Spontaneous Interfacial Alloying in Au@Ag Core@Shell Nanoparticles: Unraveling the Impact on Plasmonic Absorption

Gold–silver core@shell nanoparticles (NPs) have been extensively studied due to their tunable plasmonic spectra, which can be controlled by adjusting the core and shell sizes. However, the nature of the Au–Ag interface in the core@shell NPs and its influence on the plasmonic spectra have remained largely unclear. This study establishes that the plasmonic spectra of spherical Au@Ag NPs are highly sensitive to interfacial characteristics. Spontaneous room-temperature alloying occurs in small core–shell NPs, significantly influencing their optical response. Through systematic experiments and theoretical modeling, Au@Ag NPs with core sizes ranging from 25 to 4 nm and precisely controlled Ag shell thicknesses have been analyzed to elucidate the interfacial interdiffusion effects on plasmonic behavior. For larger cores (25 nm), the impact of interfacial alloying on the plasmonic spectra is negligible. However, as the core size decreases, the effect becomes increasingly pronounced, with interdiffusion between the core and shell atoms dominating the optical behavior. This interface-mediated transformation is further modulated by the Ag shell thickness, revealing a complex interplay between size, composition, and optical properties. This study demonstrates that the unique optical absorption features of these Au@Ag NPs are highly dependent on dimensional factors, allowing for tunability through size and interface engineering.