The non-equilibrium phase among the polymorphs of the materials provides a new solution to harness the unique properties for various applications. However, advanced synthetic routes need to be developed to experimentally realize the novel phases to overcome the thermodynamic stability of their equilibrium counterparts. Here, we synthesized cubic Ni3Sn bimetallic nanoparticles (NPs), known as a non-equilibrium polymorph, by the sequential exsolution of Ni and Sn metallic cation species from the perovskite stannate lattice framework. Following the prediction from the Ellingham diagram, Sn species decompose at a higher onset temperature than Ni species, enabling sequential formation of Ni (600°C) and Ni-Sn alloy (800°C) exsolved NPs by increasing the annealing temperature. Strikingly, due to the kinetic guidance of pre-formed cubic Ni NPs, subsequently exsolved Ni3Sn alloy NPs exhibited the unconventional cubic Fm 3 ¯ $\bar{3}$ m space group, which has a higher formation energy compared to the thermodynamically stable hexagonal Ni3Sn with P63/mmc. Interestingly, the cubic Ni3Sn NPs show unique catalytic pathways in CO* dissociation, which leads to CO-free methanol dissociation. This discovery offers a fresh perspective on the synthetic routes of non-equilibrium bimetallic polymorphs to harness distinct catalytic pathways.

The metal-insulator transition (MIT) in VO2, accompanied by a significant refractive index modulation, has attracted considerable attention for the development of Si-based photonic integrated circuits. Previous approaches to integrating VO2 films into Si photonics, however, have been limited by the loss of a steep transition due to structural and chemical disorders in VO2 films on Si. Herein, we demonstrate steep modulation of optical resonance in VO2-Si hybrid modulators at telecom wavelengths using heterogeneous integration of single-crystalline VO2 films on Si. Based on theoretical simulations, we confirmed that nonlinear optical modulation occurs in the resonant wavelength (lambda res) by a steep phase transition in the epitaxial VO2 films integrated on Si ring resonators. Interestingly, the unprecedented figure of merit for optical modulation strength (BM = 0.6) and transition sharpness ( = 1.65 ) were achieved in our hybrid ring resonator, which outperforms previous results using polycrystalline VO2-Si ring resonators. These findings highlight the potential of phase-transforming materials for silicon photonics and offer a novel strategy for the unrestricted integration of high-quality functional materials onto mature Si photonics platforms, advancing next-generation optical communication and computing systems.

Annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging is often called as the Z-contrast, where the element with a higher atomic number gives rise to the brighter contrast. ADF-STEM imaging on the self-assembled (Ti,V)O2 heterostructure, exhibiting the alternating V-rich and Ti-rich layers, unexpectedly reveals the higher contrast in V-rich layers in spite of the nearly identical atomic numbers of Ti (Z = 22) and V (Z = 23). Our analyses of local strain mapping and electron energy loss spectroscopy (EELS) confirm that oxygen vacancies are formed dominantly in the V-rich and thus the higher contrast in the V-rich layers is attributable to the accumulation of oxygen vacancies facilitated by the inherent [001] channel pathways of the rutile structure. Our results highlight the critical role of crystallographic pathways in guiding oxygen vacancy diffusion within rutile-based heterostructures.

Atomically flat SrTiO3 (001) surfaces with well-defined step-terrace structures are essential for high-quality oxide heterostructures, but their high-temperature evolution is strongly affected by surface segregation and defect formation. Here, we investigated SrTiO3 (001) surfaces annealed at 1200 °C in dry-air, Ar, and humid-air using atomic force microscopy and thermodynamic analysis. Dry-air annealing initially promoted step formation, but prolonged annealing induced phase inhomogeneity, indicating the formation of a thermodynamically stable SrO-rich surface layer. Ar annealing did not suppress this problem and instead produced a disordered surface, likely due to oxygen-vacancy-related instability. In contrast, humid-air annealing delayed SrO segregation by increasing the volatility of excess Sr species through hydroxide formation, enabling a well-defined step-terrace structure within an intermediate annealing window. However, SrO segregation reappeared after extended humid annealing, showing that humidity provides kinetic suppression rather than thermodynamic elimination. These results suggest that precise control of atmosphere chemistry can serve as a general strategy for balancing surface segregation and volatility-driven cation removal in complex oxide substrate preparation.

An oxygen-diffusive V 2 O 5 interlayer promotes oxygen redistribution in Hf 0.5 Zr 0.5 O 2 , leading to higher remnant polarization, lower coercive field, and excellent reliability.

The non‐equilibrium phase among the polymorphs of the materials provides a new solution to harness the unique properties for various applications. However, advanced synthetic routes need to be developed to experimentally realize the novel phases to overcome the thermodynamic stability of their equilibrium counterparts. Here, we synthesized cubic Ni 3 Sn bimetallic nanoparticles (NPs), known as a non‐equilibrium polymorph, by the sequential exsolution of Ni and Sn metallic cation species from the perovskite stannate lattice framework. Following the prediction from the Ellingham diagram, Sn species decompose at a higher onset temperature than Ni species, enabling sequential formation of Ni (600°C) and Ni‐Sn alloy (800°C) exsolved NPs by increasing the annealing temperature. Strikingly, due to the kinetic guidance of pre‐formed cubic Ni NPs, subsequently exsolved Ni 3 Sn alloy NPs exhibited the unconventional cubic Fm m space group, which has a higher formation energy compared to the thermodynamically stable hexagonal Ni 3 Sn with P6 3 /mmc. Interestingly, the cubic Ni 3 Sn NPs show unique catalytic pathways in CO * dissociation, which leads to CO‐free methanol dissociation. This discovery offers a fresh perspective on the synthetic routes of non‐equilibrium bimetallic polymorphs to harness distinct catalytic pathways.

Unlike the extrinsic defects using intentionally introduced foreign atoms, oxygen vacancies (V-O) as a ubiquitous intrinsic n-type dopant for many oxides can supply electrons using intrinsic defect chemistry during high temperature annealing. Here, we report a comparative study of two n-type BaSnO3 hetero-epitaxial films (extrinsic La doping and intrinsic V-O doping). Interestingly, unlike typical transparent conducting oxides, direct band gap increases with free carrier concentration based on accurate modeling with non-parabolic bands in both BaSnO3 films, which represents direct evidence of electrostatically-driven band gap renormalization in this new transparent conducting oxide. Despite this dopant-independent optical response, V-O-doped BaSnO3-delta films show higher room-temperature electron mobility (similar to 63 cm(2)V(-1)s(-1) at n(3D) = 5 x 10(19) cm(-3)) than as-grown (La, Ba) SnO3 films. The structural characterization using bright-field transmission electron microscopy (TEM) with twobeam condition directly reveals that the mobility enhancement in BaSnO3-delta is attributed to healing of threading dislocations by mutual interactions between point and extended defects during thermal treatment for V-O generation.

A delicate interplay between lattice and charge provides a unique strategy to control the strain-driven stability of the electronic phase in quantum materials featuring metal-insulator transition (MIT). The strain-induced modification of the ionic bonding network has been utilized to tune the transition temperature (TMI) of VO2 by changing the band occupancy of d electrons. Still, previous approaches prevent the maintenance of a steep phase transition due to the introduction of structural disorders. Herein, strain partitioning is exploited using freestanding tri-layers for the unrestricted TMI change without losing transition quality. Using the strategy to release freestanding TiO2/VO2/TiO2 tri-layers with different thickness ratios (tVO2/tTiO2${t_{{\mathrm{V}}{{\mathrm{O}}_2}}}/{t_{{\mathrm{Ti}}{{\mathrm{O}}_2}}}$), strain partitioning by the thickness of TiO2 layers (tTiO2${t_{{\mathrm{Ti}}{{\mathrm{O}}_2}}}$) systematically control electronic phase stability in sandwiched VO2 layers without the introduction of the disorder; remarkably, extensive resistance modulation (Delta R/R similar to 1.0 x 103) and steep transition (Delta Th, Delta Tc = 2.5 K) are achieved in TMI-tuned VO2 layer in terms of both electrical and optical characteristics. The freestanding tri-layer design will provide a new strategy to unrestrictedly tune TMI in the VO2 layer by simply changing the thickness of strain-partitioning layers (tTiO2${t_{{\mathrm{Ti}}{{\mathrm{O}}_2}}}$) and offer a unique opportunity to design strain-tunable functionality using emergent materials.

The threshold switch operated by a field-driven insulator-to-metal transition in VO2 has attracted considerable interest for emerging devices due to its nonlinear and sensitive response to external voltage. However, the nucleation barrier intrinsic to the first-order phase transition causes a finite time delay before the abrupt rise in currents under voltage pulses, thereby hindering energy-efficient device operation. Here, we demonstrate that the strain-graded VO2 epilayer on Pt nanoislands (NIs) enables percolation-limited threshold switching by promoting the nucleation process of metallic phases during voltage-triggered phase transitions. Unlike constantly strained VO2, the Pt NIs locally disrupt lattice coherency at the VO2/TiO2 interface, facilitating gradual relaxation of misfit strain energy; tailoring the spatial strain distribution in the strain-graded VO2 films effectively lowers the activation barrier for the nucleation events of metallic domains, achieving one-twentieth lower incubation time (τinc) compared to constantly strained VO2 films. Moreover, this percolation-limited phase evolution stabilizes an intermediate metastable phase (i.e., negative differential resistance), enabling robust self-oscillatory behavior across a wide current range with enhanced tunability and dynamic controllability. These findings tailor the phase transition dynamics for ultrafast and energy-efficient switching applications.

Light-driven energy conversion devices call for the atomic-level manipulation of defects associated with electronic states in solids. However, previous approaches to produce oxygen vacancy ( V O ) as a source of sub-bandgap energy levels have hampered the precise control of the distribution and concentration of V O . Here, a new strategy to spatially confine V O at the homo-interfaces is demonstrated by exploiting the sequential growth of anatase TiO 2 under dissimilar thermodynamic conditions. Remarkably, metallic behavior with high carrier density and electron mobility is observed after sequential growth of the TiO 2 films under low pressure and temperature (L-TiO 2 ) on top of high-quality anatase TiO 2 epitaxial films (H-TiO 2 ), despite the insulating properties of L-TiO 2 and H-TiO 2 single layers. Multiple characterizations elucidate that the V O layer is geometrically confined within 4 unit cells at the interface, along with low-temperature crystallization of upper L-TiO 2 films; this 2D V O layer is responsible for the formation of in-gap states, promoting photocarrier lifetime (≈300%) and light absorption. These results suggest a synthetic strategy to locally confine functional defects and emphasize how sub-bandgap energy levels in the confined imperfections influence the kinetics of light-driven catalytic reactions.

The ultimate scaling limit in ferroelectric switching has been attracting broad attention in the fields of materials science and nanoelectronics. Despite immense efforts to scale down ferroelectric features, however, only few materials have been shown to exhibit ferroelectricity at the unit-cell level. Here we report a controllable unit-cell-scale domain in brownmillerite oxides consisting of alternating octahedral/tetrahedral layers. By combining atomic-scale imaging and in situ transmission electron microscopy, we directly probed sub-unit-cell-segmented ferroelectricity and investigated their switching characteristics. First-principles calculations confirm that the phonon modes related to oxygen octahedra are decoupled from those of the oxygen tetrahedra in brownmillerite oxides, and such localized oxygen tetrahedral phonons stabilize the sub-unit-cell-segmented ferroelectric domain. The unit-cell-wide ferroelectricity observed in our study could provide opportunities to design high-density memory devices using phonon decoupling.

High-quality growth manifests the exotic properties of correlated oxides (e.g., VO2) in thin film forms, but the defects and cracks that relax the misfit strain energy deteriorate the quality of the metal-insulator transition (MIT) in VO2 epilayers on thick TiO2 substrates above the critical thickness (tc). A new approach must be developed to overcome the fundamental degradation by strain relaxation during the pseudomorphic growth of VO2 films. Herein, we utilize thin TiO2 nanomembranes (NM) as a strain-sharing layer to allow the formation of crack-free VO2 epitaxial films exceeding tc. While the inhomogeneous strain relaxation induced by cracks occurs in VO2 films on thick TiO2 substrates (∼0.5 mm), the homogeneous and relaxed 50-nm-thick VO2 films are grown by simply converting thick TiO2 substrates to thin TiO2 NM (∼8 nm) as a growth template. Atomic-scale characterization reveals that a strong strain gradient was observed in underlying TiO2 NM as well as VO2 epilayers at the interface; unlike VO2 on a thick TiO2 substrate, this strain sharing by compliant TiO2 NM suppresses the formation of catastrophic cracks in VO2 epitaxial layers. Due to the absence of the cracks, excellent MIT steepness (ΔTH = 4.5 K) and cycle endurance without resistance degradation were achieved in VO2 films above tc on TiO2 NM. Our design of thin film growth will provide a new strategy to utilize a compliance effect to release misfit strain energy and offer a novel platform for advanced epitaxial growth techniques to unrestrictedly design multifunctional heterostructures for advanced electronics.

Although metal exsolution from perovskite materials effectively improves catalytic activity, its high temperature and prolonged operation time present challenges for industrial synthesis efficiency and scalability, emphasizing the necessity for straightforward and scalable fabrication methods at lower temperatures. Herein, a novel catalyst fabrication approach through the bead milling of perovskite nanoparticles are introduced to achieve low‐temperature (300 °C) metal exsolution. The bead milling process reduces the particle size to less than 50 nm and diminishes the degree of crystallinity, thus lowering the exsolution temperature. Consequently, the oxygen evolution reaction (OER) mass activity of Co nanoparticles exsolved from La 0.6 Sr 0.4 CoO 3‐δ perovskites exhibits nearly a sixfold increase compared with pristine materials, demonstrating the enhanced OER activity achieves through bead milling and exsolution processes.

AbstractBioinspired sensory systems based on spike neural networks have received considerable attention in resolving high energy consumption and limited bandwidth in current sensory systems. To efficiently produce spike signals upon exposure to external stimuli, compact neuron devices are required for signal detection and their encoding into spikes in a single device. Herein, it is demonstrated that Mott oscillative spike neurons can integrate sensing and ceaseless spike generation in a compact form, which emulates the process of evoking photothermal sensing in the features of biological photothermal nociceptors. Interestingly, frequency‐tunable and repetitive spikes are generated above the threshold value (Pth = 84 mW cm−2) as a characteristic of “threshold” in leaky‐integrate‐and‐fire (LIF) neurons; the neuron devices successfully mimic a crucial feature of biological thermal nociceptors, including modulation of frequency coding and startup latency depending on the intensity of photothermal stimuli. Furthermore, Mott spike neurons are self‐adapted after sensitization upon exposure to high‐intensity electromagnetic radiation, which can replicate allodynia and hyperalgesia in a biological sensory system. Thus, this study presents a unique approach to capturing and encoding environmental source data into spikes, enabling efficient sensing of environmental sources for the application of adaptive sensory systems.

Analog in-memory computing, leveraging resistive switching cross-point devices known as resistive processing units (RPUs), offers substantial improvements in the performance and energy efficiency of deep neural network (DNN) training. Among the promising candidates for RPU devices, the capacitor-based synaptic circuit stands out due to its near-ideal switching characteristics. However, despite its potential, challenges such as large cell areas and retention issues remain to be addressed. In this work, we study the three-transistors-one-capacitor synaptic cell design, aiming to enhance computing performance and scalability. Through comprehensive device-level modeling and system-level simulation, assessment is done on how the transistor characteristics influence DNN training accuracy and reveal critical design strategies. A novel cell design methodology that optimizes computing performance while minimizing cell area is proposed, thereby enhancing scalability. Additionally, development guidelines for cell components are provided, identifying oxide-based semiconductors as a promising channel material for transistors. This research contributes valuable insights for the development of future analog DNN training accelerators using capacitor-based synaptic cell, with a focus on addressing the current limitations and maximizing efficiency.

Electrochemical reduction of CO2 to chemical fuels with a transition metal-based single atom catalyst (SAC) offers a promising strategy to reduce CO2 with high catalytic selectivity. To date, the study of atomically dispersed SACs has been mainly conducted by using a conventional H-type cell system with limited solubility of CO2 in aqueous electrolytes, resulting in large overpotentials and low current density. Here, we reported a pyrrolic N-stabilized Ni SAC with low-coordinated Ni-N-x sites by thermal activation of Ni ZIF-8, which was tested in a 3-compartment microfluidic flow cell system at the industrial level. When the pyrolysis temperature increased from 800 degrees C (Ni SAC-800) to 1000 degrees C (Ni SAC-1000), the content ratio of pyrrolic N/pyridinic N increased from 0.37 to 1.01 as well as the coordination number of Ni in Ni-N-x sites decreased from 3.14 to 2.63. Theoretical calculations revealed that the synergistic effect between the high content ratio of pyrrolic N and low-coordinated Ni can decrease the energy barrier for the desorption of *CO during the CO2RR. Therefore, Ni SAC-1000 exhibited superior catalytic performances with high CO selectivity (FECO = 98.24% at -0.8 V-RHE) compared to that of Ni SAC-800 (FECO = 40.76% at -0.8 V-RHE). Moreover, Ni SAC-1000 based on the flow cell system showed a higher current density (similar to 200 mA cm(-2)) compared to that of the H-type cell system (similar to 20 mA cm(-2)). As a result, this study experimentally demonstrated that the pyrrolic N-stabilized and low-coordinated Ni SAC-1000 in the microfluidic flow cell reactor provides great chances for scaling up the productivity of the CO2RR at the industrial level.

The octahedral symmetry in ionic crystals can play a critical role in atomic nucleation and migration during solid-solid phase transformation. Similarly, octahedron distortion, which is characterized by Goldschmidt tolerance factor, strongly influences the exsolution kinetics in the perovskite lattice framework during high-temperature annealing. However, a fundamental study on manipulating the exsolution process by octahedron distortion is still lacking. In this study, we accelerate Ni metal exsolution on the surface of perovskite stannates by increasing the [BO6] octahedron distortion in the lattices. Decreasing the A-site ionic radius (rBa2+ = 161 pm → rSr2+ = 144 pm → rCa2+ = 134 pm) increased the density of exsolved Ni nanoparticles by up to 640% (i.e., 47 particles μm-2 of Ba(Sn, Ni)O3 → 304 particles μm-2 of Ca(Sn, Ni)O3) after the identical exsolution process. Based on the theoretical calculation and experimental characterization, the decrease in crystal symmetry by octahedral distortion promoted the Ni exsolution owing to the boosted Ni migration by weakening the bond strength and generating domain boundaries. The findings highlight the importance of octahedral distortion to control atomic migration through the perovskite lattice framework and provide a strategy to tailor the density of uniformly populated nanoparticles in nanocomposite oxides for multifunctional material design.