Precisely Fabricating Minute Nanoclusters
What comes to mind when you hear the term one nanometer (1 nm = one-billionth of a meter)? It is larger than an atom (about 0.1 nm) but much smaller than a virus (100 nm) or a bacterium (1000 nm), and many molecules (0.5 to several nm) fall within this size range. This size regime represents the smallest functional unit for many materials. The "nanoclusters" that we are studying belong to this very size regime and are formed by assembling a few to several hundred atoms or molecules (Figure 1).
By assembling a finite number of atoms, up to several hundred, properties not observed in bulk solids or individual atoms (complexes) emerge. This is because all constituent atoms contribute their valence electrons within the single container of the atomic assembly, forming a new superstructure. Due to its resemblance to a large atom, it is called a "superatom." Just as adjacent elements in the periodic table exhibit different properties, the properties of a superatom also change significantly with just a single change in size. In other words, to leverage these unique characteristics of superatoms for fabrication, it is crucial to produce (synthesize) materials by precisely controlling the number of atoms.
To synthesize minute nanoclusters, a bottom-up approach is used, which involves gathering atoms to form an assembly. This method is based on a self-assembly reaction where atoms associate with each other, and it is an extremely fast reaction. To control this reaction and create the desired nanoclusters, it is necessary to proceed with a molecular-level understanding of phenomena such as chemical reactions and molecular diffusion. Based on this perspective, we are advancing the precise synthesis of nanoclusters by designing and building synthesis apparatuses while devising new synthesis methodologies.
The dry process, which synthesizes nanoclusters in a vacuum, generates them using a localized high-temperature reaction field, such as plasma, in a clean environment where only the atoms of the target element are present. This makes it possible to produce nanoclusters of silicon and aluminum, which are difficult to create in solution. However, the yield was low, making it unsuitable for material synthesis. We have enhanced the intensity of the dry process, developed the sub-nanocluster precise synthesis apparatus nanojima®, and enabled the mass synthesis of M@Si 16 .
On the other hand, with the wet process, which synthesizes nanoclusters in solution, the yield is high, but controlling the size was difficult because the reaction field in the liquid tends to be microscopically non-uniform. We have developed an ultrafine microreactor and, by promoting the microscopic homogenization of the reaction field, have enabled the precise synthesis of nanoclusters via the wet process.
We are advancing "nanocluster materials science" by tailoring synthesis methods to the type and required size of the nanoclusters, as well as through the evaluation of their size-specific physical properties.