Electric-Field-Controlled Alignment of Rod-Shaped Fluorescent Nanocrystals in Smectic Liquid Crystal Defect Arrays

Periodic micro-arrays of straight linear defects containing nanoparticles can be created over large surface areas at the transition from the nematic to smectic-A phase in a nanoparticle-liquid crystal (LC) composite material confined under the effect of conflicting anchoring conditions (unidirectional planar vs normal) and electric fields. Anisomeric dichroic dye molecules and rod-shaped fluorescent semiconductor nanocrystals (dot-in-rods) with large permanent electric dipole and high linearly polarized photoluminescence quantum yield align parallel to the local LC molecular director and follow its reorientation under application of the electric field. In the nano-sized core regions of linear defects, where the director is undefined, anisotropic particles align parallel to the defect whereas spherical quantum dots do not show any particular interaction with the defect. Under application of an electric field, ferroelectric semiconductor nanoparticles in the core region align along the field, perpendicular to the defect direction, whereas dichroic dyes remain parallel to the defect. This study provides useful insights into the complex interaction of anisotropic nanoparticles and anisotropic soft materials such as LCs in the presence of external fields, which may help the development of field-responsive nanoparticle-based functional materials.