  Designing and fabricating self-assembled open colloidal crystals have become
one major direction in soft matter community because of many promising
applications associated with open colloidal crystals. However, most of the
self-assembled crystals found in experiments are not open but close-packed.
Here by using computer simulation, we systematically investigate the
self-assembly of oppositely charged colloidal hard spheres confined between two
parallel hard walls, and we find that the confinement can stabilize multi-layer
NaCl-like (simple cubic) open crystals. The maximal layers of stable NaCl-like
crystal increases with decreasing the inverse screening length. More
interestingly, at finite low temperature, the large vibrational entropy can
stabilize some multi-layer NaCl-like crystals against the most energetically
favoured close-packed crystals. In the parameter range studied, we find upto
4-layer NaCl-like crystal to be stable in confinement. Our photonic calculation
shows that the inverse 4-layer NaCl-like crystal can already reproduce the
large photonic band gaps of the bulk simple cubic crystal, which open at low
frequency range with low dielectric contrast. This suggests new possibilities
of using confined colloidal systems to fabricate open crystalline materials
with novel photonic properties.
