Yung-Chang Lin

Bilayer graphene (BLG) exhibits unique physical properties that strongly depend on the twist angle, such as superconductivity at magic angles, moiré superlattices, and quasicrystal structures. However, the influence of the twist angle on the chemical properties remains unclear. Here, we investigated the chemical reactivity of twist-angle-controlled BLG toward ultraviolet (UV)/ozone oxidation. For the systematic study of the angle dependence, two sheets of monolayer graphene were stacked with controlled stacking angles from 0° to 30° with 5° intervals. Raman spectroscopy revealed that the degree of oxidation is strongly dependent on the twist angle and it increases with the twist angle, indicating a strong influence of the interlayer coupling on the reactivity. Scanning transmission electron microscopy and X-ray photoelectron spectroscopy revealed that the oxidation proceeds through the formation of localized oxygen functional groups in the graphene lattice. Density functional theory calculations suggested that functionalization becomes energetically more favorable at higher twist angles. Layer-specific 12C/13C isotope labeling demonstrated that the oxidation occurs mostly at the top graphene layer, while the bottom layer remains almost intact. Furthermore, the generality of the twist-angle-dependent reactivity was confirmed via molecular functionalization with 4-nitrobenzenediazonium molecules. Our work highlights the importance of twist angle in the chemistry of graphene, paving a way to tune its electronic structure and carrier transport properties.

Molten-salt chemical vapor deposition (CVD), using water-soluble metal salts such as Na2MoO4 and Na2WO4 as precursors, has emerged as an important strategy for growing high-quality transition metal dichalcogenides (TMDCs). Although generally regarded as a liquid-mediated process, the role of precursor volatilization remains unclear. Here, an in situ optical visualization CVD platform directly tracks Na2WO4 precursor evolution during monolayer WS2 growth. By tuning the growth temperature, two distinct growth regimes, namely weak-volatilization and strong-volatilization regimes, are identified. Under the weak-volatilization, persistent liquid droplets drive nonuniform lateral expansion, producing irregular domain morphologies and high substrate coverage. In contrast, enhanced vapor-phase precursor transport produces smoother domain edges but lower coverage. Atomic-scale characterization further reveals regime-dependent edge structure evolution and structural uniformity. These findings demonstrate that Na2WO4-mediated WS2 growth is governed by the competition between precursor retention and volatilization rather than retained liquid intermediates alone, establishing a general framework for understanding crystal growth in molten-salt CVD.

The physical properties of single-walled carbon nanotubes (CNTs) are highly sensitive to their alignment; therefore, making precise alignment control a central requirement for high-performance device architectures. However, conventional alignment techniques offer limited spatial programmability and remain inadequate for integrating heterogeneous CNT structures within a single film. Here, we report an optical post-processing technique to locally control the alignment of CNTs. We demonstrate that femtosecond-pulsed laser processing enables polarisation-guided alignment control of CNTs after film formation (post-processing). This process produces sub-micrometre scale locally aligned CNTs with thickness exceeding 200 nm and a nematic order parameter of 0.93 (strong directional order), demonstrating a high degree of orientational order and addressing a key bottleneck in spatially heterogeneous CNT integration. Additionally, these laser-aligned CNTs exhibit a direction-dependent optical and electrical response, functioning as a terahertz polariser with a polarisation ratio of 24.7; therefore, this technique enables the applications of CNT devices integrating heterogeneous structures like division-of-focal-plane polarisation cameras and on-chip polarisation routing.

Using bilayer graphene (BLG) as Å‐scale reaction cavity and focused electron beam as both a stimulus and imaging tool, we report the controlled synthesis of a new class of two‐dimensional (2D) alkali‐iron‐chloride compounds (M@FeCl ₃ , M = Li, Na, K) and lateral heterostructures formed by monolayer alkali halides (LiCl, NaCl, KCl) and iron chlorides. The electron beam locally decomposes alkali carbonate precursors deposited on the BLG surface, stimulating ion migration into pre‐intercalated FeCl ₃ layers, inducing redox‐mediated phase transitions, and enabling confined crystallization. All these processes are recorded with atomic resolution using scanning transmission electron microscopy (STEM). The resulting monolayer heterostructures feature narrow, insulating halide nanostripes embedded within a magnetic FeCl ₂ matrix. First‐principles calculations reveal spin‐polarized ground states in M@FeCl ₃ compounds and tunable quantum‐confined behaviors modulated by the insulating halide domains. This work introduces a versatile platform for selective solid‐state synthesis and programmable interface engineering at the atomic scale, with implications for quantum information, ultrathin spintronic devices, and solid‐state ionic circuitry.

Two-dimensional semiconductors offer a pathway toward ultrascaled electronics, yet achieving strong electrostatic gate control without sacrificing low-resistance contacts remains a fundamental challenge. Here, we report a selective defect-engineering strategy that addresses this gate-contact trade-off in Bi₂O₂Se transistors. Low-temperature nitrogen incorporation passivates selenium vacancies through robust N-Bi bonding, suppressing intrinsic self-doping while preserving the intrinsic band dispersion without introducing midgap states. Density functional theory and scanning tunnelling spectroscopy reveal that nitrogen provides acceptor-like compensation by neutralizing vacancy-induced donor states, rather than through conventional substitutional doping. As a result, the Fermi level shifts toward midgap, enabling precise carrier-density modulation while maintaining band-like transport. By spatially confining nitrogen incorporation to the channel region, Bi₂O₂Se field-effect transistors are converted from depletion to enhancement mode, achieving high electron mobility and on/off ratios up to 10⁹ while preserving ohmic, contact-transparent injection. This selective defect-engineering approach decouples channel electrostatics from contact properties and provides a potentially scalable, thermally benign route toward gate-controllable, contact-transparent two-dimensional transistors compatible with integrated logic architectures.

Atom-vacancy defects existing in various materials yield numerous physical phenomena, hindering high performance in electronic and photonic devices in some cases. These become more significant in two-dimensional (2D) (atomically) thin van der Waals (vdW) layers. Meanwhile, their applications to novel devices have gathered significant attention. Neuromorphic (NM) computation, which directly mimics the probabilistic operation of the human brain through ion transport, is such an effective example due to its potential in developing power- and size-efficient artificial intelligence applications. While magnetic tunnel junctions currently represent the mainstream approach in NM computation, they require local magnetic fields, posing integration challenges with CMOS circuits. Here, we demonstrate that sigmoid circuits—representative NM circuits—can be operated only by leveraging low-frequency noises [noise currents (IN) and power spectrum density (PSD)]—originating from the stochastic capture and emission of electrons through atom-vacancy defects—in few-atom-layer semiconductor molybdenum disulfide field-effect transistors. Modulating the back-gate voltage allows control of the magnitude of electrical hysteresis loops in the current and IN (PSDs), thereby enabling and regulating the stochastic electron behaviors and emergence of the observed sigmoid operation. This work paves the way for the development of NM circuits through defect engineering in vdW 2D semiconductors, eliminating the need for magnetic fields.

Moiré superlattices, arising from overlaying atomic layers with slight mismatch or rotation, have transformed the study of emergent electronic and quantum phenomena beyond those of the constituent materials. Expanding this paradigm, here we demonstrate moiré superlattice formation at the interface between strongly correlated oxides and two-dimensional layered materials. The integration of complex oxides, a classic family of strongly correlated electron systems with transition metal dichalcogenides, enables the realization of an emerging class of moiré-engineered heterostructures that may potentially extend beyond conventional van der Waals systems. Herein, we reveal the presence of moiré superlattices in oxide-WS₂ heterostructures across varying twist angles and demonstrate highly tunable moiré periodicity as well as ultrafast charge transfer in these oxide-transition metal dichalcogenide systems. Direct observation of moiré exciton minibands confirms the emergence of moiré electronic structures, enabling twist-tunable discrete quantum states and unconventional charge dynamics. In combination with continuum modeling and density functional theory, our results elucidate the intricate interplay between moiré periodicity, quantum confinement, and band-flattening effects. By harnessing the synergy between complex oxides and layered materials, this work establishes a versatile platform for engineering artificial quantum states, providing previously inaccessible insights into correlated quantum phenomena and quantum material engineering.

Phase engineering is currently a prominent topic in 2D materials research. In particular, metastable 1T′‐phase WS ₂ has attracted considerable interest owing to its unique properties, including spin‐Hall insulating states, superconductivity, and catalytic activity in hydrogen evolution reaction (HER). The 1T′‐phase WS ₂ possesses a monoclinic structure, which suggests the presence of anisotropic properties; however, these characteristics have not yet been thoroughly explored. In this study, polarized Raman spectroscopy investigations of 1T′‐phase WS ₂ are performed to examine its anisotropic behavior and establish a method to identify its principal crystallographic axes. Combined with transmission electron microscopy measurements and density functional theory calculations, the irreducible representations of the Raman peaks are successfully assigned and it is revealed that the anisotropy of the peak at ≈406.8 cm ⁻¹ can be used to determine the principal axis, corresponding to the direction of the W─W zigzag chain in 1T′‐phase WS ₂ . Understanding the anisotropic properties of 1T′‐phase WS ₂ is essential for controlling its performance in future applications.

Antiferroelectric (AFE) materials, characterized by their antiparallel dipole alignments, offer distinct ferroic functionalities desirable for high‐energy‐density storage and next‐generation low‐power electronic devices. However, the intrinsic degeneracy among antipolar domain variants poses a critical obstacle to their deterministic, nonvolatile control and practical integration into functional architectures. Here, selective and reversible manipulation of AFE domain configurations in freestanding PbZrO ₃ membranes via electron beam irradiation are demonstrated. By tailoring the electron beam–induced built‐in electric field, controlled writing and erasure of both in‐plane and out‐of‐plane antipolar domain variants are enabled. Complementary phase‐field simulations uncover the underlying energetic landscape and domain evolution under varying irradiation conditions, offering mechanistic insight into the observed transformations. These findings demonstrate a promising strategy for domain‐level engineering of antiferroelectrics, establishing a generic and programmable approach to deterministically control antipolar ordering in perovskite systems—suggesting a pathway toward controlled, reversible switching of AFE domains in nanoscale oxide architectures.