Gwan Yeong Jung

Flat phonon bands in fluorite ferroelectrics ( or ) shrink polar domains laterally to an irreducible half‐unit‐cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. Here, we report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in thin films consisting of closely spaced head‐to‐head (HH) and tail‐to‐tail (TT) charged 180 walls, each exhibiting a distinct bulk‐like structure. These charged domain walls (CDWs) further compress the irreducibly narrow, laterally stacked domains vertically to a thickness of 1–2.75 nm, yielding in‐plane domains with sub‐ footprints—among the smallest ever reported for any ferroelectric material. The HH and TT walls form due to their flat longitudinal optical (LO) polar bands, and the former are electrostatically stabilized by bound‐charge compensation via interstitial oxygen atoms, which act as natural structural defects at the HH walls. Moreover, these walls are predicted to be conducting and to exhibit ultralow propagation barriers, with HH walls (1.6 meV) being far more mobile than TT walls (22.3 meV), indicating strong potential for low‐voltage, domain‐wall‐based nanoelectronics.

Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy (Δn = 2.1) was reported in a quasi-one-dimensional chalcogenide, Sr_9/8TiS₃. Compared to SrTiS₃, the excess Sr in Sr_9/8TiS₃ leads to periodic structural modulations and introduces additional electrons, which undergo charge ordering on select Ti atoms to form a highly polarizable cloud oriented along the c-axis, hence resulting in the colossal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to Δn = 2.5 in Sr_8/7TiS₃ is reported through control over the periodicity of the atomic-scale modulations. The role of structural modulations in tuning the optical properties in a series of Sr_xTiS₃ compounds with x = [1, 9/8, 8/7, 6/5, 5/4, 4/3, 3/2] is investigated using density-functional-theory (DFT) calculations. The structural modulations arise from various stacking sequences of face-sharing TiS₆ octahedra and twist-distorted trigonal prisms and are found to be thermodynamically stable for 1 < x < 1.5. As x increases, an indirect-to-direct band gap transition is predicted for x ≥ 8/7 along with an increased occupancy of Ti-d_z² states. Together, these two factors result in a theoretically predicted maximum birefringence of Δn = 2.5 for Sr_8/7TiS₃. Single crystals of Sr_8/7TiS₃ were grown using a molten-salt flux method. Single-crystal X-ray diffraction measurements confirm the presence of long-range order with a periodicity corresponding to Sr_8/7TiS₃, which is further corroborated by atomic-scale observations using scanning transmission electron microscopy. Polarization-resolved Fourier-transform infrared spectroscopy of Sr_8/7TiS₃ crystals shows Δn ≈ 2.5, in excellent agreement with the theoretical predictions. Overall, these findings demonstrate the compositional tunability of optical properties in Sr_xTiS₃ compounds by control over atomic scale modulations and suggest that similar strategies could be extended to other compounds having modulated structures.

Reactive oxygen species or superoxide (O₂^–), which damages or ages biological cells, is generated during metabolic pathways using oxygen as an electron acceptor in biological systems. Superoxide dismutase (SOD) protects cells from superoxide-triggered apoptosis by converting superoxide to oxygen and peroxide. Lithium–oxygen battery (LOB) cells have the same aging problems caused by superoxide-triggered side reactions. We transplanted the function of SOD of biological systems into LOB cells. Malonic acid-decorated fullerene (MA-C₆₀) was used as a superoxide disproportionation chemocatalyst mimicking the function of SOD. As expected, MA-C₆₀ as the superoxide scavenger improved capacity retention along charge/discharge cycles successfully. A LOB cell that failed to provide a meaningful capacity just after several cycles at high current (0.5 mA cm^–2) with 0.5 mAh cm^–2 cutoff survived up to 50 cycles after MA-C₆₀ was introduced to the electrolyte. Moreover, the SOD-mimetic catalyst increased capacity, e.g., more than a 6-fold increase at 0.2 mA cm^–2. The experimentally observed toroidal morphology of the final discharge product of oxygen reduction (Li₂O₂) and density functional theory calculation confirmed that the solution mechanism of Li₂O₂ formation, more beneficial than the surface mechanism from the capacity-gain standpoint, was preferred in the presence of MA-C₆₀.