Spherical Colloidal Photonic Crystals

Abstract

CONSPECTUS: Colloidal photonic crystals (PhCs), periodically arranged monodisperse nanoparticles, have emerged as one of the most promising materials for light manipulation because of their photonic band gaps (PBGs), which affect photons in a manner similar to the effect of semiconductor energy band gaps on electrons. The PBGs arise due to the periodic modulation of the refractive index between the building nanoparticles and the surrounding medium in space with subwavelength period. This leads to light with certain wavelengths or frequencies located in the PBG being prohibited from propagating. Because of this special property, the fabrication and application of colloidal PhCs have attracted increasing interest from researchers. The most simple and economical method for fabrication of colloidal PhCs is the bottom-up approach of nanoparticle self-assembly. Common colloidal PhCs from this approach in nature are gem opals, which are made from the ordered assembly and deposition of spherical silica nanoparticles after years of siliceous sedimentation and compression. Besides naturally occurring opals, a variety of manmade colloidal PhCs with thin film or bulk morphology have also been developed. In principle, because of the effect of Bragg diffraction, these PhC materials show different structural colors when observed from different angles, resulting in brilliant colors and important applications. However, this angle dependence is disadvantageous for the construction of some optical materials and devices in which wide viewing angles are desired. Recently, a series of colloidal PhC materials with spherical macroscopic morphology have been created. Because of their spherical symmetry, the PBGs of spherical colloidal PhCs are independent of rotation under illumination of the surface at a fixed incident angle of the light, broadening the perspective of their applications. Based on droplet templates containing colloidal nanoparticles, these spherical colloidal PhCs can be generated by evaporation-induced nanoparticle crystallization or polymerization of ordered nanoparticle crystallization arrays. In particular, because microfluidics was used for the generation of the droplet templates, the development of spherical colloidal PhCs has progressed significantly. These new strategies not only ensure monodispersity, but also increase the structural and functional diversity of the PhC beads, paving the way for the development of advanced optoelectronic devices. In this Account, we present the research progress on spherical colloidal PhCs, including their design, preparation, and potential applications. We outline various types of spherical colloidal PhCs, such as close-packed, non-close-packed, inverse opal, biphasic or multiphasic Janus structured, and core-shell structured geometries. Based on their unique optical properties, applications of the spherical colloidal PhCs for displays, sensors, barcodes, and cell culture microcarriers are presented. Future developments of the spherical colloidal PhC materials are also envisioned.

A colloidoscope of colloid-based porous materials and their uses

“…A colloidal dispersion is confined to an emulsion droplet and left to evaporate, leading to the assembly of micron-sized spherical colloidal crystals ( Figure 4D,iii). 105,[156][157][158] Scalability. One of the great advantages of colloidal self-assembly is the potential to marry an inexpensive and parallel fabrication process with the high resolution features necessary for the plethora of applications discussed below.…”

“…204,205 For more information, a recent review covers many of the aspects of these types of photonic crystals. 158 Figure 10 shows inkjetprinted photonic hemispheres ( Figure 10A-C) and arrays of buckled-sphere photonic superparticles embedded in a flexible material ( Figure 10D), both of which show angleindependent color. It should be noted that the structural color of these photonic superparticles is only independent of viewing angle in a diffusely lit environment due to the near-normal incidence peaks being brighter than the blue-shifted, higher incidence angle peaks.…”

Responsive graphene oxide hydrogel microcarriers for controllable cell capture and release

“…Among them, microfluidic emulsified microcarriers are the most attractive due to the unprecedented control over their size, shape, and morphology [14][15][16][17][18]. And thus various biocompatible microcarriers have been generated for biomedical applications by employing different types of biopolymers in the microfluidic emulsion templates [19][20][21][22][23]. However, despite their merits, most of the current microcarriers are only suitable for the capture and proliferation of cells because they comprise simple material composites; whereas the function of on-demand cell release is rarely reached, which restricts the further applications of microcarriers in cell transfer and reuse.…”

Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies

“…interesting confinement for the crystallization process is an emulsion droplet that creates colloidal assemblies known as photonic balls (16)(17)(18)(19). Curvature imposed by the spherical confinement is a unique structural element of photonic balls.…”