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Ryo Ishikawa1 Rohan Mishra2 Peng Gao3 Andrew Lupini6 Takashi Taniguchi4 Stephen Pennycook5 Naoya Shibata1 Yuichi Ikuhara1

1, The University of Tokyo, Tokyo, , Japan
2, Washington University in St. Louis, St. Louis, Missouri, United States
3, Peking University, Beijing, , China
6, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
4, National Institute for Materials Science, Tsukuba, , Japan
5, National University of Singapore, Singapore, , Singapore

The properties of semiconductors and insulators are very sensitive to the presence and location of impurities and defects, meaning that it is therefore important to image these defects down to the level of single atoms. Scanning transmission electron microscopy (STEM) is one of the leading techniques to directly identify stable atomic structures and has solved a wide variety of materials problems. Recent improvements in the optics and mechanical stability of the electron microscope now allow us to observe dynamics at the atomic-scale, although the time resolution is still in the order of seconds. In this presentation, we show the dynamic observation of single atom diffusion in a luminescent aluminum nitride (AlN) and the phase transition from spinel to rocksalt in lithium manganese oxide (LiMn2O4) by atomic-resolution dynamic STEM imaging.

When a specimen is illuminated by a high-energy electron beam, the incident electron transfers energy to the atoms, promoting dopant diffusion in bulk materials and, moreover, enabling the stimulation of local phase transitions without heating or cooling the sample. One good example is the case of single dopant diffusion in AlN, which is important because the dopant atoms contribute to the luminescence color. The doped Ce atom has unusually large ionic radius in AlN and this size mismatch enhances the mobility of Ce dopants in AlN, enabling us to track the movement of single Ce dopants via dynamic Z-contrast STEM imaging. The second example is the phase transition of spinel LiMn2O4. Following some charge-discharge electrochemical processes, this material will locally form rock-salt type structures through cation mixing between lithium and transition metals. We have observed this gradual transition at the atomic-scale by using time-sequential imaging and spectroscopy. The presented dynamic STEM observations could open the way to in-depth understanding of the atomistic phenomena in this and other energy-relevant materials. A part of this work was supported by JSPS KAKENHI Grant Number JP17H06094.

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