Metallene: Ångström-Scale 2D Metals

Atomically thin 2D metals, also termed metallenes, constitute a distinctive class of 2D materials in which metallic bonding is preserved at the ångström scale. Quantum confinement imparts ultrahigh carrier mobility, tunable plasmonic resonances, and exposed surfaces composed of low-coordination active sites. Although "2D metals" have historically encompassed various metallic nanostructures, recent breakthroughs have enabled the isolation of structurally well-defined metallenes with ambient stability and quantum-confined properties not observed in their bulk counterparts. This review provides a comprehensive overview of metallene research, focusing on their synthetic chemistry, low-dimensional metrics, and structure-function relationships. This unified framework provides cross-disciplinary insights for rational design in catalysis, plasmonics, electronics, and biomedical applications. Rigorous criteria are first established to distinguish true monolayer metals from quasi-2D nanosheets, emphasizing bonding anisotropy, lattice continuity, and spectroscopic fingerprints. State-of-the-art fabrication strategies are then benchmarked for scalability and technology readiness. Next, the engineering toolbox, including doping, hierarchical hetero-structuring, and defect/phase/strain modulation, is surveyed, which tailors these intrinsic traits and translates them into record performances across diverse applications. Finally, outstanding challenges, including thermodynamic metastability, limited synthetic precision, unclear dynamic structure-function relationships, and device integration, and delineate research directions aimed at accelerating the rational design and practical implementation of metallenes are outlined.