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 sp 3 ‐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.