Phase Engineering of Atomically Precise Nanoclusters (APNCs) of Gold and Beyond

In solid state and nanoscale materials, a phase refers to a distinct entity that can be identified in X-ray diffraction (XRD) analysis. For bulk metals, common phases include the face-centered cubic (fcc), hexagonal close-packed (hcp), and body-centered cubic (bcc) structures. When the dimensions of a metal are shrunk to the nanometer scale (1-100 nm), new phases that are not observed in bulk metals can be attained due to the increasing importance of surface energy as a new handle for phase control. In this review, we summarize the recent progress in phase engineering of atomically precise nanoclusters (APNCs, typically 1-3 nm) of coinage metals such as gold and silver. Five primary types of phases are categorized, including the fcc, hcp, bcc, multi-twinned phases (both the icosahedron and decahedron), as well as mixed phases in APNCs. The attainment of non-fcc phases at the nanoscale is intriguing because bulk gold and silver are exclusively fcc; this underscores the importance of tailoring surface energy by protecting ligands as well as the kinetic control and other synthetic strategies as discussed in this review. Following the structural discussion, we illustrate the distinct physical and chemical properties from phase engineering of APNCs, including the optical absorption and related photothermal application, photoluminescence in the near-infrared (NIR) and related bioimaging application, as well as the catalytic reactivity. Finally, we put forth some future perspectives.