velocity in a first approximation. This allows higher sample thicknesses to be transmitted as the voltage increases. There was a third point: as the accelerating voltage increased, to achieve the same optical effect, the dimensions of the objective lens polepieces necessarily became larger. This allowed, as a by- product, the accommodation of miniaturized experimental apparatus within the lens and around the sample position. Although the first physical and chemical experiments had been done earlier with 100 kV microscopes, and Manfred von Ardenne and Ernst Ruska had already observed (at 70 kV) gas reactions in the STEM and CTEM in 1942, this resulted in an actually unexpected first boom of in situ experiments. v vi Foreword As the first major review paper by Paul Butler (E. P. Butler (1979) Rep. on Prog. in Phys. 42(5), 833) shows, almost all of the in situ techniques still in use today were invented and used during this period. Heating stages for temperatures of up to 1000 °C, helium gas cooled stages for temperatures down to below 9 K (the superconductivity transition point of Nb) both for ultrastable operation under double tilt conditions, deformation stages, as well as environmental and gas reaction cells, were constructed in top-entry and side-entry geometry. The often spectacular experimental results were greeted with enthusiasm. In situ experiments developed into a separate branch of electron microscopy, which organized its own conferences and whose results were widely discussed at international meetings. Topics were, among others, solid-state phase transformations, melting, recovery and recrystallization, plastic deformation and creep, chemical reactions, oxidation, and reduction, as well as magnetic investigations and superconductor flux-line studies in Lorentz mode. At last, it was possible to see, study, and measure directly what had previously been inferred indirectly only from static sequences of images and almost never for the same object with respect to dynamics. I remember well the movie Toru Imura and his collaborators produced on the motion of dislocations during plastic deformation. One could see Frank–Read sources in action for the first time. From then on, the film was used as teaching material in material science lectures all over the world, and it was awarded the “Grand Prix of the International Science and Technology Festival 1978.” There were also lessons to be learned from these experiments. These mainly concerned the special conditions under which they are carried out in the electron microscope, in particular the effects of the thin electron-transparent films and the intense electron irradiation. Quite a few of the results of the measurements made in the initial euphoria, for example, of the dislocation velocity, had to be revised later. It turned out that metallic alloys changed their composition at high temperatures by selective evaporation so that the observed phase transformations were in some cases not typical of bulk behavior. And of course, it was not surprising that such elementary important variables as temperature, pressure, or current chemical composition could either not be determined at all or only with great effort and rarely with the accuracy that was standard in bulk material experiments. And it became immediately clear that the fact that the experiments are performed under intense electron irradiation requires special attention. A wide range of energies is transmitted in electron–atom collisions, from values above the displacement threshold energy of the atoms, producing interstitial atoms and vacancies in the crystal, to very high rates of lower energies comparable to the diffusion activation energies for atoms and vacancies. The former lead to damage and other effects, e.g., to irradiation-enhanced or accelerated phase transformations, and the latter lead to irradiation-induced diffusion, where the energy for the atomic site changes origi- nates from subthreshold electron–atom collisions. While this type of electron–atom interaction increases with electron energy, the second type of interaction, radiol- ysis, resulting from interaction of the incident electrons with the electrons involved in chemical bonding in the material, in general decreases with increasing electron Foreword vii energy. On the one hand, one learned to reduce the irradiation effects by appropriate choice of electron acceleration voltage or, if this was not possible, to take them into account in the interpretation of experimental results. On the other hand, with regard to the development of materials for the construction of new fission and fusion reactors, high-voltage electron microscopes were widely used as a tool for studying the behavior of reactor materials under high irradiation dose rates and at high temperatures. By comparing the generation of high-voltage electron microscopes with the appearance of dinosaurs in the early geological ages, one implicitly considers that they have become extinct and that the habitat they filled then is now occupied by species better adapted to the requirements of today. The time for the high-voltage electron microscopes had come in the nineties. Ze Zhang, one of the authors of this book, and I were among the last to carry out a plastic deformation experiment in a double-tilt straining stage in a 1000 kV high-voltage electron microscope at 800 °C. Using this experiment, in 1993, after finding dislocations in quasicrystals to our own surprise and that of our colleagues in the quasicrystal community, we were able to show together with our colleagues that these dislocations are mobile at high temperature and that they carry the plastic deformation in these strange materials. Immediately thereafter, the microscope was dismantled and scrapped along with the fantastically working object goniometer, an all-time jewel. The exact reasons for the disappearance of the high-voltage electron microscopes and with them, except for a few installations in institutes that specialized in them, also of the in situ experiments will have to be investigated one day by the history of science. However, we can already say that, among other causes, the following points played a role. The time when high-voltage electron microscopes disappeared overlapped with the time when it became finally possible to compensate for the optical aberrations of electron lenses by using multiple lenses. This permitted, for the first time on a broad scale, transmission electron microscopy of materials with atomic resolving power. This book appears on the 25th anniversary of the publication, in 1997, in which this was reported in the literature. The new generation of aberration- corrected electron microscopes provided, for much less money, a level of resolving power previously thought scarcely possible. It had been learned by now that for most material problems, reducing the accelerating voltage to a maximum of 300 kV was a good compromise in terms of transmissivity and electron beam damage. This reduced the equipment requirements considerably. This shifted research interest, and modern electron microscopy turned to new, challenging topics. However, a price had to be paid for the new technology. The lens usually employed today for atomic resolution, the single-field condenser-objective lens after Riecke and Ruska (1965), restricts the space around the specimen to such an extent that object goniometers manufactured using classical technology, not to mention object manipulation devices such as were possible with the large dimensions of the lenses of the high-voltage electron microscopes, can no longer be used. This, fortunately, did not mean the end of in situ experiments in the electron microscope. In the more than three decades that had passed since the first such viii Foreword experiments, progress in the sciences and in miniaturization, especially in semi- conductor technology, had created a new field, micro-and nanotechnology. In fact, since the early 2000s, we have seen an accelerated increase in the development and commercial production of high-precision goniometers for electron microscopy using MEMS (microelectromechanical systems) technology. These contain object manipulation devices of the type previously used for experiments in the high-voltage electron microscope, with the crucial difference that their dimensions are much smaller and their special design allows the processing of samples in nanodimensions, and often in atomic dimensions. In addition, there are a variety of new ideas, including the production of miniaturized devices that mimic the function of real components during their operation. An interesting example (out of many) is the observation of chemical reactions in a model of a Li-ion battery during charging and discharging. A new term was even introduced for this, the observation “in operando”. The new capabilities of MEMS technology, often combined with the great optical properties of aberration-corrected electron microscopes have brought about a new in situ electron microscopy in the last decades, which now occupies a prominent place in modern electron microscopy. The new in-situ electron microscopy, which is mostly advantageous for observing temporal dynamics, is mainly performed in CTEMs (conventional transmission electron microscopes). However, there is also increasing work employing STEMs (scanning transmission electron microscopes). The user can choose between closed cells and open differentially pumped systems. The type of operation used depends on the problem to be studied. As this preface is being written, it is state of the art for microscope manufacturers, in cooperation with the customer and stage manufacturers, to provide platforms optimized for specific types of experiments. And it is possible to operate the instrument as a dedicated instrument essentially for in situ experiments only, or alternatively in alternating mode for conventional static studies and for dynamic studies. It is a general principle of nature that the universal experiment, which leaves no questions unanswered, does not exist. On the other hand, experiments performed in the electron microscope under direct observation with a resolution down to the atomic level offer a unique access to understanding the nature of physical and chemical processes and the observation and quantitative measurement of their course and dynamics. In their now 50-odd-year history, we have gained a huge amount of insight that allows us to understand the specific conditions under which these experiments are conducted and how their results can be interpreted and understood. There is no better way to gain access to a field of research than to look at its type of questions, its type of results and answers, and how these were obtained, using typical examples. In a field as large and constantly growing as that represented by experiments in situ in the electron microscope, it is of elementary importance to make a qualified selection of the work to be discussed for an appropriate overview. This selection has to be made with the target audience in mind and on the background of knowledge of what is being done or has been done in the field as a whole. The same holds for the manner in which the selected topics are discussed. The two editors and co-authors of this book have decades of experience in the field of in situ electron microscopy. The selected contributions to this volume are from research groups that Foreword ix belong to the top group of the field. The articles follow a common structural scheme that makes them easier to read. They are written for beginners who want to get a first overview as well as for those scientists who have been working in this field for a longer time and are searching for a handy reference or want to extend their knowledge, in case of a planned new experiment or possibly in preparation for the acquisition of a new instrument. Prof. Dr. Dr. h.c. Knut W. Urban Senior (Distinguished) Professor of Physics Jülich-Aachen Research Alliance (JARA) RWTH Aachen University and Forschungszentrum Jülich GmbH Ernst Ruska Center for Microscopy and Spectroscopy with Electrons (ER-C) Jülich, Germany Preface It is natural that human beings believe what they see before the object is well understood, which promotes the development of instruments and methodologies to visu- alize objects with sizes beyond the resolving power of the unaided human eyes. Transmission electron microscopy permits not only real space images revealing morphologies and atomic structures of objects but also chemical analysis, which has become indispensable in the study of materials. Because of the lack of dynamic observation, conventional transmission electron microscopy cannot meet the growing demand in new application fields such as nanocatalysts, nanoelectronics, and nanomechanics in which structural and/or property responses to external stimuli are key information to learn. Characteriza- tion of these complex processes requires the use of advanced in situ methodologies without losing any information about intermediate states. In situ transmission electron microscopy refers to the experiments where the specimen is changed while it is observed in a transmission electron microscope, which requires that the microscope should be not only an imaging tool but also a miniaturized laboratory where some form of stimulus or/and exotic environment can be applied deliberately to the specimen and corresponding changes can be simul- taneously monitored. In fact, the idea of in situ transmission electron microscopy was proposed at the very dawn of electron microscopy, it was just the dramatic advancements achieved in the last 20 years. Nowadays, thermal excitation, mechan- ical force, electric field, optical excitation, magnetic field, as well as liquid and/or gaseous environment, can be applied directly to the specimen area in a controllable manner by modifications of the microscope, which enable in situ transmission electron microscopy as one of the most powerful approaches for revealing physical and chemical process dynamics at atomic resolution. In this book, we have attempted to give an overview of in situ transmission electron microscopy including historical background, modern techniques, application achievements, and development trends. This book is intended for advanced under- graduate and graduate students and professional researchers in materials science, chemistry, physics, environmental and energy, electronics, and any subjects that want to explore nanoworld. xi