Optical Properties of Tipless Gold Nanopyramids

Abstract

Controlling the size and shape of plasmonic nanoparticles (NPs) is a convenient approach to tune their optical properties for a broad range of applications. For example, metal NPs can function as discrete refractive index elements in optical sensing devices[1] as well as contrast agents in biological imaging.[2] An emerging area in plasmonics that has received less attention, however, is the tailoring of specific structural features of a NP (e.g., sharp asperities) to manipulate their optical properties. Systems of interest that could display signatures of such nanoscale geometrical changes are large (>100 nm) anisotropic NPs because they can support tunable dipole, quadrupole, and higher-order plasmon modes.[3] Although synthetic methods can access different shapes by changing the morphology of the seed (hard) or molecular (soft) templates[4] or by introducing metal ions and other molecules into the growth solution,[5] the overall sizes are usually too small to access higher-order plasmons. One important shape is Au nanorods, which can be grown to various lengths by keeping the width constant and increasing the length[6] or to various widths by preferential overgrowth on their sides.[7] In general, fabrication methods offer a more systematic approach to control different aspects of large particle shapes.[8] Also, the shapes of fabricated NPs tend to have a higher degree of monodispersity because their structures are governed by hard templates (e.g., anodized alumina membranes).[3e, 9]