Interactions Between Topological Defects and Nanoparticles

Abstract

Liquid Crystal (LC) topological defects have been shown to trap nanoparticles (NPs) in the defect cores. The LC topological defects may thus be used as a matrix for new kinds of NP organizations templated by the defect geometry. We here study composites of LC smectic dislocations and gold NPs. Straight NP chains parallel to the dislocations are obtained leading to highly anisotropic optical absorption of the NPs controlled by light polarization. Combining Grazing Incidence Small Angle X-ray scattering (GISAXS), Rutherford Back Scattering (RBS), Spectrophotometry and the development of a model of interacting NPs, we explore the role of the Np size regarding the dislocation core size. We use NPs of diameter D = 6 nm embedded in an array of different kinds of dislocations. For dislocation core larger than the NP size, stable long chains are obtained but made of poorly interacting NPs. For dislocation core smaller than the NP size, the disorder is induced outside the dislocation cores and the NP chains are not equilibrium structures. However we show that at least half of these small dislocations can be filled, leading to chains with strongly enhanced electromagnetic coupling between the NPs. These chains are more probably stabilized by the elastic distortions around the defect cores, the distortion being enhanced by the presence of the grain boundary where the dislocations are embedded.

Liquid Crystal Films as Active Substrates for Nanoparticle Control

“…The inter-NP spacing in the hexagonal network has been measured on rubbed PVA surface (without a LC) by combined spectrophotometric measurements and synchrotron Grazing Incidence X-ray Scattering (GISAXS). It is equal to 1.9 nm 20,[30][31][32] . If we analyze the red-shift of the plasmonic absorption of Figure 4 using the dipolar approximation (see "Materials and methods"), we find that the inter-NP gap decreases from 1.9 nm without LC 20,30-32 to 1.1nm on top of PSNLC, leading to a shortening of 0.8 nm.…”

“…For small AuNPs, of diameter D = 4 nm with the same dodecanthiol ligands (with a size smaller than the core of the defects) no shortening is observed 39 . The AuNPs used here, of diameter D = 6 nm, have also been embedded in 2D smectic topological defects 20,30 . There, the 2D confinement induces formation of hexagonal networks similar to the one observed at the 5CB surface but instead embedded within the 2D defects.…”

“…The AuNP dielectric function was recalculated based on Johnson and Christy data to take into account the small size effect and the influence of the chemical interface, leading to broadening and blue-shifted localized surface plasmon resonance of the isolated NPs in toluene . ε m is taken as equal to 2.28, previously shown to be mostly induced by the dodecanethiol ligands for these same AuNPs …”

“…Efforts to use a LC matrix with a low density of defects as a substrate for NP self-assembly at the LC surface without the formation of uncontrolled clusters are therefore another interesting challenge. It has been reported that a cholesteric LC film, with a molecular pitch of the order of microns, exhibits a fingerprint texture which guides the NPs into ribbon-like assemblies, revealing the presence of more favorable areas at the surface of these modulated LC matrices. − However, a deep study of the local NP structure that drives the NP optical properties, in particular the light absorption when plasmonic NPs are concerned, and its relationship with the LC matrix specificity, is still lacking.…”

Defect Structures of Magnetic Nanoparticles in Smectic A Liquid Crystals

“…Generally, the interaction between particles and LC molecules depends on nanoparticle size, shape, and physical properties of both partners, NPs and organic molecules. Within the last two decades, the systems of LCs and nanoparticles have been intensively studied [ 2 ] and it has been confirmed that the orientational and positional ordering of mesogenic molecules can induce unexpected effects in the presence of nanoparticles [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ].…”