Designing Colloidal Open Crystals for Multifunctional Materials

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Multifunctional materials are in great demand for the development of 21st-century disruptive technologies for their ability to integrate different functionalities within a single material. However, these materials pose a fundamental design challenge. Open crystals, built from low-coordinated colloidal particles in the size range of hundreds of nanometres, provide an exciting platform for integrating photonic, phononic and mechanical properties by design, and thus for multifunctional materials 1. Some of these properties and their interplay can be of topological origin, providing a rich playground for exploring topological physics 2. Colloidal open lattices can act as both photonic and phononic crystals, which can be exploited for light and sound management 3,4, also providing mechanisms for phonon-photon interactions 5. These phononic crystals can also be designed to support topologically protected mechanical states 6, which can be coupled to light waves. While lithography-based fabrication techniques have been used to realise certain colloidal open crystals to date, such top-down approaches are too expensive and time-consuming, especially for fabricating 3D crystal structures.Self-assembly of colloidal building blocks offers a low-cost, scalable fabrication route to colloidal crystals, but these tend to be close-packed. This project builds on the remarkable progress achieved in recent years by the Chakrabarti group in establishing versatile bottom-up routes for triblock patchy particles to yield colloidal open crystals 7-9, thereby addressing a long-standing challenge. Our computational approach takes into consideration synthetic feasibility of designer colloidal particles 10,11. We use the versatility of two-stage self-assembly schemes to establish bottom-up routes towards optimally designed colloidal open crystals. The aim of the project is to establish colloidal open crystals as a platform for light-weight multifunctional materials integrating optical, acoustic and mechanical properties. To this end, the objectives are as follows:1. to optimally design colloidal open crystals for simultaneous management of light and sound; 2. to explore topological physics of colloidal open crystals in connection with their photonic, phononic and mechanical properties; 3. to establish bottom-up fabrication routes to optimally designed crystal structures, by exploiting self-assembly pathways for designer triblock patchy particles. The research programme employs a variety of computational techniques, implemented in software packages developed by the Chakrabarti group (GlOSP and PaSSion) as well as in open source and/or commercially available software. We use global optimisation to predict crystal structures for patchy particles, especially formed via trimers, tetrahedra or octahedra. The voids in colloidal open crystals allow for a remarkable structural diversity arising from interpenetrating lattices, which can be exploited to tailor and optimise properties. We compute photonic and phononic band structures for various crystal structures and perform multi-physics analysis using COMSOL and other open-source software as appropriate to explore topological physics. We use advanced Monte Carlo methods including free energy calculations as implemented in PaSSion to establish self-assembly pathways to yield optimally designed target crystals.1. J. Bauer et al., Adv. Mater. 29, 1701850 (2017)2. M. Fruchart et al. 115, E3655 (2018)