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 a?ect photons in a manner similar to the e?ect 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 ?lm or bulk morphology have also been developed. In principle, because of
the e?ect of Bragg di?raction, these PhC materials show di?erent structural colors when observed from di?erent 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 ?xed 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 micro?uidics was used for the generation of the droplet templates, the
development of spherical colloidal PhCs has progressed signi?cantly. 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.Sample Text