Growth Dynamics of Colloidal Silver-Gold Core-Shell Nanoparticles Studied by Second Harmonic Generation and Extinction Spectroscopy.

The growth dynamics of colloidal silver-gold core-shell (Ag@Au CS) nanoparticles (NPs) in water are monitored in a stepwise synthesis approach using time-dependent second harmonic generation (SHG) and extinction spectroscopy. Three sequential additions of chloroauric acid, sodium citrate, and hydroquinone are added to the silver nanoparticle solution to grow a gold shell around a silver core. The first addition produces a stable urchin-like surface morphology, while the second and third additions continue to grow the gold shell thickness as the surface becomes more smooth and uniform, as determined using transmission electron microscopy. The extinction spectra after each addition are compared to finite-difference time-domain (FDTD) calculations, showing large deviations for the first and second additions due to the bumpy surface morphology and plasmonic hotspots while showing general agreement after the third addition reaches equilibrium. The SHG signal is dominated by the NP surface, providing complementary information on the growth time scales due to changes to the surface morphology. This combined approach of synthesis and characterization of Ag@Au CS nanoparticles with SHG spectroscopy, extinction spectroscopy, and FDTD calculations provides a detailed foundation for investigating complex colloidal nanoparticle growth mechanisms and dynamics in developing enhanced plasmonic nanomaterial technologies.