We show that dye-doped polymers open an interesting route to controlling light at the nanoscale. Just as for the much better known metal-based plasmonic systems, propagating and localized modes are possible. We show that the attractive features offered by plasmonics, specifically enhanced optical fields and sub-wavelength field confinement, are also available with these materials. They thus open a new opportunity in nanophotonics in which fabrication and functionality might be achieved by harnessing molecular and supramolecular chemistry.Much is expected of plasmonics, with applications being pursued over a wide range of fields from on-chip plasmonic circuits 1 to nanoantennae for the emission of light. 2 The key to this wideranging interest is that plasmonics enables the control of light deep into the sub-wavelength regime, right down to the nanoscale. 3 Noble metals such as gold and silver have fuelled the plasmonics

CONSPECTUS: For decades, plasmonic nanoparticles have been extensively studied due to their extraordinary properties, related to localized surface plasmon resonances. A milestone in the field has been the development of the so-called seedmediated growth method, a synthetic route that provided access to an extraordinary diversity of metal nanoparticles with tailored size, geometry and composition. Such a morphological control came along with an exquisite definition of the optical response of plasmonic nanoparticles, thereby increasing their prospects for implementation in various fields. The susceptibility of surface plasmons to respond to small changes in the surrounding medium or to perturb (enhance/quench) optical processes in nearby molecules, has been exploited for a wide range of applications, from biomedicine to energy harvesting. However, the possibilities offered by plasmonic nanoparticles can be expanded even further by their careful assembly into either disordered or ordered structures, in 2D and 3D. The assembly of plasmonic nanoparticles gives rise to coupling/hybridization effects, which are strongly dependent on interparticle spacing and orientation, generating extremely high electric fields (hot spots), confined at interparticle gaps. Thus, the use of plasmonic nanoparticle assemblies as optical sensors have led to improving the limits of detection for a wide variety of (bio)molecules and ions. Importantly, in the case of highly ordered plasmonic arrays, other novel and unique optical effects can be generated. Indeed, new functional materials have been developed via the assembly of nanoparticles into highly ordered architectures, ranging from thin films (2D) to colloidal crystals or supercrystals (3D). The progress in the design and fabrication of 3D supercrystals could pave the way toward next generation plasmonic sensors, photocatalysts, optomagnetic components, metamaterials, etc. In this Account, we summarize selected recent advancements in the field of highly ordered 3D plasmonic superlattices. We first analyze their fascinating optical properties, for various systems with increasing degrees of complexity, from an individual metal nanoparticle through particle clusters with low coordination numbers, to disordered self-assembled structures and finally to supercrystals. We then describe recent progress in the fabrication of 3D plasmonic supercrystals, focusing on specific strategies but without delving into the forces governing the self-assembly process. In the last section, we provide an overview of the potential applications of plasmonic supercrystals, with a particular emphasis on those related with surface-enhanced Raman scattering (SERS) sensing, followed by a brief highlight of the main conclusions and remaining challenges.

This review provides an overview of current progress into Pd nanoparticles supporting localized surface plasmon resonance and their applications. We begin by analyzing briefly the optical properties of Pd putting...