Two-dimensional (2D) materials, including graphene and transition metal dichalcogenides (TMDs), provide atomically thin platforms for next-generation electronic, optoelectronic, and van der Waals (vdW) heterostructure devices. In realistic materials, however, device-relevant properties are often governed by one-dimensional (1D) structural boundaries that arise during crystal growth, grain coalescence, or layer assembly. Unlike point defects, these boundaries remain structurally continuous while introducing localized changes in crystallographic orientation, symmetry, or stacking registry, giving rise to characteristic atomic motifs and boundary-confined electronic states. In this topical review, we establish a unified conceptual framework for understanding 1D boundaries in 2D transition metal dichalcogenides (TMDs) by classifying them according to their governing structural order parameters: grain boundaries (GBs) defined by crystallographic misorientation, mirror twin boundaries (MTBs) by symmetry transition, and domain walls (DWs) by stacking-registry variation. For each class, we discuss the relationships among formation mechanisms, atomic structures, electronic properties, deterministic synthesis strategies, and emerging device concepts. By comparing these boundaries in TMD monolayers and in moiré and sliding-ferroelectric TMD bilayers, with bilayer graphene as a reference system for stacking domain walls, we show that their electronic functionalities are governed by distinct physical mechanisms, including atomic stitching, defect-core motifs, symmetry-defined metallicity, stacking topology, strain relaxation, and local electrostatic reconstruction. Finally, we highlight recent progress in deterministic boundary engineering and demonstrate how GBs, MTBs, and DWs provide complementary design concepts for programmable switching interfaces, atomically precise metallic interconnects, correlated quantum wires, and electrically reconfigurable functional devices. We conclude that the field is undergoing a transition from defect engineering toward boundary architecture engineering, in which the arrangement, interaction, and functionality of 1D boundaries become programmable design elements for next-generation 2D electronics.
Twistronics in two-dimensional (2D) materials uses relative rotational alignment between adjacent layers to tailor band structures, excitonic behavior, and emergent quantum states through Moiré superlattices. This approach has enabled the discovery of flat-bands, correlated insulating states, unconventional superconductivity, and Moiré excitons, highlighting that small variations in twist angle can profoundly alter material properties. Reliable interpretation of these phenomena therefore depends on characterization methods that can identify rotational geometry, visualize moiré structures, and probe the associated electronic responses across relevant length scales. Nevertheless, available techniques differ substantially in spatial resolution, structural sensitivity, throughput, sample preparation requirements, and material compatibility, making method selection a nontrivial task in twistronics research. This review summarizes multiscale characterization techniques for twisted 2D materials, focusing on three central tasks: precise determination of twist angle, real-space imaging of moiré superlattices, and characterization of twist-induced local electronic properties. We compare the working principles, information output, advantages, limitations, and representative applications of major methods, with particular emphasis on their complementary roles in large-area screening, local structural analysis, and electronic-state mapping. On this basis, we propose a practical selection framework to help researchers choose suitable techniques according to the material system, spatial scale, and target physical property. Finally, we outline future opportunities in AI-assisted data analysis, in-situ characterization, and multimodal correlative measurements.
Interfacing two-dimensional bismuth with a magnetic substrate has been proposed as a promising route for exploring magnetically tunable topological states. Here, we report the experimental realization of epitaxial bismuthene-magnetic insulator heterostructure by growing ultrathin Bi films on ferromagnetic EuO(111). Tunneling spectroscopy exhibits a robust energy gap of about 400 meV, consistent with a quantum spin Hall insulating phase persisting at room temperature. Spatially resolved spectroscopy on the $\alpha$-phase bismuthene islands further reveals enhanced edge-localised states at the island boundaries. To elucidate the measured electronic states, we construct a sixteen-band tight-binding model based on the experimentally determined Bi(012) lattice, incorporating Slater-Koster hopping and atomic spin-orbit coupling. The calculation reproduces a gapped $Z_2$-nontrivial phase with helical edge modes, providing a microscopic basis for the quantum spin Hall interpretation. Our results provides a promising platform for future experimental investigations of magnetism-enabled higher-order topology.
Square-planar NiO 2 planes are key structural and electronic building blocks of nickelate superconductors, yet atomically defined single-NiO 2 -plane model systems remain difficult to realize and characterize in real space. Here, we report the growth of an atomically defined single NiO 2 plane on Cu(111) by molecular beam epitaxy (MBE) and its atomic, chemical, and interfacial electronic characterization. Scanning tunneling microscopy (STM) and qPlus non-contact atomic force microscopy (qPlus nc-AFM) reveal an ordered nearly square NiO 2 lattice with stripe-like moiré modulations. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) work-function analysis support Cu-to-NiO 2 charge transfer, suggesting substrate-assisted charge compensation in the high-formal-valence NiO 2 plane. Scanning tunneling spectroscopy (STS) and energy-dependent dI/dV mapping show that the NiO 2 overlayer shifts the Cu(111) Shockley surface-state onset upward by approximately 0.17 eV while largely preserving its free-electron-like parabolic dispersion. This shift can be rationalized by modification of the surface-potential barrier induced by the NiO 2 overlayer, with charge-transfer-related electrostatic effects potentially contributing to the observed shift. These findings establish a single NiO 2 plane on Cu(111) as an atomically defined model platform for studying square-planar NiO 2 planes and provide experimental access to a key building block of nickelate superconductors.
Triboelectric nanogenerators (TENGs) provide a promising route toward self-powered wearable sensors by converting low-frequency human motion into electrical signals. However, the practical use of soft polymer TENGs in human-machine interfaces remains limited by inefficient contact formation, low charge density, and restricted power delivery. Here, a graphene-enabled microstructured PDMS TENG is developed for self-powered gesture sensing. The device combines two complementary design strategies. Micropatterning the PDMS surface improves interfacial contact, pressure localisation and contact-separation recovery, while graphene incorporation introduces 2D polymer-filler interfaces that enhance dielectric polarisation, interfacial charge storage and charge induction within the elastomeric matrix. The optimised graphene@PDMS tribolayer delivers an open-circuit voltage of 150 V, transferred charge of 34 nC, and a power density of 1.92 W m⁻², showing a 16-fold improvement over pristine microstructured PDMS. The enhanced output is attributed to an effective pre-percolation graphene network that strengthens interfacial polarisation while avoiding dominant leakage and electrostatic screening. Integrated into a five-channel wearable glove, the TENG array generates finger-specific voltage patterns for proof-of-concept self-powered sign-language recognition. This work establishes a simple 2D-material-enabled microstructure-composite engineering strategy for high-output flexible TENGs and wearable human-machine interfaces.
