The recent discovery of structural coloration in aqueous graphene-oxide (GO) dispersions has increased the applicability of carbon-based two-dimensional materials. However, the origin of the photonic band-gap is still poorly understood, and its practical manipulation is still in an early developmental stage. Here, we demonstrate full-color reflection with first- and second-order Bragg reflections in a GO dispersion, and we use two fundamental approaches to manipulate GO photonic crystals, namely, bottom-up and top-down manipulation by controlling the Debye length and using shear or surface fields, respectively. Nanoscopic tailoring of the electrostatic effective thickness and macroscopic smoothing of the curvature of the GO sheet result in similar modifications of the quality and pitch of the photonic crystallinity. Direct observation of the GO particle alignments reveals excellent electrostatic layer-to-layer packing assembly and rather poor in-layer assembly. These results elucidate the mechanism that governs the nematic nature of GO (rather than its lamellar mesophase) and the origin of its photonic crystalline periodicity and provide new methodologies for exploiting these attractive features in actual applications.

Liquid-solid contact electrification is a useful mechanism to harvest wasted micromechanical energy. In this study, we investigate how the surface properties of a solid substrate affect contact electrification efficiency. Substrate surfaces were modified from hydrophilic to hydrophobic by changing the density of self-assembled monolayers (SAMs) on a SiO₂ surface. A substrate with a partially-covered SAM exhibited superior performance. The partially-covered SAM substrate is hydrophobic enough to induce quick dewetting of water from the surface and sufficiently electronegative to induce a high charge density on the surface. The quick dewetting results from the aliphatic tail groups of the SAM and -OH groups make the SiO₂ surface electronegative; these two competing properties can be simultaneously obtained by optimizing the SAM density. Our findings contribute to the understanding of contact electrification in liquid-solid-type energy-harvesting devices and advance the strategies to maximize the electrification efficiency by optimizing surface geometries and properties.

In order to use graphene oxide (GO) dispersions for electro-optical applications, both a high GO concentration and a high electrical sensitivity are essential; however, these have not been achieved to date. Here, we report that by optimizing the mean size of GO particles to approximately 0.5 μm, one can obtain a high GO concentration of up to 2 wt% and high electrical sensitivity simultaneously. By reducing the mean GO-particle size, the interparticle interaction and the rotational viscosity can be significantly reduced, and a high-concentration isotropic phase can be obtained. As a result, the maximum birefringence increases and the dynamic response becomes faster. However, further decrease of the mean size below 0.1 μm causes a decrease in the anisotropy of electrical polarizability, resulting in the reduction of the electrical sensitivity of GO dispersions.