June 23, 2025
Keio University
A research group from Keio University, led by Professor Atsushi Nakajima of the Department of Chemistry, Faculty of Science and Technology, along with Miwa Tokita (a second-year master's student) and then Research Associate (Non-tenured) Tomoya Inoue from the Graduate School of Science and Technology, has revealed that the minimum unit required for the emergence of localized surface plasmon resonance (LSPR) is 21 atoms. They achieved this by using gold (Au) nanoclusters with the number of atoms precisely controlled to a single unit and performing a detailed analysis of the photoelectron emission process from light irradiation on a deposited solid surface.
The plasmon phenomenon refers to the collective oscillation of free electrons in a metal excited by an electromagnetic field such as light. LSPR is expected to contribute to performance improvements in photonic devices such as solar cells, optical sensing, and nano-optical circuits. However, the fundamental question of how LSPR emerges at the atomic scale—that is, what its "smallest unit" is—has long remained unanswered in plasmonics research. Previously, the same research group reported that LSPR emerges from 9 atoms in silver nanoclusters, suggesting that the conditions for plasmon emergence may vary significantly depending on the element and electronic structure.
In this study, the researchers used a uniquely developed high-intensity metal cluster ion source to deposit gold nanoclusters with precisely selected numbers of constituent atoms onto a solid surface. They then analyzed the photoresponse and electron emission with high precision using two-photon photoelectron spectroscopy. As a result, they determined that the LSPR response is first observed in gold nanoclusters with 21 or more atoms.
This achievement is a crucial milestone in understanding the origin and characteristics of plasmon emergence for each element, and it is expected to become an effective foundational technology for the development of nanodevices, such as enhancing the photoelectric conversion process in solar cells and creating plasmonic optical circuits for high-speed communications. The results of this research were published in the American Chemical Society's journal "ACS Photonics" on June 16, 2025 (US time).
For the full press release, please see below.