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

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Vol. 11; no. 28; pp. 9686 - 9694
• Main Authors: Khatun, Nurjahan, Sridurai, Vimala, Csorba, Katalin F, Nair, Geetha G
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 20.07.2023
• Subjects: Ambient temperature; Crystal defects; Dimers; Display devices; Elastic deformation; Elastic properties; Free energy; Liquid crystals; Low molecular weights; Molecular weight; Nematic crystals; Optical microscopy; Optical properties; Optics; Quantum dots; Response time; Thermal stability; Topology
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/d3tc00861d

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. Adding an NTB material to BPLC increases the thermal range of BPI. Further doping with QDs leads to the induction of BPIII, with a wide thermal range of 34 °C, spanning ambient and below/above ambient temperatures with enhanced display parameters.