Smectic Liquid Crystalline Titanium Dioxide Nanorods: Reducing Attractions by Optimizing Ligand Density

Abstract

Crystalline titanium dioxide (TiO2) semiconductor nanorods (NRs) feature several optical properties, such as birefringence combined with high refractive indexes and a wide bandgap precluding optical absorption in visible-light spectrum, making them attractive for many applications such as optoelectronics. Dispersing these NRs in suitable solvents creates inorganic liquid crystals (LCs) offering enhanced collective and orientation-dependent properties, which can additionally be utilized to manipulate optical behavior. Herein, a synthetic procedure from literature is scaled up and coupled with an important post-synthesis-treatment step such that self-assembled NRs dried onto a liquid interface and in bulk can be investigated. Comprehensive characterizations confirm the vital role of surface ligand density of the NRs in reducing the effects of attractions between them and thus increasing the range of volume fractions in which these dispersions can be exploited. Various symmetries (hexagonal or tetragonal) can be achieved in the smectic layers of NRs by tuning the aspect ratios of the NRs from 4.8 to 8.5. Experiments show that external fields such as shear flow or electric fields can easily either induce a reversible nematic order in isotropic dispersions or order existing LC phases over much longer regions, opening many opportunities to manipulate light for optical applications.