Yoshifumi Oshima

Direct imaging of lithium atoms in LiV2O4 by spherical aberration-corrected electron microscopy

We visualized lithium atom columns in LiV₂O₄ crystals by combining scanning transmission electron microscopy with annular bright field (ABF) imaging using a spherical aberration-corrected electron microscope (R005) viewed from the [110] direction. The incident electron beam was coherent with a convergent angle of 30 mrad (semi-angle), and the detector collected scattered electrons over 20-30 mrad (semi-angle). The ABF image showed dark dots corresponding to lithium, vanadium and oxygen columns.

Measurement method of aberration from Ronchigram by autocorrelation function

Aberrations up to the fifth-order were successfully measured using an autocorrelation function of the segmental areas of a Ronchigram. The method applied to aberration measurement in a newly developed 300kV microscope that is equipped with a spherical aberration corrector for probe-forming systems. The experimental Ronchigram agreed well with the simulated Ronchigram that was calculated by using the measured aberrations. The Ronchigram had an infinite magnification area with a half-angle of 50mrad, corresponding to the convergence angle of a uniform phase.

Achieving 63 pm Resolution in Scanning Transmission Electron Microscope with Spherical Aberration Corrector

The performance of a newly developed high-resolution 300 kV microscope equipped with a spherical aberration corrector for probe-forming systems is reported. This microscope gave the highest resolution for the distance between atomic columns, as determined by a high-angle annular dark field imaging method using a GaN[211] crystalline specimen, where the distance between the neighboring columns of Ga was 63 pm.

Solid-liquid phase transition of tin particles observed by UHV high resolution transmission electron microscopy: pseudo-crystalline phase

Solid-liquid transition of fine tin particles having diameter of 2–10 nm is studied in-situ by high-resolution transmission electron microscopy under a ultra-high vacuum condition. Melting temperature is confirmed to decrease with the decrease of particle diameter. The particles less than the critical size, 2rc⋍5 nm, are found to have a specific phase between the solid and the liquid phase. The particle in this “pseudo-crystalline” phase contains crystalline embryos in it. Particles larger than the critical size have sharp liquid-solid transition, which completed within the time resolution of our microscope observation, 33 ms upon heating or cooling process. Large solid particles have Wulffs polyhedron, while particles around the critical diameter have rather spherical shape. Structural anomaly at the critical size occurs all over the outer most surface layer slightly below the melting temperature. Origin of the “pseudo-crystalline” phase and surface pre-melting phenomena are discussed.

Atomic Resolution Imaging of Gold Nanoparticle Generation and Growth in Ionic Liquids

Recent advances in in situ transmission electron microscopy (TEM) techniques have provided unprecedented knowledge of chemical reactions from a microscopic viewpoint. To introduce volatile liquids, in which chemical reactions take place, use of sophisticated tailor-made fluid cells is a usual method. Herein, a very simple method is presented, which takes advantage of nonvolatile ionic liquids without any fluid cell. This method is successfully employed to investigate the essential steps in the generation of gold nanoparticles as well as the growth kinetics of individual particles. The ionic liquids that we select do not exhibit any anomalous effects on the reaction process as compared with recent in situ TEM studies using conventional solvents. Thus, obtained TEM movies largely support not only classical theory of nanoparticle generation but also some nonconventional phenomena that have been expected recently by some researchers. More noteworthy is the clear observation of lattice fringes by high-resolution TEM even in the ionic liquid media, providing intriguing information correlating coalescence with crystal states. The relaxation of nanoparticle shape and crystal structure after the coalescence is investigated in detail. The effect of crystal orientation upon coalescence is also analyzed and discussed.

Direct Observation of an Anomalous Spinel-to-Layered Phase Transition Mediated by Crystal Water Intercalation

The phase transition of layered manganese oxides to spinel phases is a well-known phenomenon in rechargeable batteries and is the main origin of the capacity fading in these materials. This spontaneous phase transition is associated with the intrinsic properties of manganese, such as its size, preferred crystal positions, and reaction characteristics, and it is therefore very difficult to avoid. The introduction of crystal water by an electrochemical process enables the inverse phase transition from spinel to a layered Birnessite structure. Scanning transmission electron microscopy can be used to directly visualize the rearrangement of lattice atoms, the simultaneous insertion of crystal water, the formation of a transient structure at the phase boundary, and layer-by-layer progression of the phase transition from the edge. This research indicates that crystal water intercalation can reverse phase transformation with thermodynamically favored directionality.

Counting lithium ions in the diffusion channel of an LiV2O4 crystal

As a new microscopic method to reveal lithium ion behavior in lithium ion batteries, we demonstrated that lithium atoms in the diffusion channel of the spinel structure (LiV2O4 crystal) were visualized and their number was countable one-by-one by using annular bright field imaging method in conjunction with a spherical aberration corrected electron microscope: the lithium column intensity varied by a step of single lithium atom in correlation with the thickness change of the LiV2O4 crystal, in accordance with theoretical image simulations.

A stable lithium-rich surface structure for lithium-rich layered cathode materials

Lithium ion batteries are encountering ever-growing demand for further increases in energy density. Li-rich layered oxides are considered a feasible solution to meet this demand because their specific capacities often surpass 200 mAh g−1 due to the additional lithium occupation in the transition metal layers. However, this lithium arrangement, in turn, triggers cation mixing with the transition metals, causing phase transitions during cycling and loss of reversible capacity. Here we report a Li-rich layered surface bearing a consistent framework with the host, in which nickel is regularly arranged between the transition metal layers. This surface structure mitigates unwanted phase transitions, improving the cycling stability. This surface modification enables a reversible capacity of 218.3 mAh g−1 at 1C (250 mA g−1) with improved cycle retention (94.1% after 100 cycles). The present surface design can be applied to various battery electrodes that suffer from structural degradations propagating from the surface.

Scanning moiré fringe imaging for quantitative strain mapping in semiconductor devices

The development of a method for the precise measurement of strain fields in semiconductor devices has become a critical requirement because the electrical performances of the devices are greatly influenced by the strain induced in their structures. We applied scanning moire fringe imaging to demonstrate the quantitative strain mapping of a Si/Si1−xGex interfacial layer. The strain field was measured at a nano-meter scale spatial resolution, with a detection precision of 0.1%. The maximum value of the strain was measured to be 1.1% ± 0.1%, which is consistent with the direct measurement by high-resolution scanning transmission electron microscopy image.

Reversible contrast in focus series of annular bright field images of a crystalline LiMn2O4 nanowire

A through-focus series of annular bright field (ABF) images were observed simultaneously with high-angle annular dark field (HAADF) images of very thin lithium manganese oxide (LiMn₂O₄), a typical cathode material used in lithium ion batteries, using a spherical aberration corrected electron microscope with a 50 pm resolution (R005). The ABF images showed dark dips at the positions of Li and Mn--O atomic columns, which reversed to bright peaks when the defocus sign was changed, as commonly observed in phase contrast images. The optimal defocus for the ABF images was about 2 nm of over-focus, while that for the HAADF images was 2 nm of under-focus for an incident probe with a convergent semi-angle of 30 mrad. These experimental results are interpreted based on a weak-phase-object approximation.