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

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Scopus

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

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PubMed

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1016/j.scriptamat.2023.115685

Language：English   Publisher：The Ceramic Society of Japan  

<p>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.</p>

DOI： 10.2109/jcersj2.23060

CiNii Research

Language：English   Publisher：The Ceramic Society of Japan  

<p>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(4<span style="text-decoration: overline;">5</span>10)/[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.</p>

DOI： 10.2109/jcersj2.23044

CiNii Research

Language：English   Publisher：The Ceramic Society of Japan  

<p>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.</p>

DOI： 10.2109/jcersj2.23055

CiNii Research

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

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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

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Publisher：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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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

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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

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevmaterials.7.013603

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.7.013603/fulltext

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

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

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Language：Japanese   Publisher：Physical Chemistry Chemical Physics  

To accurately predict grain boundary (GB) atomic structures and their energetics in CdTe, the present study constructs an artificial-neural-network (ANN) interatomic potential. To cover a wide range of atomic environments, large amounts of density functional theory (DFT) data are used as a training dataset including point defects, surfaces and GBs. Structural relaxation combined with the trained ANN potential is applied to symmetric tilt and twist GBs, many of which are not included in the training dataset. The relative stability of the relaxed structures and their GB energies are then evaluated with the DFT level. The ANN potential is found to accurately predict low-energy structures and their energetics with reasonable accuracy with respect to DFT results, while conventional empirical potentials critically fail to find low-energy structures. The present study also provides a way to further improve the transferability of the ANN potential to more complicated GBs, using only low-Σ GBs as training datasets. Such improvement will offer a way to accurately predict atomic structures of general GBs within practical computational cost. This journal is

DOI： 10.1039/d1cp04329c

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PubMed

Language：Japanese  

DOI： 10.1016/j.ssi.2021.115793

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Language：Japanese   Publisher：Applied Physics Letters  

Controlling thermal conductivity in nanocrystalline materials is of great interest in various fields such as thermoelectrics. However, its reduction mechanism has not been fully given due to the difficulty to assess local thermal conduction at grain boundaries (GBs) and grain interiors. Here, we calculated spatially decomposed thermal conductivities across and along MgO symmetric GBs using perturbed molecular dynamics, varying the GB separation from 2.1 to 20.0 nm. This reveals the different length scale of GB scattering for two directions: over hundreds of nanometers across GBs while within a few nanometers along GBs. Numerical analyses based on the spatially decomposed thermal conductivities demonstrate that the former is dominant upon suppressing thermal conductivity in polycrystalline materials, whereas the latter has a non-negligible impact in nanocrystalline materials because of a large reduction of intragrain thermal conductivity along GBs. These insights provide the exact mechanisms of heat transport in nanocrystalline materials toward more precise control of thermal conductivity.

DOI： 10.1063/5.0075854

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

DOI： 10.1021/acs.jpcc.1c07973

Language：Japanese  

DOI： 10.1016/j.jcrysgro.2021.126313

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Language：Japanese  

DOI： 10.1039/d1ra06311a

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Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/27660400.2021.1969701

Language：Japanese  

DOI： 10.1038/s41563-020-00879-z

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DOI： 10.1039/d1cp00790d

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DOI： 10.1021/acs.nanolett.0c04337

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DOI： 10.1021/acs.inorgchem.0c03240

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DOI： 10.1007/s10853-020-05488-4

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ssi.2020.115367

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1111/jace.17263

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2020.06.016

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Language：Japanese   Publishing type：Research paper (scientific journal)  

© 2020 Inorganic semiconductors show interesting mechanical behavior in response to their surrounding lighting environments. In fact, we recently found out that zinc sulfide (ZnS) shows the light-illumination controlled plasticity at room temperature. However, its mechanism is poorly understood. Here we report systematic density functional theory results of electronic and atomic structures of glide partial dislocations in ZnS. We have revealed that partial dislocations in ZnS have excess electrostatic fields localized around their cores and can trap electrons or holes, depending on excess ionic species at the cores. Such carrier-dislocation interactions can induce energetically more stable bond reconstruction at their cores, as compared with the case before carrier trapping. Reconstructed bonds at the dislocation cores should be broken upon dislocation glide, and thus can restrict the dislocation motion significantly. Such reduced dislocation motion due to core reconstruction should give rise to increased plastic deformation stresses, namely, hardening of the materials under light illumination. The present results provide a critical understanding of the experimentally observed light-illumination controlled plasticity in ZnS at the electronic level, which is opposed to previously reported plasticity and dislocation motion in other elemental and III-V semiconductors.

DOI： 10.1016/j.actamat.2020.06.010

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41467-020-15619-9

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.nanolett.9b05298

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jeurceramsoc.2019.10.060

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Language：Japanese  

DOI： 10.1103/PhysRevMaterials.4.026002

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Language：Japanese  

DOI： 10.1103/PhysRevMaterials.4.014605

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Language：Japanese   Publishing type：Research paper (scientific journal)  

© 2019 Acta Materialia Inc. Grain boundaries often have a decisive effect on the macroscopic properties of polycrystalline materials, but the wide variety of their atomic structures, interfacial lengths, and compositions makes them difficult to characterize all-encompassingly. An indispensable example in which an understanding of the relationship between grain boundary structures and properties would greatly facilitate development of superior materials is thermal transport, especially with respect to microstructure evolution, thermoelectrics and thermal barrier coatings. To contribute to a more comprehensive understanding, we performed a systematic study of lattice thermal conduction across a wide range of symmetric tilt grain boundaries in MgO using perturbed molecular dynamics. It was found that thermal conductivities vary significantly with grain boundary structure but are strongly correlated with excess volume, which is a measure of the number density of atoms in the vicinity of the grain boundary planes. Real-space analysis revealed that dislocation densities determine the phonon transport paths and thermal conductivity in low-angle boundaries whereas it is the amount of open volume rather than the shape of structural units in high-angle boundaries that determine thermal conductivity. We also found that low-angle boundaries mainly reduce phonon transports at low frequencies whereas high-angle boundaries also reduce it at intermediate and high frequencies effectively, regardless of the shape of structural units. These insights are expected to be applicable to other close-packed oxide systems, and should aid the design of next-generation thermal materials through tailoring of grain boundaries.

DOI： 10.1016/j.actamat.2019.04.009

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.2320/matertrans.M2018253

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

DOI： 10.1039/c8ta02859a

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

DOI： 10.7567/APEX.11.061303

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Language：Japanese   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier B.V. This study focuses on transformation of grain boundary (GB) structures under high pressures up to 60 GPa by using a simulated annealing technique with molecular dynamics and lattice statics calculations for various symmetric tilt GBs (STGBs) of MgO. It is found that except for the Σ3(111)/[11¯0] that is a rather stable GB, all the STGBs studied transform into a metastable structure more than once at threshold pressures. In addition, the GBs with an open-core structure and small tilt angle are found to be more “flexible” to transform into different structures than the GBs with a dense structure. For polycrystalline MgO, therefore, GBs may also exhibit GB transformation under high pressures and flexible GBs may govern overall transformation and deformation. These findings also suggest that polycrystals sintered at high pressures consist of more pressure-resistant GBs than those at normal pressures.

DOI： 10.1016/j.physb.2017.03.014

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

DOI： 10.3390/cryst8030127

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DOI： 10.1080/14786435.2018.1478146

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

DOI： doi:10.1038/ncomms11079

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.2320/matertrans.MA201567

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.scriptamat.2015.02.021

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/la503338x