Size, Composition, and Phase-Tunable Plasmonic Extinction in Au–Sn Alloy Nanoparticles

The synthesis of Au–Sn nanoparticles with tailorable localized surface plasmon resonances (LSPR) is explored, with a focus on size dependence, composition, and phase formation. Au–Sn nanoparticles were synthesized starting from Au seeds ranging in diameter from 5 to 30 nm. UV–visible spectroscopy revealed controllable blueshifting of the LSPR, from 520 to 460 nm, as Sn incorporation increased. X-ray diffraction (XRD) confirmed the formation of Au₅Sn and AuSn intermetallic phases, with intermetallic formation dependent on both nanoparticle size and Sn content. Elemental analysis through energy-dispersive X-ray spectroscopy (EDS), total reflectance X-ray fluorescence (TXRF), and inductively coupled plasma optical emission spectroscopy (ICP-OES) provided further insight into the incorporation of Sn into Au nanoparticle seeds. We show that this approach allows one to create Au–Sn alloy nanoparticles of varying radii and crystalline phase contents all with the same LSPR (500 nm). Additionally, the size-dependent formation of intermetallic phases provides new physical insight into their impact on the LSPR. Formation of Au_xSn_1–x is associated with minimal blueshifting and broadening and Au₅Sn is associated with linear blueshifting and a small amount of broadening, while AuSn formation leads to rapid blueshifting, broadening, and plasmon damping. This understanding enables precise control over the size, structure, and optical properties of Au–Sn nanoparticles, paving the way for the design of new plasmonic materials for applications in sensing, imaging, and catalysis.