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

• Main Authors: Sankaewtong, Krongtum, Lei, Qun-li, Ni, Ran
• Other Authors: School of Chemical and Biomedical Engineering
• Format: Article
• Language: English
• Published: 2021
• Subjects: Engineering::Chemical engineering; Crystalline Materials; Energy Gap
• Online Access: https://hdl.handle.net/10356/150226
• Institution: Nanyang Technological University

Designing and fabricating self-assembled open colloidal crystals have become one major direction in the 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 number of layers of stable NaCl-like crystals increases with decreasing 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 up to 4-layer NaCl-like crystals 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 in the low frequency range with a low dielectric contrast. This suggests new possibilities of using confined colloidal systems to fabricate open crystalline materials with novel photonic properties.