担当区分：筆頭著者,　責任著者   記述言語：英語  

記述言語：英語  

出版者・発行元：Journal of Materials Science and Technology  

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness.

DOI： 10.1016/j.jmst.2023.02.017

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Scopus

出版者・発行元：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

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：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

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Physics and Chemistry of Solids  

The authors regret the inclusion of author Hirotaka Kato. Professor Hirotaka Kato name has been removed from the article. The authors would like to apologise for any inconvenience caused.

DOI： 10.1016/j.jpcs.2023.111273

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Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Acta Materialia  

Light environment drastically alters the plasticity of zinc sulfide (ZnS) single crystals: they exhibit ductility in darkness while brittleness in lights. This study investigated their microscopic deformation modes of the light-environment-dependent plasticity, by using synchrotron X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning TEM (STEM) measurements. The XRD analyses disclosed that the ZnS crystals originally have two domains with almost the same volume fraction with a twin relationship and plastically deform in darkness, accompanying changes in the domain volume ratio (namely extinction of twins). It was also observed that the domain volume ratios become constant to be 0.2 at more than 10% plastic strains. The STEM analyses indicated that the deformation mode in darkness switches at around 10% plastic strain from slip deformation by glide of isolated partial dislocations to by simultaneous glide of paired partials inside the major domains. Although ZnS single crystals exhibited only a few percent of plastic strains in lights, localized glide of isolated partials and the resultant changes in domain volume ratios similar to in darkness were confirmed. That is, the deformation modes of ZnS at the initial stage were similar regardless of light environments although the macroscopic deformation behaviors were quite different.

DOI： 10.1016/j.actamat.2023.118738

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the American Ceramic Society  

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

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Scopus

出版者・発行元：Journal of Physics and Chemistry of Solids  

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

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review Materials  

It was experimentally reported that II-VI group inorganic semiconductor crystals become harder and brittle by external light illumination. In order to reveal essential factors of the light illumination effect, systematic GGA+U calculations are performed for 30° partial dislocations in ZnSe and ZnTe, and the obtained results are discussed together with those for ZnS reported previously. It is found in these three crystal systems that the cores of pristine partial dislocations have unreconstructed atomic structures whereas their cores undergo atomic reconstruction energetically more favorably by trapping excess carriers. Such carrier trapping and atomic reconstruction of the dislocation cores are ascribed to the presence of excess electrostatic potentials at the cores due to ionic bonding characters of the host crystals. The dislocation core reconstructions decrease potential energies of the partial dislocations and can in turn increase Peierls-potential barriers for dislocation glide, corresponding to the observed hardening and brittleness by light illumination. The energy gains due to the dislocation-core reconstructions also depend on energy positions of the defect-induced levels that appear within the band gaps.

DOI： 10.1103/PhysRevMaterials.7.013603

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記述言語：英語  

記述言語：英語  

記述言語：英語  

担当区分：筆頭著者,　責任著者   記述言語：英語  

記述言語：英語  

記述言語：日本語   出版者・発行元：公益社団法人 日本金属学会  

DOI： 10.2320/materia.61.629

CiNii Research

記述言語：日本語   出版者・発行元：公益社団法人 日本顕微鏡学会  

<p>近年，脆く壊れやすいと考えられていた半導体材料が室温でも高い可塑性を示しうることが明らかとなっている．例えば，チタン酸ストロンチウム（SrTiO₃）結晶では，点欠陥濃度を制御することで室温での塑性変形能（可塑性）が向上する．また，硫化亜鉛（ZnS）結晶では，光照射下では壊れやすいが，暗闇の中では大きな可塑性を発現する．このように従来想像すらされていなかった結晶の異常な可塑性が見出され，大きく注目を集めている．これら半導体ともセラミックスとも言える材料系は，脆いがゆえに構造材としての適正がなく，結果として専ら機能材料として利用されてきた．材料の脆さを克服できるメカニズムが見つかれば，そのメカニズムの理解は様々な材料系にとって非常に有用であり，構造材として利用できる材料系の拡大が期待される．本稿では，結晶の形状変化のその場観察および電子顕微鏡による転位組織観察を元に，SrTiO₃よびZnSの可塑性向上に関する最新の研究を議論する．</p>

DOI： 10.11410/kenbikyo.56.3_110

CiNii Research

記述言語：日本語   出版者・発行元：一般社団法人 粉体粉末冶金協会  

The science and technology related with light has revolutionized modern society, and understanding the effects of light on semiconducting materials has become crucial to current science and technology. Although much research has been done on the effects of light on the electronic and optical properties of materials, the effects of light on the mechanical properties of materials are not well understood. It was recently found that extraordinarily large plasticity appears in bulk compression of single-crystal ZnS in complete darkness even at room-temperature. This is believed to be due to the less interactions between dislocations and photo-excited electrons and/or holes. However, methods for evaluating dislocation behavior in such semiconductors with small dimensions under a particular light condition had not been well established. Here we show a new nanoindentation method that incorporates well designed lighting system for exploring dislocation behavior depending on the light conditions in advanced semiconductors. We used single-crystal ZnS as a model material because its bulk deformation behavior has been well investigated. It is confirmed that the decrease of dislocation mobility with light observed in conventional bulk deformation tests can be understood even by the nanoindentation tests at room-temperature. It is remarkable that we experimentally demonstrate that dislocation mobility appears to be more sensitive to light exposure than dislocation nucleation.

DOI： 10.2497/jjspm.68.469

Scopus

CiNii Research

記述言語：日本語   出版者・発行元：Applied Physics Letters  

An enormous change in the dislocation-mediated plasticity has been found in a bulk semiconductor that exhibits the photoplastic effect. Herein, we report that UV (365 nm) light irradiation during mechanical testing dramatically decreases the fracture toughness of ZnS. The crack tip toughness on a (001) single-crystal ZnS, as measured by the near-tip crack opening displacement method, is increased by ∼45% in complete darkness compared to that in UV light. The increase in fracture toughness is attributed to a significant increase in the dislocation mobility in darkness, as explained by the crack tip dislocation shielding model. Our finding suggests a route toward controlling the fracture toughness of photoplastic semiconductors by tuning the light irradiation.

DOI： 10.1063/5.0047306

Open Access

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Scopus

記述言語：英語   出版者・発行元：Nano Letters  

It was recently found that extremely large plasticity is exhibited in bulk compression of single-crystal ZnS in complete darkness. Such effects are believed to be caused by the interactions between dislocations and photoexcited electrons and/or holes. However, methods for evaluating dislocation behavior in such semiconductors with small dimensions under a particular light condition had not been well established. Here, we propose the "photoindentation"technique to solve this issue by combining nanoscale indentation tests with a fully controlled lighting system. The quantitative data analyses based on this photoindentation approach successfully demonstrate that the first pop-in stress indicating dislocation nucleation near the surface of ZnS clearly increases by light irradiation. Additionally, the room-temperature indentation creep tests show a drastic reduction of the dislocation mobility under light. Our approach demonstrates great potential in understanding the light effects on dislocation nucleation and mobility at the nanoscale, as most advanced technology-related semiconductors are limited in dimensions.

DOI： 10.1021/acs.nanolett.0c04337

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Scopus

PubMed

記述言語：日本語   出版者・発行元：公益社団法人 応用物理学会  

<p>光環境が半導体材料の電気的性質に影響することは広く知られているが，力学的性質にどのような影響があるかはあまり理解されていなかった．筆者らは最近，無機半導体の1つである硫化亜鉛（ZnS）結晶が，暗闇下であれば室温でも金属並みの可塑性（変形能，柔軟性）を示すことを発見した．暗闇下における塑性変形量は塑性ひずみ量45%に達しており，無機半導体はもろいというこれまでの常識を覆すものだった．興味深いことに，暗闇下で塑性変形を受けたZnS結晶はもともと大きくバンドギャップが変化し，従来知られていなかった新奇な光物性を示すことも明らかになりつつある．このような物性の変化は，変形により導入された金属並みの高密度な転位が無機半導体中に特異な電子構造を形成していることによる．つまり，従来は悪者にすぎなかった結晶欠陥の転位を逆に有効利用することで，新奇な半導体物性の開拓が可能となると期待される．</p>

DOI： 10.11470/oubutsu.90.3_176

CiNii Research

記述言語：日本語   出版者・発行元：公益社団法人 日本金属学会  

DOI： 10.2320/materia.60.30

CiNii Research