Homoepitaxy-like heteroepitaxy via monolayer interface achieves grain-boundary-free ultraflat silver thin films.

Wafer-scale growth of metallic films into single crystals is challenging due to large lattice mismatches and uncontrollable atomic stacking during deposition. Here, single-crystalline Ag(111) films are grown on atomically flat Cu(111) buffer layers using atomic sputtering epitaxy, despite the significant Ag/Cu lattice mismatch (about 13%). The mismatch strain is confined to the first Ag monoatomic interface layer and does not spread into the adjacent Ag layers. This effective strain relaxation occurs through regulated in-plane displacements of Ag atoms where Ag and Cu atoms meet periodically. Although the grain-boundary-free Ag thin films initially exhibited twin boundaries, we successfully demonstrated conditions that significantly reduced them-a feat considered challenging in thin film growth technology. The resulting films have inherently flat surfaces with occasional monoatomic steps, making them ideal for use as reflectors and plasmonic devices.