Gold nanoparticles in a cholesteric liquid crystal matrix: self-organization and localized surface plasmon properties

By combining experimental and calculated optical extinction spectra, scanning electron microscopy and optical microscopy, we investigate the optical properties of a hybrid system consisting of a cholesteric liquid crystal and gold nanoparticles. Close to the air-exposed surface, the liquid crystal film exhibits a structural modulation. In this article, we explore how to use this modulation to drive the formation of gold nanoparticle assemblies and thus to control their localized surface plasmon properties. We found that the penetration of the gold nanoparticles within the liquid crystal modulation depends on the initial concentration of nanoparticles in the colloidal solution. Two distinct regimes are pointed out: (i) at low concentration the nanoparticles are weakly interacting and embedded in a homeotropic environment at the film surface and (ii) at high concentration the penetration of the NPs occurs within the modulation, and their localized surface plasmon resonance is strongly red-shifted due to nanoparticles packing and electromagnetic interactions between the nanoparticles. This surface plasmon resonance shows sensitivity to the light polarization attributed to the formation of anisotropic aggregates oriented by the structural modulation of the liquid crystal.