arXiv subjects

Young-Hoon Kim

Publications and source records attributed to Young-Hoon Kim.

7 recordsLinked to original sources

arXiv2026

Deterministic fabrication of large-area, high-crystallinity oxide moire superlattices

Oxide twistronics extends moire engineering beyond van der Waals materials, offering a promising platform for accessing emergent interfacial phenomena arising from the strong coupling of lattice, charge, and orbital degrees of freedom in complex oxides. However, deterministic fabrication of high-crystallinity oxide moire superlattices over large lateral dimensions remains challenging due to the three-dimensional bonding network of oxides. Here, we demonstrate a scalable, generalized fabrication strategy that enables the formation of high-crystallinity oxide moire superlattices with clean, chemically bonded interfaces and precisely controlled twist angles down to nominal values of 0.1 degree, achieving sub-degree twist-angle accuracy across large contiguous lateral dimensions approaching the millimeter scale. Using NaNbO3 as a model system, we show that the resulting interlayer coupling drives pronounced structural reconstruction that modifies both the phase structure and ferroelectric domain configuration. Synchrotron-based X-ray 3D reciprocal space mapping reveals the emergence of a single-phase state in twisted bilayers, in contrast to the mixed-phase structure observed in single-layer membranes prior to twist assembly. The structural signatures are further consistent with gradual lattice rotation distributed along the thickness direction that may accommodate interfacial shear strain, distinct from reconstruction observed in van der Waals moire systems, which primarily occurs through in-plane stacking rearrangement. This collective lattice response is correlated with twist-dependent nanoscale electromechanical modulations observed by piezoresponse force microscopy. These results establish a scalable materials platform for oxide twistronics and open new pathways towards integrating twist-engineered complex oxides into practical, macroscale device architectures.

cond-mat.mtrl-sci↗

arXiv2026

Establishing Atomic Coherence in Twisted Oxide Membranes Containing Volatile Elements

Twisted oxide membranes represent a promising platform for exploring moire physics and emergent quantum phenomena. However, the presence of amorphous interfacial dead layers in conventional oxide heterostructures impedes coherent coupling and suppresses moire-induced interactions. While high-temperature thermal treatments can facilitate interfacial bonding, additional care is needed for materials containing volatile elements, where elevated temperatures may cause elemental loss. This study demonstrates the realization of atomically coherent, chemically bonded interface in twisted NaNbO3 heterostructures through controlled oxygen-annealing treatment. Atomic-resolution imaging and spectroscopy reveal ordered perovskite registry accompanied by systematic lattice contraction and modified electronic structure at the twisted interface, providing signatures of chemical reconstruction rather than physical adhesion. This reconstructed interface mediates highly asymmetric strain propagation in which the bottom membrane remains nearly relaxed while the top membrane accommodates substantial shear strain, thereby establishing a strain gradient that enables long-range electromechanical coupling throughout the twisted oxide membranes. By resolving the nature of the reconstructed interface, these findings establish a robust pathway for achieving coherent and strain-tunable oxide moire superlattices, opening pathways to engineer emergent ferroic and electronic functionalities.

cond-mat.mtrl-sci↗

arXiv2025

Ultralow-Temperature Cryogenic Transmission Electron Microscopy Using a New Helium Flow Cryostat Stage

Advances in cryogenic electron microscopy have opened new avenues for probing quantum phenomena in correlated materials. This study reports the installation and performance of a new side-entry condenZero cryogenic cooling system for JEOL (Scanning) Transmission Electron Microscopes (S/TEM), utilizing compressed liquid helium (LHe) and designed for imaging and spectroscopy at ultra-low temperatures. The system includes an external dewar mounted on a vibration-damping stage and a pressurized, low-noise helium transfer line with a remotely controllable needle valve, ensuring stable and efficient LHe flow with minimal thermal and mechanical noise. Performance evaluation demonstrates a stable base temperature of 6.58 K measured using a Cernox bare chip sensor on the holder with temperature fluctuations within 0.04 K. Complementary in-situ electron energy-loss spectroscopy (EELS) via aluminum bulk plasmon analysis was used to measure the local specimen temperature and validate cryogenic operation during experiments. The integration of cryogenic cooling with other microscopy techniques, including electron diffraction and Lorentz TEM, was demonstrated by resolving charge density wave (CDW) transitions in NbSe2 using electron diffraction, and imaging nanometric magnetic skyrmions in MnSi via Lorentz TEM. This platform provides reliable cryogenic operation below 7 K, establishing a low-drift route for direct visualization of electronic and magnetic phase transformations in quantum materials.

cond-mat.str-el↗

arXiv2025

Stray light in 3D porous nanostructures of single crystalline copper film

In the design of optical devices and components, geometric structures and optical properties of materials, such as absorption, refraction, reflection, diffraction, scattering, and trapping, have been utilized. Finding the ideal material with certain optical and geometric characteristics is essential for a customized application. Here, we fabricated unoxidizable achromatic copper films (ACFs) on Al2O3 substrates utilizing an atomic sputtering epitaxy apparatus. ACFs are made up of two regions vertically: a comparatively flat layer region and a three-dimensional (3D) porous nanostructured region on top of the flat region. The measured specular reflectance displayed low-pass filter behaviour with a sharp cutoff frequency in the infrared spectrum. Furthermore, the measured diffusive reflectance spectra showed light-trapping behaviour in the spectral region above the cutoff frequency, where there are no known absorption mechanisms, such as phonons and interband transitions. A focused ion beam scanning electron microscope was utilized to study the thin film's nanostructured region through 3D tomographic analysis in order to comprehend the phenomena that were observed. This work will shed fresh light on the design and optimization of optical filters and light-trapping employing porous nanostructured metallic thin films.

physics.optics↗

arXiv2025

Rapid Vapor-Assisted Solution Process of Metal-Organic Chalcogenides for High-Performance Light-Emitting Diodes

Metal-organic chalcogenides (MOCs), robust crystalline assemblies composed of coinage metals, chalcogens and organic ligands, are typically synthesized via prolonged, high temperature tarnishing of vacuum-deposited metal films with organochalcogen precursors. The prolonged exposure to high temperatures and the necessity for direct vacuum deposition of silver can induce damage to the underlying films, posing significant challenges to the fabrication of optoelectronic devices, despite their cost-effectiveness and chemical robustness. This study introduces vapor-assisted solution processing, a novel chemical vapor deposition method, enabling remarkably rapid fabrication of luminescent MOC films. Furthermore, the first MOC-based light-emitting diodes (MOCLEDs) are realized, achieving an external quantum efficiency (EQE) approaching 0.1% and electroluminescence peaking at 633 nm. These results highlight the potential of MOCs as next-generation emitters for displays and solid-state lighting. This work offers a promising fabrication strategy and insights for advancing MOCLEDs and expanding their optoelectronic potential.

cond-mat.mtrl-sci↗

arXiv2023

Roadmap on Perovskite Light-Emitting Diodes

In recent years, the field of metal-halide perovskite emitters has rapidly emerged as a new community in solid-state lighting. Their exceptional optoelectronic properties have contributed to the rapid rise in external quantum efficiencies (EQEs) in perovskite light-emitting diodes (PeLEDs) from <1% (in 2014) to approaching 30% (in 2023) across a wide range of wavelengths. However, several challenges still hinder their commercialization, including the relatively low EQEs of blue/white devices, limited EQEs in large-area devices, poor device stability, as well as the toxicity of the easily accessible lead components and the solvents used in the synthesis and processing of PeLEDs. This roadmap addresses the current and future challenges in PeLEDs across fundamental and applied research areas, by sharing the community's perspectives. This work will provide the field with practical guidelines to advance PeLED development and facilitate more rapid commercialization.

physics.optics↗

arXiv2021

Color of Copper/Copper oxide

Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. We demonstrate coherent propagation of an oxidation front in single-crystal Cu thin film to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain boundary-free and atomically flat films prepared by atomic-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full-color RGB indices is realized by precise control of oxide-layer thickness; our samples covered ~50.4% of the sRGB color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from oxide pigment (complex dielectric functions of Cu2O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. We further demonstrate laser-oxide lithography with micron-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.

cond-mat.mtrl-sci↗