Gold nanoparticle self-assembly moderated by a cholesteric liquid crystal

We show that the study of gold nanoparticle self-assemblies induced by a liquid crystal matrix reveals the intimate distorted structure of the liquid crystal existing prior to nanoparticles' incorporation. We also show how this intimate structure controls the spacing between nanoparticles in the self-assemblies. We have created hybrid films of cholesteric liquid crystal (CLC) and gold nanoparticles, the CLC being deformed by competing anchorings at its two interfaces. Whereas previous results have evidenced formation of only slightly anisotropic clusters for large nanoparticles (diameter 20 nm), we now demonstrate for smaller nanoparticles (diameter 4.2 nm) formation of long needles of lengths larger than 50 nanoparticles and widths smaller than 5 nanoparticles, on average oriented perpendicular to the anchoring direction. The difference between the two kinds of nanoparticle aggregations is interpreted by a modification of the balance between aggregation between nanoparticles and trapping by the defects, favoured by the disorder induced by the alkylthiol molecules grafted around the nanoparticles. This leads to a well-defined, anisotropic Localized Surface Plasmonic Resonance (LSPR) of the 4.2 nm embedded nanoparticles. Interpretation of these optical properties using generalized Mie theory allows for a comparison between CLC/gold nanoparticles and the same nanoparticles trapped within smectic topological defects or deposited on the same substrate without a liquid crystal. A smaller spacing between nanoparticles is demonstrated in the CLC system with an attraction between nanoparticles induced by the CLC matrix, related to the additional disorder associated with the nanoparticles' presence. The experimental observations allow us to estimate the disordered size of the liquid crystal shell around the nanoparticles in the CLC to be of some nanometers. They also suggest that the CLC distorted by competing anchorings is characterized by the presence of arrays of defects with topological cores of width smaller than 5 nm that act as efficient anisotropic traps for the nanoparticles.