Nanoparticles Confined in Liquid Crystalline Coatings with Smectic Focal-Conic Textures: Implications for Precise Nanoparticle Positioning

Composite systems prepared by dispersing mineral particles in molecular liquid crystals (LCs) have attracted much interest due to the prospects of improving the LC properties thanks to the particles and of precisely positioning the particles on a substrate thanks to the spontaneous organization of the LC. However, to achieve these goals, the strong tendency of particles to aggregate in LCs must be mitigated. We describe a simple and inexpensive process to prepare thin (≈4 μm) coatings consisting of metallic (gold) or semiconducting (quantum dots) nanoparticles (NPs), of ≈5 nm diameter, dispersed in the common smectic LC 8CB. Thanks to the regular lattice of focal conic domains (FCDs) that appears under hybrid anchoring conditions, nanoparticle aggregation is limited to the micrometer scale, allowing us to fabricate large-area (≈100 cm2) homogeneous NP/8CB coatings. Polarized-light, dark-field, fluorescence, and confocal optical microscopies, together with atomic force microscopy, were used to determine the location of the NPs in the coatings. These techniques revealed that most of the NPs cluster at the coating top surface, forming small (1 μm) aggregates located not only at defect lines but also at the center of each FCD. Moreover, after several months, the NPs adsorb onto the polymer-coated substrate to form a honeycomb lattice. Furthermore, upon heating in the nematic phase, the NPs aggregate in regularly spaced lines. These observations show that the issue of NP localization in LC coatings is more complex than previously thought. Because our process for preparing nanocomposite coatings is robust to variations in both substrate type (glass or silicon wafer) and NP type and size, it could be used for applications that require a macroscopically homogeneous distribution of metallic or semiconducting particles that are precisely positioned on a substrate, such as the fast parallel screening of pesticides or biological markers by surface-enhanced Raman scattering.