Synthesis of Novel Anisotropic Gold Nanoparticles for Optimization of Electric-field Enhancement

Gold nanostars are one of the most fascinating anisotropic plasmonic nanoparticles, owing in particular to the intense E-field enhancement that they produce. The morphology of a nanostar can be controlled by changing various synthetic parameters; however, the detailed growth mechanism is still not fully understood. Therefore, we are interested in investigating this process, focusing in particular on the properties of the crystalline seeds, which evolve to include penta-twinned defects as the gateway to anisotropic growth into the 6-branched morphology. In our most recent work, we have developed a high-yield seed-mediated protocol for the synthesis of 6-branched nanostars with high dimensional monodispersity in the presence of Triton-X, ascorbic acid, and AgNO3. Detailed spectroscopic and microscopic analyses have allowed the identification of several key intermediates in the growth process, revealing that it proceeds via penta-twinned intermediate seeds. Importantly, we have, for the first time, shown evidence with sub-nanometer resolution that silver forms a uniform monolayer on the surface of these nanostars with a role as a stabilizing agent. Our results indicate that metallic silver on the spikes stabilizes the nanostar morphology and the remaining silver, present when AgNO3 is added at a high concentration, deposits on the core and between the bases of neighboring spikes. Importantly, we also demonstrate the possibility of achieving dimensional monodispersity, reproducibility, and tunability in colloidal gold nanostars that are substantially higher than those previously reported, which we then leveraged to carry out holistic computational–experimental studies to understand, predict, and tailor their plasmonic response. Our concerted computational and experimental efforts prove that these nanostars combine the unique advantages of nanostructures fabricated from the top-down and those synthesized from the bottom-up, showcasing a unique plasmonic response that remains largely unaltered on going from the single particle to the ensemble. Furthermore, they display multiple, well-separated, narrow resonances, the most intense of which extends in space much farther than that observed before for any plasmonic mode localized around a colloidal nanostructure. Importantly, the unique close correlation between morphology and plasmonic response leads the resonant modes of these particles to be tunable between 600 and 2000 nm, a unique feature that could find relevance in cutting edge technological applications.