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

Plasmonic crystals are well known to have band structure including a band gap, enabling the control of surface plasmon propagation and confinement. The band dispersion relation of bulk crystals has been generally measured by momentum-resolved spectroscopy using far field optical techniques while the defects introduced in the crystals have separately been investigated by near field imaging techniques so far. Particularly, defect related energy levels introduced in the plasmonic band gap have not been observed experimentally. In order to investigate such a localized mode, we performed electron energy-loss spectroscopy (EELS) on a point defect introduced in a plasmonic crystal made up of flat cylinders protruding out of a metal film and arranged on a triangular lattice. The energy level of the defect mode was observed to lie within the full band-gap energy range. This was confirmed by a momentum-resolved EELS measurement of the band gap performed on the same plasmonic crystal. Furthermore, we experimentally and theoretically investigated the emergence of the defect states by starting with a corral of flat cylinders protrusions and adding sequentially additional shells of those in order to eventually form a plasmonic band-gap crystal encompassing a single point defect. It is demonstrated that a defectlike state already forms with a crystal made up of only two shells.

DOI： 10.1103/PhysRevB.100.245402

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

Plasmonic resonator arrays have attracted a great interest as a platform to enhance light-matter interaction and have been examined for their applicability to various types of optical devices, such as sensors, light emitter, and photocatalyst, to name a few. In a plasmonic resonator array, localized and propagating plasmon modes can hybridize, which is known to result in an anticrossing of the plasmon bands in the dispersion curves. However, it was so far unclear how the modal symmetry affects such a hybridization, especially when it occurs at a specific reciprocal lattice point with a high degree of symmetry, for example, the Î" point. In this work, we used momentum-resolved cathodoluminescence-scanning transmission electron microscopy to comprehensively characterize the modal hybridization at the Î" point. Our study reveals theoretically and experimentally the existence of mode symmetry selection rules that specify hybrid pairs of the lattice mode and localized mode.

DOI： 10.1021/acsphotonics.9b00977

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

Electron beam spectroscopy has recently attracted much attention in the modal analysis of nanophotonic and plasmonic systems. In principle, electron energy-loss spectroscopy (EELS) provides information about the electromagnetic local density of states of all sorts of electromagnetic modes as well as nonradiative modes. However, there have not been many examples related to the EELS analyses for electromagnetic Bloch modes. Herein, EELS measurements are performed for the characterization of nonradiative band-edge modes facing the first bandgap in a plasmonic crystal with a triangular lattice, which is well-known for its full bandgap formation. The obtained spectrum images clearly determine the characteristics of the lower and upper band-edge modes, compared with an analytical model based on group theory and a numerical simulation by the finite-difference time-domain method. The EELS spectra also reveal differences in the plasmonic density of states between the lower and upper band-edges, which provides information about the band deformation near the bandgap.

DOI： 10.1021/acsphotonics.8b00936

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

In conventional transmission electron microscopy, specimens to be observed are placed in between the objective lens pole piece and therefore exposed to a strong magnetic field about 2 T. For a ferromagnetic specimen, magnetization of the specimen causes isotropic and anisotropic defocusing, deflection of the electron beam as well as deformation of the specimen, which all become more severe when the specimen tilted. Therefore electron tomography on a ferromagnetic crystalline specimen is highly challenging because tilt-series data sets must be acquired without changing the excitation condition of a specific diffraction spot. In this study, a scanning transmission electron microscopy (STEM) tomography method without magnetizing a ferromagnetic specimen has been developed for three-dimensional (3D) visualization of dislocations in α-Fe, which is a typical ferromagnetic material. Magnetic-field-free environment down to 0.38 ± 0.07 mT at the specimen position is realized by demagnetizing the objective lens pole piece of a commercial STEM instrument. By using a spherical aberration corrector with the magnetic-field-free environment, an “aberration corrected Low-Mag STEM mode” with no objective lens field reaches a convergence semi angle ∼1 mrad and a spatial resolution ∼5 nm, and shows an adequate performance of imaging dislocations under a two-beam excitation condition for a low-index diffracted beam. The illumination condition for the aberration corrected Low-Mag STEM mode gives no overlap between the direct beam disk (spot) and neighboring diffraction disks. An electron channeling contrast imaging technique, in which an annular detector was located at a doughnut area between the direct beam and the neighboring diffracted beams, was effectively employed with the aberration corrected Low-Mag STEM mode to keep image intensity high enough even at large specimen-tilt angles. The resultant tomographic observation visualized 3D dislocation arrangements and active slip planes in a deformed α-Fe specimen.

DOI： 10.1016/j.ultramic.2017.07.016

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

Next generation on-chip optical devices require light manipulation in time and space, that is, control of group velocity of light in subwavelength dimensions. A waveguide in plasmonic crystal fulfills such requirements offering nanoscale light confinement in the dispersion-tunable plasmonic crystal matrix. However, there has been no direct access to the local dispersion of the waveguide mode itself, and the group velocity of light could not be evaluated. Herein, for the first time, we experimentally clarify the dispersion of the waveguide modes by use of angle-resolved cathodoluminescence scanning transmission electron microscopy. Their group velocity can be extremely slowed down by the existence of a bandgap formed in the waveguide in the energy range of the plasmonic crystal bandgap.

DOI： 10.1021/acsphotonics.6b00943

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

Square lattice plasmonic crystals (SQ-PlCs) composed of silver pillars generate large bandgaps for surface plasmon polaritons (SPPs). SPP confinement is demonstrated using one- and two-dimensional heterostructures of SQ-PlCs comprised of cylindrical pillars with different diameters in a common square lattice. Two kinds of localized modes are observed to appear in the heterostructures by photon map imaging using cathodoluminescence (CL) technique combined with a scanning transmission electron microscopy (STEM). Angle-resolved CL spectroscopy reveals contrasting characteristics of the two localized modes in their emission distributions, indicating that they originate from the band-edge A and E modes of the matrix SQ-PlC.

DOI： 10.1021/acs.nanolett.5b02623

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

Surface plasmon-polaritons (SPPs) localized inside a plasmonic crystal (PlC) cavity are probed by STEM-CL technique to characterize the influence of the surface shape parameters on the cavity modes. The results elucidate the formation mechanism of the cavity mode in terms of the symmetry and quality factor, which provide a clear design guide for the PlC cavity to control the coupling between SPPs and photons in plasmonic devices and future integrated circuits.

DOI： 10.1021/acs.nanolett.5b01719

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

The optical properties of surface plasmon polaritons (SPPs) are investigated at the Γ point in a two-dimensional plasmonic crystal with a hexagonal lattice (Hex-PlC). The cathodoluminescence (CL) technique combined with a scanning transmission electron microscope (STEM) are used to produce spectral images of the SPP standing waves at the Γ point and identify the four types of band-edge modes predicted by group theory. The systematic measurement of the band-edge energies employed here is used to determine the characteristic dependence of each band-edge mode on the structure parameters, which provides some criteria for the design of plasmonic devices with Hex-PlCs.

DOI： 10.1364/OE.23.002524

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

The coupled surface plasmon-polariton (SPP) modes excited in an Al/SiO₂/ Al multilayer structure were analyzed using angle-resolved electron energy-loss spectroscopy (AREELS) with a relativistic electron probe. The dispersion relations for the coupled SPP modes were then directly observed and compared with predicted relations obtained via calculations. Good agreement was noted between the experimental and calculated results. In the multilayer structures, the dispersion relation for the coupled SPP modes was found to be sensitive to the thickness of each film, which could be interpreted qualitatively by the electron energy-loss probability calculated for thin aluminum (Al) films and narrow Al gaps using Kröger’s formula. It was demonstrated that significant differences in the excitation probability for SPPs could be observed depending on the coupling modes.

DOI： 10.1093/jmicro/dft047

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

Retardation effects in the valence electron energy-loss spectrum (EELS) of a Si slab are analyzed by angular-resolved EELS. The dispersion curves of the valence spectra excited in a slab are directly observed from a specimen area with several different thicknesses and are interpreted by performing a calculation of the dispersion relation using Krögers formula. The dispersion curves observed below about 3 eV are attributed to guided modes coupled with Čerenkov radiation (ČR). The coupling between guided modes and ČR is found to be dependent on the sample thickness (t). For the sample with t > 150 nm, the intensity of the guided modes increased linearly with thickness, revealing the coupling with ČR. For t 150 nm, however, the intensity of the guided modes rapidly decreased due to a diminished coupling with ČR, resulting from the thickness-dependent dispersion curves of the guided modes.

DOI： 10.1063/1.4796140

DOI： 10.1039/d4ta08485c

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DOI： 10.1039/d4ta08137d

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DOI： 10.1016/j.snb.2024.137176

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DOI： 10.1021/acsaem.4c02958

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DOI： 10.1021/acsanm.4c07224

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Publisher：Advanced Materials Technologies  

Atomic layer deposition (ALD) in the presence of organic template molecules on substrates (molecular-templated ALD) is a promising technique for designing a molecular selectivity with thermal robustness onto the surfaces of metal oxides, which are required for heterogeneous catalysts and molecular sensing. However, self-aggregations of template molecules significantly impair the selectivity of the resulting molecules, which is difficult to prevent or observe. Here, a rational method is presented for uni-molecularly dispersing template molecules and revealing their state in the molecular-templated ALD process. The environment-sensitive fluorescence of pyrene is utilized as a probe to visualize the microenvironments around template molecules. The dispersion of pyrene moieties in OH-terminated SAMs allowed the pyrene molecules to be 1) covalently immobilized, 2) isolated from other pyrene molecules, and 3) surrounded by OH groups that act as ALD initiation sites. Systematic spectroscopic measurements of pyrene probes revealed the successful ALD of metal oxides surrounding template molecules without their undesired aggregations. Furthermore, emission enhancements are observed when Al2O3 is used as the surrounding metal oxide. The amplification of pyrene fluorescence intensity, lifetime, and quantum yield is attributed to the suppression of non-radiative decay, indicating that the Al2O3 layer has grown closely around a single organic molecule.

DOI： 10.1002/admt.202401639

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Publisher：ACS Applied Nano Materials  

Molecular-templated atomic layer deposition (ALD) is defined as a deposition process in the presence of target molecules on surfaces. The resulting nanocavities after removal of the template molecules have unique molecular recognition abilities and are promising for various applications, including volatile organic compound sensors. However, few studies have investigated the nanocavity formation process, mainly due to its complexity. In particular, the complicated reconstructions of metal oxide surfaces in solution-phase processes have hindered facile control of the molecular template formation process. Here, we developed a molecular-templated ALD system performed entirely in the gas phase. The key to this system is a QCM chip covered by a metal oxide (ZnO) nanowire array, which amplifies the QCM signals to enable minute amounts of molecular monitoring. The suppression of metal oxide (TiO2) deposition in the ALD low cycle region (less than 20 cycles) was confirmed by QCM, indicating that the molecule significantly affects the formation of metal oxide nanosurfaces. The effect of the template molecule on the nanosurface formation in this region was also suggested by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Furthermore, the molecular capture ability after removing the template molecule was investigated by exposure to hexanal vapor. The amount of molecular adsorption was dependent on the ALD cycle number, with the highest value obtained near the cycle number, where the template molecule influenced the ALD growth mode. These results suggest that TiO2 grew around the template molecule to form a nanostructure influenced by the presence of the molecule, enhancing its ability to capture molecules of similar size as the template molecules. The technology developed in this study is expected to pave the way for the development of molecular sensors that can selectively adsorb and detect specific substances in gas mixtures.

DOI： 10.1021/acsanm.4c03810

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Publisher：Acta Materialia  

Experimental observation methods for understanding industrially important solid-state sintering are essential for the development of new materials and devices. To experimentally characterize solid-state sintering, the limitations posed by the complexity of target materials, experimental equipment, and observation conditions must be overcome. Therefore, hybrid techniques for predicting sintering behavior based on experimental datasets and physics-based simulation models are highly sought after. Herein, we propose a new technique for combining a physics-based model and experimental observation results from solid-state sintering using a nonsequential Bayesian data assimilation (DA) method. The proposed technique assimilates experimental data—obtained using in situ electron tomography/scanning transmission electron microscopy—into the corresponding phase-field (PF) model to enable high-fidelity PF simulations by estimating multiple material parameters included in the PF model. This study demonstrates the inverse estimation of seven parameters, including temperature-dependent diffusion coefficients, from the time-series information on the morphology of sintered nanoparticles observed in situ. The estimated parameters provide the high-fidelity PF simulation to capture the observed solid-state sintering of copper nanoparticles. Thus, this study contributes to the construction of digital twins for solid-state sintering based on DA-integrated PF simulations and in situ observation datasets and deepens our understanding of the sintering process.

DOI： 10.1016/j.actamat.2024.120251

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Publisher：Meteoritics and Planetary Science  

The surface morphology of regolith grains from the C-type asteroid Ryugu was studied in search of evidence of impact events on the asteroid. Scanning electron microscopy revealed that ~8% of C0105-042 Ryugu grains have a smooth surface on one side of the grains. One of these grains has striated linear grooves (striations) on its smooth surface. Transmission electron microscopy of the grain showed that a porous fine-grained Mg-Fe phyllosilicate assemblage, which is the main component of Ryugu grains, is compacted near the smooth surface. The smooth surface with striations closely resembles a slickenside, a characteristic texture found in terrestrial fault rocks formed by shear deformation. There is no evidence of melting/decomposition in the Mg-Fe phyllosilicates near the smooth surface, indicating that the shear heating temperature is less than ~1100 K. Assuming that the average length of the striations corresponds to the minimum displacement of the micro-fault, the shock pressure recorded in the C0105-042 Ryugu grain is estimated to be <~4.5 GPa by a fault mechanics calculation. The shock pressures of C0105-042, together with those of C0014 (~2 GPa) and C0055 (>~3.9 GPa) in previous studies suggest that the impact velocities recorded in these grains are < ~0.89–1.63 km s−1. Based on the impact velocities, these grains may record an impact event that occurred when asteroid Ryugu was in the orbit in Main Belt.

DOI： 10.1111/maps.14271

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Language：Japanese   Publisher：The Japanese Society of Microscopy  

<p>Scanning transmission electron microscopy (STEM) is more suitable for visualizing the internal structure of thick samples compared to conventional transmission electron microscopy whose resolution is limited by the chromatic aberration of the imaging lens system, and it is often used for three-dimensional structural analysis using electron tomography. However, STEM image quality is seriously degraded by noise and artifacts, especially when pursuing rapid imaging on the order of milliseconds per frame or faster. In this paper, we report that deep learning-based denoising is effective for rapid STEM imaging, and can be applicable to rapid STEM tomography. By acquiring tilt-series images in just 5 seconds, the three-dimensional dislocation arrangement in a thick (300 nm) steel sample can be determined with sufficient accuracy. This method has enormous potential on improving in-situ or operando observation of samples in relatively thick media including liquid cells.</p>

DOI： 10.11410/kenbikyo.59.2_52

CiNii Research

Publisher：NPG Asia Materials  

Superconducting magnets based on high-temperature superconductors (HTSs) have become critical components in cutting-edge technologies such as advanced medical applications. In HTSs, weak links of superconductivity are inevitable at high-angle grain boundaries (GBs). Thus, two adjacent grains should be crystallographically aligned within the critical angle (θc), for which the intergrain critical current density (Jc) starts to decrease exponentially. The θc of several iron-based superconductors (IBSs) is larger than that of cuprates. However, the decreases in both θc and intergrain Jc under magnetic fields for IBSs are still substantial, hampering their applications in polycrystalline forms. Here, we report that potassium-doped BaFe2As2 (Ba122:K) exhibits superior GB performance to that of previously reported IBSs. A transport Jc of over 0.1 MA/cm2 across [001]-tilt GBs with misorientation angles up to θGB = 24° was recorded even at 28 K, which is a required level for practical applications. Additionally, even in an applied magnetic field, θc was unaltered, and the decay of the intergrain Jc was small. Our results highlight the exceptional potential of Ba122:K for polycrystalline applications and pave the way for next-generation superconducting magnets.

DOI： 10.1038/s41427-024-00561-9

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Publisher：Meteoritics and Planetary Science  

Transmission electron microscopy analyses of Hayabusa2 samples show that Ryugu organic matter exhibits a range of morphologies, elemental compositions, and carbon functional chemistries consistent with those of carbonaceous chondrites that have experienced low-temperature aqueous alteration. Both nanoglobules and diffuse organic matter are abundant. Non-globular organic particles are also present, and including some that contain nanodiamond clusters. Diffuse organic matter is finely distributed in and around phyllosilicates, forms coatings on other minerals, and is also preserved in vesicles in secondary minerals such as carbonate and pyrrhotite. The average elemental compositions determined by energy-dispersive spectroscopy of extracted, demineralized insoluble organic matter samples A0107 and C0106 are C100N3O9S1 and C100N3O7S1, respectively, with the difference in O/C slightly outside the difference in the standard error of the mean. The functional chemistry of the nanoglobules varies from mostly aromatic C=C to mixtures of aromatic C=C, ketone C=O, aliphatic (CHn), and carboxyl (COOH) groups. Diffuse organic matter associated with phyllosilicates has variable aromatic C, ketone and carboxyl groups, and some localized aliphatics, but is dominated by molecular carbonate (CO3) absorption, comparable to prior observations of clay-bound organic matter in CI meteorites.

DOI： 10.1111/maps.14128

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Publisher：Meteoritics and Planetary Science  

Samples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small-sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine-grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S- and Ni-rich ferrihydrite is abundant. The coarse-grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x-ray microscopy at the Fe-L3 edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe3+) than in Ryugu (<50% Fe3+). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine-grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe-based nanophases. On the other hand, the coarse-grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long-term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.

DOI： 10.1111/maps.14101

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Publisher：Meteoritics and Planetary Science  

Regolith samples returned from asteroid 162173 Ryugu by the Hayabusa2 mission provide direct means to study how space weathering operates on the surfaces of hydrous asteroids. The mechanisms of space weathering, its effects on mineral surfaces, and the characteristic time scales on which alteration occurs are central to understanding the spectroscopic properties and the taxonomy of asteroids in the solar system. Here, we investigate the behavior of the iron monosulfides mineral pyrrhotite (Fe1−xS) at the earliest stages of space weathering. Using electron microscopy methods, we identified a partially exposed pyrrhotite crystal that morphologically shows evidence for mass loss due to exposure to solar wind ion irradiation. We find that crystallographic changes to the pyrrhotite can be related to sulfur loss from its space-exposed surface and the diffusive redistribution of resulting excess iron into the interior of the crystal. Diffusion profiles allow us to estimate an order of magnitude of the exposure time of a few thousand years consistent with previous estimates of space exposure. During this interval, the adjacent phyllosilicates did not acquire discernable damage, suggesting that they are less susceptible to alteration by ion irradiation than pyrrhotite.

DOI： 10.1111/maps.14176

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Abstract

In-situ observation has expanded the application of transmission electron microscopy (TEM) and has made a significant contribution to materials research and development for energy, biomedical, quantum, etc. Recent technological developments related to in-situ TEM have empowered the incorporation of three-dimensional observation, which was previously considered incompatible. In this review article, we take up heating as the most commonly used external stimulus for in-situ TEM observation and overview recent in-situ TEM studies. Then, we focus on the electron tomography (ET) and in-situ heating combined observation by introducing the authors’ recent research as an example. Assuming that in-situ heating observation is expanded from two dimensions to three dimensions using a conventional TEM apparatus and a commercially available in-situ heating specimen holder, the following in-situ heating-and-ET observation procedure is proposed: (i) use a rapid heating-and-cooling function of a micro-electro-mechanical system holder; (ii) heat and cool the specimen intermittently and (iii) acquire a tilt-series dataset when the specimen heating is stopped. This procedure is not too technically challenging and can have a wide range of applications. Essential technical points for a successful 4D (space and time) observation will be discussed through reviewing the authors’ example application.

DOI： 10.1093/jmicro/dfae008

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Other Link： https://academic.oup.com/jmicro/article-pdf/73/2/133/57186420/dfae008.pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Time-resolved or time-correlation measurements using cathodoluminescence (CL) reveal the electronic and optical properties of semiconductors, such as their carrier lifetimes, at the nanoscale. However, halide perovskites, which are promising optoelectronic materials, exhibit significantly different decay dynamics in their CL and photoluminescence (PL). We conducted time-correlation CL measurements of CsPbBr3 using Hanbury Brown-Twiss interferometry and compared them with time-resolved PL. The measured CL decay time was on the order of subnanoseconds and was faster than PL decay at an excited carrier density of 2.1 × 1018 cm-3. Our experiment and analytical model revealed the CL dynamics induced by individual electron incidences, which are characterized by highly localized carrier generation followed by a rapid decrease in carrier density due to diffusion. This carrier diffusion can play a dominant role in the CL decay time for undoped semiconductors, in general, when the diffusion dynamics are faster than the carrier recombination.

DOI： 10.1021/acs.nanolett.4c00483

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Language：English   Publisher：Sustainable Energy and Fuels  

This study investigates the hydrogen evolution reaction (HER) efficiency of two photosystems incorporating an all-inorganic molecular thiomolybdate [Mo3S13]2− cluster as a HER catalyst. First, we delve into the performance of a homogeneous [Mo3S13]2−/[Ru(bpy)3]2+ (Mo3/Ru) dyad which demonstrates high turnover frequencies (TOFs) and apparent quantum yields (AQYs) at 445 nm approaching the level of 0.5%, yet its performance is marked by pronounced deactivation. In contrast, a heterogeneous approach involves anchoring [Mo3S13]2− onto graphitic carbon nitride (GCN) nanosheets through weak electrostatic association with its triazine/heptazine scaffold. [Mo3S13]2−/GCN (Mo3/GCN) displays effective H2 generation under visible light, with TOF metrics on par with those of its homogeneous analog. Although substantial leaching of [Mo3S13]2− species from the Mo3/GCN surface occurs, the remaining {Mo3}-based centers demonstrate impressive stability, leading to enduring HER performance, starkly distinguishing it from the homogeneous Mo3/Ru photosystem. Photoluminescence (PL) quenching experiments confirm that the performance of Mo3/GCN is not limited by the quality of the inorganic interface, but could be optimized by using higher surface area supports or a higher concentration of [Mo3S13]2− sites. Our findings showcase complexities underlying the evaluation and comparison of photosystems comprising well-defined catalytic centers and pave the way for developing analogous surface-supported (photo)catalysts with broad use in energy applications.

DOI： 10.1039/d3se01658g

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

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

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

Language：English   Publisher：Springer Nature  

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

CiNii Research

Publisher：Nature Astronomy  

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

DOI： 10.1038/s41550-023-02137-z

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Publisher：Meteoritics and Planetary Science  

The Hayabusa2 mission from the Japan Aerospace Exploration Agency (JAXA) returned to the Earth samples of carbonaceous asteroid (162173) Ryugu. This mission offers a unique opportunity to investigate in the laboratory samples from a C-type asteroid, without physical or chemical alteration by the terrestrial atmosphere. Here, we report on an investigation of the mineralogy and the organo-chemistry of Hayabusa2 samples using a combination of micro- and nano-infrared spectroscopy. Particles investigated with conventional FTIR spectroscopy have spectra dominated by phyllosilicate-related absorption, as observed for samples of CI-chondrites, selected ungrouped carbonaceous chondrites, and selected hydrated micrometeorites. Ryugu samples show smaller sulfate-related absorption than CI-chondrites. Our samples that were only briefly exposed to the Earth atmosphere show absorptions related to molecular water, revealing fast terrestrial contamination of the spectral signature at 3 μm. Overall, our FTIR data are in agreement with other work done on Ryugu samples, revealing a low degree of mineralogical variability across Ryugu samples. AFM-IR mapping of the grains shows the presence of a micrometer-sized organic globule in one of our analyzed grains. The AFM-IR spectra obtained on this globule are similar to IR spectra obtained on IOM suggesting that it is constituted of refractory organic matter. This globule may host silicate in its interior, with a different mineralogy than bulk Ryugu phyllosilicate. The shape, presence of peculiar silicate, and the nature of organic constituting the globule point toward a pre-accretionary origin of this globule and that at least part of Ryugu organics were inherited from the protosolar nebulae or the interstellar media. Altogether, our results show the similarities between Ryugu samples and CI chondrites.

DOI： 10.1111/maps.14122

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Publisher：Meteoritics and Planetary Science  

Ryugu asteroid grains brought back to the Earth by the Hayabusa2 space mission are pristine samples containing hydrated minerals and organic compounds. Here, we investigate the mineralogy of their phyllosilicate-rich matrix with four-dimensional scanning transmission electron microscopy (4D-STEM). We have identified and mapped the mineral phases at the nanometer scale (serpentine, smectite, pyrrhotite), observed the presence of Ni-bearing pyrrhotite, and identified the serpentine polymorph as lizardite, in agreement with the reported aqueous alteration history of Ryugu. Furthermore, we have mapped the d-spacings of smectite and observed a broad distribution of values, ranging from 1 to 2 nm, with an average d-spacing of 1.24 nm, indicating significant heterogeneity within the sample. Such d-spacing variability could be the result of either the presence of organic matter trapped in the interlayers or the influence of various geochemical conditions at the submicrometer scale, suggestive of a range of organic compounds and/or changes in smectite crystal chemistry.

DOI： 10.1111/maps.14124

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Publisher：Applied Physics Express  

Highly efficient green emission of Cs4PbBr6 has been attributed to intermediate states formed by embedded CsPbBr3 nanocrystals or defects. However, direct experimental confirmation of the presence of such nano-emitters is not straightforward and the emission mechanism remains elusive. By using cathodoluminescence (CL) imaging with a high spatial resolution, we demonstrate that CsPbBr3 nanocrystals within the Cs4PbBr6 matrix contribute to the green emission, exhibiting optical behavior distinct from the matrix. Additionally, we explore its potential as an electron beam scintillator, given its high CL intensity and exceptionally short lifetime.

DOI： 10.35848/1882-0786/ad1bc4

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

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

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

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

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

Publisher：International Journal of Applied Glass Science  

Herein, the crystallization behavior of a Li2O–Al2O3–SiO2 (LAS) glass system with the addition of ZrO2 and SnO2 as nucleating agents was investigated using X-ray diffraction, differential scanning calorimetry, four-dimensional scanning transmission electron microscopy, and X-ray absorption fine-structure measurements. At lower heat-treatment temperatures, the addition of ZrO2 and SnO2 afforded a ZrSnO4 solid solution (SS), whereas at higher heat-treatment temperatures, the ZrSnO4 SS decomposed, affording tetragonal ZrO2 and tetragonal SnO2. LAS-based crystalline phases, such as β-quartz and β-spodumene phases SS, were formed after the formation of the ZrSnO4 SS. ZrSnO4 SS particles a few nanometers in size were present in contact with the β-quartz SS particles a few dozen nanometers in size. This suggests that the ZrSnO4 SS served as a crystal nucleus for the β-quartz SS, promoting its growth.

DOI： 10.1111/ijag.16644

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Language：Japanese   Publisher：The Japanese Association for Crystal Growth  

<p>  Metal nanoparticles undertake sintering at a lower temperature compared to their bulk counterparts, attracting great attention in industrial applications such as printed electronics. Since nanoparticles are easy to aggregate, the three-dimensional (3D) visualization of their sintering process plays a vital role in quantifying the morphological changes during heating. However, the nanoparticles are sensitive to surface contamination, resulting in poor affinity to the 3D visualization with conventional electron beam intensity because the visualization technique requires a long beam exposure time, potentially making beam induced contamination significant. In this paper, we demonstrate our developed 3D observation scheme, including advanced image processing techniques. A sample transfer system, which enables to prevent nanoparticles from being exposed to air, and an ultra-low-electron dose observation protocol prevents surface contamination during the observation. The low-signal-to-noise ratio of acquired images is compensated by the advanced image processing techniques. The obtained 3D images of nanoparticles enable to measure the neck growth and variation of particle distance in 3D during the sintering in a quantitative way.</p>

DOI： 10.19009/jjacg.51-1-03

CiNii Research

Language：English   Publisher：The Asahi Glass Foundation  

DOI： 10.50867/afreport.2024_060

CiNii Research

Publisher：Meteoritics and Planetary Science  

Samples returned from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 mission revealed that Ryugu is composed of materials consistent with CI chondrites and some types of space weathering. We report detailed mineralogy of the fine-grained Ryugu samples allocated to our “Sand” team and report additional space weathering features found on the grains. The dominant mineralogy is composed of a fine-grained mixture of Mg-rich saponite and serpentine, magnetite, pyrrhotite, pentlandite, dolomite, and Fe-bearing magnesite. These grains have mineralogy comparable to that of CI chondrites, showing severe aqueous alteration but lacking ferrihydrite and sulfate. These results are similar to previous works on large Ryugu grains. In addition to the major minerals, we also find many minerals that are rare or have not been reported among CI chondrites. Accessory minerals identified are hydroxyapatite, Mg-Na phosphate, olivine, low-Ca pyroxene, Mg-Al spinel, chromite, manganochromite, eskolaite, ilmenite, cubanite, polydymite, transjordanite, schreibersite, calcite, moissanite, and poorly crystalline phyllosilicate. We also show scanning transmission electron microscope and scanning electron microscope compositional maps and images of some space-weathered grains and severely heated and melted grains. Although our mineralogical results are consistent with that of millimeter-sized grains, the fine-grained fraction is best suited to investigate impact-induced space weathering.

DOI： 10.1111/maps.14093

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

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

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Electron microscopy based on high-energy electrons allows nanoscopic analytical imaging taking advantage of secondarily generated particles. Especially for cathodoluminescence, the correlation between primary incident electrons and emitted photons includes information on the entire interaction process. However, electron-photon time correlation tracking the relaxation dynamics of luminescent materials has so far not been achieved. In this work, we propose time-correlated electron and photon counting microscopy, where coincidence events of primary electrons and generated photons are counted after interaction. The electron-photon time correlation enables extracting a unique lifetime of the emitter independent of the photon state, accounting for coherent and incoherent photon generation processes. We also introduce a correlation factor and discuss the correlation between electrons and generated coherent photons. Through momentum selection, we observe correlation changes indicating the presence of pair correlation originated from the electron-photon entanglement. The present work lays the foundation for developing next-generation electron microscopy based on quantum correlation.

DOI： 10.1038/s42005-023-01371-1

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Other Link： https://www.nature.com/articles/s42005-023-01371-1

Language：Japanese   Publisher：The Society of Materials Science, Japan  

DOI： 10.2472/jsms.72.631

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CiNii Research

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Copper-based sulfide semiconductors with the structure based on a sphalerite-like network can provide environmentally sound solutions to thermoelectric and photovoltaic energy conversion at a low cost. In this study, we report the synthesis and characterization of a copper-based sulfide, Cu30Ti6Sb2S32, discovered within the pseudobinary Cu3SbS4-Cu4TiS4 system. This new compound crystallizes in a colusite-like cubic structure, which is characterized by “interstitial” Ti atoms forming tetrahedral-octahedral [TiS4]Cu6 complexes linearly arranged within the Cu-S corner-sharing matrix. The [TiS4]Cu6 unit arrangement distinguishes the crystal structures of Cu26Ti2Sb6S32 (isolated units, 0D linkage), Cu30Ti6Sb2S32 (chain of linked units, 1D), and Cu4TiS4 (plane of linked units, 2D). We emphasize the semiconducting nature and the low thermal conductivity. The electronic structure and vibrational properties are investigated by the combination of experiments and first-principles calculations. We demonstrate that a design approach based on the pseudobinary composition, here, with the generic formula (Cu3SbS4)1-x(Cu4TiS4)x, is a fruitful direction for the discovery of new copper-based sulfides with tunable transport properties.

DOI： 10.1021/acs.chemmater.3c01135

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

Publisher：IEEE Transactions on Applied Superconductivity  

For obtaining microscopic knowledge toward a fabrication process of (Bi,Pb)2Sr2Ca2Cu3Ox (Bi,Pb-2223) single-phase material, we fabricated a multilayered film on a SrTiO3(001) substrate by a sputtering method using (Bi,Pb)2Sr2CaCu2Oy (Bi,Pb-2212) and Pb-Ca-Cu-O targets alternately at 650 °C. The as-grown film was an epitaxially grown multilayered film consisting of Bi,Pb-2212 and Pb-Ca-Cu-O layers. Coarse grains of impurity phases were also formed within the multilayered film, and these impurity phases were difficult to eliminate even after heat treatments. After 10 h heat treatment at 840 °C with Bi,Pb-2223 pellets, the phase transition from Bi,Pb-2212 to Bi,Pb-2223 occurred in the multilayered film. The phase transition to Bi,Pb-2223 proceeded preferentially at the interface with the impurity phases and the Pb-Ca-Cu-O layers. These observation results suggest that the impurity phases promote three-dimensional atomic diffusion in the multilayered film to accelerate the phase transition to Bi,Pb-2223, as well as consume constituent elements, Ca, Cu and O. Even after the 100 h heat treatment at 840 °C with Bi,Pb-2223 pellets, the phase transition from Bi,Pb-2212 to Bi,Pb-2223 did not proceed completely, and the fraction of the Bi,Pb-2223 phase in the whole superconducting phases was ∼50%. It is suggested that controlling the formation and microstructure of the impurity phases is a key to further increasing the fraction of the Bi,Pb-2223 phase.

DOI： 10.1109/TASC.2023.3259920

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Publisher：Advanced Energy Materials  

Metal-organic frameworks (MOFs) are promising materials for photocatalytic water splitting reactions, but examples of visible light-responsive, catalytically active, and stable MOFs are still rare. A detailed investigation is conducted for COK-47 – a recently described MOF comprising 2D Ti-O6 secondary building units (SBUs) – toward a photocatalytic hydrogen evolution reaction (HER), showing how overall particle morphology, surface area, and missing ligand defects are central parameters governing the material's ultimate performance. The newly synthesized COK-47ISO is among the most active MOFs to date, yielding HER-rates of 8.6 µmol h−1, and an apparent quantum yield (AQY) of 0.5% under visible light illumination. Optoelectronic and photoluminescence investigations, supported by theoretical calculations, enable the unraveling of its electronic structure along with charge transfer and recombination kinetics. A wavelength-dependent reaction mechanism is proposed involving ligand to metal charge transfer (LMCT) and the main challenges for visible or UV photoexcitation are identified, demonstrating that the unique 2D layered structure aids charge separation and is key to the high performance. This work introduces COK-47 as a promising alternative to the well-known MIL-125 family and offers directions for future studies.

DOI： 10.1002/aenm.202300961

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

The nanoscale characterization of thermally activated solid reactions plays a pivotal role in products manufactured by nanotechnology. Recently, in situ observation in transmission electron microscopy combined with electron tomography, namely four-dimensional observation for heat treatment of nanomaterials, has attracted great interest. However, because most nanomaterials are highly reactive, i.e., oxidation during transfer and electron beam irradiation would likely cause fatal artefacts; it is challenging to perform the artifact-free four-dimensional observation. Herein, we demonstrate our development of a novel in situ three-dimensional electron microscopy technique for thermally activated solid-state reaction processes in nanoparticles (NPs). The sintering behaviour of Cu NPs was successfully visualized and analyzed in four-dimensional space-time. An advanced image processing protocol and a newly designed state-of-the-art MEMS-based heating holder enable the implementation of considerably low electron dose imaging and prevent air exposure, which is of central importance in this type of observation. The total amount of electron dose for a single set of tilt-series images was reduced to 250 e− nm−2, which is the lowest level for inorganic materials electron tomography experiments. This study evaluated the sintering behaviour of Cu NPs in terms of variations in neck growth and particle distance. A negative correlation between the two parameters is shown, except for the particle pair bound by neighbouring NPs. The nanoscale characteristic sintering behavior of neck growth was also captured in this study.

DOI： 10.1039/d3nr00992k

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

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

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

The current study reveals the Hall-Petch relationship, or mean grain size control, and flow properties of equiatomic FeCoNi alloy having different grain sizes at room temperature (298 K) and cryogenic temperature (77 K). For the first time, various mean grain sizes ranging from the ultra-fine regime (0.7 μm) to the coarse grain regime (145.3 μm) were achieved in a FeCoNi alloy by high-pressure torsion and subsequent annealing. The tensile yield strength depends strongly on temperature and grain size. Hall-Petch plots demonstrate that the grain boundary strength coefficient is insensitive to the temperature, whereas friction stress increases as the temperature decreases. For all the grain sizes both strength and ductility were increased with decreasing the temperature down to 77 K. Dislocation slip is responsible for the room temperature mechanical properties. At cryogenic temperature, on the other hand, nano twinning appears as an additional deformation mechanism in addition to the dislocation slip.

DOI： 10.1016/j.scriptamat.2023.115392

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Language：English   Publisher：ACS Applied Materials and Interfaces  

Artificially programming a sequence of organic-metal oxide multilayers (superlattices) by using atomic layer deposition (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here, we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy, in situ quartz crystal microbalance measurements, and Fourier-transformed infrared spectroscopy. These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the −OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive −OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.

DOI： 10.1021/acsami.3c04470

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

Electron tomography (ET) is a powerful tool for elucidating the properties and functionalities of materials. The innovative development of aberration-corrected electron microscopy in the early 21st century and the remarkable progress in the development of detectors, equipment and devices related to ET have resulted in substantial improvements in resolution. However, not only advances in hardware but also remarkable developments in reconstruction algorithms and related three-dimensional (3D) analysis methods have contributed to the resolution improvements. ET has its own problems, including the missing-wedge problem due to the limited tilt-angle range and the need to acquire numerous specimen-tilt images, the latter of which is time-consuming and can potentially damage the specimen. This review paper aims to (i) describe the established basic theories and definitions regarding 3D resolution of ET and practical 3D resolution measurement methods, (ii) discuss various reconstruction algorithms that effectively overcome the aforementioned problems and (iii) describe recent progress in the core of ET applications in materials science with respect to atomic ET, analytical ET and in-situ ET. The aforementioned ET problems have been addressed with each method developed in each field of application. Notably, in terms of aim (ii), recently developed reconstruction algorithms can reduce the number of projection images (specimen-tilt images) needed to attain a certain resolution without violating the Nyquist criterion. This approach is interpreted as a novel non-linear sampling theorem.

DOI： 10.1093/jmicro/dfac071

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Publisher：Geochimica et Cosmochimica Acta  

We have investigated several particles collected during each of two touchdowns of the Hayabusa2 spacecraft at the surface of the C-type asteroid 162173 Ryugu using various electron microscope techniques. Our detailed transmission electron microscopy study shows the presence of magnetite with various morphologies coexisting in close proximity. This is characteristic of CI chondrite-like materials and consistent with the mineral assemblages and compositions in the Ryugu parent body. We describe the microstructural characteristics of magnetite with different morphologies, which could have resulted from the chemical conditions (growth vs. diffusion rate) during their formation. Furthermore, we describe the presence of magnetites with a spherulitic structure composed of individual radiating fibers that are characterized by pervasive, homogeneously distributed euhedral to subhedral pores that have not been described in previous chondrite studies. This particular spherulitic structure is consistent with crystallization under nonequilibrium conditions. Additionally, the presence of a high density of defects within the magnetite fibers, the high surface/volume ratio of this morphology, and the presence of amorphous materials in several pores and at the edges of the acicular fibers further support their formation under nonequilibrium conditions. We suggest that the growth processes that lead to this structure result from the solution reaching a supersaturated state, resulting in an adjustment to a lower free energy condition via nucleation and rapid growth.

DOI： 10.1016/j.gca.2023.02.003

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

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

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

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

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

Language：English   Publisher：Springer Nature  

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe³⁺ to Fe²⁺ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

DOI： 10.1038/s41550-022-01841-6

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CiNii Research

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

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Application of scanning transmission electron microscopy (STEM) to in situ observation will be essential in the current and emerging data-driven materials science by taking STEM’s high affinity with various analytical options into account. As is well known, STEM’s image acquisition time needs to be further shortened to capture a targeted phenomenon in real-time as STEM’s current temporal resolution is far below the conventional TEM’s. However, rapid image acquisition in the millisecond per frame or faster generally causes image distortion, poor electron signals, and unidirectional blurring, which are obstacles for realizing video-rate STEM observation. Here we show an image correction framework integrating deep learning (DL)-based denoising and image distortion correction schemes optimized for STEM rapid image acquisition. By comparing a series of distortion corrected rapid scan images with corresponding regular scan speed images, the trained DL network is shown to remove not only the statistical noise but also the unidirectional blurring. This result demonstrates that rapid as well as high-quality image acquisition by STEM without hardware modification can be established by the DL. The DL-based noise filter could be applied to in-situ observation, such as dislocation activities under external stimuli, with high spatio-temporal resolution.

DOI： 10.1038/s41598-022-17360-3

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Other Link： https://www.nature.com/articles/s41598-022-17360-3

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

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

Language：English   Publisher：ACS Materials Au  

The ongoing transition to renewable energy sources and the implementation of artificial photosynthetic setups call for an efficient and stable water oxidation catalyst (WOC). Here, we heterogenize a molecular all-inorganic [CoIIICoII(H2O)W11O39]7-({CoIIICoIIW11}) Keggin-type polyoxometalate (POM) onto a model TiO2surface, employing a 3-aminopropyltriethoxysilane (APTES) linker to form a novel heterogeneous photosystem for light-driven water oxidation. The {CoIIICoIIW11}-APTES-TiO2hybrid is characterized using a set of spectroscopic and microscopic techniques to reveal the POM integrity and dispersion to elucidate the POM/APTES and APTES/TiO2binding modes as well as to visualize the attachment of individual clusters. We conduct photocatalytic studies under heterogeneous and homogeneous conditions and show that {CoIIICoIIW11}-APTES-TiO2performs as an active light-driven WOC, wherein {CoIIICoIIW11} acts as a stable co-catalyst for water oxidation. In contrast to the homogeneous WOC performance of this POM, the heterogenized photosystem yields a constant WOC rate for at least 10 h without any apparent deactivation, demonstrating that TiO2not only stabilizes the POM but also acts as a photosensitizer. Complementary studies using photoluminescence (PL) emission spectroscopy elucidate the charge transfer mechanism and enhanced WOC activity. The {CoIIICoIIW11}-APTES-TiO2photocatalyst serves as a prime example of a hybrid homogeneous-heterogeneous photosystem that combines the advantages of solid-state absorbers and well-defined molecular co-catalysts, which will be of interest to both scientific communities and applications in photoelectrocatalysis and CO2reduction.

DOI： 10.1021/acsmaterialsau.2c00025

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

Language：English   Publisher：ACS Catalysis  

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo3S13]2-clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo3S13]2-to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S22-groups, upon which Mo-O-Ti bonds with the hydroxylated TiO2surface are established. The heterogenized [Mo3S13]2-clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO2surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N2. Our data demonstrate the importance of the trinuclear core of the [Mo3S13]2-cluster and suggest bridging S22-and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.

DOI： 10.1021/acscatal.2c00972

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Language：English   Publisher：Nanophotonics  

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), possessing unique exciton luminescence properties, have attracted significant attention for use in optical and electrical devices. TMDCs are also high refractive index materials that can strongly confine the electromagnetic field in nanoscale dimensions when patterned into nanostructures, thus resulting in complex light emission that includes exciton and dielectric resonances. Here, we use cathodoluminescence (CL) to experimentally visualize the emission modes of single molybdenum disulfide (MoS2) nanoflakes and to investigate luminescence enhancement due to dielectric resonances in nanoscale dimensions, by using a scanning transmission electron microscope. Specifically, we identify dielectric modes whose resonant wavelength is sensitive to the shape and size of the nanoflake, and exciton emission peaks whose energies are insensitive to the geometry of the flakes. Using a four-dimensional CL method and boundary element method simulations, we further theoretically and experimentally visualize the emission polarization and angular emission patterns, revealing the coupling of the exciton and dielectric resonant modes. Such nanoscopic observation provides a detailed understanding of the optical responses of MoS2 including modal couplings of excitons and dielectric resonances which play a crucial role in the development of energy conversion devices, single-photon emitters, and nanophotonic circuits with enhanced light-matter interactions.

DOI： 10.1515/nanoph-2021-0643

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

Other Link： https://pubs.acs.org/doi/full/10.1021/acscatal.2c00972

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

Other Link： https://pubs.rsc.org/en/content/articlehtml/2022/ra/d2ra01199a

Language：English   Publisher：RSC Advances  

All-solid-state Li batteries have attracted significant attention because of their high energy density and high level of safety. In a solid-state Li-ion battery, the electrodes contain a solid electrolyte that does not contribute directly to the capacity. Therefore, a battery that does not require a solid electrolyte in its electrode mixture should exhibit a higher energy density. In this study, a MgH2 electrode was used as the negative electrode material without a solid electrolyte in its mixture. The resultant battery demonstrated excellent performance because of the formation of an ionic conduction path based on LiH in the electrode mixture. LiH and Mg clearly formed upon lithiation and returned to MgH2 upon delithiation as revealed by TEM-EELS analysis. This mechanism of in situ electrolyte formation enables the development of a solid-state battery with a high energy density.

DOI： 10.1039/d2ra01199a

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Language：English   Publisher：ACS Sensors  

Humidity and moisture effects, frequently called water poisoning, in surroundings are inevitable for various molecular sensing devices, strongly affecting their sensing characteristics. Here, we demonstrate a water-selective nanostructured dehumidifier composed of ZnO/TiO2/CaCl2core-shell heterostructured nanowires for molecular sensing spaces. The fabricated nanostructured dehumidifier is highly water-selective without detrimental adsorptions of various volatile organic compound molecules and can be repeatedly operated. The thermally controllable and reversible dehydration process of CaCl2·nH2O thin nanolayers on hydrophilic ZnO/TiO2nanowire surfaces plays a vital role in such water-selective and repeatable dehumidifying operations. Furthermore, the limitation of detection for sensing acetone and nonanal molecules in the presence of moisture (relative humidity ∼90%) was improved more than 20 times using nanocomposite sensors by operating the developed nanostructured dehumidifier. Thus, the proposed water-selective nanostructured dehumidifier offers a rational strategy and platform to overcome water poisoning issues for various molecular and gas sensors.

DOI： 10.1021/acssensors.1c02378

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Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.61.84

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Other Link： https://www.jstage.jst.go.jp/article/materia/61/2/61_84/_pdf

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

Other Link： https://pubs.acs.org/doi/full/10.1021/acssensors.1c02378

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

Other Link： https://www.degruyter.com/document/doi/10.1515/nanoph-2021-0643/html?lang=en

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

Other Link： https://www.sciencedirect.com/science/article/pii/S0921883121004659

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Other Link： https://www.nature.com/articles/s41427-021-00337-5

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

DOI： 10.1021/acs.chemmater.1c00872

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DOI： 10.1038/s41598-021-85216-3

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DOI： 10.1103/PhysRevMaterials.5.014801

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

Compound semiconductors derived from ZnS (zincblende and wurtzite) with tetrahedral framework structures have functions for various applications. Examples of such materials include Cu–S-based materials with zincblende-derivative structures, which have attracted attention as thermoelectric (TE) materials over the past decade. This study illuminates superior TE performance in polycrystalline samples of enargite Cu₃P_{1− x}Ge_xS₄ with a wurtzite-derivative structure. The substitution of Ge for P dopes holes into the top of the valence band composed of Cu-3d and S-3p, whereby its multiband characteristic leads to a high TE power factor. Furthermore, a reduction in the grain size to 50–300 nm can effectively decrease phonon mean free paths, leading to low thermal conductivity. These features result in a dimensionless TE figure of merit ZT of 0.5 at 673 K for the x = 0.2 sample. Environmentally benign and low-cost characteristics of the constituent elements of Cu₃PS₄, as well as its high-performance thermoelectricity, make it a promising candidate for large-scale TE applications. Furthermore, this finding extends the development field of Cu–S-based TE materials to those with wurtzite-derivative structures.

DOI： 10.1002/adfm.202000973

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

Copper-based chalcogenides with tetrahedral framework structures have been attracting increasing attention as environmentally friendly thermoelectric materials. A representative group of such thermoelectric chalcogenides is the Cu ₂₆ A ₂ M ₆ S ₃₂ (A = V, Nb, Ta; M = Ge, Sn) family of colusites, which exhibit low electrical resistivity, a large Seebeck coefficient, and low thermal conductivity; these properties are necessary for efficient thermal-to-electronic energy conversion. Here, we show the impact of crystal structure on the lattice thermal conductivity of colusite with A = Nb, M = Sn. The crystal structure can be modified by controlling the cationic compositions and the deficiency in the sulfur content as Cu _26-x Nb ₂ Sn _6+x S _32-δ . The Cu/Sn ratio is found to be the key parameter for exsolution into distinct phases with ordered and disordered arrangements of cations. For the ordered-structure phase, sulfur sublimation induces atomic-scale defects/disordered states including interstitial defects, anti-site defects, and site splitting, which function as strong phonon scatterers, and the lowest lattice thermal conductivity of ∼0.5 W K ^-1 m ^-1 is achieved for the modified ordered structure. This finding provides a simple approach to modifying the crystal structure of thermoelectric chalcogenides via the loss of anions to reduce their lattice thermal conductivity.

DOI： 10.1039/c8ta08248k

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

To investigate the precipitation behavior of super duplex stainless steel in its weld by friction stir welding (FSW) at a low welding speed, we carried out microstructural observation and analysis. An intermetallic compound phase, ·, was observed in the heat affected zone (HAZ). The · phase precipitated at the interfaces between ¤-Fe (ferrite) and £ -Fe (austenite) grains 12 mm away from the stir zone (SZ)/HAZ boundary. On the other hand, the Cr₂N was observed together with the belt-like £ -Fe grain aggregates in the vicinity of the advancing side (AS) of the SZ. The other intermetallic phase,», was also observed at a triple junction of £ -Fe grains. This demonstrates that the precipitation of the Cr₂N and» phases correlates with the transformation of ¤-Fe to £A-Fe (secondary austenite).

DOI： 10.2320/matertrans.M2019069

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

Development of An In-Situ Straining and Tomography System in TEM

DOI： 10.11410/kenbikyo.54.1_44

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

Placticals and Trends of Electron Tomography for Materials Research

DOI： 10.2320/materia.57.589

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

Control of the optical properties of nano-plasmonic structures is essential for next-generation optical circuits and high-throughput biosensing platforms. Realization of such nano-optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as dark modes, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well studied, couplings of inverse structures, that is, holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti-symmetric coupling modes has not been performed. Here we present, for the first time, a systematic study of the symmetric and anti-symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy and electromagnetic simulation. The symmetric coupling mode, approximated as a pair of facing dipoles, is observed at a lower energy than that of the anti-symmetric coupling mode, indicating that the facing dipoles attract each other. The anti-symmetric coupling mode splits into the inner- and outer-edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as inverses of coupled disks. Symmetric and anti-symmetric coupling modes are also observed in a short-range ordered pore array, where one pore supports multiple local resonance modes, depending on the distance to the neighboring pore. Accessibility to the observed symmetric modes by far field is also discussed, which is important for nanophotonic device applications.

DOI： 10.1038/lsa.2016.146

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

Perovskite-type oxides have been widely applied in heterogeneous catalytic reactions, such as soot oxidation. However, a poor contact point between the catalyst and solid reactant (soot) often limits the catalytic performance. Here, we report La
_{1 - x}
Sr
_x
Co
_0.2
Fe
_0.8
O
_{3 - δ}
perovskite oxide catalysts with a unique three-dimensional (3D) fiber web structure that increases the high-contact area by trapping soot in the unique pore structure for effective catalytic activity. This feature was carefully analyzed using scanning transmission electron microscopy (STEM) tomography to investigate the location of the soot on the web. The structure of the web, with a thickness of approximately 55 μm, indicated that the soot particles were caught by the 3D pores between the fibers. The relationship between the Sr amount and activate oxygen was also characterized by means of XPS. The results show that the Sr amount of 0.4 produced the highest amount of active oxygen species (O
^-
) that are essential for soot oxidation reaction. The developed catalyst exhibited a good catalytic performance due to the optimized perovskite chemical structure and the greatly increased number of the contact points owing to the 3D inter-fiber spaces. Hence, our proposed approach is reasonable for application to real soot combustion processes and can also be easily extended to numerous other catalytic processes to enhance the catalytic activity.

DOI： 10.1016/j.apcatb.2016.03.001

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

The microstructure of an AlN template after high-temperature annealing was investigated by transmission electron microscopy (TEM). The AlN template was prepared by depositing an AlN layer of about 200nm thickness on a sapphire (0001) substrate by metal-organic vapor phase epitaxy. The AlN template was annealed under (N₂ + CO) atmosphere at 1500-1650 °C. TEM characterization was conducted to investigate the microstructural evolution, revealing that the postannealed AlN has a two-layer structure, the upper and lower layers of which exhibit Al and N polarities, respectively. It has been confirmed that postannealing is an effective treatment for controlling the microstructure.

DOI： 10.7567/APEX.9.065502

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

Microwave-assisted heterogeneous catalytic oxidation of CO over ABO₃ type perovskite oxides (A: La; B: Mn, Fe, Co) was carried out and the factors controlling the catalytic properties were investigated. The heating behavior of perovskite oxides depended on the physical state of catalyst particles and catalyst compositions. Catalytic activity of the perovskite oxide decreased in the order of LaCoO₃>LaMnO₃>LaFeO₃ under conventional heating, although LaMnO₃ exhibited similar activity to LaCoO₃ under microwave heating. The partial substitution of A site cation La by Sr enhanced the heating and catalytic properties under microwave heating conditions. Catalytic properties of the perovskite oxides under microwave heating were compared with those under conventional external heating.

DOI： 10.1016/j.cej.2015.07.051

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

Hybrid waveguides, i.e., dielectric waveguides combined with plasmonic waveguides, have great potential for concomitantly exhibiting subwavelength confinement and long range propagation, enabling a highly integrated photonic circuit. We report the characterization of hybrid waveguide modes excited in Si/SiO₂/Al films, by dispersion measurement using angle-resolved electron energy-loss spectroscopy. This experiment directly verifies the formation of the hybrid waveguide mode with a strongly localized electromagnetic field in a 6-nm-thick SiO₂ layer. The results clearly describe the characteristic behavior of the hybrid waveguide mode, which depends on the effective index of the constituent dielectric waveguide and the surface plasmon-polariton modes.

DOI： 10.1063/1.4916800

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

Codeposition of metal chlorides with Pt colloidal nanoparticles on TiO₂ followed by oxidation at 400 °C and reduction at 200 °C improved the activity for Pt-catalyzed CO oxidation with stoichiometric amount of O₂. The highest activity was obtained when FeCl₃ was used as the additive with the Fe/Pt ratio of 0.2. XAFS, X-EDS, FTIR, and catalytic studies revealed that the deposition of Fe species on the reduced Pt sites gave active sites, which exhibited CO oxidation with high efficiency.

DOI： 10.1246/cl.150903

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

A scanning transmission electron microscope (STEM) -cathodoluminescence (CL) technique is used to investigate the size dependence of the band structures in two-dimensional plasmonic crystals with a square lattice (SQ-PlCs) composed of cylindrical pillars and holes. The experimentally determined and calculated dependences of the band edge energies of the three SPP modes at the γ point on the diameter of the cylindrical structure agree well. The photon maps reveal the field strength distributions of the standing SPP waves of the three eigenmodes. Additionally, a mechanism is proposed to explain the dependence of the contrast on the detected light polarization.

DOI： 10.1364/OE.22.029761

Language：English   Publishing type：Research paper (other academic)  

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

We investigated the degree to which the dispersion relation of surface plasmon-polaritons excited on silver nanoantennas depends on length. To accomplish this, dispersion measurements for individual silver nanoantennas with lengths from 220 nm to 2.5 μm were performed using angle-resolved electron energy-loss spectroscopy (AREELS) and spatially resolved EELS (SREELS). AREELS enabled measurements of the dispersion relation extending into the high-energy region of 2.6 eV for a 2.5-μm-long silver nanoantenna, without the need to excite Fabry–Perot-type resonances. Our experiments showed that the dispersion relation of silver nanoantennas that have the same diameter is independent of their individual lengths.

DOI： 10.1093/jmicro/dft048

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

High-resolution transmission electron microscopy (HRTEM) and high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM) were applied to investigate the precipitates formed during age hardening at 170 ± 5 °C for up to 48. h of cast A319 aluminium alloy (Al-4.93. wt%Si-3.47. wt%Cu). The precipitates at the peak-aged condition have been identified as mainly θ" together with a smaller amount of θ'. It is proposed that the θ" is responsible for hardening at peak ageing at 170 °C of the cast A319 aluminium alloy.

DOI： 10.1016/j.micron.2012.10.009

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

In this study, the surface geometric structures of epitaxial (0001) ZnO films treated by NaOH solution are investigated using photoassisted scanning tunneling microscopy (STM). By illuminating ultraviolet (UV) light on the epitaxial (0001) ZnO film, the tunneling current can be significantly enhanced to construct the well-defined STM images. Polarity identification of the epitaxial (0001) ZnO film by convergent-beam electron diffraction indicates that the epitaxial (0001) ZnO film exhibits the Zn-polar surface. Two types of topographic features, i.e., hexagonal pyramid and flat plane, are observed in the AFM images of the as-grown epitaxial (0001) ZnO film. UV-assisted STM images reveal the anisotropic etching behaviors of the epitaxial (0001) ZnO films in NaOH solution. The faceted and symmetrically layered hexagonal-pyramid feature gets asymmetrical and rounded with increasing etching time. On the other hand, few small hexagonal pits on the as-grown flat ZnO(0001)-Zn surface are developed to asymmetrically hexagonal cavities with flat terraces and steps after NaOH treatments. In addition, triangular reconstruction of the NaOH-treated ZnO(0001)-Zn surface and evidently layer-stacking feature on a faceted ZnO surface with a step height resolved in the atomic scale are also demonstrated in ambience using the photoassisted STM.

DOI： 10.1021/jp302045j