Periodic defect patterns and guided nanoparticle assembly in smectic liquid crystal films


Periodic micropatterns due to the nucleation and self-assembly of defect domains can be obtained in smectic liquid crystal films confined under conflicting boundary conditions. The domain dimensionality (1D/2D), symmetry and size, ranging from less than 0.5 µm to several µm, can be controlled by varying the film thickness and boundary conditions (anchoring), and applying an external electric field. Smectic micropatterns are useful for various applications, such as templates for soft lithography and microlens arrays, where pattern regularity over large areas, rapid self-assembly and a ‘soft’ responsive character are demanded.

We focus on the ability of such micropatterns to organize the spatial distribution of nanoparticle dispersions owing to a strong interaction between nanoparticles and the core of liquid crystal defects. Namely, 1D patterns induce the formation of nanochains trapped within linear defects with a cross-sectional size comparable to the nanoparticle diameter. Moreover, chains of gold nanoparticles show polarization-dependent optical plasmon resonance due to optical coupling of nanoparticles along the chains. Understanding nanoparticle-defect interactions is complicated by the lack of knowledge on the nanoscale structure of the defect domains. By combining the results of synchrotron X-ray diffraction, polarized fluorescence confocal microscopy and atomic force microscopy, we show that defect domains and patterns are the result of a complex balance between boundary interactions and distortions of the smectic layered structure, achieved through anchoring and structure frustrations and/or creation of uncommon defect types such as curvature walls and virtual singularities.