Gold nanoparticle self-assembly moderated by a cholesteric liquid crystal

Abstract

International audienceWe 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 monitors 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 of large nanoparticles (diameter 20nm), we now demonstrate for smaller nanoparticles (diameter 4.2nm) formation of long needles of length larger than 50 nanoparticles and width 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 4.2nm 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 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 additionnal 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 5nm that act as efficient anisotropic traps for the nanoparticles

Trapping of gold nanoparticles within arrays of topological defects: evolution of the LSPR anisotropy

“…Acc. Lincei similar to the one obtained with the same NPs embedded in a cholesteric matrix (Pendery et al 2013). In cholesteric LCs, the LC matrix induces a compression, leading to s ¼ 0:6 nm.…”

“…k LSPR ¼ 516 nm in toluene. Their synthesis is described elsewhere (Pendery et al 2013). The solution was deposited on the substrate and left evaporating (for 2 min.…”

Helically Twisted Chiral Arrays of Gold Nanoparticles Coated with a Cholesterol Mesogen

“…14 So far efforts to obtain chiral nematics filled with NPs have either relied on adding hydrocarbon groupcapped NPs to a chiral nematic matrix or, alternatively, on functionalizing gold NPs with chiral groups and dispersing them in a nematic liquid crystal. 15,16,17,18,19,20,21,22 In the main it has been found that NPs aggregate in the defects of the LC texture. This is possibly mediated by the pinning of features of defects such as fingerprint or schlieren textures to surfaces, providing thus a means of spatial NP assembly.…”

Alignment of Rod‐Shaped Single‐Photon Emitters Driven by Line Defects in Liquid Crystals

“…[36,[43][44] This aggregation is due to the distortions and disorder induced in the liquid crystal around most of the nanoparticles, which become reduced if the nanoparticles are aggregated. [36,45] In nematic liquid crystals, the best method to prevent aggregation of nanorods is to graft specific ligandsaround the nanorodsin order to allow for a weak anchoring of the liquid crystal molecules. [46,47] Weak anchoring indeed prevents the formation of liquid crystal defects and thus prevents aggregation.…”