Gold Nano-Earbuds: Seed-mediated Synthesis and the Emergence of Three Plasmonic Peaks.

We demonstrate the first successful synthesis of reasonably monodisperse and single crystalline gold nano-earbuds (Au NEBs) using a binary surfactant mixture of cetyltrimethylammonium chloride (CTAC) and benzyldimethylhexadecylammonium chloride (BDAC) in a seed-mediated growth method. We have focused on the key chemical parameters behind the formation and growth of Au NEBs to result in tunable dimensions (lengths 37-77 nm; widths 4-6 nm and aspect ratios 7-19), as a consequence of which the longitudinal surface plasmon resonance (LSPR) peak could be tuned beyond 1200 nm. The achievement of LSPR beyond 1200 nm while maintaining the dimension well below 100 nm is a challenging accomplishment in the realm of 1D Au nanostructures. This earbud-like morphology additionally exhibits three plasmonic peaks, rather uncommon for 1D nanostructure, which were analyzed theoretically based on finite element method. The new resonance peak of Au NEB was assigned as an additional longitudinal mode intensified by the bulbous ends as well as the high-aspect ratio, thereby providing conclusive evidence that it is indeed a new morphology.