Plasmonic Polymorphs by Combining Shape Anisotropy and Soft Interactions in Bipyramid Thin Films.

Thin-film plasmonic supercrystals of pentagonal gold nanobipyramids (AuBP) exhibit a diverse range of packing structures that influence the near-field distribution of the enhanced electric field and the far-field response. By varying the molecular weight of the coating ligands, the softness of the anisotropic building blocks is changed. A thorough structural characterization reveals that this affects the resulting superstructures from self-assembly more intricately than with isotropic building blocks. Softer coatings lead to smaller aligned domains in monolayers, while bilayers exhibit more crystalline domains with dominant interlayer twist angles near 0° and 90°. The far-field distribution and near-field response are measured using micro-absorbance and electron energy loss spectroscopy (EELS). Correlating these data with high-resolution transmission electron microscopy (HR-TEM) structural analysis enabled the identification of the longitudinal and transverse individual and collective plasmonic modes. Notably, for large crystalline bilayer domains, a strong polarization-dependent optical response is observed. These features underline the potential of these superstructures for applications in surface-enhanced spectroscopies, plasmonic photocatalysis, and advanced optical manipulation in switchable optical metamaterials.