Effects of gold nanoparticle film morphology on the alignment of a nematic liquid crystal

We report the alignment of liquid crystal (LC) 4-cyano-4'-pentylbiphenyl (5CB) to well-defined films of alkanethiol-capped gold nanoparticles, residing at the LC/water interface and in optical sandwich cell configurations. Gold nanoparticles (AuNPs) of two core sizes (2.6 and 4.1nm) were synthesised with a variety of alkyl chain lengths (CnH2n+1SH, for n=5-18). Langmuir films of the nanoparticles were compressed to 10mN/m and introduced to the LC/water interface via Langmuir-Schaefer transfer onto an similar to 20m thick film of LC. The 4.1nm AuNP films consistently yield homeotropic alignment of the LC while the 2.6nm AuNP films yield mixed alignments. These observations reflect differences in the AuNP film morphology, surface coverage and relatively weak anchoring strength of 5CB to the nanoparticle films. We determine the anchoring of 5CB to these alkanethiol-capped AuNP films at high coverage. This method can be applied to other types of nanoparticles and ligand shells for determining the LC anchoring to condensed films of water-insoluble nanoparticles with the goal of controlling nanoparticle-LC interactions.