Institute of Engineering Innovation, School of Engineering, The University of Tokyo
Research Topics
Method Development
Material Applications
Magnetic-Field-Free Electron Microscope
MARS (Magnetic field free Atomic Resolution STEM) is the next generation atomic resolution electron microscope that enables direct observation of atoms in a magnetic field environment for the first time in the world. Conventional atomic resolution electron microscopes use a magnetic field as a lens for electron beams (magnetic lens), so it was necessary to introduce the sample into the lens magnetic field. This lens magnetic field is very strong (about 2 to 3T) and destroys the magnetic and magnetic domain structure and physical structure of the sample itself, so atomic resolution observation of magnetic materials has been impossible for many years. MARS has developed a new objective lens to overcome this problem. Furthermore, by combining this objective lens with the latest DELTA corrector, we achieved the world's first sub-Å spatial resolution in a magnetic-free environment in 2019. This MARS overcomes the long-standing problems of electron microscopes and provides a completely new analysis method for magnetic materials and devices that were previously impossible to observe with an atomic resolution electron microscope.
40-segment Detector
SAAF 40(Segmented Annular All Field Detector 40) is a segment detector based on the existing SAAF technology and boasting the largest number of segments in the world with 40 segments. SAAF development started in 2006, adopting the scintillator + PMT system, and has already been developed with 16 segmented version and 8 segmented version (commercial machine SAAF OCTA made by JEOL). In recent years, pixelated detectors using CMOS and CCD technology have been actively developed, but SAAF has a performance that greatly exceeds that of pixel detectors in terms of detection speed, real-time observation, and dynamic range. By dividing it into 40 segments, quantitative performance, detection efficiency, and image control flexibility have been greatly improved compared to existing SAAF detectors. Using this detector, we are trying to observe various phenomena at the atomic level and extract information that was previously impossible.
Atomic Resolution Electromagnetic Field Imaging
The DPC STEM (Differential Phase Contrast Scanning Transmission Electron Microscopy) has recently attracted a great deal of attention as a method for observing the electromagnetic field distribution inside materials and devices in real space. This group has succeeded in observing the electric field inside the atom by making the DPC STEM method at atomic resolution in 2012 [1]. In addition, we have succeeded in direct observation of electron clouds by converting electric field information into charge density information in 2018. This group has been applying DPC STEM to various material and device research fields and opening up new possibilities.
Quantitative Electromagnetic Field Imaging at Crystal Interfaces
Electric and magnetic fields at crystal interfaces, such as heterointerfaces and grain boundaries, are critical to material properties and device performance. However, quantitative observation of these fields at crystal interfaces has been challenging due to overlapping signals from structural changes. Our group has developed an advanced imaging technique that enables precise, quantitative observation of electromagnetic fields at these interfaces. Using this method, we are investigating the mechanisms by which interfacial electromagnetic fields impact material properties.
Development of Ultra-high Sensitive STEM Imaging
Lithium-ion battery materials, zeolites, and metal-organic complexes are very weak materials to electron beams, and severe irradiation damages have made it extremely difficult to observe them by electron microscopy at atomic resolution. In this group, an ultra-high sensitive STEM imaging technique has been developed by processing multiple signals simultaneously obtained using the SAAF detector for maximizing the signal-to-noise ratio theoretically. We are working on the observation of electron beam sensitive materials at atomic resolution using the developed method in order to understand the mechanism of their functional properties from the atomistic level.[1][2]
Atomic-Scale Analysis of Grain Boundaries in Steel
The properties of steel are controlled by its microstructure. In particular, understanding the atomic structure of grain boundaries—the interfaces between crystalline grains—is a critical challenge in materials design. However, conventional transmission electron microscopes make such observations extremely difficult in ferromagnetic materials like steel, due to magnetic fields generated by the objective lens.
By using a magnetic-field-free electron microscope developed in our laboratory, we have succeeded in directly observing the atomic structure of grain boundaries in steel. This has led to the discovery of a previously unanticipated grain boundary structure that exhibits dynamic atomic fluctuations over long periodicities.
This finding not only provides practical insight into the development of advanced steels but also challenges conventional assumptions about grain boundary structures, offering new possibilities for future fundamental and applied research.
Quantitative Observation of Local Electric Field Distribution in Electronic Devices
In modern high-speed, energy-efficient semiconductor devices, precise control of local electric field distribution is crucial to performance. However, direct, quantitative observation of nano-scale electric fields has been highly challenging. We have developed a DPC STEM method that quantifies local electric field distribution with very high spatial resolution. This technique has enabled us to observe electric field distributions in GaAs p-n junctions and nitride semiconductors, including heterointerfaces, quantitatively. This new method is anticipated to greatly advance electronic device research and development.
Magnetic Domain Observation and Microstructure Design of Pollycrystalline Magnets
High-performance hard magnets are developed by controlling their microstructures. It is thus necessary to fundamentally understand the active role of microstructures on the properties of magnets. DPC STEM is expected to contribute to the true understanding of the origin of physical properties in magnetic materials. Therefore, our group has developed a new DPC imaging technique that can significantly reduce the diffraction contrast (right figure) in DPC images. Using this technique, we are aiming to establish the microstructure design principles for high-performance magnets demanded in modern society.
Observation and Control of Magnetic Skyrmion
We are studying the structure of magnetic skyrmions, which are expected to be applied as magnetic memories, and developing control methods for them. Recently, we succeeded in confining skyrmions by using minute defect structures on the surface.
Local Structure Analysis of Heterogeneous Nanocatalyst
Noble metal nanoparticles (Pd, Au, and Pt) dispersed on metal oxide surfaces are widely used in commercial heterogeneous catalysis for the purification or detoxification of CO/NOx gases. However, fundamental questions remain, such as the atomistic origin of catalytic activity and the mechanisms of degradation. These questions are crucial for advancing the development of new catalytic materials. By combining atomic-resolution STEM imaging with spectroscopy, we have successfully observed the evolution of local atomic structures and clarified the atomistic mechanisms responsible for degradation.[1][2]
Space charge observation at oxide grain boundaries
Yttria-stabilized cubic zirconia is an oxygen ion conductor used as a solid electrolyte in solid oxide fuel cells. The ionic conductivity of the electrolyte is significantly influenced by the charges at the grain boundaries. By advancing quantitative methods for local electric field analysis at interfaces, we successfully visualized the space charge layer within zirconia grain boundaries. Additionally, we revealed the correlation between the space charge layer and the atomic structure of the grain boundary.
Charge observation of ferroelectric domain wall
Ferroelectric materials are key for next-generation devices, but it has been challenging to directly observe the nanoscale charges at domain walls, which determine device performance.
Therefore, we locally analyzed ferroelectric domain walls by combining atomic-resolution structural observation via OBF-STEM with electric field observation via tDPC-STEM . As a result, we successfully performed precise measurements of the charge state and its distribution width at both head-to-head and tail-to-tail domain walls.
