To inorganic nanoparticles via nanoclusters: Nonclassical nucleation and growth pathway

Nanoclusters, intermediates in size between atoms and nanoparticles, have been a topic of great interest because of their unique molecular structures and optical properties as compared to those of nanoparticles and bulk counterparts. Recent mechanistic studies have shown that the nanoclusters appear in the initial stages of the nanoparticle growth. The formation of nanoclusters is supported by the nonclassical nucleation theory, revealing that the nanoclusters with extremely large surface areas can be stabilized by the ligands. In this review, we first provide the theoretical background of the classical and nonclassical nucleation mechanisms in the nanoparticle formation, which helps understand the stability of nanoclusters. We then focus on the synthesis and characterization of nanoclusters of noble metals, semiconductors, metal oxides, and their alloys. Furthermore, the potential applications in bioimaging, sensing, optoelectronics, and catalysis, enabled by unique optical and chemical properties of nanoclusters, are discussed.