Yeong-Gi So

Direct Atomic-Resolution Observation of Two Phases in the Li1.2Mn0.567Ni0.166Co0.067O2 Cathode Material for Lithium-Ion Batteries†

About phase: The coexistence of rhombohedral LiTMO2 (TM=Ni, Co, or Mn) and monoclinic Li2MnO3-like structures inside Li1.2Mn0.567Ni0.166Co0.067O2 is revealed directly at atomic resolution. The hetero-interface along the [001]rh/[103]mon zone axis direction is demonstrated, indicating the two-phase nature of these lithium-rich cathode materials (green Li, blue Mn, red O, cyan TM).

Spontaneous Formation of Wurzite-CdS/Zinc Blende-CdTe Heterodimers through a Partial Anion Exchange Reaction

Ion exchange of ionic semiconductor nanoparticles (NPs) is a facile method for the synthesis of type-II semiconductor heterostructured NPs with staggered alignment of band edges for photoelectric applications. Through consideration of the crystallographic orientation and strain at the heterointerface, well-designed heterostructures can be constructed through ion exchange reactions. Here we report the selective synthesis of anisotropically phase-segregated cadmium sulfide (CdS)/ cadmium telluride (CdTe) heterodimers via a novel anion exchange reaction of CdS NPs with an organic telluride precursor. The wurtzite-CdS/zinc blende-CdTe heterodimers in this study resulted from spontaneous phase segregation induced by the differences in the crystal structures of the two phases, accompanying a centrosymmetry breaking of the spherical CdS NPs. The CdS/CdTe heterodimers exhibited photoinduced spatial charge separation because of their staggered band-edge alignment.

Direct observation of Σ7 domain boundary core structure in magnetic skyrmion lattice

Advanced electron microscopy reveals a unique structural relaxation mechanism in a magnetic skyrmion domain boundary core. Skyrmions are topologically protected nanoscale magnetic spin entities in helical magnets. They behave like particles and tend to form hexagonal close-packed lattices, like atoms, as their stable structure. Domain boundaries in skyrmion lattices are considered to be important as they affect the dynamic properties of magnetic skyrmions. However, little is known about the fine structure of such skyrmion domain boundaries. We use differential phase contrast scanning transmission electron microscopy to directly visualize skyrmion domain boundaries in FeGe1−xSix induced by the influence of an “edge” of a crystal grain. Similar to hexagonal close-packed atomic lattices, we find the formation of skyrmion “Σ7” domain boundary, whose orientation relationship is predicted by the coincidence site lattice theory to be geometrically stable. On the contrary, the skyrmion domain boundary core structure shows a very different structure relaxation mode. Individual skyrmions can flexibly change their size and shape to accommodate local coordination changes and free volumes formed at the domain boundary cores. Although atomic rearrangement is a common structural relaxation mode in crystalline grain boundaries, skyrmions show very unique and thus different responses to such local lattice disorders.

Strongest π–metal orbital coupling in a porphyrin/gold cluster system

Face-on and close configuration of a π-conjugated molecule on an Au cluster generates a strong π–metal coupling between the π and metal orbitals. We successfully synthesized porphyrin-coordinated Au clusters in face-on configuration at the closest distance. The interaction between the Au cluster and porphyrin was investigated by UV-vis-NIR and transient absorption measurement.

Mechanical properties of dual-phase alloys of B2 and icosahedral phases in the Al–Cu–Fe system

Intermetallic alloys with B2 structure and dual-phase alloys of the B2 phase and an icosahedral (i-) phase have been produced, and their mechanical properties have been investigated. Vickers hardness measurements and compression experiments have shown that the dual-phase alloys have considerably higher strength than that of the B2 single-phase alloy up to a high temperature of around 750 K. The dual-phase alloys have also exhibited much better ductility than i-single-phase alloys. One possibility to be pointed out is that we can further design a high-strength and good-ductility alloy by tuning the volume fraction, size and morphology of the i-phase precipitates.

Jointed magnetic skyrmion lattices at a small-angle grain boundary directly visualized by advanced electron microscopy

The interactions between magnetic skyrmions and structural defects, such as edges, dislocations, and grain boundaries (GBs), which are all considered as topological defects, will be important issues when magnetic skyrmions are utilized for future memory device applications. To investigate such interactions, simultaneous visualization of magnetic skyrmions and structural defects at high spatial resolution, which is not feasible by conventional techniques, is essential. Here, taking advantages of aberration-corrected differential phase-contrast scanning transmission electron microscopy, we investigate the interaction of magnetic skyrmions with a small-angle GB in a thin film of FeGe1−xSix. We found that the magnetic skyrmions and the small-angle GB can coexist each other, but a domain boundary (DB) was formed in the skyrmion lattice along the small-angle GB. At the core of the DB, unexpectedly deformed magnetic skrymions, which appear to be created by joining two portions of magnetic skyrmions in the adjacent lattices, were formed to effectively compensate misorientations between the two adjacent magnetic skyrmion lattices. These observations strongly suggest the flexible nature of individual magnetic skyrmions, and also the significance of defect engineering for future device applications.

Formation of Tsai-type 1/1 approximants in In-Pd-RE (RE: rare earth metal) alloys

The formation of the 1/1 crystal approximant phase (1/1 phase) to the icosahedral phase (i phase) in In-Pd-RE (RE: rare earth metal) systems has been investigated. A new series of 1/1 phases were found in In53Pd33RE14 (RE; Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) alloys. For Y, Sm, Gd, Tb, Dy, and Ho, the 1/1 phases were found in annealed alloys, indicating that they are thermodynamically stable. The atomic structure of the 1/1 phases was directly observed by high-angle annular dark-field imaging performed via scanning transmission electron microscopy, revealing that the 1/1 phases consisted of a periodic arrangement of Tsai-type icosahedral clusters. Further, the atomic size effect on i phase formation, as well as formation conditions previously reported for other Tsai-type i and 1/1 phases were examined. It was found that the ratio of the atomic radius of base metals such as In and Pd affects i phase formation. Moreover, the appropriate range of the radius ratio for i phase formation was narrower than that for 1/1 phase formation.

Spectroscopic study of Ni-rich Al–Co–Ni quasicrystal

The valence-band electronic structure of a decagonal Ni-rich Al–Co–Ni quasicrystal, Al72Co8Ni20, was investigated using soft X-ray photoelectron spectroscopy. In particular, the energy distributions of the transition metal 3d bands were determined from the Co and Ni 2p–3d resonance photoemission and compared with those calculated by the discrete variational Xα method for a model cluster based on a proposed Ni-rich Al–Co–Ni approximant, as well as the 3d band observed in the Co-rich Al–Co–Ni quasicrystal, Al72Co16Ni12. In the Ni-rich Al–Co–Ni, the transition metal 3d band exhibits a peak at a binding energy, E B, of 2.3 eV, which is higher than that of the Co-rich Al–Co–Ni. The observed Ni 3d band has a single-peaked distribution around E B ∼ 2.4 eV, in contrast to the calculated bimodal and wide-spread distribution for the proposed Ni-rich Al–Co–Ni model cluster, whereas the Co 3d band is located at E B ∼ 1.7 eV, consistent with the model calculation.

Internal friction of an Al–Cu–Fe icosahedral quasicrystal and its crystal approximant

The internal friction was measured for an Al–Cu–Fe icosahedral phase (i-phase) and an Al–Cu–Fe–Si 1/1 approximant phase (1/1-phase) by using a dynamical mechanical analyzer (DMA) in the frequency range 0.05–10 Hz and the temperature range 300–873 K. For the i-phase, four absorption peaks were observed at around 420, 580, 620 and 800 K, respectively. These were of the thermally activated relaxation type, and their activation parameters were evaluated. The parameters evaluated for the peak at 800 K indicate a relaxation process involving a collective atomic motion, which can be attributed to correlated phason jumps. This peak was absent for the 1/1-phase. These results are similar to the results of the internal friction measurements previously reported for an Al–Pd–Mn i-phase and its 1/1 approximant phase. Based on the phonon–phason dynamics model, the magnitudes of phason elastic constants were estimated for the Al–Cu–Fe i-phase.

Formation of crystal approximants to icosahedral quasicrystal in Sc-based alloys

The composition range for the formation of a 1/1 cubic approximant phase to icosahedral quasicrystal has been investigated in various Sc based alloys of Sc-Rh-TM (TM: transition metal) type. The composition area of the approximant phase differs depending on the TM element and can be classified into three groups: in one group it extends roughly along a Rh constant line, in another it extends roughly along a Sc constant line and in the other it extends two-dimensionally in the diagram. The shape of the composition area correlates well with the position of the TM element in the periodic table, although it has no apparent correlation with the number of valence electrons per atom ratio (e/a ratio).