Stable colloidal suspension of graphene oxide liquid crystals: optical characteristics and electro-optic switching for sample
Résumé
Graphene oxide liquid crystal has very interesting optical and electro-optic properties. In fact, reorientation occurs already at few volts per millimeter and the disk-like graphene oxide flakes, typically suspended in water, can form liquid crystal phases at very low concentration due to the large aspect ratio of the flakes. The liquid crystal formation depends on the flake size, their concentration but also on the magnitude of the repulsive and attractive inter-flakes forces. However, the spatial confinement has relevance too on the macroscopic alignment and even in inducing birefringence as visible in filaments and in more confining configurations. Unlike graphene, graphene oxide has a very small light absorption in the visible, in some regions actually negligible. However, even if the colloidal characteristics are preserved and there is still birefringence, the optical characteristic can change due to near-UV exposure, resulting in an increase of optical absorption. This has the consequence that the imaginary part has to be included in the general refractive index expression and in the optical transmission evaluation. Stability is an important aspect for graphene oxide suspensions either for changes in the optical characteristics or the destabilization of the suspension due to partial or total reduction leading to the formation of reduced graphene oxide, but also for the isotropic – nematic phase separation occurring for concentrations in the bi-phase region. All these aspects are very relevant also for electro-optic applications. It will be shown here that some issues can be solved and improved if the chosen phase is the nematic phase.