No Substrate Required: Electron-Beam-Enabled, Single-Atom-Thick 2D Metals and Metal Oxides

Freestanding, single-atom-thick metals and metal oxides, defined here as monolayers suspended without an underlying solid substrate and supported only at their perimeter, represent an extreme two-dimensional (2D) limit that cannot be accessed by exfoliation of van der Waals solids. This review argues that their stability and formation are fundamentally interfacial: the most convincing realizations occur in edge-confined geometries (notably graphene pores) where perimeter anchoring, atom capture and boundary conditions stabilize low-coordination configurations that otherwise relax toward three-dimensional (3D) packing. In this context, the transmission electron microscope operates as an interface reactor, coupling atomic-resolution imaging to beam-driven diffusion, selective sputtering and reconstruction, such that fabrication and characterization are inseparable. We organize the literature into three experimentally relevant routes: (I) graphene-pore templating yielding membranes, planar patches and ultranarrow nanoribbons; (II) beam-driven top-down thinning and alloy-enabled dealloying concepts that bias reconstruction toward monoatomic remnants; and (III) subtractive beam chemistry converting compound precursors into new monolayers (e.g., oxyhalide → oxide and MoSe₂ → Mo). We set out evidence and reporting standards (quantitative thickness assignment and dose-history reporting) and describe how automation and closed-loop control can translate these atomically thin interfaces from demonstrations to reproducible platforms.