(276c) Pressure–Tunable Photonic Band Gaps in an Entropic Colloidal Crystal

Materials adopting the diamond structure possess useful properties, both in atomic and colloidal systems. They are a popular target for synthesis on the colloidal length scale, where a photonic band gap in the range of visible light is possible. Photonic crystals reflect different regions of the electromagnetic spectrum based solely on their crystal structure. This represents an exciting avenue for materials innovation, but the synthesis of these colloidal crystals remains challenging. The desirable photonic properties of the diamond structure pose an interesting opportunity for reconfigurable matter: can we create a colloidal crystal that switches reversibly to and from the diamond structure? Drawing inspiration from high-pressure phase transitions of diamond-forming atomic systems, we design a system of particles with polyhedral shapes that transitions from diamond to a tetragonal diamond derivative upon a small change in pressure. The transition can alternatively be triggered by altering the shape of the particle in-situ, and it coincides with a noteworthy modulation of the photonic properties of the crystal. We propose that the transition provides a reversible reconfiguration process for a potential new colloidal material, and we draw parallels between this transition and the phase behavior of equivalent atomic materials from which we take inspiration.