Maximizing Photoelectrochemical Performance in Metal-Oxide Hybrid Composites via Amorphous Exsolution-A New Exsolution Mechanism for Heterogeneous Catalysis.

Exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle-perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond's low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

Wafer-recyclable, environment-friendly transfer printing for large-scale thin-film nanoelectronics.

Transfer printing of thin-film nanoelectronics from their fabrication wafer commonly requires chemical etching on the sacrifice of wafer but is also limited by defects with a low yield. Here, we introduce a wafer-recyclable, environment-friendly transfer printing process that enables the wafer-scale separation of high-performance thin-film nanoelectronics from their fabrication wafer in a defect-free manner that enables multiple reuses of the wafer. The interfacial delamination is enabled through a controllable cracking phenomenon in a water environment at room temperature. The physically liberated thin-film nanoelectronics can be then pasted onto arbitrary places of interest, thereby endowing the particular surface with desirable add-on electronic features. Systematic experimental, theoretical, and computational studies reveal the underlying mechanics mechanism and guide manufacturability for the transfer printing process in terms of scalability, controllability, and reproducibility.

Ultrafast Infrared Photoresponse from Heavily Hydrogen-Doped VO2 Single Crystalline Nanoparticles.

Infrared photodetectors are sought for diverse applications and their performance relies on photoactive materials and photocurrent generation mechanisms. Here, we fabricate IR photodetectors with heavily hydrogen-doped VO2 (i.e., HVO2) single-crystalline nanoparticles which show two orders greater resistivities than pure VO2. The I-V plots obtained under IR light irradiation are expressed by space charge limited current mechanism and the increase in photocurrent occurs due to the increase in the number of photoinduced trap sites. This phenomenon remarkably improves the key parameters at λ = 780 nm of high responsivity of 35280 A/W, high detectivity of 1.12 × 1013 Jones, and strikingly fast response times of 0.6-2.5 ns, that is, 3 orders of magnitude faster than the best records of two-dimensional structures and heterostructures. Density functional theory calculations illustrate that the generation of photoinduced trap sites is attributed to the movement of hydrogen atoms to less stable interstitial sites in VO2 under light exposure.

Reversible Polymorphic Transition and Hysteresis-Driven Phase Selectivity in Single-Crystalline C8-BTBT Rods.

Organic semiconductors (OSCs) are highly susceptible to the formation of metastable polymorphs that are often transformed by external stimuli. However, thermally reversible transformations in OSCs with stability have not been achieved due to weak van der Waals forces, and poor phase homogeneity and crystallinity. Here, a polymorph of a single crystalline 2,7-dioctyl[1] benzothieno[3,2-b][1]benzothio-phene rod on a low molecular weight poly(methyl methacrylate) (≈120k) that limits crystal coarsening during solvent vapor annealing is fabricated. Molecules in the polymorph lie down slightly toward the substrate compared to the equilibrium state, inducing an order of greater resistivity. During thermal cycling, the polymorph exhibits a reversible change in resistivity by 5.5 orders with hysteresis; this transition is stable toward bias and thermal cycling. Remarkably, varying cycling temperatures leads to diverse resistivities near room temperature, important for nonvolatile multivalue memories. These trends persist in the carrier mobility and on/off ratio of the polymorph field-effect transistor. A combination of in situ grazing incident wide angle X-ray scattering analyses, visualization for electronic and structural analysis simulations, and density functional theory calculations reveals that molecular tilt governs the charge transport characteristics; the polymorph transforms as molecules tilt, and thereby, only a homogeneous single-crystalline phase appears at each temperature.

Polar Metal Phase Induced by Oxygen Octahedral Network Relaxation in Oxide Thin Films.

ABO3 perovskite materials and their derivatives have inherent structural flexibility due to the corner sharing network of the BO6 octahedron, and the large variety of possible structural distortions and strong coupling between lattice and charge/spin degrees of freedom have led to the emergence of intriguing properties, such as high-temperature superconductivity, colossal magnetoresistance, and improper ferroelectricity. Here, an unprecedented polar ferromagnetic metal phase in SrRuO3 (SRO) thin films is presented, arising from the strain-controlled oxygen octahedral rotation (OOR) pattern. For compressively strained SRO films grown on SrTiO3 substrate, oxygen octahedral network relaxation is accompanied by structural phase separation into strained tetragonal and bulk-like orthorhombic phases, and the asymmetric OOR evolution across the phase boundary allows formation of the polar phase, while bulk metallic and ferromagnetic properties are maintained. From the results, it is expected that other oxide perovskite thin films will also yield similar structural environments with variation of OOR patterns, and thereby provide promising opportunities for atomic scale control of material properties through strain engineering.

Recoverable Electrical Breakdown Strength and Dielectric Constant in Ultralow-k Nanolattice Capacitors.

The dielectric reliability of low-k materials during mechanical deformation attracts tremendous attention, owing to the increasing demand for thin electronics to meet the ever-shrinking form factor of consumer products. However, the strong coupling between dielectric/electric and mechanical properties limits the use of low-k dielectrics in industrial applications. We report the leakage current and dielectric properties of a nanolattice capacitor during compressive stress cycling. Electrical breakdown measurements during the stress cycling, combined with a theoretical model and in situ mechanical experiments, provide insights to key breakdown mechanisms. Electrical breakdown occurs at nearly 50% strain, featuring a switch-like binary character, correlated with a transition from beam bending and buckling to collapse. Breakdown strength appears to recover after each cycle, concomitant with nanolattice's shape recovery. The compressive displacement at breakdown decreases with cycling due to permanently buckled beams, transforming the conduction mechanism from Schottky to Poole-Frankel emission. Remarkably, our capacitor with 99% porosity, k ∼ 1.09, is operative up to 200 V, whereas devices with 17% porous alumina films breakdown upon biasing based on a percolation model. Similarly with electrical breakdown, the dielectric constant of the capacitor is recoverable with five strain cycles and is stable under 25% compression. These outstanding capabilities of the nanolattice are essential for revolutionizing future flexible electronics.

Deterministic Nanoassembly of Quasi-Three-Dimensional Plasmonic Nanoarrays with Arbitrary Substrate Materials and Structures.

Guided manipulation of light through periodic nanoarrays of three-dimensional (3D) metal-dielectric patterns provides remarkable opportunities to harness light in a way that cannot be obtained with conventional optics yet its practical implementation remains hindered by a lack of effective methodology. Here we report a novel 3D nanoassembly method that enables deterministic integration of quasi-3D plasmonic nanoarrays with a foreign substrate composed of arbitrary materials and structures. This method is versatile to arrange a variety of types of metal-dielectric composite nanoarrays in lateral and vertical configurations, providing a route to generate heterogeneous material compositions, complex device layouts, and tailored functionalities. Experimental, computational, and theoretical studies reveal the essential design features of this approach and, taken together with implementation of automated equipment, provide a technical guidance for large-scale manufacturability. Pilot assembly of specifically engineered quasi-3D plasmonic nanoarrays with a model hybrid pixel detector for deterministic enhancement of the detection performances demonstrates the utility of this method.

Growth Kinetics of Individual Au Spiky Nanoparticles Using Liquid-Cell Transmission Electron Microscopy.

Precise control over the size and morphology of the Au spiky nanoparticle (SNP) is essential to obtain narrow and tunable surface plasmon resonance (SPR). However, these challenges require a fundamental understanding of the particle growth mechanism and kinetics as well as its morphological transition, which can only be achieved by real-time observation at nanometer resolution. Here, we report in situ liquid cell transmission electron microscopy studies of single and multiple Au SNP growth at various conditions of such parameters as size and dose rate of electron beam and HAuCl4 solution concentration. The particle evolves via a mixture of reaction and Au formation-limited growth, followed by Au formation-limited growth-a transition from faceted to roughened surfaces, confirmed by the analysis with different beam sizes and the UV-vis spectra that feature a unique trend of short- and long-wavelength plasmon band shift. Quantitative analyses combined with a theoretical model determine the transition time (tc) of the two regimes and estimate the surface concentration (ci) of the particle with time. Interestingly, tc can be manipulated by the particle density, which alters the surface roughening rate, and the density is modulated by tuning the aforementioned parameters based on DLVO theory. These results suggest a new method for synthesizing a Au SNP whose size, morphology, SPR, and density can be sensibly manipulated without adding reducing or capping agents.

Growth Kinetics of Individual Co Particles Ex-solved on SrTi0.75Co0.25O3-δ Polycrystalline Perovskite Thin Films.

A precise control of the size, density, and distribution of metal nanoparticles dispersed on functional oxide supports is critical for promoting catalytic activity and stability in renewable energy and catalysis devices. Here, we measure the growth kinetics of individual Co particles ex-solved on SrTi0.75Co0.25O3-δ polycrystalline thin films under a high vacuum, and at various temperatures and grain sizes using in situ transmission electron microscopy. The ex-solution preferentially occurs at grain boundaries and corners which appear essential for controlling particle density and distribution, and enabling low temperature ex-solution. The particle reaches a saturated size after a few minutes, and the size depends on temperature. Quantitative measurements with a kinetic model determine the rate limiting step, vacancy formation enthalpy, ex-solution enthalpy, and activation energy for particle growth. The ex-solved particles are tightly socketed, preventing interactions among them over 800 °C. Furthermore, we obtain the first direct clarification of the active reaction site for CO oxidation-the Co-oxide interface, agreeing well with density functional theory calculations.

Enabling Simultaneous Extreme Ultra Low-k in Stiff, Resilient, and Thermally Stable Nano-Architected Materials.

Low dielectric constant (low-k) materials have gained increasing popularity because of their critical role in developing faster, smaller, and higher performance devices. Their practical use has been limited by the strong coupling among mechanical, thermal, and electrical properties of materials and their dielectric constant; a low-k is usually attained by materials that are very porous, which results in high compliance, that is, silica aerogels; high dielectric loss, that is, porous polycrystalline alumina; and poor thermal stability, that is, Sr-based metal-organic frameworks. We report the fabrication of 3D nanoarchitected hollow-beam alumina dielectrics which k is 1.06-1.10 at 1 MHz that is stable over the voltage range of -20 to 20 V and a frequency range of 100 kHz to 10 MHz. This dielectric material can be used in capacitors and is mechanically resilient, with a Young's modulus of 30 MPa, a yield strength of 1.07 MPa, a nearly full shape recoverability to its original size after >50% compressions, and outstanding thermal stability with a thermal coefficient of dielectric constant (TCK) of 2.43 × 10-5 K-1 up to 800 °C. These results suggest that nanoarchitected materials may serve as viable candidates for ultra low-k materials that are simultaneously mechanically resilient and thermally and electrically stable for microelectronics and devices.

Real-Time Structural and Electrical Characterization of Metal-Insulator Transition in Strain-Modulated Single-Phase VO2 Wires with Controlled Diameters.

Single-crystal VO2 wires have gained tremendous popularity for enabling the study of the fundamental properties of the metal-insulator transition (MIT); however, it remains tricky to precisely measure the intrinsic properties of the transitional phases with controlled wire-growth properties, such as diameter. Here, we report a facile method for growing VO2 wires with controlled diameters by separating the formation of the liquidus V2O5 seed droplets from the evolution of the VO2 wire using oxygen gas. The kinetic analyses suggest that the growth proceeds via the VS (vapor-solid) mechanism, whereas the droplet determines the size and the location of the wire. In situ Raman spectroscopy combined with analyses of the electrical properties of an individual wire allowed us to construct a diameter-temperature phase diagram from three initial phases (i.e., M1, T, and M2), which were created by misfit stress from the substrate and were preserved at room temperature. We also correlated this relation with resistivity-diameter and activation energy-diameter relations supported by theoretical modeling. These carefully designed approaches enabled us to elucidate the details of the phase transitions over a wide range of stress conditions, offering an opportunity to quantify relevant thermodynamic and electronic parameters (including resistivities, activation energies, and energy barriers of the key insulating phases) and to explain the intriguing behaviors of the T phase during the MIT.

Dual detection of ultraviolet and visible lights using a DNA-CTMA/GaN photodiode with electrically different polarity.

We demonstrated the dual-detectable DNA-CTMA/n-GaN photodiode (DG-PD) for ultraviolet and visible lights. Halogen and UV lamps are employed to recognize the visible and UV wavelength, respectively. The DG-PD under dark condition has a negative-bias shift of current-voltage (I-V) curves by 0.78 V compared to reference diode without DNA. However, the I-V curves move towards positive bias side by 0.75 V and 1.02 V for the halogen- and UV-exposed photodiode, respectively. These cause electrically different polarity and amount for halogen- and UV-induced photocurrents, indicating that the DNA-CTMA on n-GaN is quite effective for recognizing visible and UV lights as a dual-detectable photodiode. The formation and charge transport mechanisms are also discussed.

Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams.

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.

Simulational investigation of self-aligned bilayer linear grating enabling highly enhanced responsivity of MWIR InAs/GaSb type-II superlattice (T2SL) photodetector.

Linear gratings polarizers provide remarkable potential to customize the polarization properties and tailor device functionality via dimensional tuning of configurations. Here, we extensively investigate the polarization properties of single- and double-layer linear grating, mainly focusing on self-aligned bilayer linear grating (SABLG), serving as a wire grid polarizer in the mid-wavelength infrared (MWIR) region. Computational analyses revealed the polarization properties of SABLG, highlighting enhancement in TM transmission and reduction in TE transmission compared to single-layer linear gratings (SLG) due to optical cavity effects. As a result, the extinction ratio is enhanced by approximately 2724-fold in wavelength 3-6 µm. Furthermore, integrating the specially designed SABLG with an MWIR InAs/GaSb Type-II Superlattice (T2SL) photodetector yields a significantly enhanced spectral responsivity. The TM-spectral responsivity of SABLG is enhanced by around twofold than the bare device. The simulation methodology and analytical analysis presented herein provide a versatile route for designing optimized polarimetric structures integrated into infrared imaging devices, offering superior capabilities to resolve linear polarization signatures.

Concurrent Amorphization and Nanocatalyst Formation in Cu-Substituted Perovskite Oxide Surface: Effects on Oxygen Reduction Reaction at Elevated Temperatures.

The activity and durability of chemical/electrochemical catalysts are significantly influenced by their surface environments, highlighting the importance of thoroughly examining the catalyst surface. Here, Cu-substituted La0.6Sr0.4Co0.2Fe0.8O3-δ is selected, a state-of-the-art material for oxygen reduction reaction (ORR), to explore the real-time evolution of surface morphology and chemistry under a reducing atmosphere at elevated temperatures. Remarkably, in a pioneering observation, it is discovered that the perovskite surface starts to amorphize at an unusually low temperature of approximately 100 °C and multicomponent metal nanocatalysts additionally form on the amorphous surface as the temperature raises to 400 °C. Moreover, this investigation into the stability of the resulting amorphous layer under oxidizing conditions reveals that the amorphous structure can withstand a high-temperature oxidizing atmosphere (≥650 °C) only when it has undergone sufficient reduction for an extended period. Therefore, the coexistence of the active nanocatalysts and defective amorphous surface leads to a nearly 100% enhancement in the electrode resistance for the ORR over 200 h without significant degradation. These observations provide a new catalytic design strategy for using redox-dynamic perovskite oxide host materials.

Enabling Durable Ultralow-k Capacitors with Enhanced Breakdown Strength in Density-Variant Nanolattices.

Ultralow-k materials used in high voltage devices require mechanical resilience and electrical and dielectric stability even when subjected to mechanical loads. Existing devices with organic polymers suffer from low thermal and mechanical stability while those with inorganic porous structures struggle with poor mechanical integrity. Recently, 3D hollow-beam nanolattices have emerged as promising candidates that satisfy these requirements. However, their properties are maintained for only five stress cycles at strains below 25%. Here, we demonstrate that alumina nanolattices with different relative density distributions across their height elicit a deterministic mechanical response concomitant with a 1.5-3.3 times higher electrical breakdown strength than nanolattices with uniform density. These density-variant nanolattices exhibit an ultralow-k of ≈1.2, accompanied by complete electric and dielectric stability and mechanical recoverability over 100 cyclic compressions to 62.5% strain. We explain the enhanced insulation and long-term cyclical stability by the bi-phase deformation where the lower-density region protects the higher-density region as it is compressed before the higher-density region, allowing to simultaneously possess high strength and ductility like composites. This study highlights the superior electrical performance of the bi-phase nanolattice with a single interface in providing stable conduction and maximum breakdown strength.

Segmentation and Morphology Computation of a Spiky Nanoparticle Using the Hourglass Neural Network.

Morphological measurements of nanoparticles in electron microscopy images are tedious, laborious, and often succumb to human errors. Deep learning methods in artificial intelligence (AI) paved the way for automated image understanding. This work proposes a deep neural network (DNN) for the automated segmentation of a Au spiky nanoparticle (SNP) in electron microscopic images, and the network is trained with a spike-focused loss function. The segmented images are used for the growth measurement of the Au SNP. The auxiliary loss function captures the spikes of the nanoparticle, which prioritizes the detection of spikes in the border regions. The growth of the particles measured by the proposed DNN is as good as the measurement in manually segmented images of the particles. The proposed DNN composition with the training methodology meticulously segments the particle and consequently provides accurate morphological analysis. Furthermore, the proposed network is tested on an embedded system for integration with the microscope hardware for real-time morphological analysis.

Transparent Intracellular Sensing Platform with Si Needles for Simultaneous Live Imaging.

Vertically ordered Si needles are of particular interest for long-term intracellular recording owing to their capacity to infiltrate living cells with negligible damage and minimal toxicity. Such intracellular recordings could greatly benefit from simultaneous live cell imaging without disrupting their culture, contributing to an in-depth understanding of cellular function and activity. However, the use of standard live imaging techniques, such as inverted and confocal microscopy, is currently impeded by the opacity of Si wafers, typically employed for fabricating vertical Si needles. Here, we introduce a transparent intracellular sensing platform that combines vertical Si needles with a percolated network of Au-Ag nanowires on a transparent elastomeric substrate. This sensing platform meets all prerequisites for simultaneous intracellular recording and imaging, including electrochemical impedance, optical transparency, mechanical compliance, and cell viability. Proof-of-concept demonstrations of this sensing platform include monitoring electrical potentials in cardiomyocyte cells and in three-dimensionally engineered cardiovascular tissue, all while conducting live imaging with inverted and confocal microscopes. This sensing platform holds wide-ranging potential applications for intracellular research across various disciplines such as neuroscience, cardiology, muscle physiology, and drug screening.

Electrical and structural properties of antimony-doped p-type ZnO nanorods with self-corrugated surfaces.

We report on p-type conductivity in antimony (Sb)-doped ZnO (ZnO:Sb) nanorods which have self-corrugated surfaces. The p-ZnO:Sb/n-ZnO nanorod diode shows good rectification characteristics, confirming that a p-n homojunction is formed in the ZnO nanorod diode. The low-temperature photoluminescence (PL) spectra of the ZnO:Sb nanorods reveal that the p-type conductivity in p-ZnO:Sb is related to the Sb(Zn)-2V(Zn) complex acceptors. Transmission electron microscopy (TEM) analysis of the ZnO:Sb nanorods also shows that the p-type conductivity is attributed to the Sb(Zn)-2V(Zn) complex acceptors which can be easily formed near the self-corrugated surface regions of ZnO:Sb nanorods. These results suggest that the Sb(Zn)-2V(Zn) complex acceptors are mainly responsible for the p-type conductivity in ZnO:Sb nanorods which have corrugated surfaces.