Impact of tip curvature and edge rounding on the plasmonic properties of gold nanorods and their silver-coated counterparts

Colloidal solutions of gold nanorods and silver-coated gold nanorods were prepared. The seeded growth synthesis protocols were improved by adding a flocculation purification step. The resulting populations of pure gold nanorods and Au@Ag core–shell cuboids were characterized by very low dispersion in size and shape. UV–vis–near-infrared absorption measurements were performed on several batches of well-calibrated nano-objects, supported by calculations based on the discrete dipole approximation, allowed to highlight the impact of various morphological features on the optical response. In addition to the well-known effect of the nanorod aspect ratio on the shift of the longitudinal surface plasmon resonance mode, special attention was paid to changing either the rounding of the nanorod end-caps or that of the edges of the coating silver shell. Nanorods and cuboids were modeled as superellipsoids. This approach enabled us to model precisely their complex shapes using just a few simple parameters and analyze the evolution of their extinction spectra as a function of the rounding of their tips and edges. Such nano-objects are widely used for various applications in fields such as biomedical, biosensing, or surface-enhanced Raman spectroscopy, thus making it crucial to precisely assess the impact of each morphological feature for optimizing their performance.