Combining Experiments and Theoretical Modeling To Interrogate the Anisotropic Growth and Structure-Plasmonic Property Relationships of Gold Nanostars

We present a combined strategy of experiments and theoretical modeling for understanding the evolution of the morphology and plasmonic properties of gold nanostars (GNSs) in the seed-mediated synthesis by changing the poly(vinylpyrrolidone) (PVP) molecular weight, PVP concentration, and synthesis temperature. A dramatic change of the morphology of GNSs as a function of these synthesis parameters is observed that is related to variations of the plasmonic properties and thus surface-enhanced Raman spectroscopy (SERS) enhancement. We observe the favorable growth of anisotropic GNS structures with sharp protruding tips using PVP of low molecular weight and of rounded GNSs with short protruding tips using PVP of high molecular weight. The PVP concentration has less influence on the core size than on the tip length of GNSs. The high synthesis temperature causes the rounding of the GNS structure. Finite-difference time-domain (FDTD) simulations reveal a remarkable correlation of the GNS morphology with the plasmonic properties as well as the SERS enhancement. The maximum local electric field enhancement occurs at the apex of the sharp protruding tips of the GNSs. The weak plasmonic coupling is observed between the protruding tips of GNSs because of their large separation distance, and increasing the number of protruding tips beyond two only increases the extinction cross section without further red-shifting the plasmon peak. A resonance overlap of the plasmon band with the incident laser wavelength is responsible for the morphology-dependent plasmonic properties and SERS enhancement. The present work demonstrates that a mechanistic understanding of the structural evolution of GNSs along with their morphology-plasmonic property correlation can be achieved through the combination of experimental investigations and FDTD-based theoretical modeling.