Doped-Up: Bio-Inspired Assembly of Single Crystal Nanocomposites

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The ability to tune the physical properties of materials is extremely attractive. All too often, the performance of a material is a compromise between two important properties such as high transparency and high conductivity or low thermal conductivity and high electrical conductivity. The obvious solution to this problem is to combine materials to generate composite structures. However, the creation of a new hybrid material by simply mixing materials with complementary properties rarely results in a net advantage. The key is to exert control over the assembly of the component materials over multiple length scales.The goal of this project is to develop a robust and general methodology for the synthesis of a unique class of functional nanocomposites - single crystals containing a uniform distribution of inorganic nanoparticles. Our approach takes its inspiration from biominerals, such as bones, teeth and seashells, where these are invariably inorganic/ organic composites with hierarchical structures. Indeed, even single crystal biominerals are composites in which organic molecules are embedded within the crystal lattice. Nature therefore demonstrates that although crystallisation is a common means of purification, it is entirely possible to occlude additives within a crystal lattice given the appropriate pairing of the crystal and additive. Using the biologically-important mineral calcite (calcium carbonate) as a test system, we have made the exiting discovery that this biogenic strategy can be translated to synthetic systems to achieve efficient nanoparticle occlusion in single crystals. We now wish to build on these preliminary results to develop our bio-inspired crystallisation strategy - in which copolymer-stabilised nanoparticles are used as simple crystal growth additives - for the synthesis of functional nanoparticle/ single crystal nanocomposites. This strategy delivers a number of key features. We are creating nanocomposites in which the nanoparticles are embedded within a single crystal, rather than the typical amorphous or polycrystalline matrix, and the nanoparticles are not aggregated. This provides a unique structure where the absence of grain boundaries is expected to enhance many physical properties. It is experimentally straightforward and amenable to scale-up, and we can easily produce sufficient material to determine structure/property relationships. We also benefit from the vast knowledge that is available concerning the crystallisation of traditional ionic compounds to control the size, shape and porosity of the nanocomposites. Judicious design of the copolymer will provide control over the structures of the nanocomposites at the nano- and meso- length scales, and we will establish a tool-kit for controlling the nanoparticle loading, the inter-particle separations and the interfaces between the nanoparticles and the crystal host. As a suitable test-system we will focus on functional metal oxides containing noble metal nanoparticles/ quantum dots and study their transport and photocatalytic properties. Particular emphasis will be placed on evaluating the structure/property relationships, where the absence of grain boundaries and our ability to tune the structures of our materials is expected to provide us with unique information about their material properties. Our synthetic method is quite general however, and it is envisaged that it can be used as a platform for creating a broad spectrum of materials including capacitors, batteries, thermoelectrics and electrochromics. Finally, while significant efforts have been made to identify the strategies by which organisms control crystallisation, these have seldom been applied to functional materials. This project will demonstrate the feasibility and potential of this approach, and will hopefully inspire other researchers to use bio-inspired crystallisation strategies to control the structure and properties of advanced materials.