Nanoparticle Self-Assembly and Domain Growth Kinetics in BPI: A Synchrotron Ultra-Small Angle X-ray Scattering Study

Abstract

Soft-stimuli-responsive cubic blue phases, BPI, and BPII, that occur in highly chiral liquid crystals, are 3D photonic crystals with an in-situ tunable photonic band gap. However, the inherent low thermal stability and polycrystallinity hinder their potential in photonic applications. Earlier studies indicate that nanoparticles of suitable size confined within disclination cores, that make up the blue phases, lower the free energy, thus enhancing the thermal stability. Nevertheless, the factors governing the effective confinement of nanoparticles are not well understood. The Synchrotron Ultra-Small Angle X-ray Scattering study described here investigates the self-assembly of nanoparticles in BPI exhibited by quantum dots-doped liquid crystalline system as the concentration of quantum dots is varied. The results indicate effective confinement of quantum dots for concentrations of 0.1 and 1 wt.%, with the particles mimicking the BCC lattice of BPI. The particles agglomerate at higher concentrations, as apparent from the signature scattering profiles. The study also explores the kinetics of domain growth of BPI to get direct evidence of lattice expansion/reorientation. The data show that the lattice planes predominantly orient along (211) as the domains grow with time. On the other hand, temperature-dependent studies reveal that the lattice planes align along (220) as the band gap redshifts.