Shape control with atomic precision: anisotropic nanoclusters of noble metals

When plasmonic metal nanoparticles become smaller and smaller, a new class of nanomaterials-metal nanoclusters of atomic precision-comes to light and has become an attractive research topic in recent years. These ultrasmall nanoparticles (or nanoclusters) are unique in that they are molecularly uniform and pure, often possess a quantized electronic structure, and can grow into single crystals as do protein molecules. Exciting achievements have been made by correlating their properties with the precise structures at the atomic level, which has provided a profound understanding of some mysteries that could not be elucidated in the studies on conventional nanoparticles, such as the critical size at which plasmons are emergent. While most of the reported nanoclusters are spherical or quasi-spherical owing to the reduced surface energies (and hence stability), some anisotropic nanoclusters of high stability have also been obtained. Compared to the anisotropic plasmonic nanoparticles, the nanocluster counterparts such as rod-shaped nanoclusters can provide insights into the growth mechanisms of plasmonic nanoparticles at the early stage (i.e., nucleation), reveal the evolution of properties (e.g., optical), and offer new opportunities in catalysis, assembly, and other themes. In this Review, we highlight the anisotropic nanoclusters of atomic precision obtained so far, primarily gold, silver, and bimetallic ones. We focus on several aspects, including how such nanoclusters can be achieved by kinetic control, and how the anisotropy gives rise to new properties over the isotropic ones. The anisotropic nanoclusters are categorized into three types, (i) dimeric, (ii) rod-shaped, and (iii) oblate-shaped nanoclusters. For future research, we expect that anisotropic nanoclusters will provide exciting opportunities for tailoring the physicochemical properties and thus lead to new developments in applications.