Silver nanoclusters (Ag NCs) are nanomaterials with atomically determined structures that possess unique optical properties arising from their quantized electronic states. Their photoluminescence properties in particular hold promise for applications in sensors and optical devices, but low luminescence efficiency at room temperature has been a significant challenge.
A research group consisting of Professor Yuichi Negishi and Assistant Professor Sourav Biswas of the Institute of Multidisciplinary Research for Advanced Materials at Tohoku University, Professor Masahiro Ehara of the Research Center for Computational Science, National Institutes of Natural Sciences (NINS) and The Graduate University for Advanced Studies, SOKENDAI, Professor Junpei Yuasa of the Faculty of Science Division I at Tokyo University of Science, and their colleagues successfully synthesized two types of high-nuclear silver nanoclusters (Ag78 and Ag79) containing 78 and 79 atoms. Although they have nearly identical frameworks, Ag79, which has just one more silver atom in its outer shell than Ag78, was found to achieve a room-temperature photoluminescence quantum yield 77 times higher. This new finding that "a difference of just one atom dramatically changes luminescence properties" provides important guidance for designing high-efficiency luminescent materials and optical devices. The research was published in the Journal of the American Chemical Society.
The research group first synthesized high-nuclear Ag NCs using an anion-templated synthesis method. By arranging Ag atoms and thiolate ligands around an Ag18 core templated with sulfate ions, they successfully obtained two types of NCs with different atom counts, Ag78 and Ag79. Ag78 has a trigonal crystal structure with 60 Ag atoms in its outer shell, while Ag79 forms a monoclinic crystal structure with 61 Ag atoms in its outer shell, lacking the symmetry of Ag78.
Comparison of optical properties revealed that Ag78 had an extremely low photoluminescence quantum yield of 0.001 and hardly emitted light at room temperature, whereas Ag79 showed strong red emission of around 710 nanometers with a photoluminescence quantum yield of 0.0773-a 77-fold improvement. Furthermore, excited state lifetime measurements showed that while Ag78 exhibited only short-lived emission on the nanosecond timescale, Ag79 showed long-lived emission on the microsecond timescale, revealing pronounced phosphorescent properties originating from triplet excited states.
To elucidate this difference, theoretical analysis using density functional theory was performed. The results showed that in Ag79, the addition of an outer shell Ag atom changes the electron density distribution, promoting localization that makes radiative transitions more favorable. It was also revealed that differences in the ligand environment strengthen Ag-S and Ag-Ag bonds, making the overall structure more rigid, which was confirmed to lead to significant suppression of nonradiative decay pathways. Moreover, in Ag79, intersystem crossing from singlet to triplet states is enhanced, providing theoretical support for the experimentally observed long-lived emission.
The research demonstrated that improvements in luminescence efficiency, previously achievable only through large-scale core reconstruction or heterometal doping, can be accomplished through precise atomic-level control of the outer shell. This opens new pathways for rational design of luminescent materials while symbolically demonstrating the concept of "minimal structural changes producing maximal functional changes" in quantum materials science.
Looking ahead, based on the insights gained from this research, efficient luminescence design is expected to become possible not only for silver nanoclusters but also for other metal clusters. This has diverse applications, including luminescent devices, sensing, and bioimaging, making significant contributions to the development of next-generation photofunctional materials and devices.
Journal Information
Publication: Journal of the American Chemical Society
Title: Triggering Photoluminescence in High-Nuclear Silver Nanoclusters via Extra Silver Atom Incorporation
DOI: 10.1021/jacs.5c10289
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