Plasmon Resonances in Bimetallic Nanostructures: The Role of Temperature and Core/Shell Architecture

Bimetallic nanostructures are of significant technological interest due to their ability to synergistically combine properties of two distinct metals giving rise to multimodal characteristics.  In this work, we have synthesized Au@Ag bimetallic nanostructures of three different geometries (nanocubes, nanopyramids, and nanorods) and investigated the role of thermal induction in controlling the nanostructure morphological evolution and the corresponding impact on the optical resonances.  By increasing the reaction temperature from 28 °C to 150 °C we observe a red-shift in the plasmon resonance of the nanopyramids followed by a blue-shift at temperatures above boiling point of water; however, no appreciable shift was observed in the optical resonance of the nanocubes.  The observed behavior was attributed to a combination of phase retardation effects, stability and critical micelle concentration of ligands, and energetics of the different crystal planes.  We also investigated the formation dynamics and observed a sigmoidal growth at lower temperatures relative to an exponential growth at higher temperatures.  The activation energy was calculated from the growth dynamics and the rate of nucleation was correlated to the reductive ability of ascorbic acid. Finally we also investigated the effect of core to shell ratio in determining the morphological evolution of bimetallic nanostructures.