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The nanoscale characterization of thermally activated solid reactions plays a pivotal role in products manufactured by nanotechnology. Recently, in situ observation in transmission electron microscopy combined with electron tomography, namely four-dimensional observation for heat treatment of nanomaterials, has attracted great interest. However, because most nanomaterials are highly reactive, i.e., oxidation during transfer and electron beam irradiation would likely cause fatal artefacts; it is challenging to perform the artifact-free four-dimensional observation. Herein, we demonstrate our development of a novel in situ three-dimensional electron microscopy technique for thermally activated solid-state reaction processes in nanoparticles (NPs). The sintering behaviour of Cu NPs was successfully visualized and analyzed in four-dimensional space-time. An advanced image processing protocol and a newly designed state-of-the-art MEMS-based heating holder enable the implementation of considerably low electron dose imaging and prevent air exposure, which is of central importance in this type of observation. The total amount of electron dose for a single set of tilt-series images was reduced to 250 e(-) nm(-2), which is the lowest level for inorganic materials electron tomography experiments. This study evaluated the sintering behaviour of Cu NPs in terms of variations in neck growth and particle distance. A negative correlation between the two parameters is shown, except for the particle pair bound by neighbouring NPs. The nanoscale characteristic sintering behavior of neck growth was also captured in this study. JSPS KAKENHI [JP18H05479, JP20H02426, JP20K21093, JP21K20491, JP 22K14466]; Iketani Science and Technology Foundation; CREST Nanomechanics [JPMJCR1994]; Japan Science and Technology Agency (JST) CREST [JPMJCR18J4]; Five-Star Alliance; Nano-scale Characterization and Fabrication Laboratory (NCFL), Institute for Critical Technology and Applied Science (ICTAS); Virginia Tech; NSF [ECCS 2025151] Published version This study was supported by JSPS KAKENHI Grant Number (JP18H05479, JP20H02426, JP20K21093, JP21K20491, JP 22K14466), Iketani Science and Technology Foundation, CREST Nanomechanics (JPMJCR1994), Japan Science and Technology Agency (JST) CREST (JPMJCR18J4), and Five-Star Alliance. This study was partly supported by Nano-scale Characterization and Fabrication Laboratory (NCFL), Institute for Critical Technology and Applied Science (ICTAS), Virginia Tech and used shared facilities at the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure (NanoEarth), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), supported by NSF (ECCS 2025151).

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) enable the visualization of three-dimensional (3D) microstructures ranging from atomic to micrometer scales using 3D reconstruction techniques based on computed tomography algorithms. This 3D microscopy method is called electron tomography (ET) and has been utilized in the fields of materials science and engineering for more than two decades. Although atomic resolution is one of the current topics in ET research, the development and deployment of intermediate-resolution (non-atomic-resolution) ET imaging methods have garnered considerable attention from researchers. This research trend is probably not irrelevant due to the fact that the spatial resolution and functionality of 3D imaging methods of scanning electron microscopy (SEM) and X-ray microscopy have come to overlap with those of ET. In other words, there may be multiple ways to carry out 3D visualization using different microscopy methods for nanometer-scale objects in materials. From the above standpoint, this review paper aims to (i) describe the current status and issues of intermediate-resolution ET with regard to enhancing the effectiveness of TEM/STEM imaging and (ii) discuss promising applications of state-of-the-art intermediate-resolution ET for materials research with a particular focus on diffraction contrast ET for crystalline microstructures (superlattice domains and dislocations) including a demonstration of in situ dislocation tomography. Published version

X線CT(computed tomography : コンピューター断層撮影法)の原理を透過電子顕微鏡(TEM: transmission electron microscope)に応用した電子線トモグラフィーは、物質のナノ構造を3次元で観察・解析する方法として、また、人間の直感に訴える新しいナノ構造可視化技術として、様々な材料への応用が進んできている。電子線トモグラフィーについては本誌で既に解説がなされており、本稿ではその後の進展として、結晶内部の格子欠陥を3次元可視化する取り組みを紹介する。