Free-standing two-dimensional metal oxides at the single-atomic-layer limit

Two-dimensional metal oxides offer a versatile platform for exploring quantum phenomena and enable functionalities beyond those of their bulk counterparts. Despite advances in synthesizing ultrathin and free-standing oxides, controlled fabrication at the single-atomic-layer limit has remained a formidable challenge. Here we report the controlled fabrication of diverse free-standing two-dimensional metal oxides (MO), including CrO, ErO and BiO, down to the single-atomic-layer limit via electron-beam-induced dehalogenation of layered metal oxyhalides (MOX). Using monolayer CrO as a representative system, which features a single-atomic-layer square lattice, we obtain free-standing single-crystalline domains exceeding 10⁴ nm² and demonstrate controllable patterning at the micrometre scale. Density functional theory confirms both the thermodynamic and kinetic stability of free-standing monolayer CrO, predicting a Néel antiferromagnetic order below 230 K. Combined experimental and theoretical analysis reveals that ionization-driven halogen selective removal and lattice relaxation underpin the CrOCl-to-CrO transformation, a conversion mechanism potentially extendable to other metal oxyhalides. This approach further enables the fabrication of MOX-MO lateral heterostructures as well as aligned and twisted metal oxide bilayers, which exhibit pronounced excitonic responses with an unusually large exciton binding energy. Our work opens opportunities for two-dimensional metal oxide monolayers, heterostructures and moiré superlattices, and for exploring emergent excitonic phenomena in low-dimensional oxides.