# Self-assembled multi-layer simple cubic photonic crystals of oppositely charged colloids in confinement

## TL;DR

This study uses computer simulations to demonstrate that confinement stabilizes multi-layer open NaCl-like colloidal crystals, which have promising photonic properties, expanding possibilities for fabricating novel photonic materials.

## Contribution

It reveals that confinement and temperature effects can stabilize multi-layer open colloidal crystals, specifically NaCl-like structures, which are typically not observed in experiments.

## Key findings

- Up to 4-layer NaCl-like crystals are stable in confinement.
- Confinement and vibrational entropy stabilize open crystals over close-packed ones.
- Photonic calculations show large band gaps in 4-layer NaCl-like crystals.

## Abstract

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.