Topological defects stabilized by a soft twist-bend dimer and quantum dots lead to a wide thermal range and ultra-fast electro-optic response in a liquid crystalline amorphous blue phase

Abstract

Amorphous Blue phase, or BPIII, a mesophase exhibited by highly chiral liquid crystals, is increasingly being investigated for next-generation displays due to its attractive electro-optical properties, such as sub-millisecond response time, high contrast ratio and wide viewing angle. However, obtaining a fast-responding, thermally stable BPIII with commercial usability is still a challenge due to the frustrated nature of the phase. The study presented here investigates the thermal and electro-optic properties of the BPIII exhibited by a low molecular weight liquid crystalline system. Adding a twist-bend nematic dimer to a mixture comprising a nematic liquid crystal and chiral dopant helps stabilize the BPI, the cubic blue phase, due to the ultra-low bend elastic constant and saddle-splay deformation inherent to the dimer. Doping small concentrations of surface-functionalized quantum dots reduces the free energy associated with the topological defects leading to a complete transformation of the cubic blue phase to an amorphous one, with the latter exhibiting a wide thermal range. Polarizing optical microscopy, and electro-optical studies confirm the existence of BPIII over 34 °C spanning ambient and below/above ambient temperatures. The response time associated with the switching between achromatic dark and bright states is ∼200 μs, a value faster than that reported for low molecular weight systems and on par with polymeric ones. Furthermore, the BPIII exhibits a hysteresis-free optical transmittance with low operating voltages and high contrast ratio. A prototype device fabricated using the BPIII composite is found to be thermally, temporally and electrically stable, making it highly promising for display device applications.