Plasmonic Nanoparticles-Liquid Crystal Composites

We report on the plasmonic properties of 6 nm gold nanoparticles that form highly stable solutions in the nematic liquid crystal 4-cyano-4-n-pentylbiphenyl (5CB). The nanoparticles were covalently functionalized with 4-sulfanylphenyl-4-[4-(octyloxy)phenyl]benzoate, which resembles the structure of the 5CB molecules. The solubility of these nanoparticles in 5CB was significantly higher than that of conventional alkanethiol-terminated nanoparticles. An 8 nm shift of the surface plasmon resonance was observed when the gold nanoparticles were dissolved in the nematic phase of 5CB, as compared to the isotropic solution in methylene chloride. Good agreement of the experimental surface plasmon resonance shift with Mie calculations using an adjusted dielectric function for a reduced electron mean free path in small nanoparticles confirmed that the gold nanoparticles are solvated by the liquid crystal molecules. The stability of this composite was verified by repeated temperature cycling between the isotropic and nematic phases. We also investigated the nematic-to-isotropic phase transition temperature and the threshold voltage for the Freedericksz transition in gold-nanoparticle-doped and undoped liquid crystal devices.