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 _x Sn_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.