(Invited) Plasmonic Nano-Intermetallics
Authors: Sato Ryota , Takekuma Haruka , Teranishi Toshiharu 
Published: December 2024

One of the unique optical responses of nano-sized metals is localized surface plasmon resonance (LSPR), a phenomenon in which the collective vibration of free electrons is excited by incident light of a specific wavelength, generating a strong enhanced electric field in the vicinity of the nanoparticle surface and forming hot carriers in the relaxation process. LSPR has a wide range of applications, including pigments, surface-enhanced Raman scattering (SERS), and increasing the efficiency of light-energy conversion systems such as artificial photosynthesis snd solar cells.
Coinage metals, namely, Au, Ag, and Cu, of group 11 elements are commonly used as practical metallic nanomaterials that exhibit LSPR in the visible region due to their chemical stability and ease of synthesis, and have led the development of visible nanoplasmonics. On the other hand, the search for alternative nanomaterials for coinage metals in binary alloys, much less multiple alloys, that do not contain group 11 elements still leaves a lot of unexplored space. One of the reasons for this is the difficulty in synthesizing monodisperse alloy nanoparticles with a particle size (> 10 nm) that clearly shows LSPR absorption (scattering) and contains a high ratio of base metals that are not easily chemically reduced in the liquid phase. In our study, we are working on developing the scientific principle of plasmonic ordered nanoalloys that can contribute to the creation of novel alloy nanomaterials "beyond coinage metals" through the co-creation between experimental synthesis of alloy nanoparticles containing base metals that show LSPR in and near the visible region and theoretical simulation of photo-excited electron dynamics.
As a result of establishing a synthetic method for base metal-containing alloy nanoparticles with various crystal structures, we found that several types of colloidal ordered alloy (intermetallic compound) nanoparticles with B2 (CsCl-type) and C1 (CaF2-type) structures show distinct absorption peaks in the visible region. These absorption peaks were attributed to LSPR by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) analysis. To verify the LSPR mechanism by theoretical calculations, we analyzed the photoexcited electron dynamics of nanoparticle models (less than 3 nm) and found similarities and differences between alloys and coinage metals. As an example, when Au561 and Pt249In432 were compared,1) similarities were observed: a large electron polarization originating from collective vibration of free electrons was observed on the surface of the particle, while the polarization of d electrons bound to each atom was observed in opposite phase. This polarization of bound electrons in the opposite phase is called "screening",2) which contributes to the energy reduction of the LSPR. In Au561, all 5d electrons are uniformly involved in screening, while in Pt249In432, In 4d electrons with higher bound energy have very small vibrational displacements and are almost uninvolved in screening, and Pt 5d electrons with bound energy equivalent to Au 5d electrons are polarized with a larger displacement. These results indicate that screening with large displacement by d electrons of Pt, which has a small number of constituent atoms, efficiently lowers the LSPR energy, and straightforwardly show that the crystal structure and composition in alloy nanoparticles can be a controlling factor for the LSPR nature.
We believe that deepening these studies will greatly contribute to the creation of diversity and superior functionality in plasmonic nanomaterials. Furthermore, the utilization of plasmonic nano-intermetallics is expected to lead to the development of unique plasmonic electrocatalysts and photocatalysts that take advantage of alloy-specific multifunctional surfaces not found in monometals.

References

1) H. Takekuma, R. Sato, K. Iida, T. Kawawaki, M. Haruta, H. Kurata, K. Nobusada, T. Teranishi, Adv. Sci.
2024, 11, 2307055.
2) K. Iida, M. Noda, K. Ishimura, K. Nobusada, J. Phys. Chem. A
2014, 118, 11317.