Jun Kikkawa

Growth rate of silicon nanowires

We have measured the growth rate of silicon nanowires (SiNWs), which were grown at temperatures between 365 and 495 °C via the vapor-liquid-solid (VLS) mechanism. We grew SiNWs using gold as catalysts and monosilane (SiH4) as a vapor phase reactant. Observing SiNWs by means of transmission electron microscopy, we have found that SiNWs with smaller diameters grow slower than those with larger ones, and the critical diameter at which growth stops completely exists. We have estimated the critical diameter of SiNWs to be about 2 nm. We have also measured the temperature dependence of the growth rate of SiNWs and estimated the activation energy of the growth of SiNWs to be 230kJ∕mol.

Participation of Oxygen in Charge/Discharge Reactions in Li1.2Mn0.4Fe0.4O2: Evidence of Removal/Reinsertion of Oxide Ions

We have investigated the charge-discharge mechanism in the first cycle and the origin of its high charge―discharge capacity for Li 1.2 Mn 0.4 Fe 0.4 O 2 (0.5Li 2 MnO 3 ·0.5LiFeO 2 ) positive electrode material of lithium ion batteries. Results reveal that oxygen loss occurs in the entire region of the Li 1.2 Mn 0.4 Fe 0.4 O 2 particles composed of Mn-rich (Fe-substituted Li 2 MnO 3 ) and Fe-rich (Mn-substituted LiFeO 2 ) nanodomains during the first charge. Nanodomains of Mn-Li ferrites with a spinel structure start to be formed along the particle surfaces. During the first discharge, the extracted oxygen is partially reinserted preferentially into the Fe-rich nanodomains as oxide ions rather than in the Mn-rich nanodomains, and the proportion of the spinel nanodomains decreases. The origin of the high charge―discharge capacity might be ascribed to the participation of the oxide ions and neutral oxygen species in charge compensation by incorporation of the LiFeO 2 component into Li 2 MnO 3 . Irreversible capacity at the first cycle can be caused by the irreversible loss of oxygen during the charge and irreversible structural changes throughout the cycle: the movements of transition metal ions inducing random cation-site occupation throughout the cycle, associated with the formation and incomplete disappearance of the spinel ferrite nanodomains which is almost electrochemically-inactive under the applied voltage range.

Fe-rich and Mn-rich nanodomains in Li1.2Mn0.4Fe0.4O2 positive electrode materials for lithium-ion batteries

The authors investigated the distribution and local valence states of transition metal ions in a positive electrode material for lithium-ion batteries, Li1.2Mn0.4Fe0.4O2 nanoparticles, by electron energy-loss spectroscopy combined with scanning transmission electron microscopy. The experiments clarified the coexistence of Mn-rich and Fe-rich nanodomains in each single particle, and it is found that Fe-rich nanodomains contain Mn3+ ions which should be active in a redox reaction in spite of previous views of inactive Mn4+ ions in this material. The authors discuss a redox mechanism associated with the nanodomains.

Assembly of Na3V2(PO4)3 Nanoparticles Confined in a One‐Dimensional Carbon Sheath for Enhanced Sodium‐Ion Cathode Properties

Structural and morphological control is an effective approach for improvement of electrochemical properties in rechargeable batteries. One-dimensionally assembled structure composed of NASICON-type Na3 V2 (PO4 )3 nanoparticles were fabricated through an electrospinning method to meet the requirements for the development of efficient electrode materials in Na-ion batteries. High-temperature treatment of electrospun precursor fibers under an argon flow provides a nonwoven fabric of nanowires comprising crystallographically oriented nanoparticles of NASICON-type Na3 V2 (PO4 )3 within a carbon sheath. The mesostructure comprising NASICON-type Na3 V2 (PO4 )3 and carbon give a short sodium-ion transport pass and an efficient electron conduction pass. Electrochemical properties of NASICON-type Na3 V2 (PO4 )3 are improved on the basis of one-dimensional nanostructures designed in the present study.

Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode

Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na2MO3 (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na2MO3 are poorly established. Here using two polymorphs of Na2RuO3, we demonstrate the critical role of honeycomb-type cation ordering in Na2MO3. Ordered Na2RuO3 with honeycomb-ordered [Na1/3Ru2/3]O2 slabs delivers a capacity of 180 mAh g−1 (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAh g−1 (1.0-electron reaction). We clarify that the large extra capacity of ordered Na2RuO3 is enabled by a spontaneously ordered intermediate Na1RuO3 phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na2MO3 cathodes.

Coexistence of layered and cubic rocksalt structures with a common oxygen sublattice in Li1.2Mn0.4Fe0.4O2 particles: A transmission electron microscopy study

Local crystal structures associated with nanoscale variations of the concentration ratio of Fe ions to Mn ions in each single Li1.2Mn0.4Fe0.4O2 particle have been studied by nanobeam electron diffraction, high-resolution transmission electron microscopy, and electron energy-loss spectroscopy combined with scanning transmission electron microscopy. We have found the direct evidence that the Li1.2Mn0.4Fe0.4O2 particle is comprised of Mn-rich nanodomains with the layered rocksalt structure and Fe-rich nanodomains with the cubic rocksalt structure featured by the cation short-range order, within a common oxygen lattice framework of the cubic close-packed structure. Possible oxygen-centered octahedra in the present material are suggested. We discuss both origin and stability of the chemical-nanodomain structure.

Formation and Disappearance of Spinel Nanograins in Li1.2 − x Mn0.4Fe0.4O2 ( 0 ≤ x ≤ 0.99 ) during Extraction and Insertion of Li Ions

The authors investigated the changes in local structures and magnetic property of the Li 1.2 Mn 0.4 Fe 0.4 O 2 positive electrode material for lithium-ion batteries during the first charge―discharge cycle. Although Li 1.2 Mn 0.4 Fe 0.4 O 2 has a composite structure of the layered and disordered cubic rocksalt before the electrochemical testing, nanometer-sized grains with spinel structures are formed during the extraction of Li ions up to Li 1.2 Mn 0.4 Fe 0.4 O 2 (0 ≤ x ≤ 0.99). The spinel nanograins have weak spontaneous magnetization (ca. 0.3 G cm 3 /g), which is associated with the movements of Fe ions from the octahedral to the tetrahedral sites in both the layered and disordered rocksalt structures. The fraction of spinel nanograins decreased after the subsequent insertion of Li ions.

Understanding Li-K edge structure and interband transitions in LixCoO2 by electron energy-loss spectroscopy

The authors clarified fine structures of Li-K edge of LiCoO2 reflecting core–hole effects, using monochromated transmission electron microscopy—electron energy-loss spectroscopy (TEM–EELS) and first-principles calculations. Variation of interband transitions into empty Co 3d states hybridized with O 2p states with decrease in x of LixCoO2 was also presented. A reduced peak of interband transitions at 3.2 eV in low-loss EELS spectrum with decrease in x was related to reduction in the original empty Co eg states for LiCoO2 and appearance of empty bands just below the eg band.The authors clarified fine structures of Li-K edge of LiCoO2 reflecting core–hole effects, using monochromated transmission electron microscopy—electron energy-loss spectroscopy (TEM–EELS) and first-principles calculations. Variation of interband transitions into empty Co 3d states hybridized with O 2p states with decrease in x of LixCoO2 was also presented. A reduced peak of interband transitions at 3.2 eV in low-loss EELS spectrum with decrease in x was related to reduction in the original empty Co eg states for LiCoO2 and appearance of empty bands just below the eg band.

Electrical Characterization of Wafer-Bonded Germanium-on-Insulator Substrates Using a Four-Point-Probe Pseudo-Metal--Oxide--Semiconductor Field-Effect Transistor

The electrical characteristics of wafer-bonded non-doped germanium-on-insulator (GOI) substrates were investigated using a four-point-probe pseudo-metal–oxide–semiconductor field-effect transistor. Annealing the wafer-bonded GOI substrates in vacuum strongly influenced their electrical characteristics. GOI samples annealed at temperatures below 500 °C exhibited n-channel depletion transistor operation, whereas GOI samples annealed at temperatures between 550 and 600 °C exhibited p-channel depletion transistor operation. The carrier mobility strongly depended on the sweep direction of the gate voltage; this characteristic disappeared after annealing at temperatures above 550 °C. The dependence of the electrical characteristics on the annealing temperature is explained in terms of the influence of the defect states on energy band bending near the interface.

Quantitative annular dark-field imaging of single-layer graphene-II: atomic-resolution image contrast.

We have investigated how accurately atomic-resolution annular dark-field (ADF) images match between experiments and simulations to conduct more reliable crystal structure analyses. Quantitative ADF imaging, in which the ADF intensity at each pixel represents the fraction of the incident probe current, allows us to perform direct comparisons with simulations without the use of fitting parameters. Although the conventional comparison suffers from experimental uncertainties such as an amorphous surface layer and specimen thickness, in this study we eliminated such uncertainties by using a single-layer graphene as a specimen. Furthermore, to reduce image distortion and shot noises in experimental images, multiple acquisitions with drift correction were performed, and the atomic ADF contrast was quantitatively acquired. To reproduce the experimental ADF contrast, we used three distribution functions as the effective source distribution in simulations. The optimum distribution function and its full-width at half-maximum were evaluated by measuring the residuals between the experimental and simulated images. It was found that the experimental images could be explained well by a linear combination of a Gaussian function and a Lorentzian function with a longer tail than the Gaussian function.