  Photonic crystals -- a class of materials whose optical properties derive
from their structure in addition to their composition -- can be created by
self-assembling particles whose sizes are comparable to the wavelengths of
visible light. Proof-of-principle studies have shown that DNA can be used to
guide the self-assembly of micrometer-sized colloidal particles into fully
programmable crystal structures with photonic properties in the visible
spectrum. However, the extremely temperature-sensitive kinetics of
micrometer-sized DNA-functionalized particles has frustrated attempts to grow
large, monodisperse crystals that are required for photonic metamaterial
applications. Here we describe a robust two-step protocol for self-assembling
single-domain crystals that contain millions of optical-scale
DNA-functionalized particles: Monodisperse crystals are initially assembled in
monodisperse droplets made by microfluidics, after which they are grown to
macroscopic dimensions via seeded diffusion-limited growth. We demonstrate the
generality of our approach by assembling different macroscopic single-domain
photonic crystals with metamaterial properties, like structural coloration,
that depend on the underlying crystal structure. By circumventing the
fundamental kinetic traps intrinsic to crystallization of optical-scale
DNA-coated colloids, we eliminate a key barrier to engineering photonic devices
from DNA-programmed materials.
