Seeing Ligands on Nanoclusters and in Their Assemblies by X-ray Crystallography: Atomically Precise Nanochemistry and Beyond.

Ligands are of tremendous importance for colloidal nanoparticles (NPs) in terms of surface protection, size and shape control, tailoring properties, self-assembly, and applications. However, it is very challenging to obtain unambiguous information on the ligands and their interactions and patterning on NPs. The recent advent of atomically precise nanochemistry has opened new horizons. One can now see ligands with atomic resolution and understand their behavior on the surface of ultrasmall NPs (1-3 nm) and also in their assemblies. Such atomically precise NPs (or nanoclusters, NCs) bridge up with conventional NPs by providing unprecedented opportunities to reveal the specific patterns formed by intra- and inter-particle ligand interactions. In this Perspective, we first discuss how to achieve atomically precise NCs and determine their total structures. Then, we highlight the intra-particle ligand interactions (i.e., the ligand shell), including the various patterns formed on the NCs, the ligand modes on facets and edges, and some aesthetic patterns assembled by ligands that are akin to biomolecular organization. The inter-particle ligand interactions and their roles in directing the self-assembly of NCs into coherent superlattices are also discussed, which provides deep understanding of assembly mechanisms, with insights from atomically precise NCs hinting for the assembly of conventional NPs. Overall, the success in achieving atomically precise NCs is expected to bring new opportunities to fields beyond nanochemistry, especially to the materials design, engineering and applications.