Defect-Templated Phase Engineering in Atomically Thin Metals.

Achieving deterministic control over crystal phase at the atomic limit remains a fundamental challenge for atomically thin metals, where subtle differences in atomic registry can produce large changes in electronic and optical functionality. Here, we establish interfacial defect templating as a general materials-design strategy for phase engineering in confined two-dimensional metals, using monolayer silver as a model system. By tailoring the defect chemistry of a graphene overlayer, we selectively stabilize two competing crystalline phases of two-dimensional Ag at the graphene/SiC interface: a near-commensurate phase promoted by vacancy and line defects in epitaxial graphene, and a denser phase favored beneath intrinsically sp3-rich zero-layer graphene. Multimodal characterization reveals distinct lattice registries, electronic structures, and charge transfer to the graphene overlayer for each phase. First-principles calculations show that phase selectivity arises from a competition between kinetically favored nucleation pathways and thermodynamically preferred packing configurations, explaining both controlled phase formation and long-term evolution. The defect-programmed Ag phases exhibit strongly contrasting linear and nonlinear optical responses, enabling phase-tunable optical functionality at atomic thickness. More broadly, this work reframes defects as deliberate design elements for programming structure-property relationships in confined two-dimensional metals.