Editorial Cite This: Nano Lett. 2018, 18, 657−659
pubs.acs.org/NanoLett
Dynamic Nanomaterials Phenomena Investigated with in Situ Transmission Electron Microscopy: A Nano Letters Virtual Issue
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recent years to study phase transformations in battery materials and electrochemical processes at interfaces. The first involves the use of a probing/biasing in situ holder on which an electrochemical cell with a solid-state or ionic liquid-based electrolyte is constructed that contains nanoscale active material. By applying a bias in situ to operate this cell, this technique allows for highresolution imaging of the reaction of Li or Na (for Li-ion or Naion batteries) with various nanoscale materials, and it is exceedingly useful for tracking atomic-scale transformation processes and reaction pathways.1,3,5,7,9 This technique has also recently been used to reveal interfacial chemical and structural changes between electrodes and solid-state electrolytes in allsolid-state batteries;2,4 interfacial changes can cause high impedance in these emerging systems and are important to understand. The second type of in situ TEM technique for battery research involves the use of a liquid cell/biasing in situ holder, which enables the creation and imaging of nanoscale battery cells that contain liquid electrolytes (like in most real batteries).6,8,10 Within papers included in this Virtual Issue, liquid cells were used to investigate the electrodeposition process of Li metal (a high-capacity electrode material), as well as to understand the growth of the solid electrolyte interphase (SEI) layer on electrode surfaces.6,10 Electrode dynamics in Li-air batteries have also been studied.8 These studies demonstrate the importance of in situ TEM for determining nanoscale reaction mechanisms in batteries. For future work, it is necessary to focus on defining links between measured current/voltage and observed nanoscale physicochemical dynamics, as well as to combine in situ TEM discoveries with other in situ methods for a holistic understanding of how battery processes are linked across length scales. Liquid-Phase Materials Growth. Improved understanding of the nucleation, growth, and self-assembly dynamics of nanomaterials in liquid has been an area in which in situ TEM has made groundbreaking contributions.11−14 In situ cells featuring a thin liquid layer sandwiched between electrontransparent membranes (either silicon nitride or graphene) enable TEM imaging of processes in liquid. In nanocrystal growth experiments, the nucleation and growth of particles is usually induced by the electron beam itself, where radiation damage to the liquid will form a variety of reactive species that may reduce dissolved precursors (depending on the pH of the solution). Studies included here have investigated the details of the growth of faceted Au nanocrystals,12,14 as well as the growth of more complex bimetallic nanocrystals.13 In addition, the selfassembly of nanoparticles, which is important for bottom-up fabrication of larger-scale structures, has been imaged in situ to reveal the magnitude of forces between individual nanoparticles.11 Beyond this work, graphene-encapsulated liquid cells have received attention in recent years, as they allow for high resolution imaging of materials and biological samples due their
o engineer nanomaterials for various applications, it is critical to understand their dynamic, time-dependent behavior under realistic stimuli and environmental conditions. Processes such as nucleation and growth, phase transformations at high pressures or temperatures, and chemical reactions at surfaces often define the properties and behavior of nanomaterials but conventional experiments fail to elucidate these processes at the nanoscale. Over the past decade, significant effort has been dedicated to developing in situ and operando instrumentation and techniques to probe such processes in materials. In situ experiments offer more tangible information for relevant applications than traditional ex situ analysis, and they provide a unique real-time glimpse into fascinating and complex materials mechanisms. In situ transmission electron microscopy (TEM) and scanning TEM (STEM) are among the most informative techniques for studying the dynamic behavior of nanomaterials, as they provide the nanoscience community with temporally resolved structural and chemical analysis capabilities at the atomic-scale. In situ TEM has progressed rapidly in recent years with the commercial introduction of a variety of in situ sample holders that allow for electrical biasing, heating, physical probing, magnetic response, and mechanical deformation of nanomaterials. Furthermore, although conventional TEMs operate under high vacuum conditions, special in situ holders or environmental TEMs allow for investigation of materials behavior in liquid or gaseous environments. Along with the development of these in situ TEM methods, (S)TEM imaging itself has undergone significant recent advancements that allow for imaging at higher spatial and temporal resolution (e.g., lens and chromatic aberration correction, direct electron detectors), as well as improved chemical analysis. Thus, the concurrent development of in situ TEM techniques and improved imaging capabilities have revolutionized this field to enable investigation of a wide variety of dynamic nanomaterials phenomena. In situ TEM has enabled rapid recent progress in understanding the dynamic behavior of nanomaterials under realistic conditions, and Nano Letters has been the forum for many impactful papers on this topic. In this Virtual Issue, we have selected 30 papers published in Nano Letters since 2015 that represent the state-of-the-art of in situ TEM investigations of nanomaterials. These studies reveal new scientific understanding of complex nanoscale behavior in different environments, and at the same time, they show the steady advance of in situ TEM capabilities that have enabled these insights. The remainder of this editorial introduces these papers and discusses their impact on progress in this field. Battery Electrochemistry. In situ TEM has made a significant impact on understanding nanoscale behavior of materials for next-generation batteries.1−10 Batteries rely on phase transformations within electrode materials to operate, and structural changes during these transformations strongly influence the charge storage capacity and lifetime of batteries. Two different in situ TEM techniques have been developed in © 2018 American Chemical Society
Published: February 14, 2018 657
DOI: 10.1021/acs.nanolett.8b00266 Nano Lett. 2018, 18, 657−659
Nano Letters
Editorial
study reported in situ tensile testing of small silica nanowires, and the findings showed a significant increase in fracture strength and ductility when nanowire diameters decrease below 18 nm.28 These experiments were enabled by the use of a TEM holder that allows for precise nanoscale manipulation of samples while measuring stress and strain. Another recent study was among the first to demonstrate and observe the effects of high cycle fatigue loading in situ on nanocrystalline thin films.29 Tensile loading and controlled fatigue loading of individual nanoscale samples have only recently been achieved with in situ TEM, and these results illustrate the new knowledge that can be gained from such experiments. Ultrafast TEM and Dynamic TEM. An emerging and exciting area is the investigation of dynamic materials phenomena with much higher temporal resolution than in traditional in situ TEM, which usually features video frame rates of ∼30 frames per second. Ultrafast TEMs are unique instruments that use two incoming laser pulses to separately excite a sample and release a packet of imaging electrons from an electron source; such a setup can allow for imaging of reversible or irreversible processes with femtosecond to nanosecond temporal resolution. This capability allows for imaging of processes that have never been previously captured, as well as transient states which are difficult to detect in these time regimes. This technique is illustrated here in the paper by McKenna et al., in which strain waves in individual MoS2 nanoflakes are excited and imaged over hundreds of picoseconds.30 Ultrafast TEM holds enormous promise for uncovering short-time-scale, sitespecific behavior at the nanoscale that until now has been hidden from view, and it is expected to be an important technique for many years to come. Conclusions. This Virtual Issue is intended to showcase the significant progress made in recent years in understanding the dynamic, real-time behavior of nanomaterials under various environments and stimuli via the use of in situ TEM. This collection of groundbreaking papers demonstrates the leadership of Nano Letters in this field, and continued advancement is solicited with a particular focus on steadily increasing the degree of quantification and reproducibility of in situ experiments for a robust mechanistic understanding of various nanoscale processes. In addition, future development of new instrumental capabilities, such as low-dose imaging techniques, are expected to gain increased importance. Low-dose capabilities allow for imaging of nontraditional materials that may be unstable under electron beam irradiation, and they are also necessary for improved temporal resolution. In-situ TEM has already enabled significant advances in many areas of nanoscience, and researchers new to the field can gain access to equipment and expertise through local microscopy facilities or via international user facilities (for example, in the U.S.A. worldwide collaboration is sought at various national laboratories). As in situ TEM techniques continue to advance toward elucidating the fundamental mechanisms in more complex systems, the realworld impact of this field on the science and engineering of nanomaterials will only increase.
minimal thickness, and they recently have been shown to scavenge reactive beam-induced species to enable stable imaging of beam-sensitive specimens.15,16 Insights gained through atomic-scale imaging in graphene liquid cells will likely spur this field to even greater discoveries in the near future. Bias-Induced Solid-State Transformations. Another area of recent focus has been the study of electrical bias-induced transformations and atomic/ionic migration in materials for (opto)electronic devices.17−20 Resistive switching devices for memory and neuromorphic applications that operate via metal filament growth through a dielectric are an example of such devices; in situ TEM has provided novel insight into the dynamics of filament growth.18,20 Other work has focused on defect dynamics in ferroelectrics,17 as well as ion motion-induced degradation in organic/inorganic metal halide perovskite solar cell materials.19 In many of these devices, both ion and electron motion play key roles in defining electrical behavior, which is different than in conventional electronic devices. As these ionic/ electronic devices continue to progress, in situ TEM will remain an important technique for uncovering fundamental nanoscale dynamics. Gas-Phase Reactions and Catalysis. The direct reaction of nanomaterials with gases is being increasingly studied with in situ TEM methods.21−24 Specialized gas-flow sample holders, or environmental TEMs in which the region near the sample can sustain gas flow at elevated pressure, are used for investigation of gas/material interactions. The dynamic interactions between catalysts and oxide support materials under reducing gaseous conditions have been investigated by Zhang et al., and this study found that the support material can migrate to form an atomically thin coating on catalyst nanoparticles.23 Such an effect is important because this coating layer could dramatically influence catalytic rates. Atomic-scale oxidation pathways of various nanomaterials, such as carbon nanotubes, have also been revealed with in situ environmental TEM.21 Finally, vapor− liquid−solid (VLS) growth of semiconductor nanowires has been investigated extensively using in situ TEM instruments that allow for introduction of gaseous semiconductor precursors.24 The paper by Gamalski et al. included here,24 along with other related work, have revealed unprecedented atomic-scale insight into dynamic nanowire growth processes, which is necessary for engineering tailored nanowires for electronic applications. Solid-State Chemical Transformations. Heating of nanoscale specimens inside a TEM during imaging was one of the first in situ techniques developed. Recent years have seen important progress in the development of precision in situ heating holders that overcome sample drift issues during heating; these holders allow for atomic-scale imaging during rapid heating and cooling of nanoscale specimens. Solid-state reactions at interfaces between semiconductors and metallic contacts at elevated processing temperatures are a concern in the electronics industry; recent work in Nano Letters has revealed the atomicscale reaction kinetics and pathways of such reactions.25,26 Other specialized TEM techniques, such as electron holography, have also been used to investigate phase transformations in nanoscale thin films.27 Mechanical Behavior. Nanomaterials often feature divergent mechanical behavior compared to bulk materials because of the influence of surfaces and interfaces. The controlled application of strains and simultaneous quantitative measurement of forces within a TEM offer a unique view of the mechanical behavior of nanomaterials. Two papers that highlight recent progress in this area are included in this Virtual Issue. One
Matthew T. McDowell, Guest Editor
G. W. W. School of Mechanical Engineering and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
Katherine L. Jungjohann, Guest Editor
Center for Integrated Nanotechnologies, Sandia National Laboratory, Albuquerque, New Mexico 87185, United States 658
DOI: 10.1021/acs.nanolett.8b00266 Nano Lett. 2018, 18, 657−659
Nano Letters
Editorial
Nanocrystal Growth Kinetics through Solution Chemistry. Nano Lett. 2015, 15, 5314−5320. (15) Cho, H.; Jones, M. R.; Nguyen, S. C.; Hauwiller, M. R.; Zettl, A.; Alivisatos, A. P. The Use of Graphene and Its Derivatives for LiquidPhase Transmission Electron Microscopy of Radiation-Sensitive Specimens. Nano Lett. 2017, 17, 414−420. (16) Park, J.; Park, H.; Ercius, P.; Pegoraro, A. F.; Xu, C.; Kim, J. W.; Han, S. H.; Weitz, D. A. Direct Observation of Wet Biological Samples by Graphene Liquid Cell Transmission Electron Microscopy. Nano Lett. 2015, 15, 4737−4744. (17) Li, L.; Zhang, Y.; Xie, L.; Jokisaari, J. R.; Beekman, C.; Yang, J. C.; Chu, Y. H.; Christen, H. M.; Pan, X. Atomic-Scale Mechanisms of Defect-Induced Retention Failure in Ferroelectrics. Nano Lett. 2017, 17, 3556−3562. (18) Jang, M. H.; Agarwal, R.; Nukala, P.; Choi, D.; Johnson, A. T.; Chen, I. W.; Agarwal, R. Observing Oxygen Vacancy Driven Electroforming in Pt-TiO2-Pt Device Via Strong Metal Support Interaction. Nano Lett. 2016, 16, 2139−2144. (19) Jeangros, Q.; Duchamp, M.; Werner, J.; Kruth, M.; DuninBorkowski, R. E.; Niesen, B.; Ballif, C.; Hessler-Wyser, A. In Situ TEM Analysis of Organic-Inorganic Metal-Halide Perovskite Solar Cells under Electrical Bias. Nano Lett. 2016, 16, 7013−7018. (20) Hubbard, W. A.; Kerelsky, A.; Jasmin, G.; White, E. R.; Lodico, J.; Mecklenburg, M.; Regan, B. C. Nanofilament Formation and Regeneration During Cu/Al2O3 Resistive Memory Switching. Nano Lett. 2015, 15, 3983−3987. (21) Koh, A. L.; Gidcumb, E.; Zhou, O.; Sinclair, R. Oxidation of Carbon Nanotubes in an Ionizing Environment. Nano Lett. 2016, 16, 856−863. (22) Yuan, W.; Wang, Y.; Li, H.; Wu, H.; Zhang, Z.; Selloni, A.; Sun, C. Real-Time Observation of Reconstruction Dynamics on TiO2(001) Surface under Oxygen Via an Environmental Transmission Electron Microscope. Nano Lett. 2016, 16, 132−137. (23) Zhang, S.; Plessow, P. N.; Willis, J. J.; Dai, S.; Xu, M.; Graham, G. W.; Cargnello, M.; Abild-Pedersen, F.; Pan, X. Dynamical Observation and Detailed Description of Catalysts under Strong Metal-Support Interaction. Nano Lett. 2016, 16, 4528−4534. (24) Gamalski, A. D.; Tersoff, J.; Kodambaka, S.; Zakharov, D. N.; Ross, F. M.; Stach, E. A. The Role of Surface Passivation in Controlling Ge Nanowire Faceting. Nano Lett. 2015, 15, 8211−8216. (25) Chen, R.; Jungjohann, K. L.; Mook, W. M.; Nogan, J.; Dayeh, S. A. Atomic Scale Dynamics of Contact Formation in the Cross-Section of InGaAs Nanowire Channels. Nano Lett. 2017, 17, 2189−2196. (26) Fauske, V. T.; Huh, J.; Divitini, G.; Dheeraj, D. L.; Munshi, A. M.; Ducati, C.; Weman, H.; Fimland, B. O.; van Helvoort, A. T. In Situ HeatInduced Replacement of GaAs Nanowires by Au. Nano Lett. 2016, 16, 3051−3057. (27) Gatel, C.; Fu, X.; Serin, V.; Eddrief, M.; Etgens, V.; WarotFonrose, B. In Depth Spatially Inhomogeneous Phase Transition in Epitaxial MnAs Film on GaAs(001). Nano Lett. 2017, 17, 2460−2466. (28) Luo, J.; Wang, J.; Bitzek, E.; Huang, J. Y.; Zheng, H.; Tong, L.; Yang, Q.; Li, J.; Mao, S. X. Size-Dependent Brittle-to-Ductile Transition in Silica Glass Nanofibers. Nano Lett. 2016, 16, 105−113. (29) Bufford, D. C.; Stauffer, D.; Mook, W. M.; Syed Asif, S. A.; Boyce, B. L.; Hattar, K. High Cycle Fatigue in the Transmission Electron Microscope. Nano Lett. 2016, 16, 4946−4953. (30) McKenna, A. J.; Eliason, J. K.; Flannigan, D. J. Spatiotemporal Evolution of Coherent Elastic Strain Waves in a Single MoS2 Flake. Nano Lett. 2017, 17, 3952−3958.
Umberto Celano, Guest Editor
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IMEC, 3001, Leuven, Belgium
AUTHOR INFORMATION
ORCID
Matthew T. McDowell: 0000-0001-5552-3456 Umberto Celano: 0000-0002-2856-3847 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.nanolett.8b00266 Nano Lett. 2018, 18, 657−659
