Abnormal Hypsochromic Shifts of Surface Plasmon Scattering by Atomic Ordering in Gold–Copper Intermetallic Nanoparticles

Alloy nanoparticles are of importance to tailor their physical and chemical characteristics, which can overcome the limits of monometallic materials. In particular, the plasmonic properties of metal alloys are primarily tuned by varying their alloy composition and electronic structure. The alloys tend to be described as a linear combination of the constituent elements, and their plasmonic characteristics are also expected to follow Vegard’s rule. In this study, we demonstrated the importance of atomic ordering in Au–Cu alloy nanoparticles, which exhibited a rather opposite trend of photophysical properties against such rough estimation for random alloys. The intermetallic Au₃Cu, AuCu, and AuCu₃ nanoparticles with a diameter of 20 nm were synthesized using a modified co-reduction method under identical aqueous environments. The alloys were characterized by X-ray spectroscopic techniques precisely, confirming their compositions and atomic arrangements in crystals. Notably, plasmon light scattering on intermetallic nanoparticles collected by single-particle dark-field spectroscopy exhibited unexpected abnormal hypsochromic shifts with the Cu content increase, in contrast to linear bathochromic shifts of random alloys. Density functional theory calculations considered the combined effect of both intraband and interband transitions in the intermetallic compounds and successfully confirmed the hypsochromic shifts of the plasmon bands by atomic ordering in the Au–Cu alloys. Our findings suggest that atomic engineering in alloy nanoparticles, by modulating their intrinsic complex permittivities, provide a new way for regulating plasmonic optical properties and developing various alloy systems with desired photophysical properties.