Metal clusters smaller than 2 nm exhibit novel physicochemical properties that are very different from those of the bulk state and strongly depend on their size and structure. The essential requirement for the study of their basic properties and for practical applications is to synthesize clusters as stable compounds while controlling their size with atomic precision. This account summarizes the state-of-the-art methods of size-controlled synthesis and structure determination of gold and silver clusters whose surfaces are passivated by monolayers of thiolates. The origin of the high stabilities of these isolated clusters is explained in terms of the closure of electronic and geometric shells.
The development of dendrimer reactors opened the new research field by the production of metal subnanoparticles with definite atomicities. These precise platinum subnanoparticles exhibited the true catalytic properties, which have been hidden by the conventional synthetic method due to the substantial size distribution. One significant finding was that the several subnanoparticles exhibited much higher oxygen reduction reaction catalytic activity than the conventional platinum nanoparticle (∼3 nm). The result was completely opposite to the common notice that the most catalytically active particle size is ca. 3 nm. Despite of this inconsistence, it was finally concluded that some subnanoparticles have specific surfaces which have higher activities originated from the unique geometric structures.
Multinary I-III-VI2 group semiconductor nanocrystals, such as CuInS2, Cu(InGa)Se2, AgInS2 and their solid solutions with ZnS, have been attracting increasing attention due to their low toxicity, wide range of absorption from UV to visible or near-infrared regions, and tunable energy gap. In this review, we introduce recent progress of solution-phase syntheses of high-quality multinary nanocrystals, which are candidates for alternative materials of Cd- and Pb-based binary nanocrystals showing high toxicity. Physicochemical properties of multinary nanocrystals are tunable depending on their crystal size and chemical composition. For example, nanocrystals of solid solution between AgInS2 and ZnS exhibited strong photoluminescence, the color of which was controlled from red to green with an increase in the ZnS fraction. The controllability of their photochemical properties is useful to develop novel photofunctional materials for the applications, such as photocatalyst and biomolecule marker.
Magnetic nanoparticles (MNPs) have become readily available thanks to the development of a range of synthetic techniques. In addition, various multifunctional hybrid MNPs have been recently developed for biomedical applications. We review the progress of research on biomedical multifunctional MNPs.