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

Abstract 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.