Effect of CsPbBr3 perovskite quantum dots on molecular alignment, dielectric and memory effect of a cyanobiphenyl based liquid crystal

We demonstrate here the impact of doping of CsPbBr₃ perovskite quantum dots (CsPbBr₃ PQDs, diameter ∼ 7–9 nm) on the molecular alignment, dielectric and memory effect of nematic (N) and smectic A (SmA) phases of a thermotropic liquid crystal material, namely 4-octyl-4′-cyanobiphenyl (8CB) using polarising optical microscope and dielectric spectroscopy. There is no significant change in the alignment of N and SmA phases of 8CB by the doping of CsPbBr₃ PQDs, except the color change attributed to the tuned birefringence of 8CB by PQDs. The slight aggregation of PQDs is observed in both LC phases of 1.0 wt% CsPbBr₃ PQDs-8CB composite and which could be due to the higher dopant concentration in 8CB matrix. The dielectric parameters (permittivity, loss and anisotropy) of N and SmA phases of 8CB are found to be strongly dependent on the dopant concentration. The dielectric anisotropy (Δε′) of 8CB is observed to decrease with increase in the dopant concentration except 0.5 wt% composite. The temperature dependent Δε′ further confirms that there is no significant change in the clearing temperature of composites as compared to pure 8CB. The short axis molecular relaxation observed in both N and SmA phases of pure 8CB and its composites on the application of 40 V bias. The slight spectral shift of this relaxation towards lower frequency side and decrease in the amplitude of ɛ″ in both SmA and N phases of 1.0 wt% composite as compared to the pure 8CB and composites with concentration lower than 1.0 wt%. The partial memory effect is observed in SmA phase of pure 8CB through voltage-dependent dielectric permittivity and cross-polarised optical textures. The observed memory effect in SmA phase is found to improve with increase in the concentration of PQDs. Most importantly, the complete memory effect is observed in SmA phase of 1.0 wt% CsPbBr₃ PQDs-8CB composite. This could be attributed to the holding of smectic layers reorientation achieved at 40 V bias for longer duration after the removal of 40 V bias voltage. A point to be noted here is that no memory effect is observed in the N phase of pure 8CB and its composites with CsPbBr₃ PQDs. Our results on CsPbBr₃ PQDs-8CB composites are clearly indicative towards their potential applications in next generation tunable electro-optical and long-lasting soft memory devices. Moreover, results on these composites would also serve as the testbeds for theoretical and computational studies to explore the mechanism responsible for tunable optical, dielectric and electro-optical properties of 8CB by the dopant CsPbBr₃ PQDs.