Bottom-up and top-down manipulations for multi-order photonic crystallinity in a graphene-oxide colloid

Abstract

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 secondorder 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. INTRODUCTIONGraphene oxide (GO) flakes, that is, 1-nm-thick disk-like particles, can be well-dispersed in water to form a stable colloid with spontaneous nematic assembly and non-linear anisotropic rheology. [1][2][3] Nematic GO dispersions subjected to illumination typically exhibit a collective reflection of a brownish color from many GO particles, owing to the intrinsic absorption band of GO. 4 However, color reflection throughout the entire visible spectrum was recently obtained from photonic crystals in a well-prepared nematic GO dispersion. 5 Structural coloration has also been demonstrated in other colloidal systems such as in the swollen lamellar mesophase of plate-like particles with or without a supporting polymer network or surfactants, 6-8 the columnar phase of regularly sized plate-like particles [8][9][10] and the steric packing assembly of silica or polymer spheres or cubes. 11-13 Nonetheless, color reflection in the GO dispersion is quite unexpected, considering that GO particles are highly polydisperse, irregularly shaped, curved in solution, 14,15 and two orders of magnitude thinner than the photonic band-gap. From a practical point of view, photonic crystals potentially allow the use of GO in various optical applications, as long as an easy method is provided for manipulation. 16 Finding ways to manipulate the reflection color and to achieve large-scale uniformity are, therefore, highly desirable for practical advances.

Iridescence in nematics: Photonic liquid crystals of nanoplates in absence of long-range periodicity

“…2 B, Inset) (4). The uniform and single color throughout the system suggests the formation of an ordered structure that follows Bragg-Snell equation λ max = 2(d/m)(n 2 − sin 2 θ) 1/2 , where d is the average lattice spacing, m is the order of the Bragg reflection, θ is the incidence angle of light, and n is the average refractive index of the suspension (19,31). Fig.…”

“…As the discotic nematic PLC is not composed of monolayers, we expected an improved thermal stability in our PLCs. It was reported that conventional heating/drying processes of monolayerbased LC material can cause irreversible structural change to PLCs of 2D monolayers (31,42). First, we evaluated the thermal stability of unexfoliated ZrHP dispersion by multiple heatingcooling cycles.…”

Quantum dot light-emitting diodes using a graphene oxide/PEDOT:PSS bilayer as hole injection layer

“…22 The GO synthesis followed the same process recently reported. 23,24 The GO dispersion was puried using the centrifuge cleaning process approximately 10 times, in order to remove residual ions in the solvent. 25 The mean size of GO particles in the GO dispersion was $600 nm, which were conrmed by eld-emission scanning electron microscopy (FESEM) (JSM-7000F, JEOL, Japan) and particle size analysis (Zetasizer, Marvern, UK).…”

Dielectrophoretic Condensation and Tailored Phase Separation in Graphene Oxide Liquid Crystals

“…The expanded image of the L region in Figure 6e reveals highly porous GO particles, whereas the GO particles in the H region (Figure 6f) are highly packed and aligned. Although freezing may alter the initial GO particle alignment, [4,21] the GO alignment difference significantly remained in the dried samples.…”

“…Since lyotropic liquid crystallinity was discovered in graphene oxide (GO) dispersions in 2011, [1] the self-assembly property of GO particles in solution has been widely used to control the alignment of GO particles for various applications including batteries, [2,3] optoelectronics, [4][5][6][7][8][9] conductive paper and fibers, [10][11][12] and chemical filters. [13,14] In lyotropic liquid crystals, the concentration and shape of dispersed particles predominantly determine their self-assembly property and phase.…”