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

Abstract

We report the alignment of liquid crystal (LC) 4-cyano-4ʹ-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 4.1 nm) were synthesised with a variety of alkyl chain lengths (C_nH_2n+1SH, for n = 5–18). Langmuir films of the nanoparticles were compressed to 10 mN/m and introduced to the LC/water interface via Langmuir–Schaefer transfer onto an ~20 μm thick film of LC. The 4.1 nm AuNP films consistently yield homeotropic alignment of the LC while the 2.6 nm 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.

Keywords:

• nanoparticles
• Langmuir films
• film morphology
• alkanethiol
• LC–aqueous interface
• areal density

Acknowledgements

We thank Yiqun Bai and Nick Abbott for generously preparing and donating the evaporated gold substrates, and Dr Louise Weaver (University of New Brunswick) for TEM imaging.

Additional information

Funding

This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada [grant number 341933].