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Non-stoichiometric structure localized at the grain boundary, namely, segregations of impurities, dopants, and vacancies, has an important effect on a broad variety of material properties. An understanding of this behavior is therefore indispensable for further material development. Although molecular dynamics simulation and a simulation combined with randomly swapping atoms and vacancies have usually been used to investigate the segregation structures, they require more than ten thousand structures and energy calculations to reach the stable configuration. Although several mathematical or informatics approaches, for example, genetic algorithm and Bayesian optimization, have been proposed to solve such combination optimization problems, they required some hyper parameters which crucially affect efficiency and huge computations to tune these parameters. Furthermore, a parallelization of the computation task is often impossible in molecular dynamics simulation and Bayesian optimization because their structures are related to each other before and after the time or simulation steps. Here, we develop a Monte Carlo tree search algorithm for grain boundary segregation and apply it to determine the stable segregation configuration of copper Σ5[001]/(210) and Σ37[001]/(750) with silver impurities. We achieved a determination of the stable configuration by searching only 1% of all possible configurations. Furthermore, we found that the search path and the number of playouts at the branch provide important insight to comprehend the background of the search. In the present case, the search path was identical to the sites with the spatially larger sites.

Native defects and Pr dopant around grain boundaries in Pr-doped and undoped ZnO bicrystals were investigated by electron energy loss spectroscopy (EELS) with a focus on the relationship with the current–voltage characteristics. The Pr-doped bicrystal exhibited a nonlinear current–voltage characteristic, whereas the undoped bicrystal shows an ohmic characteristic. In the Pr-doped bicrystal, Pr was found to be present within 8nm around the grain boundary. EELS investigation of native defects combined with first-principles calculations indicated the presence of zinc vacancies in the vicinity of the Pr-doped grain boundary. The formation of zinc vacancies is considered to be the origin of the nonlinear current–voltage characteristic.

Electronic structures and stability of substitutional Ti3+ clusters in Al2O3 were investigated by first-principles pseudopotential calculations using large supercells. It was found that a substitutional Ti3+ defect induces an extra defect level in the band gap, and the defect levels have strong bonding interactions in the cluster formation. In addition, binding energies of Ti3+ clusters increased with increasing numbers of Ti3+ ions, indicating that cluster formation is energetically favorable. The resulting impurity band within the band gap of Al2O3 will contribute to the significant electronic conduction of Ti-doped Al2O3 observed experimentally.

X-ray absorption near edge structure (XANES) analysis is an element-specific method for proving electronic state mostly in the field of applied physics, such as battery and catalysis reactions, where the valence change plays an important role. In particular, many results have been reported for the analysis of positive electrode materials of Li-ion batteries, where multiple transition materials contribute to the reactions. However, XANES analysis has been limited to identifying the valence state simply in comparison with reference materials. When the shape of XANES spectra shows complicated changes, we were not able to identify the valence states or estimate the valence quantitatively, resulting in insufficient reaction analysis. To overcome such issues, we propose a valence state evaluation method using K- and L-edge XANES analysis with first-principles simulations. By using this method, we demonstrated that the complicated reaction mechanism of Li(Ni1/3Co1/3Mn1/3)O2 can be successfully analyzed for disti...

Density functional theory calculations with a correction of the long-range dispersion force, namely the van der Waals (vdW) force, are performed for SiC polytypes. The lattice parameters are in good agreement with those obtained from experiments. Furthermore, the stability of the polytypes in the experiments, which show 3C-SiC as the most stable, are reproduced by the present calculations. The effect of the vdW force on the electronic structure and the stability of polytypes are discussed. We observe that the vdW interaction is more sensitive to the cubic site than the hexagonal site. Thus, the influence of the vdW force increases with decreasing the hexagonality of the polytype, which results in the confirmation that the most stable polytype is 3C-SiC., 12pages, 1table, 4figures

A new lithium-niobate (LiNbO3)-type polar compound, namely, ZnPbO3 (a = 5.41605(7) A and c = 14.33151(3) A), with closed-shell ions only was synthesized under high pressure and high temperature (8 GPa and 1273 K). A point-charge-model calculation based on atomic positions refined by Rietveld analysis of synchrotron X-ray diffraction data gave an electrical ionic polarization of 77 μC/cm2 along the hexagonal c-axis. Detailed structural analysis indicated that the contribution of Pb4+ in ZnPbO3 to the polarization was almost twice as large as that of Sn4+ in ZnSnO3. Transport measurement showed metallic behavior of ZnPbO3 from room temperature to low temperature despite the fact that both cations are closed-shell ions.

We investigated the migration energy and vacancy formation energy of La and Sr ions at a LaAlO3/SrTiO3 heterointerface using first-principles calculations. Our study reveal that the migration energies at the p-type interface are lower than those at the n-type interface, and the formation energies of Sr and La vacancies are relatively high when we assume a reduction atmosphere and insulator conditions. To explain the experimental evidence that intermixing is preferentially taking place at the n-type interface, considering the Fermi energy is critical. We find that the presence of electron carriers plays an important role in the intermixing behaviors at the LaAlO3/SrTiO3 heterointerface.

To develop a higher efficiency thin-film compound photovoltaic (PV) cell, it is crucial to understand not only the key factors determining the stability of the grain boundary (GB) but also the band gap and band bending at the GB in the photoabsorption layer. In this study, we investigate the atomic structure, band gap, and electrostatic potential at the (112)[11¯0] twin GB of CuInSe2 (CIS), which is typically used in the photoabsorption layer in thin-film compound PV cells. We utilize aberration-corrected scanning transmission electron microscopy and first-principles hybrid-functional calculations. We find that the formation energies of the twin GB in CIS are very small: 0.03–0.15 J/m2. The local coordination of the Se at the GB is a primary factor modulating the GB energy, band gap, and the electrostatic potential at the GB.

The solid solutions between ilmenite (FeTiO3) and hematite (α-Fe2O3) have recently attracted considerable attention as a spintronic material due to their interesting magnetic and electrical properties. In this study, the electronic and magnetic structures of epitaxially grown 0.6FeTiO3·0.4Fe2O3 solid solution thin films were investigated by combining x-ray absorption near-edge structure (XANES), x-ray magnetic circular dichroism (XMCD) for two different crystallographic projections, and first-principles theoretical calculations. The Fe L-edge XANES and XMCD spectra reveal that Fe is in the mixed-valent Fe2+–Fe3+ states while Fe2+ ions are mainly responsible for the magnetization. Moreover, the experimental Fe L-edge XANES and XMCD spectra change depending on the incident x-ray directions, and the theoretical spectra explain such spectral features. We also find a large orbital magnetic moment, which can originate the magnetic anisotropy of this system. On the other hand, although the valence state of Ti was interpreted to be 4+ from the Ti L-edge XANES, XMCD signals indicate that some electrons are present in the Ti-3d orbital, which are coupled antiparallel to the magnetic moment of Fe2+ ions.

The redox reaction of Mn in Li2MnO3 was studied by X-ray absorption spectroscopy and ab initio multiplet calculation. Associated with the de-intercalation of Li-ion, small but clear spectral changes were observed in Mn-L2,3 X-ray absorption near edge structure (XANES). The systematic ab initio multiplet calculations of Mn-L2,3 XANES revealed that the spectral changes in the experiment could not simply be ascribed to the change of the valency from Mn4+ to Mn5+ but can be explained well by the changes of local atomic structures around Mn4+ due to the Li de-intercalation. Our results suggest that the electronic state of oxygen should change during charging in Li2MnO3.

The distorted CuO6 octahedron in La2CuO4 was studied using aberration-corrected scanning transmission electron microscopy at atomic resolution. The near-edge structure in the oxygen K-edge electron energy-loss spectrum was recorded as a function of the position of the electron probe. After background subtraction, the measured spectrum image was processed using a recently developed inversion process to remove the mixing of signals on the atomic columns due to elastic and thermal scattering. The spectra were then compared with first-principles band structure calculations based on the local-density approximation plus on-site Coulomb repulsion (LDA + U) approach. In this article, we describe in detail not only anisotropic chemical bonding of the oxygen 2p state with the Cu 3d state but also with the Cu 4p and La 5d/4f states. Furthermore, it was found that buckling of the CuO2 plane was also detectable at the atomic resolution oxygen K-edge. Lastly, it was found that the effects of core-hole in the O K-edge were strongly dependent on the nature of the local chemical bonding, in particular, whether it is ionic or covalent.

The effects of the local coordination environment of Mn ions in perovskite manganese oxides on the Mn-L2,3 edge electron energy loss (EEL) spectra was experimentally and theoretically investigated. The Mn-L2,3 edge EEL spectra were observed for various perovskite manganese oxides, including YMnO3, LaMnO3, BaMnO3, SrMnO3, and CaMnO3, in which the Mn ions have different valence states and local coordination. The experiment revealed that the Mn L3/L2 ratio is influenced not only by the valence state but also by the local environment of the Mn ions. Furthermore, compared to the Mn L3/L2 ratios of Mn3+ compounds, the Mn L3/L2 ratios of the Mn4+ compounds are found to be much more sensitive to local distortions. The ab-initio multiplet calculation of the Mn-L2,3 edge EEL spectra revealed that the effects of local coordination on the spectral features are dependent on the local electronic structures of the Mn ions. These findings indicate that the valence state as well as the local environments of the Mn ions can be unraveled by combining experimental and theoretical investigations of Mn-L2,3 edge EEL spectra.

We investigated the effects of neighboring vacancies and their charge state on ion migration in rhombohedral-LaAlO3. Our results show that the oxygen migration energy is greatly influenced by the charge state of the oxygen vacancies, which is attributed to the neutral oxygen vacancy generating a localized electronic state at the vacancy site, while the charged oxygen vacancy has delocalized electronic state. In contrast, the charge state of the vacancy does not influence the migration of cations. Furthermore, we found that cation vacancies around the migrating ion enhance the cation migration, whereas they inhibit oxygen migration.

Electron energy-loss spectroscopy (EELS) studies in scanning transmission electron microscopy are widely used to investigate the location and bonding of atoms in condensed matter. However, the interpretation of EELS data is complicated by multiple elastic and thermal diffuse scattering of the probing electrons. Here, we present a method for removing these effects from recorded EELS spectrum images, producing visually interpretable elemental maps and enabling direct comparison of the spectral data with established first-principles energy-loss fine structure calculations.

The solution behavior of dopants, Si, La, and N, in HfO2 polymorphs, monoclinic, tetragonal, and cubic was investigated by using a first principles calculation based on density functional theory within the local density approximation. It was found that the Si and La solutions are more preferable under oxidization atmosphere than reduction atmosphere, and the most efficient nitridation condition for the HfO2 is NO/NO2 atmosphere. By comparing the energy difference between the monoclinic and the tetragonal phases, it was found that the energy difference is decreased by the Si or La doping, whereas influence of the N doping is small, indicating that the phase transition from the monoclinic to tetragonal phase would be enhanced by the Si or La doping.

The defect structures around oxygen vacancies in perovskite-type oxides play very important roles in determining material properties through the change of the electronic state. Therefore, from the viewpoint of developing condensed matter physics and technological applications, it is crucial to obtain direct images of these structures. We report the simultaneous visualization of oxygen vacancies and the cation shifts around them in perovskite-type manganite by scanning transmission electron microscopy. The defective structure of the material makes it amenable to using Fourier analysis to obtain important structural information like the oxygen vacancy locations and the shift of nearby cation columns.

La2/3−xLi3xTiO3 (LLT, x = 0.1 and 0.167) thin films were grown on (001)-oriented SrTiO3 (STO) substrates by pulsed laser deposition. The resultant LLT films were heteroepitaxially grown with atomically flat surfaces. Although the films considered to be single crystalline, they were composed of 90°-rotated multi domains (∼5 nm in size) with epitaxial relationships of (100) [001] LLT || (001) [100] STO and (100) [010] LLT || (001) [100] STO. The bulk conductivity of the LLT (x = 0.1) films was 2.5 × 10−2 S cm−1 at 190 °C, comparable to that of a bulk single crystalline LLT. Single crystalline LLT thin films with atomically flat surfaces should help clarify interfacial lithium-ion conductivity.

The atomic structure of a Σ3 [110]/(111) grain boundary in CeO2 was studied by scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principles calculations. It was revealed that this grain boundary does not promote the formation of oxygen vacancies and keeps oxygen stoichiometry, which is different from that of Σ5 CeO2 grain boundary studied previously [H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, N. Shibata, T. Yamamoto, and Y. Ikuhara, Nano Lett. 10, 4668 (2010)]. It was found that the difference in grain boundary oxygen stoichiometry is correlated with the grain boundary atomic structure.

The effect of oxygen vacancy on Ti-L2,3 electron energy-loss near-edge structures (ELNES) of BaTiO3 was theoretically investigated through ab initio multiplet calculation. The presence of an oxygen vacancy influences spectral features not only at the nearest neighbor Ti site but also at Ti sites further from the oxygen vacancy. The effects of different oxygen vacancy concentrations were also investigated. Based on this study, it was concluded that the detection limit for oxygen vacancy with Ti-L2,3 ELNES is approximately 1%.

Ruddlesden-Popper (R.P.) type faults have been investigated for Sr-excess SrTiO3 (STO) homoepitaxial thin films by pulsed laser deposition (PLD) and tuning of the laser fluence. The R.P. type fault structures were found to depend on the amount of excess Sr. The growth direction of the planer faults changed from normal to normal and parallel to the substrate surface with an increase in excess Sr. Due to the directional change of the planer faults, the out-of-plane lattice shows lattice expansion. The results suggest that we can control the planer fault structures in STO thin films simply by changing the laser fluence in the PLD method.

The atomic and electronic structures at interfaces in thin films are typically different from the bulk and are vitally important in determining the physical properties of thin films. The interface between SrVO3, chosen as a prototype for vanadium-based perovskite materials in this work, and LaAlO3 substrate is investigated by scanning transmission electron microscopy, electron energy-loss spectroscopy, and theoretical multi-electron calculations. Extra electrons have been detected on the interface layer by comparing the energy-loss near-edge structures of V-L3,2 edges to those from the film far from the interface. Monochromated EELS and theoretical calculations for SrVO3, VO2, and V2O 3 support this conclusion. The extra electrons appear to originate from a change in the local bonding configuration of V at the La-O terminated substrate surface as determined by Z-contrast imaging. © 2011 American Institute of Physics.

The defect structure of a homoepitaxial thin film of SrTiO3 containing excess Ti grown by pulsed laser deposition is investigated. The Sr/Ti ratio of the film is controlled to possess a slight excess of Ti by tuning the laser fluence. High-resolution transmission electron microscopy and electron energy loss spectroscopy studies reveal that the film contains Sr vacancies because of the presence of excess Ti. These vacancies tend to cluster as platelets several nanometers in diameter on (001) planes in the films. The SrO planes that contain the vacancy clusters exhibit lattice expansion due to Coulomb repulsion.

Threading dislocations in CeO2 thin films grown on yttria-stabilized ZrO2 substrates were investigated by transmission electron microscopy (TEM), high-resolution TEM, and scanning TEM. It is revealed that there are two kinds of threading dislocations with the Burgers vector of b=1/2⟨110⟩: one is pure edge-type and the other is mixed-type. Comparing the strain field of the mixed-type dislocations with that of the Peierls–Nabarro and the Foreman dislocation models, we find that the Foreman model better describes it in CeO2.

The atomistic behavior of migration in SrTiO3 was investigated by first-principles nudged elastic band calculations. Calculated migration energies for Sr and oxygen are consistent with experimental values. In contrast, the calculated energy for Ti with a simple Ti-vacancy mechanism is far larger than the experimental value. In examining different Ti-migration mechanisms, the Ti-migration energy is found to decrease and become comparable to the Sr-migration energy by introducing a Sr vacancy. This Sr-vacancy-mediated Ti migration, which is consistent with the experimentally proposed mechanism by Gomann et al. [Phys. Chem. Chem. Phys. 6, 3639 (2004)], is confirmed theoretically by the present calculations.

The oxidation state of Mn in cubic SrMnO(3) and orthorhombic SrMnO(2.5) was investigated by electron energy loss (EEL) spectroscopy. Change in the oxidation state of Mn produced some spectral changes in the O-K edge as well as in the Mn-L(2,3) edge EEL spectra. This study demonstrated that the oxidation state of Mn and the amount of oxygen vacancies in cubic SrMnO(3) and orthorhombic SrMnO(2.5) could be quantified by analyzing the features of the O-K edge spectrum and the Mn L(3)∕L(2) ratio in the Mn-L(2,3) edge spectrum. Our quantitative analysis showed that the spectral changes in the Mn-L(2,3) edge were mainly caused by the oxidation state of Mn, whereas those in the O-K edge could be sensitive to both the oxidation state of Mn and to lattice distortions.

Conductive nanowires were fabricated in GaN thin film by selectively doping of Al along threading dislocations. Electrical current flow localized at the nanowires was directly measured by a contact mode atomic force microscope. The current flow at the nanowires was considered to be Frenkel–Poole emission mode, suggesting the existence of the deep acceptor level along the nanowires as a possible cause of the current flow. The results obtained in this study show the possibility for fabricating nanowires using pipe-diffusion at dislocations in solid thin films.

We fabricated conductive nanowires in insulating AlN thin films by doping Mn along high-density unidirectional threading dislocations. Investigation of the dislocation microstructures by transmission electron microscopy (TEM) and high-resolution scanning TEM revealed that Mn segregates to the dislocation cores. Strain analysis around the dislocations suggests that the strong attractive interaction between the Mn ions and the strain field around the dislocations enhances the confinement of the Mn only in the vicinity of the dislocation cores. Atomic force microscopy measurements under contact-current mode detected the local electrical conduction along the Mn-doped dislocations. The present results open up the possibility for fabricating functional nanowires using dislocations in thin films.

We report direct observation of fluorine ion dopants in the recently discovered LaFeAsO1−xFx superconductor, where the doping is critical for the superconductivity. Employing spectroscopic imaging based on scanning transmission electron microscopy, the spatial distribution of the doped fluorine ions, which are invisible in Z-contrast imaging, is clearly visualized with atomic-scale resolution. The observation directly proves that fluorine substitutes into the oxygen site in the superconducting LaFeAsO1−xFx compound. Our results demonstrate the potential of the present method for revealing dopants undetectable by conventional microscopy imaging.

Transmission electron microscopy and scanning transmission electron microscopy were performed to investigate B-site cation-ordered structures in a (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (x∼0.30) single crystal. Selected-area diffraction patterns showed presence of F-spots corresponding to 1:1 order. Besides, the F-spots were elongated to be “X”-like in shape. Transmission electron microscopy dark-field images revealed that size of ordered domains was about a few nanometers, and they were dispersed within disordered matrix. Scanning transmission electron microscopy indicated the presence of 1:1 ordered domains. Presence of 1:2 ordered structures was also indicated. These are consistent with features of the selected-area diffraction patterns.

Ca-ion superstructures such as sqrt(3)a×sqrt(3)a (hexagonal) and 2a×sqrt(3)a (orthorhombic) in layered Ca_0.33CoO_2 crystalline film, which were prepared by the reactive solid-phase epitaxy and the subsequent ion-exchange treatment, were directly distinguished by using spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The fluctuation of Ca intensities in the HAADF-STEM image indicates the existence of many Ca vacancies in the 2a×sqrt(3)a orthorhombic superstructure, which is consistent with the partial occupation of Ca (x=0.33) in the cation sites of x=0.5.

Supersaturated ZnO–Al2O3 (>20at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600°C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10at.%. The expansion disappears after annealing the films at above 800°C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)3(Al2O3) as the local environment of Al in the supers...

We report two-dimensional Seebeck coefficients (|S|_2D) of [(SrTiO_3)x/(SrTi_0.8Nb_0.2O_3)y]_20 (x=1–60, y=1–20) superlattices, which were grown on the (100) face of insulating LaAlO_3 substrates to clarify the origin of the giant |S|_2D values of the SrTiO_3 superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The |S|_2D values of the [(SrTiO_3)_17/(SrTi_0.8Nb_0.2O_3)y]_20 superlattices increased proportionally to y^{−0.5} and reached 320µVK^{−1} (y=1), which is approximately five times larger than that of the SrTi_0.8Nb_0.2O_3 bulk (|S|_3D=61µVK^{−1}). The slope of the log|S|_2D-log y plots was −0.5, proving that the density of states in the ground state for SrTiO_3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25nm (16 unit cells of SrTiO_3).

Multifunctional materials promise to provide the foundation for a new class of devices in which functional properties are coupled to one another. Examples include magnetoelectric materials in which magnetic and ferroelectricproperties are coupled. Here the authors report the successful growth of single phase, fully epitaxialthin films of the multifunctional material, Pb V O 3 , using pulsed laser deposition. This growth offers an alternative means for the production of Pb V O 3 outside of high-temperature and high-pressure techniques through growth of epitaxialthin films on various substrates. The structure of this highly distorted perovskite is examined using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy.

Electron energy loss near-edge structures (ELNES) and first-principles band structure calculations were combined in order to identify native defects which are segregated by heat treatments at the vicinity of SrTiO3 grain boundary (GB). Spectral differences between the bulk and the vicinity of GB mainly appear around the second peak of OK ELNES. The spectral differences can be reproduced by calculating the theoretical OK ELNES from the first-nearest-neighbor oxygen to a Sr vacancy. It is therefore concluded that the Sr vacancies are concentrated in the vicinity of GB by the heat treatment.

AlN thin films have been grown on c-cut sapphire substrates by pulsed-laser deposition. The film epitaxially grown at 1073 K under vacuum of 5×10−4Pa was used to examine the crystallographic orientation dependence of Al K-edge x-ray absorption near-edge structures (XANES), which satisfactorily agrees with theoretical spectra obtained by first-principles calculations. The film grown at 1073 K with N2 backfill of 7×10−2Pa shows nanotextured structure with its c plane parallel to the substrate. Although the nanotexture is not evident by x-ray diffraction, XANES can unambiguously indicate the texturing. Cross-sectional high-resolution electron microscopy provides the evidence of the nanostructure.