The morphology dependent plasmonic optics of urchin-like gold nanoparticles in different silver-coating modes

In this work, the spectral properties of urchin-like gold nanoparticles (ULGNs) have been studied theoretically and experimentally. Firstly, a model with six branches evenly distributed on the spherical core was built. Its spectrum and electric field distributions are simulated and calculated using discrete dipole approximation (DDA) method. Two extinction peaks generated by plasmon resonance on spherical core and symmetrical branches were determined. What’s more, intensities between two resonance modes can be switched by regulating the number of branches since charges are more likely to accumulate at the tips of branches. Then, branch thickness has been regulated. The highest field enhancement factor |E/E0|= 36.95 can be obtained when r = 6 nm. Models with r = 2 nm branches fail to concentrate charges at the tips while two resonance modes fuse in models with r = 8 nm branches. Secondly, ULGNs@Ag were synthesized experimentally. To study the optics and the cause of three peaks appeared in spectrum, two ULGNs@Ag models (core surrounded and full coverage) were built and simulated based on the morphology from TEM pictures. Except for plasmon resonance on silver shell and ULGNs, coupling effects between exposed gold branches and silver shell were found in core surrounded ULGNs@Ag model, which is more consistent with the experimental results. While in full coverage ULGNs@Ag model, main two peaks are formed due to the silver layer and the gold-silver core-shell branches. There are another two inconspicuous peaks emerge which may ascribed to concentration of hot spots due to negative curvature surfaces and hybridization effect on gold-silver and silver-medium interface, providing a theoretical basis for spectral analysis of both ULGNs and ULGNs@Ag.