Introduction of Research
Frontier Research in Nanomaterials Science
In various materials, the atomic arrangement and electronic state localized in the surface, interface, and point defects play a decisive role. Understanding the relationships between the nanostructures and the properties of these materials should advance precise material design form a central paradigm of nanomaterials science. In recent years, experiments and theoretical calculations to obtain quantitative information on nanostructures of materials have advanced significantly. Development of new methods greatly advances nanomaterials science.
In this project, we focus on frontier research on nanomaterials science. We have combined state-of-the-art techniques such as atomic image observations and spectroscopic analyses via scanning transmission electron microscopy (STEM), atomic force microscopy, first-principles calculations, and nanostructure built-in development technology to produce many world-first achievements.
In addition, we collaborate with information science researchers to develop methods that analyze experimental data based on statistical learning. These efforts have made it possible to perform magnetic moment analyses and element/site selective spectroscopic analyses from nanostructures. These methods have been applied to develop materials, leading to significant achievements.
Nanoanalysis of Grain Boundaries of Ceramics and Grain Boundary Informatics
Prediction model to estimate the grain boundary structure by machine learning
STEM image of a zirconia grain boundary and EDS mapping result
Material properties of ceramics are closely related to grain boundaries. Countless grain boundaries exist inside materials. To develop high-performance and high-functionally ceramics, the structure–property relationship of an individual grain boundary must not only be elucidated but precisely be controlled. However, grain boundaries are at the interface where crystals with different orientation relationships are bonded at the atomic level. The essence of the structure lies in an atomic structure (superstructure) that differs from bulk crystals formed in a region of one nanometer or less.
In this project, we aim to conduct research that integrates state-of-the-art nanotechnology measurement techniques and information science methods to realize innovations essential to elucidate various phenomena of ceramic grain boundaries and efficiently predict stable structures. First, we have established a method to directly observe the atomic structures of grain boundaries as well as the presence of impurities and solute atoms enriched in the atomic structures in real space using scanning electron microscopy (STEM) with a spatial resolution of 1 Å or less. Using our STEM method in combination with energy dispersive X-ray spectroscopy (EDS), we have clarified the solute segregation behaviors at zirconia grain boundaries on the atomic level. Until now, this has been considered impossible. In addition, we have established grain boundary informatics as a method to efficiently and accurately predict complex grain boundary atomic structures.
Our method allows the structure of a grain boundary to be comprehensively predicted. This feat was once thought impossible due to the large exploration space. This achievement should lay the foundation to control the crystal grain boundary and to develop high-performance and high-functionally ceramics.
B. Feng et al. Nature comm. 7 (2016) 11079.
S. Kiyohara et al. Sci. Adv. 2 (2016) e1600746.
Elucidation of the Local Structure of Functional Elements on Functional Oxide Surfaces
Elucidating the local structure of a platinum monoatom adsorbed on a TiO2 (110) surface
A functional oxide surface carrying noble metal nanoparticles has a high catalytic activity. Since the catalytic activity is attributed to the interface interactions between the noble metal and the oxide surface, clarifying the interface atomic arrangement and the electronic state at the electron/atom level is indispensable to design catalytic activities.
In this project, we conduct research intensively using theoretical calculations and experimental electron microscopic analyses to elucidate the monomolecular adsorption structures and bonding states of gold and platinum supported on the surface of rutile-structured titanium oxide (110). Gold atoms and platinum atoms have greatly different adsorption stabilities from that of the adsorption surface site due to the different sites of oxygen vacancies on the titanium oxide surface. In particular, basal oxygen vacancies, which are the most stable adsorption sites of platinum atoms, are surface defects that have not attracted much attention in titanium oxide surface research.
Our research has elucidated a new adsorption mechanism of noble metal/titanium oxide interfaces, which should greatly impact the basic science of catalyst materials.
T.Y. Chang et al. Nano Lett. 14 (2014) 134; K. Matsunaga et al. Phys. Rev. B 90 (2014) 195303.
K. Matsunaga et al. J. Phys. Condens. Matter 28 (2016) 175002.
Hyperspectral Image Analysis by Nonnegative Tensor Factorization
Scan images (STEM images) and spectroscopic analysis data using sub-nanometer–sized electronic probes have mathematical structures, which are generally described by numerical value sequences (tensors) of three or more dimensions as numerical intensity sequences assigned to two-dimensional coordinates of a designated area. By extracting a small number of feature quantities, which constitute data without a priori information, from large-volume experimental data provided by an automated apparatus, noise can be removed. This allows component spectra to be extracted, images to be recovered, superimposed information to be separated, and spatial information to be mapped. For microscopic/spectroscopic data where the count data are always nonnegative, the method called "nonnegative tensor factorization (NTF)" is useful. Utilizing this method enables electron energy loss spectroscopy (EELS) spectra to be separated and displayed. Such spectra indicate that different chemical states overlap with each other, particularly in composite materials and heterophase interfaces. Additionally, important information can be extracted from data with a small signal/noise ratio.
In this project, applying this method to STEM-EELS spectral image data quantitatively measures the interfacial local angular momentum of ferromagnets with an atomic column resolution. This technique can clarify the relationship between lattice defects and magnetism, which is essential information for the development of a strong magnet by microstructure control.
J. Rusz, S. Muto et al. Nature. Comm. 7 (2016) 12672.
T. Thersleff, S. Muto et al. Sci. Rep. 7 (2017) 44802.
Development of New Functional Materials by High-pressure Syntheses
Cubic boron nitride (cBN)/diamond joined crystal obtained under high-pressure. HAADF-STEM image and theoretical prediction of the heterobonding interface of single crystals
C.Chen, N. Shibata, T. Taniguchi, Y. Ikuhara et al. Nat. Comm. 6 (2015) 6327.
Photocatalytic activity of alpha PbO2 type TiO2
H. Murata et al. Phys. Status Solidi RRL 8 (2014) 822.
H. Murata et al. J. Appl. Phys. 120 (2016) 142108.
High-pressure syntheses can explore a vast range of materials, including diamonds. Hence, it is necessary to efficiently explore materials with the desired properties by utilizing nanostructure information. New functions should be developed and designed based on the analyses of unique interface structures obtained by high-pressure syntheses. On the other hand, some materials undergo a phase transition to another crystal structure or an amorphous phase during the recovery process from a high-pressure/high-density structure to ordinary pressure.
In collaboration with the ultrafine structure analysis group using advanced electron microscopy and the first-principles calculation group, we have developed a high-pressure/high-temperature process as a material/substance development technology. This is the basis of built-in nanostructure technology and function generation.
We have explored new physical properties that have yet to be applied to research on high-pressure phases that change during decompression. We have also discovered highly active photocatalysts and proposed collection guidelines for high-pressure phases. These studies have realized a way to utilize a number of already known high-pressure phases.