Junwoo Son

In thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels, strategies that accelerate heating-induced shrinking often slow down subsequent re-swelling upon cooling, because the driving forces favorable for shrinking and those favorable for re-swelling are fundamentally opposed within a single homogeneous network. This kinetic asymmetry limits the reversibility and cyclability of thermoresponsive actuators. To resolve this trade-off, we propose decoupled attraction and repulsion for thermal actuation (DART), in which the shrinking and re-swelling driving forces are assigned to separate structural components of a hydrogel rather than sharing a single network. Water-swollen, negatively charged poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) nanogels, produced by planetary milling of bulk PAMPS hydrogels, were dispersed throughout a phase-separated PNIPAAm matrix to construct a composite hydrogel (PSG-Pn). The porous, phase-separated matrix enables rapid heating-induced water expulsion, while thermally modulated electrostatic repulsion among the PAMPS nanogels, governed by the temperature-dependent dielectric constant and Debye screening length, drives fast re-swelling upon cooling without impeding the shrinking process. PSG-Pn exhibited shrinking and re-swelling relaxation times of 2.24 and 219 s, respectively, approximately 1250-fold and 14-fold faster than those of a conventional non-phase-separated PNIPAAm hydrogel. PSG-Pn sustained ∼90% volume recovery over 20 heating-cooling cycles in water, and it also maintained repeated shrinking and re-swelling under open-air conditions without drying or structural failure. Building on this response, a near-infrared-responsive bilayer combining PSG-Pn with graphene oxide was steered to crawl in either direction across an unpatterned flat surface simply by switching the irradiation site, without any built-in ratchet or friction asymmetry, a behavior further supported by finite-element analysis of the localized contraction-recovery cycle. Together, these results indicate that partitioning attractive and repulsive driving forces into distinct structural elements offers a broadly applicable strategy for engineering thermoresponsive soft actuators with fast, large-amplitude, and reversible motion.

Overcoming the trade-off between strong photoresponse and fading memory remains a key challenge for efficient photonic reservoir computing based on photoactive materials. In this work, we demonstrate that Pt nanoparticle-embedded VO2 devices can simultaneously enhance photocarrier generation and prolong photocarrier lifetime through heterogeneous nanoscale VO2/Pt interfaces. A physical reservoir based on these devices resolves 16 distinct states under 4-bit optical input sequences and achieves an accuracy of 88.5% in sequence-encoded MNIST handwritten-digit classification, compared with 54.8% for pristine VO2. Ultrafast transient absorption spectroscopy further confirms that Pt NPs reshape the photoexcited carrier dynamics in VO2, leading to both improved photocarrier generation and an extended dominant carrier lifetime (3.86 ± 0.18 ns vs 2.60 ± 0.03 ns). These results suggest that nanoscale engineering of metal/oxide interfaces offers an effective materials-level strategy for improving temporal information processing in future photonic neuromorphic computing systems.

Neuromorphic sensory‐to‐motor interfaces require compact devices that can combine nonvolatile synaptic weight storage with volatile neuronal firing, yet these functions typically rely on distinct material and circuit mechanisms. Here, we report a structure‐engineered VO _y /nanoporous VO _x heterostructure that enables electrically selectable nonvolatile and volatile switching within a vanadium oxide memristor platform. Annealing‐induced interfacial diffusion and oxidation produce an asymmetric stack comprising a crystalline VO _y layer that supports threshold insulator‐to‐metal transition dynamics and an oxygen‐vacancy‐rich nanoporous VO _x region that promotes filamentary conductance modulation. In a 16 × 16 crossbar array, identically fabricated cells are reconfigured either as artificial synapses exhibiting multilevel retention and analog long‐term potentiation/depression or as artificial neurons producing relaxation‐oscillator spiking and diverse neuronal response features. By pairing two cells as a one‐synapse–one‐neuron unit, the programmed synaptic conductance modulates the neuronal firing frequency and measured current‐spike amplitude, thereby linking analog weight storage with spike‐based signal generation. Using measured device characteristics, a hardware‐informed spiking neural network recognizes rock–paper–scissors images with high accuracy, and its output commands are coupled to a memristive synergistic motor system that drives a robotic hand to generate counter‐gestures. These results suggest that structure‐engineered nanoporous vanadium oxide memristors can serve as reconfigurable building blocks for neuromorphic sensory‐to‐motor interfaces.

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.

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)O ₂ 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.

The metal‐insulator transition (MIT) in VO ₂ , accompanied by a significant refractive index modulation, has attracted considerable attention for the development of Si‐based photonic integrated circuits. Previous approaches to integrating VO ₂ films into Si photonics, however, have been limited by the loss of a steep transition due to structural and chemical disorders in VO ₂ films on Si. Herein, we demonstrate steep modulation of optical resonance in VO ₂ –Si hybrid modulators at telecom wavelengths using heterogeneous integration of single‐crystalline VO ₂ films on Si. Based on theoretical simulations, we confirmed that nonlinear optical modulation occurs in the resonant wavelength (λ _res ) by a steep phase transition in the epitaxial VO ₂ 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 VO ₂ –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.

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.

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 (τ_inc) 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.

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.

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 (t_c). 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 t_c. 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 (ΔT_H = 4.5 K) and cycle endurance without resistance degradation were achieved in VO₂ films above t_c 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.