We
studied the formation of supraparticles from nanocrystals confined
in slowly evaporating oil droplets in an oil-in-water emulsion. The
nanocrystals consist of an FeO core, a CoFe2O4 shell, and oleate capping ligands, with an overall diameter of 12.5
nm. We performed in situ small- and wide-angle X-ray
scattering experiments during the entire period of solvent evaporation
and colloidal crystallization. We observed a slow increase in the
volume fraction of nanocrystals inside the oil droplets up to 20%,
at which a sudden crystallization occurs. Our computer simulations
show that crystallization at such a low volume fraction is only possible
if attractive interactions between colloidal nanocrystals are taken
into account in the model as well. The spherical supraparticles have
a diameter of about 700 nm and consist of a few crystalline face-centered
cubic domains. Nanocrystal supraparticles bear importance for magnetic
and optoelectronic applications, such as color tunable biolabels,
color tunable phosphors in LEDs, and miniaturized lasers.
Colloidal photonic
crystals display peculiar optical properties
that make them particularly suitable for application in different
fields. However, the low packing fraction of the targeted structures
usually poses a real challenge in the fabrication stage. Here, we
propose a route to colloidal photonic crystals via a binary mixture of hard tetramers and hard spheres. By combining
theory and computer simulations, we calculate the phase diagram as
well as the stacking diagram of the mixture and show that a colloidal
analogue of the MgCu2 Laves phase—which can serve
as a precursor of a photonic band-gap structure—is a thermodynamically
stable phase in a large region of the phase diagram. Our findings
show a relatively large coexistence region between the fluid and the
Laves phase, which is potentially accessible by experiments. Furthermore,
we determine the sedimentation behavior of the suggested mixture,
by identifying several stacking sequences in the sediment. Our work
uncovers a self-assembly path toward a photonic structure with a band
gap in the visible region.
Colloidal crystals with a diamond and pyrochlore structure display wide photonic band gaps at low refractive index contrasts. However, these low-coordinated and open structures are notoriously difficult to self-assemble from colloids interacting with simple pair interactions. To circumvent these problems, arXiv:1906.10680v1 [cond-mat.soft]
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