Large graphene oxide flakes: self-assembly and electro-optical switching in liquid crystal phase

Graphene oxide (GO) flakes in aqueous suspension self-organize into liquid crystal (LC) phases. They have been studied for various applications because of their unique properties, but mostly in their bi-phasic suspensions since high concentrations was needed for their pure liquid crystal. The stability of a suspension of GO flakes over time is essential for their application. A crucial change in GO biphasic suspension is the isotropic - liquid crystal phase separation. Here I study the isotropic-nematic phase transition for large GO flakes with an average size of ~38 µm for 570 days to examine their stability. I found that large GO flakes make pure LC phases at a very low concentration of 0.7 mg/mL (0.035 vol%), completely stable over time. I observed that the equilibrium concentration for making a LC phase increases over time, and it is not constant until it reaches a critical concentration for the pure nematic phase. As typical LCs, GO LCs exhibits birefringence property and, interestingly, low-field-induced birefringence. Thus, graphene-based lyotropic LCs are an attractive class of materials for electro-optic devices. To control the electro-optic properties of these high-performance functional materials at a macroscopic scale, it is necessary to understand their self-assembly individually. I study this here by determining the large GO flakes arrangement and spatial order in the nematic phase via synchrotron small-angle X-ray scattering and direct visualization of the flakes with fluorescence confocal laser scanning microscopy. The high fluorescence of large GO flakes makes it possible to individually study the assembly of flakes in real-time when suspended in aqueous dispersions. I successfully performed an enhanced electro-optical switching of large GO flakes in their low concentration nematic phase by using a low electric field, unlike what has been reported so far. I study the optical behavior of GO LCs using polarizing optical microscopy in static and dynamic conditions to follow the flake reassembly under the application of the electric field