Unraveling the Dominant Size Effect in Polydisperse Solutions and Maximal Electric Field Enhancement of Gold Nanoparticles

In this study, we systematically investigate theoretically and experimentally the plasmonic effect and roles of big and small gold nanoparticles (Au NPs) within a mixed solution. The polydisperse solution was initially prepared by mixing small (10, 30 nm) Au NPs with larger ones (50, 80 nm), followed by measuring the extinction using ultraviolet–visible (UV-vis) spectroscopy. The experimental results clearly showed that the extinction of the mixed solution is predominantly influenced by the presence of the larger NPs, even though their quantity is small. Subsequently, we conducted simulations to explore the plasmonic properties of Au NPs of different sizes as well as their mixings and to validate the experimental results. To explain the deviation of the extinction spectra between experimental observations and simulations, we elaborated a simulation model involving the mixture of spherical Au NPs with ellipsoidal NPs, thus showing agreement between the simulation and the experiment. By performing simulations of plasmonic near-field of NPs, our investigation revealed that the maximal electric field intensity does not occur precisely at the plasmonic resonant wavelength but rather at a nearby redder wavelength. The optimal size of the Au NP dispersed in water for achieving the highest field enhancement was found to be 60 nm, with an excitation wavelength of 553.7 nm. These interesting findings not only enrich our understanding of plasmonic NPs’ optical behavior but also guide researchers for potential applications in various domains.