Designing Colloidal Open Crystals for Multifunctional Materials

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)

1. Our primary impact will be by supplying the UK knowledge economy with skilled multidisciplinary researchers, equipped with the technical and transferable skills to establish the UK as pre-eminent in topology-based future technologies. The training they receive will make them proficient in the demands of the translation of academic science (with a broad background in condensed matter physics, materials science and applied electromagnetics) to industry, with direct experience from internship and industry engagement days. With their exposure to both theoretical research (including modelling and big data-driven problems) and experimental practice, our graduates will be ideally equipped to tackle research challenges of the future and communicate to a broad audience, ready to lead teams made up of diverse specialised components. The potential impact of our researchers will be enhanced by a broad programme of transferable skills, focusing on innovation, entrepreneurship and responsible research. Beneficiaries here will include the students themselves as they embark on future careers intertwining academic research and industry, as well as the other sectors listed below.

2. The research undertaken by students in the CDT will have impact on the future direction of topological science. Related disciplines, including physics, materials science, mathematics, and information technology will benefit from the cross-disciplinary fertilisation it will enable. The CDT will not only provide an interface between research in physical sciences and engineering, but also provide a route for academia to interact effectively with industry. This will help organise researchers from different disciplines to collaborate around the needs of future technology to design materials based on topological properties.

3. Our research will enable industries to set the direction of topological research around the needs of commercial research and development, leading to wealth generation for the UK, and to influence the mindset of the next generation of future technologists. Specifically, topological design has the promise to revolutionise devices and materials relevant to communications, microwave and terahertz technologies, optical information processing, manufacturing, and cybersecurity. Through partnership with organisations from the wider knowledge sector, we will deepen the relationship between academic research and disciplines including IP law and scientific software development.

4. Our CDT will also have impact on the wider academic community. New specialist courses and training in transferable skills will be developed utilising cutting-edge multimedia technologies. Our international research collaborators, including prominent global laboratories, will benefit from placements and research visits of the CDT students. Our interdisciplinary research, combining the needs of academia and industry will be an exemplar of the effectiveness of the CDT model on an international stage.

5. The wider community will benefit from our organised public engagement activities. These will include direct interaction activities, such as demonstrating at the Birmingham Thinktank Science Centre, the Royal Society Summer Exhibition, local schools and community centres.