The discovery of new materials, or innovative uses of existing materials, is essential to making progress in many of the technological challenges we face: from collecting energy from the sun to interfacing with the human body. Research in the department ranges from the chemical synthesis of new compounds and design of superalloys, to theoretical modelling and even the computational prediction of new materials. Recent highlights include the design and synthesis of non-toxic photovoltaic materials, high temperature superalloys, discovering a way to make brittle materials plastic and a new family of glasses, biomedical scaffolds and a theoretical understanding of a record breaking superconductor.
The remarkable performance of hybrid perovskite-based solar cells has transformed photovoltaic research. Power conversion efficiencies have increased from around 4% to 20% in just six years. However the prototypical light-harvesting material, [CH3-NH3]PbI3, contains the toxic and moisture-sensitive element lead and this will probably limit its widespread commercialization. The challenge therefore is to discover a lead-free alternative which retains its favourable optoelectronic properties, ease of synthesis and practical utilization. By combining the predictive power of density functional theory with the simplicity of hydrothermal synthesis, a systematic chemical evaluation of a range of lead-free hybrid perovskites has been performed, focusing on phase stability, band structure and mechanical properties. It has been found that the performance of these materials can be optimized by an appropriate choice of organic cation and inorganic framework. The atomic structure of a hybrid double perovskite and single crystals of a lead free compound grown in the Department are shown.
Existing melt-quenched glasses are characterised as inorganic (non-metallic), organic and metallic, and widely used in DVD technology, solar cells and telecommunications. An entirely new family of hybrid glasses has been discovered, which combine all of the attributes of the existing divisions, and circumvents the existing problem of limited chemically functionality in noncrystalline materials. The glasses are formed by melting hybrid crystalline materials called metal-organic frameworks (MOFs) which contain both inorganic, and organic components. The atomic configurations of a Zn(C3H3N2)2 crystal and glass, with accompanying optical images.
The drive for more efficient civil air transportation requires the next generation of jet engines to operate at higher temperatures. Current materials are already operating at their limit and as a result it is critical to develop new materials with higher temperature capability. Research in the department is taking place to develop a range of new materials, including Ni-base and Co-base superalloys, High Entropy Alloys and Refractory Metal-base alloys. The approach combines empirical relationships, thermodynamic and first principles calculations, as well as neural networks to identify promising candidate materials prior to evaluation through rapid small scale alloy prototyping. An electron image of a novel Ni-based superalloy developed in the Department is shown.
The combined power of modern computers and robust first principles codes has made computational materials discovery possible. Ab initio random structure searching (AIRSS) can be used to predict crystal, defect or grain boundary/interface structure, with little or no experimental input. Recent applications include the discovery of two dimensional ice structures (top), and understanding the crystal and electronic structure of the record-breaking (Tc=204K) superconductor, hydrogen sulphide at high pressure (bottom).
A major difficulty in developing materials for use at high temperatures is that in most oxidation resistant materials, the predominant obstacle to dislocation motion is due to the changes in misfit energy as a dislocation moves, causing them to be brittle. At present, there is virtually no understanding of how to design brittle crystals to give easy plastic flow. Using density functional theory calculations, it has been found that small modifications to the electronic structure and elastic deformation of crystals can drastically affect plastic flow. This appears to be a general method by which dislocation properties may be controlled to allow easier flow and this concept is consistent with observations in crystals. Furthermore, it is a substantial effect suggesting that such an approach might be used as a general way of tailoring plasticity in crystals. Deforming a MAX phase crystal.
In order to direct cell adhesion and proliferation, the chemistry of biomedical scaffolds with three dimensional porous architectures is being controlled using biochemical surface modification. Triple helical peptide (THP) sequences are used to mimic defined functions. These peptides have been synthesised by collaborators in the Department of Biochemistry. Using the high integrin affinity ligand, GFOGER, cell reactivity has been shown to be promoted. The mechanism for integrin binding to peptide sequences in collagen is shown.
Class II organic-inorganic hybrids combine the best of both worlds: chemical functionality and elasticity from the organic polymer and optical transparency from the inorganic part (e.g. silica, zirconia). We are developing new class II hybrid materials with tuneable physicochemical and optical properties (see picture) for a diverse range of applications including spectral conversion for solar cells (upconversion and downshifting), optical sensing and membrane technology.
We are investigating a variety of methods (sol-gel, reprecipitation, emulsion) to fabricate diverse photoactive nanomaterials for application in light-emitting displays (e.g. perovskite nanocrystals – see image), catalysis and environmental remediation (e.g. porous inorganic oxide nanoparticles from stimuli-responsive templates) and optical sensing/theranostics (e.g. organic-inorganic hybrid polymers).