Phase Engineering of Nanogold: Non-Close Packed Square Planes in A'B' Stacking with a 0.5 Å Channel

Crystal phase engineering of inorganic materials is one of the most important strategies for achieving new material properties and functionality. Gold is well known to adopt the face-centered-cubic (fcc) structure, and it remains challenging to create new phases, but recent research on nanogold has provided exciting opportunities by achieving 2H (note: H denotes hexagonal), 4H, and body-centered-tetragonal (bct) phases. In the current work, an unusual non-compact phase in 40-atom gold nanoclusters is reported, which is in contrast to the close-packed isomer of fcc structure. The non-compact isomer exhibits exclusive square planes in a layer-by-layer stacking, hence giving rise to a unique 0.5 Å channel, whereas the fcc isomer is composed of close-packed planes with a closed structure (i.e., no 1D channel). The two Au₄₀ isomers lead to a nearly two orders of magnitude difference in electronic excited-state lifetime. The attainment of the new non-compact phase in nanogold presents potential opportunities in terahertz emission owing to the strong electron-vibration coupling, ion channeling and ion selection, artificial photosynthesis, and biocatalysis.