Single-Twinned Seed-Mediated Synthesis of Au Right Bipyramids for Uniform Near-Field Focusing

Noble metal nanoparticles with sharp edges and corners, such as triangles, tetrahedrons, bipyramids, etc., support superior plasmonic field enhancement for enhanced spectroscopy. However, high-yield synthesis of gold right bipyramids (Au RBPs) with sharp tips has remained limited because they have a single-twinned (ST) crystal structure whose growth pathway is hard to control. Here, we report the seed-mediated growth of Au RBPs using ST seeds, achieving a yield exceeding 70% by moderately increasing the size of these seeds. Au RBPs with sizes from 34 to 94 nm and controlled corner sharpness are achieved through the regulation of seed and bromide amounts. Uniform and strong near-field enhancement is generated in the Au RBPs-on-mirror configuration, which outperforms the well-established nanocube-on-mirror configuration in Raman enhancement. This work highlights the merits of ST seed-mediated growth for anisotropic nanocrystals and their potentials in plasmonic near-field focusing.