Institute of Engineering Innovation, School of Engineering, The University of Tokyo
Research Topics
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.
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.
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.
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.
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. 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.
In modern semiconductor devices that require high speed and energy saving, minute control of local electric field distribution inside the devices plays a key role for improving their performances. However, it has been extremely difficult to observe such a nano-scale local electric field distribution directly and quantitatively. Therefore, we develop a method for quantifying the local electric field distribution using DPC STEM at very high spatial resolution. We have succeeded in the quantitative observation of electric field distributions in GaAs p-n junctions and nitride semiconductors including heterointerfaces. The newly developed method is expected to significantly advance the research and development of electronic devices.
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 combined with other STEM imaging techniques can directly visualize the local magnetic field distribution and microstructures simultaneously from the same sample regions at high spatial resolution. DPC STEM is expected to contribute to the true understanding of the origin of physical properties in magnetic materials. However, since the DPC images are disturbed by the diffraction contrast derived from the diffraction of the crystalline grains, the electromagnetic field contrasts become unclear, especially in a polycrystalline samples (left figure). 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.
