Controlled Overgrowth of Five-Fold Concave Nanoparticles into Plasmonic Nanostars and Their Single-Particle Scattering Properties.

Growth of anisotropic nanostructures enables the manipulation of the optical properties across the electromagnetic spectrum by the fine morphological tuning of the nanoparticles. Among them, stellated metallic nanostructures present enhanced properties owing to their complex shape, and hence, the control over the final morphology becomes of great importance. Herein, a seed-mediated method for the high-yield production of goldrich-copper concave branched nanostructures and their structural and optical characterization is reported. The synthesis protocol enabled an excellent control and tunability of the final morphology, from concave pentagonal nanoparticles to five-fold branched nanoparticles, named "nanostars." The anisotropic shape was achieved via the kinetic control over the synthesis conditions by the selective passivation of facets using a capping agent and assisted by the presence of the copper chloride ions, both having a crucial impact over the final structure. Optical extinction measurements of nanostars in solution indicated a broad spectral response, hiding the properties of the individual nanostars. Hence, single-particle scattering measurements of individual concave pentagonal nanoparticles and concave nanostars were performed to determine the origin of the multiple plasmon bands by correlation with their morphological features, following their growth evolution. Finite-difference time-domain calculations delivered insights into the geometry-dependent plasmonic properties of concave nanostars and their packed aggregates. Our results uncover the intrinsic scattering properties of individual nanostars and the origin of the broad spectral response, which is mostly due to the z-direction packed aggregates.