Photoluminescent surface-functionalized graphene quantum dots for spontaneous interfacial homeotropic orientation of liquid crystals

Abstract

An interfacial molecular array of octadecylamine-functionalized graphene quantum dots (GQDs) was fabricated to simultaneously realize photoluminescence characteristics and molecular orientation for liquid crystals (LCs). The functionalized GQDs were synthesized using a simple bottom-up method by first grafting them with octadecylamine. The subsequent interfacial self-assembly of these functionalized GQDs in an LC medium formed a nanostructured molecular array on an indium tin oxide (ITO) electrode. Experiments revealed that the hydrophobic GQDs spontaneously covered the hydrophilic ITO surface through polar intermolecular interactions, and the resulting GQD layer induced the uniform vertical orientation of the LC molecules at the ITO interface owing to van der Waals interactions of long side chains. In addition, the unique photoluminescence of pure GQDs in the blue region was maintained even in an LC mixture with a low concentration of functionalized GQDs. A new type of homeotropically oriented LC device based on the nanostructured layer of functionalized GQDs exhibited superior electro-optical properties and a 375% improvement in response time for turn-off switching of the device relative to those of a conventional polymer layer. This functionalized GQD-based device simultaneously achieved the spontaneous alignment of the LCs, blue photoluminescence emission, and high-speed response performance for turn-off switching of the device. In addition, our method is cost-effective and encompasses a simple fabrication process for developing functional controllable surfaces without requiring a tedious multi-step manufacturing process. © 2021 Elsevier B.V.