Self-Assembled Plasmonic Pyramids from Anisotropic Nanoparticles for High-Efficient SERS

Abstract

Surface-enhanced Raman scattering (SERS) substrates play important roles for the enhancement of inelastic scattering signals. Traditional substrates such as roughened electrodes and colloidal aggregates suffer from well-known signal reproducibility issues, whereas for current dominant two-dimensional planar systems, the hot spot distributions are limited by the zero-, one- or two-dimensional plane. The introduction of a three-dimensional (3D) system such as a pyramid geometry breaks the limitation of a single Cartesian SERS-active area and extends it into the z-direction, with the tip potentially offering additional benefits of strong field enhancement and high sensitivity. However, current 3D pyramidal designs are restricted to film deposition on prepared pyramid templates or self-assembly in pyramidal molds with spherical building blocks, hence limiting their SERS effectiveness. Here, we report on the fabrication of a new class of low cost and well-defined plasmonic nanoparticle pyramid arrays from different anisotropic shaped nanoparticles using combined top-down lithography and bottom-up self-assembly approach. These pyramids exhibit novel optical scattering properties that can be exploited for the design of reproducible and sensitive SERS substrate. The SERS intensity was found to decrease drastically in accordance with a power law function as the focal planes move from the apex of the pyramid structure towards the base. In comparison to sphere-based building blocks, pyramids assembled from anisotropic rhombic dodecahedral gold nanocrystals with numerous sharp tips exhibited the strongest SERS performance. Macroscale pyramidal array films with plasmonic tunability as a new class of SERS substrate for sensitive detection of chemicals.