Controlling the crystalline phases (FCC, HCP and BCC) of thiolate-protected gold nanoclusters by ligand-based strategies

Significant advances in the research on atomically precise gold nanoclusters have led to crystallographic determination of total structures (i.e. atomic arrangements in the metal core and surface ligands) of nanoclusters. A fundamental question is what types of structures would exist in gold nanoclusters. Research in this field first revealed icosahedral and decahedral structures in gold nanoclusters, which are novel structures since these are not observed in bulk gold. The emergence of such new structures in nanoclusters has gained much attention; on the other hand, a question arises naturally: can ultrasmall gold nanoclusters adopt the face-centered cubic (FCC) packing structure that is exclusive in bulk gold or even the hexagonal-close-packing (HCP) and body-centered-cubic (BCC) structures? This question has been addressed by the recent successes in controlling gold nanoclusters with all the three types of crystalline structures (or phases). The syntheses of these nanoclusters are based on the ligand strategies. The ligand effect on the structure, which was overlooked in earlier works, has now been recognized as an essential factor. In this highlight, we introduce the diverse structures of gold nanoclusters with 30 atoms or so, such as Au-38(SR)(24), Au-36(SR)(24), Au-30(SR)(18), and Au38S2(SR)(20) as examples of the icosahedron, FCC, HCP, and BCC structures, respectively. The driving force to form these structures is discussed from the perspective of the major roles of the protecting ligands.