Screening nanopyramid assemblies to optimize surface enhanced raman scattering

This Letter describes how gold pyramidal nanoshells (nanopyramids) can be assembled into low- and high-order structures by varying the rate of solvent evaporation and surface wettability. Single-particle and individual-cluster dark field scattering spectra on isolated dimers and trimers of nanopyramids were compared. We found that the short-wavelength resonances blue-shifted as the particles assembled; the magnitude of this shift was greater for high-order structures. To test which assembled architecture supported a larger Raman-active volume, we compared their surface-enhanced Raman scattering (SERS) response of the resonant Raman molecule methylene blue (δ_ex = 633 nm). We discovered that high-order structures exhibited more Raman scattering compared to low-order assemblies. Finite difference time domain modeling of nanopyramid assemblies revealed that the highest electromagnetic field intensities were localized between adjacent particle faces, a result that was consistent with the SERS observations. Thus, the local spatial arrangement of the same number of nanoparticles in assembled clusters is an important design parameter for optimizing nanoparticle-based SERS sensors.