Faculty Profiles - YOKOI Tatsuya

Papers - YOKOI Tatsuya

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Spectrophotometric analyses on electronic structures of glide dislocation cores in ZnS

Yu Ogura, Kaito Mishina, Sena Hoshino, Kimitaka Higuchi, Tatsuya Yokoi, Atsutomo Nakamura, Katsuyuki Matsunaga

Journal of the American Ceramic Society 2024.12

Publishing type：Research paper (scientific journal) Publisher：Wiley

Abstract

It has been reported that dislocations in ZnS can trap photoexcited carriers and then undergo core reconstruction. However, electronic states of the trapped carriers at the reconstructed dislocations were not detected experimentally. In this study, unique ultraviolet (UV)–visible–infrared (IR) transmission measurements by using a spectrophotometer with UV pump beam were performed for deformed ZnS single crystals having glide dislocations. Optical transmission values of the deformed specimens decreased by the pump beam illumination apparently. Additionally, the optical transmission reductions increased more with rising plastic strains, namely, rising dislocation density. Since the UV pump beam can produce photoexcited carriers of electrons and holes, it is expected that the glide dislocations introduced in the deformed specimens undergo core reconstruction due to carrier trapping. Therefore, it is thought that the observed reduction in optical transmission of the deformed specimens is ascribed to photoabsorption due to in‐gap states localized at the reconstructed partial dislocation cores. Moreover, time‐dependent transmission measurements revealed that the core reconstruction proceed rapidly while the opposite processes are quite slow, which may be closely related to atomic structure changes of the glide dislocation cores due to the additional pump beam illumination.

DOI： 10.1111/jace.20332

Web of Science

Scopus

Machine learning interatomic potential with DFT accuracy for general grain boundaries: Analysis of grain boundary energy and atomic structure in α-Fe polycrystals Reviewed

Kazuma Ito, Tatsuya Yokoi, Katsutoshi Hyodo, Hideki Mori

npj Computational Materials Vol. 10 page： 255 2024.11

Artificial-neural-network descriptor enhancing accuracy of machine-learning interatomic potential and its application to lattice defects in Si

M. Uchida, T. Yokoi, Y. Ogura, K. Matsunaga

Physical Review Materials Vol. 8 ( 10 ) 2024.10

Multicrystalline informatics: a methodology to advance materials science by unraveling complex phenomena.

Usami N, Kutsukake K, Kojima T, Kudo H, Yokoi T, Ohno Y

Science and technology of advanced materials Vol. 25 ( 1 ) page： 2396272 2024.9

Atomic-structure changes of 30° partial-dislocation cores due to excess carriers in GaP

Hoshino S., Oi S., Ogura Y., Yokoi T., Li Y., Nakamura A., Matsunaga K.

Physical Review Materials Vol. 8 ( 9 ) 2024.9

Publisher：Physical Review Materials

It was experimentally reported that light illumination leads to reduced deformation stresses in some III-V compound semiconductors such as GaP. This phenomenon is known as the negative photoplastic effect, which is expected to originate from interactions between photoexcited carriers and glide dislocations. To clarify its physical origin at the atomic and electronic levels, density-functional-theory calculations were performed for Shockley 30° partial dislocations in GaP. In the absence of excess carriers, both Ga and P cores of the partial dislocations were found to have reconstructed structures that are energetically most stable. This can be understood by the fact that dangling-bond-like states at undercoordinated atoms of the dislocation cores are removed by core reconstruction. In the presence of excess carriers that would be formed by light illumination, the reconstructed Ga and P cores were able to trap excess holes and electrons, respectively, and were subsequently transformed to unreconstructed structures. It was also found that the unreconstructed structures due to excess carriers tend to have smaller potential barrier heights for dislocation glide, as compared to the pristine reconstructed structures without any excess carriers. This is in good agreement with the increased dislocation mobility in GaP under external light illumination that has been experimentally reported.

DOI： 10.1103/PhysRevMaterials.8.093605

Scopus

Revealing Atomic Structure of Hybrid Octacalcium-Phosphate Derivative

Susaki, N; Saito, T; Yokoi, T; Ogura, Y; Matsunaga, K

INORGANIC CHEMISTRY Vol. 63 ( 34 ) page： 15924 - 15930 2024.8

Language：English Publisher：Inorganic Chemistry

Octacalcium phosphate (OCP), which is one of the bioactive calcium phosphates, can incorporate various organic molecules in its crystal lattice, forming the organic-inorganic hybrid derivatives. However, detailed atomic arrangements of OCP hybridized with organic molecules such as dicarboxylate are still unknown, although many years have passed since the first discovery of the materials systems. In the present study, some black-box optimization methods combined with first-principles calculations were used to theoretically identify the most stable atomic structure of the OCP with the incorporation of malonate ions as a typical case study. The results showed that the calculated interplanar spacing on the (100) plane of the most stable structure agrees well with experimental data, by taking account of implicit solvent of aqueous solution. An underlying mechanism that realizes the bridging feature of the incorporated malonate ions between the apatitic layers is also discussed. The present methodology can pave the way to accurately explore reliable atomic structures of such complicated organic-inorganic hybrid biomaterials with high structural degrees of freedom.

DOI： 10.1021/acs.inorgchem.4c02259

Web of Science

Scopus

PubMed

The effect of room‐temperature plastic deformation in darkness on the photoluminescence properties of ZnS

Yu Ogura, Atsutomo Nakamura, Tatsuya Kameyama, Yasuyoshi Kurokawa, Eita Tochigi, Naoya Shibata, Tsukasa Torimoto, Sena Hoshino, Tatsuya Yokoi, Katsuyuki Matsunaga

Journal of the American Ceramic Society Vol. 107 ( 4 ) page： 2040 - 2047 2024.4

Publishing type：Research paper (scientific journal) Publisher：Wiley

Abstract

Inorganic semiconductors have been considered to be brittle at room temperature generally, but recently zinc sulfide (ZnS), a II–VI compound semiconductor, was found to exhibit extraordinarily large plasticity even at room temperature if it was deformed in darkness. Room‐temperature plastic deformation can achieve high density of glide dislocations in the crystal without significantly changing the point defect structure, and these dislocations themselves have a potential to provide unique functional properties different from those of the bulk. In this study, therefore, undoped ZnS crystals were plastically deformed at room temperature in darkness to generate only a large number of dislocations, and the changes in luminescence properties resulting from this process were investigated for the first time. As a result, we found that ZnS deformed in darkness exhibits characteristic photoluminescence and persistent luminescence emissions with a visible green color. SEM‐CL analyses also identified the emissions from the dislocations. Theoretical calculations indicated that individual dislocations in ZnS have the ability to trap photo‐excited carriers. Such changes in luminescence properties due to room temperature plastic deformation in darkness can be considered to originate from dislocations rather than point defects. In other words, the dislocations themselves serve as the significant recombination centers, realizing visible light emission.

DOI： 10.1111/jace.19564

Web of Science

Scopus

Multicrystalline Informatics Applied to Multicrystalline Silicon for Unraveling the Microscopic Root Cause of Dislocation Generation

Kenta Yamakoshi, Yutaka Ohno, Kentaro Kutsukake, Takuto Kojima, Tatsuya Yokoi, Hideto Yoshida, Hiroyuki Tanaka, Xin Liu, Hiroaki Kudo, Noritaka Usami

Advanced Materials Vol. 36 ( 8 ) page： e2308599 2023.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley

Abstract

A comprehensive analysis of optical and photoluminescence images obtained from practical multicrystalline silicon wafers is conducted, utilizing various machine learning models for dislocation cluster region extraction, grain segmentation, and crystal orientation prediction. As a result, a realistic 3D model that includes the generation point of dislocation clusters is built. Finite element stress analysis on the 3D model coupled with crystal growth simulation reveals inhomogeneous and complex stress distribution and that dislocation clusters are frequently formed along the slip plane with the highest shear stress among twelve equivalents, concentrated along bending grain boundaries (GBs). Multiscale analysis of the extracted GBs near the generation point of dislocation clusters combined with ab initio calculations has shown that the dislocation generation due to the concentration of shear stress is caused by the nanofacet formation associated with GB bending. This mechanism cannot be captured by the Haasen‐Alexander‐Sumino model. Thus, this research method reveals the existence of a dislocation generation mechanism unique to the multicrystalline structure. Multicrystalline informatics linking experimental, theoretical, computational, and data science on multicrystalline materials at multiple scales is expected to contribute to the advancement of materials science by unraveling complex phenomena in various multicrystalline materials.

DOI： 10.1002/adma.202308599

Web of Science

Scopus

PubMed

Anharmonicity in grain boundary energy for Al: thermodynamic integration with artificial-neural-network potential Reviewed International journal

Scripta Materialia Vol. 236 page： 115685 2023.11

Photoplastic effect in MgO single crystals

Ogura Yu, Tsuchiya Yuki, Hoshino Sena, Yokoi Tatsuya, Matsunaga Katsuyuki

Journal of the Ceramic Society of Japan Vol. 131 ( 10 ) page： 628 - 631 2023.10

Language：English Publisher：The Ceramic Society of Japan

The photoplastic effect, phenomena that flow stress and/or hardness change by light illuminations, has been known mainly in II–VI compound semiconductors. In this study, it was revealed for the first time that magnesium oxide (MgO) single crystals exhibit increase in flow stress by light illumination, namely, the positive photoplastic effect. Scanning transmission electron microscope (STEM) observations demonstrated that the plastic deformation of MgO is realized by generation of glide dislocations on the slip system of {110}〈110〉. Therefore, the observed positive photoplastic effect of MgO is likely due to interactions of the glide dislocations and photo-excited carriers. According to the theoretical calculations, individual dislocations in MgO had a specific band structure that differs from the bulk. This also indicates possible interactions between dislocations and carriers. It is expected that light illumination can reduce dislocation mobility in MgO, leading to the increase in flow stress under light illuminations.

DOI： 10.2109/jcersj2.23060

CiNii Research

Grain boundary segregation of Y and Hf dopants in α-Al2O3: A Monte Carlo simulation with artificial-neural-network potential and density-functional-theory calculation

Yokoi Tatsuya, Hamajima Akihiro, Ogura Yu, Matsunaga Katsuyuki

Journal of the Ceramic Society of Japan Vol. 131 ( 10 ) page： 751 - 761 2023.10

Language：English Publisher：The Ceramic Society of Japan

Artificial-neural-network (ANN) interatomic potentials for Al–Y–O and Al–Hf–O systems are constructed using density-function-theory (DFT) data and combined with Monte Carlo (MC) simulations in order to predict Y and Hf segregation behavior at the ∑7(4510)/[0001] grain boundary (GB) in α-Al2O3. The ANN potentials are demonstrated to accurately predict preferential substitutional sites of not only an isolated but also multiple dopant ions. This enables us to circumvent DFT calculations for MC trial moves, thereby greatly reducing computational cost. There is a tendency that both Y and Hf ions substitute for 6-fold Al ions with elongated Al–O bonds at the GB and have coordination numbers greater than 6 after structural relaxation. This may suggest that even at the GB, Y and Hf ions prefer atomic environments in Y- and Hf-containing oxides with 7- and 8-fold coordination. Furthermore, effects of dopant species and concentrations on band-gap reduction at the GB are elucidated by analyzing partial density of states for the dopant-segregated GBs. The ANN-MC method with DFT analysis will pave the way for systematically determining atomic and electronic structures of GBs involving dopants, as demonstrated in this work.

DOI： 10.2109/jcersj2.23044

CiNii Research

Electronic and atomic structures of Shockley-partial dislocations in CdX (X = S, Se and Te)

Hoshino Sena, Yokoi Tatsuya, Ogura Yu, Matsunaga Katsuyuki

Journal of the Ceramic Society of Japan Vol. 131 ( 10 ) page： 613 - 620 2023.10

Language：English Publisher：The Ceramic Society of Japan

II–VI semiconductors, including Cd compounds, become brittle under light illumination. This phenomenon is known as the photoplastic effect (PPE) and is thought to arise from interactions between glide dislocations and photoexcited carriers. The present study investigated atomic structures of 30° Shockley-partial dislocations with and without excess carriers in CdX (X = S, Se and Te), by density-functional-theory (DFT) calculations. It was found that both Cd and anion cores favor unreconstructed atomic structures when excess carriers are absent. In the presence of excess carriers, on the other hand, reconstructed atomic structures were more stable at the anion cores while the unreconstructed ones were still energetically more favorable at the Cd cores. It is thus expected that only the anion cores change their atomic structures by light illumination, which can retard glide-dislocation motion by forming like-atom bonds. Analyses of local densities of states (LDOSs) revealed that the reconstructed Cd and anion cores form shallow and deep defect states within the band gaps, respectively. This determines the possible atomic reconstructions at the dislocation cores in the presence of excess carriers excited by external light.

DOI： 10.2109/jcersj2.23055

CiNii Research

Grain-boundary thermodynamics with artificial-neural-network potential: Its ability to predict the atomic structures, energetics, and lattice vibrational properties for Al

Yokoi Tatsuya, Matsuura Maya, Oshima Yu, Matsunaga Katsuyuki

PHYSICAL REVIEW MATERIALS Vol. 7 ( 5 ) 2023.5

Publisher：Physical Review Materials

An artificial neural network (ANN) potential for Al, trained with density-functional-theory (DFT) data, is constructed to accurately predict lattice vibrational properties and thermodynamics of grain boundaries (GBs) in Al. The ANN potential is demonstrated to accurately predict not only atomic structures and energetics of the GBs at 0 K but also partial phonon densities of states and vibrational entropies, even for GBs absent in the training data sets. In addition, their total potential energies and atomic forces by DFT at elevated temperatures up to 800 K can also be well reproduced by molecular dynamics with the ANN potential. In contrast, a modified embedded atom method (MEAM) potential shows larger errors in phonon frequencies and atomic forces for atoms at GBs, as well as in the bulk, than the ANN potential. The MEAM potential is thus likely to be inadequate to quantitatively predict thermodynamic properties of GBs, particularly at high temperature. The present ANN potential is also applied to systematically examine thermodynamic stability of asymmetric tilt GBs. It is predicted that for the ς9 system, the GB free-energy profile as a function of inclination angle exhibits a cusp at elevated temperatures, due to its larger vibrational entropies of asymmetric tilt GBs than those of ς9 symmetric tilt GBs.

DOI： 10.1103/PhysRevMaterials.7.053803

Web of Science

Scopus

Atomic and electronic structure of grain boundaries in a-Al2O3: A combination of machine learning, first-principles calculation and electron microscopy

Yokoi T., Hamajima A., Wei J., Feng B., Oshima Y., Matsunaga K., Shibata N., Ikuhara Y.

SCRIPTA MATERIALIA Vol. 229 2023.5

Publisher：Scripta Materialia

To accurately determine the atomic and electronic structures of symmetric tilt grain boundaries (GBs) in α-Al2O3, this work employed an artificial-neural-network (ANN) interatomic potential, density-functional-theory (DFT) calculation and scanning transmission electron microscopy (STEM) observation. An ANN-based simulated annealing method was demonstrated to efficiently screen candidate low-energy structures with reasonably high accuracy. For Σ7 and Σ31GBs with the [0001] tilt axis, which were absent in the training datasets for the ANN potential, their lowest-energy structures predicted from ANN and DFT calculations were in quantitative agreement with STEM images in terms of both Al- and O-column positions. The exact GB structures have enabled us to analyze quantitatively the relationship between their atomic and electronic structure. This work will be an important model case where a combination of machine-learning, theoretical calculation and experiment has successfully solved the problem of determining complicated GB structures and their electronic structures in α-Al2O3.

DOI： 10.1016/j.scriptamat.2023.115368

Web of Science

Scopus

Atomic structures of grain boundaries for Si and Ge: A simulated annealing method with artificial-neural-network interatomic potentials (vol 173, 111114, 2023)

Yokoi Tatsuya, Kato Hirotaka, Oshima Yu, Matsunaga Katsuyuki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS Vol. 176 2023.5

Ca-vacancy effect on the stability of substitutional divalent cations in calcium-deficient hydroxyapatite

Tatsushi Saito, Yuto Ishikawa, Yusuke Noda, Tatsuya Yokoi, Yu Oshima, Atsutomo Nakamura, Katsuyuki Matsunaga

JOURNAL OF THE AMERICAN CERAMIC SOCIETY Vol. 106 ( 2 ) page： 1587 - 1596 2023.2

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY

First-principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M2+ (M = Mg, Zn, Sr) in Ca-deficient hydroxyapatite (dHAp). M2+ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M2+ are lower in dHAp than in stoichiometric HAp (sHAp) at all M2+ concentrations. This indicates that these M2+ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca-O bond lengths over a wide area so that Ca-O polyhedrons with various sizes are produced. As a result, M2+ ions can predominantly occupy Ca sites at which M2+ fits better, depending on the ionic radii of M2+. For Zn2+ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp.

DOI： 10.1111/jace.18853

Web of Science

Scopus

Atomic structures of grain boundaries for Si and Ge: A simulated annealing method with artificial-neural-network interatomic potentials

Tatsuya Yokoi, Hirotaka Kato, Yu Oshima, Katsuyuki Matsunaga

Journal of Physics and Chemistry of Solids Vol. 173 2023.2

Publishing type：Research paper (scientific journal)

To accurately predict low-energy structures for symmetric tilt grain boundaries (GBs) in Si and Ge, artificial-neural-network (ANN) interatomic potentials are constructed and are combined with a simulated annealing (SA) method based on molecular dynamics simulations. The ANN-driven SA method is demonstrated to predict GB structures that are in good agreement with previous electron microscopy observations, without prior knowledge about their atomic configurations. Their GB energies also reasonably agree with density-functional-theory (DFT) calculations. By contrast, a conventional empirical potential fails to predict those GB structures. For misorientation angles 2θ≥93.37°, the lowest-energy structures are found to contain atomic configurations that cannot be reproduced by one repeat unit of the perfect crystal along the tilt axis. Such GB structures cannot be obtained using the γ-surface method, although it is most commonly used for exploring low-energy GB structures. These results highlight the importance of using simulation cells with multiple repeat units along the tilt axis and of performing the SA method with high-accuracy interatomic potentials transferable to GBs.

DOI： 10.1016/j.jpcs.2022.111114

Web of Science

Scopus

DFT calculations of carrier-trapping effects on atomic structures of 30° partial dislocation cores in zincblende II-VI group zinc compounds

Sena Hoshino, Yu Oshima, Tatsuya Yokoi, Atsutomo Nakamura, Katsuyuki Matsunaga

Physical Review Materials Vol. 7 ( 1 ) 2023.1

Atomic structures and stability of finite-size extended interstitial defects in silicon: Large-scale molecular simulations with a neural-network potential

Ohbitsu M., Yokoi T., Noda Y., Kamiyama E., Ushiro T., Nagakura H., Sueoka K., Matsunaga K.

Scripta Materialia Vol. 214 2022.6

Language：Japanese Publisher：Scripta Materialia

The energetics of various extended interstitial (I) defects in crystalline Si is examined by constructing an artificial-neural-network (ANN) potential trained with density-functional-theory (DFT) data, enabling us to perform accurate large-scale simulations and to obtain well-converged formation energies (Ef). By varying the number of interstitials n to around 1,000, Ef is calculated for the compact cluster, I12-like, (001)-plane, (311)-rod-like and Frank-loop defects. For n ≤ 36, the compact cluster or (311)-rod-like defect is found to be most stable, depending on n. This trend strongly depends on simulation cell sizes, suggesting the importance of sufficiently large cells. For 36 < n ≲ 860, the (311)-rod-like defect is most stable whereas the Frank-loop defect becomes most stable for larger n. The ANN potential is demonstrated to outperform empirical potentials in prediction of Ef and defect structures. Furthermore, ANN values of Ef are fitted to analytic functions with the aim of refining macroscopic simulations for device manufacturing processes.

DOI： 10.1016/j.scriptamat.2022.114650

Scopus

First-Principles Analysis of the Oxide-Ion Conduction Mechanism in Si-Deficient Lanthanum Silicate Apatite

Yusuke Ogura, Tatsuya Yokoi, Kotaro Fujii, Masatomo Yashima, Katsuyuki Matsunaga

Journal of Physical Chemistry C Vol. 126 ( 13 ) page： 5805 - 5812 2022.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

Si-deficient lanthanum silicate apatites (La9.33+0.67xSi6-0.5xO26, LSOs) have attracted attention for their high oxide-ion conductivities. However, the carrier species and the conduction pathways responsible for the oxide-ion conduction are still unclear. In this study, the conduction mechanism of oxide ions in Si-deficient LSO was studied by first-principles calculations. It was found that Si-deficient LSO can include a small amount of excess oxide ions, which are located at the interstitial site along the c axis. The excess oxide ions in Si-deficient LSO can rapidly diffuse along the c axis by the push-pull mechanism with a potential barrier of 0.35 eV, which involves cooperative movements of two neighboring oxide ions, O4 ions, and the excess oxide ions. The potential barrier height was in good agreement with the experimentally reported activation energy. It is shown that a small amount of excess oxygen contributes critically to oxide-ion conduction in Si-deficient LSO, as was also discussed in stoichiometric La9.33Si6O26. These findings indicate the existence of a mechanism whereby a small amount of excess carriers in the ion-conduction pathways can significantly change the energy profiles for ionic conduction and enhance ion conductivity.

DOI： 10.1021/acs.jpcc.1c10486

Web of Science

Scopus

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Papers - YOKOI Tatsuya