In Situ Observation of Structural and Optical Changes of Phase-Separated Ag-Cu Nanoparticles during a Dewetting Process via Transmission Electron Microscopy.

Metallic nanoparticles with localized surface plasmon resonance have suitable optical properties for various applications such as optical filters, efficient photocatalysts, and high-sensitivity sensors. Phase-separated plasmonic nanoparticles with heterogeneous metastructures exhibit unique resonance features with separate optical field enhancements in each phase and hot electron transfer across the interface. Hence, interface engineering is crucial, particularly for catalyst applications. In this study, we investigated the evolution of the interface at high temperatures during nanoparticle formation using the dewetting method. We selected a Ag-Cu binary alloy system as a representative case and observed the nanoparticles via in situ transmission electron microscopy using a dedicated specimen heating holder. In situ elemental mapping revealed that the initial as-deposited film, which was composed of core-shell structures with Ag cores and Cu shells, converted into phase-separated Janus nanoparticles through marbled structures. A major structural change was observed at approximately 200 °C, which was in agreement with optical measurements. These results confirmed that the optical properties and metastructures of the phase-separated nanoparticles could be tuned by selecting the appropriate temperature and duration of the heat treatment.