Reversible long range network formation in gold nanoparticle - nematic liquid crystal composites

Abstract

Nanoparticles (NPs) are dispersed into liquid crystals (LCs) to create ordered NP assemblies and thereby modify the LC and NP properties. Although low NP concentrations are normally used to avoid aggregation, high concentrations can lead to new organization through coupling of the interparticle attractive forces with the LC elastic properties. Gold nanoparticles (AuNPs) with mesogenic coatings, tailored to be highly miscible in the liquid phase of n-alkyl-cyanobiphenyl LCs, form reversible micron-scale networks on cooling at the clearing point by enrichment of the NPs at the nematic-isotropic liquid interfaces. The network topology and LC director field orientation are controlled by the cooling rate, surface alignment, film thickness, AuNP concentration and ligand shell composition. Thin film networks consisted of branches and circular areas of LC enriched in AuNPs. Nucleating nematic droplets evolve into homeotropic alignment of the host nematic matrix, accompanied by birefringent disclination lines and loops. Thick film AuNP networks in LCs form complex structures with stable radial director configurations in small domains and Schlieren domains elsewhere. Controlled formation of networks via the use of LC phase transitions offers an additional approach to produce quasi-periodic NP assemblies that are both long range and reversible in nature.