Selective stabilization of blue phase liquid crystals using spherical and rod-shaped colloidal nanocrystals

Abstract

Optimizing strategies used for improving the stability and properties of blue phase (BP) liquid crystals directly impact device performance. Various factors ranging from molecular structure to sample size and substrate conditions can influence selective reflection and electro-optics of BPs. More recently, the technique of incorporating colloidal nanoparticle (NP) assemblies has been used to enhance BP ranges. In cubic BPs, disclination networks can act as trapping centers for NPs, reducing the high elastic energy cost of these regions, favoring BP stability. Organization of NPs in the defect regions can sustain stable 3D colloidal structures, widening the scope and applicability of BPs as photonic materials. Physical and chemical properties, size, and shape of the NPs can also determine the utilization of BPs for advanced applications like lasers and high quality displays. In view of this, a mixture of two calamitic chiral compounds in which all three BPs, viz., BPI, BPII, and BPIII, were induced was combined with rod-shaped CdSe/CdS quantum rods (QRs) and spherical CdSe quantum dots (QDs), which were specifically chosen due to their exceptional optical properties. This also provided an opportunity to investigate the effect of the shape of the NPs on the preferential stabilization of the BPs and on the electro-optic Kerr effect. QRs were found to be more efficient in enhancing the overall BP range, with an almost twofold increase of ∼27 °C with ∼0.5 wt. %. On the other hand, with QDs, the BP range showed an initial increase of 20 °C for ∼0.3 wt. %, which, however, decreased with a further increase in QDs. Another major difference is that the Kerr effect was active only in the BPIII in the case of QDs but is measurable in both cubic BPI and BPII in the case of QRs. The results have been described in terms of the organization of the nanocrystals within the defect lines and the lattice orientations imposed by the substrates.