Plasmonic sphere-on-plane systems with semiconducting polymer spacer layers.

The optical properties of metal-film-coupled nanoparticles (NPs) are highly sensitive to physical and optical interactions between the NPs and the spacer medium in the gap between the NP and metal film. Here, we investigate the physical and optical interactions between gold NPs (AuNPs) and semiconducting conjugated polymer thin-film spacers in a "sphere-on-plane" type metal-film-coupled NP system, and their influence on the plasmonic scattering of individual AuNPs. We choose two different conjugated polymers: one with an absorption spectrum that is resonant with the plasmonic modes of the AuNPs and another that is non-resonant. By correlating dark-field back-scattering optical images with topographic atomic force microscope images, we find that partial embedding of the AuNPs occurs in both conjugated polymers to different extents. This can lead to partial quenching of certain plasmonic scattering modes, which results in a change of the back-scattering colors from the AuNPs. Pronounced, red-shifted scattering is observed due to deep embedding of the AuNPs, particularly for thicker conjugated polymer spacers that have resonant absorption with the plasmonic modes of the AuNPs. Polarization-controlled defocused dark-field imaging is employed to visualize the emergence of horizontally-polarized scattering modes upon embedding of AuNPs into the conjugated polymer spacer. These results demonstrate the importance of nanoparticle-spacer physical interactions to the control of the color and polarization of coupled plasmonic modes in nanoparticle-film systems relevant.