Diamond Annual Review 2023/24

28 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 3 / 2 4 Structures and Surfaces Group Science Highlights From theory to confidence: building trust in twistronics models A single sheet of graphene, composed of a single layer of carbon atoms in a hexagonal pattern, is a semimetal. However, adding a second sheet of graphene, twisted at a slight angle to the first, can give rise to very different electronic properties, depending on the angle. At the ‘magic’ angle of about 1.1°, for example, a twisted bilayer sheet of graphene is a superconductor. The same effect is seen in other 2D materials, giving rise to a new field of study - twistronics - seeking to both understand and exploit the relationship between twist angles and novel electronic properties. Researchers from the University of Warwick and the National Graphene Institute at the University of Manchester used spatially-resolved angle- resolved photoemission spectroscopy (ARPES) on Diamond’s I05 beamline to study the twist-dependent band structure of twisted-bilayer, monolayer- on-bilayer, and double-bilayer graphene. Angle-resolved ARPES allows the researchers to measure directly the electronic structure of the 2D materials. It allows to determine both the energy and momentum of the electrons within the material, which directly gives the electronic structure which underpins the optical properties and the transport properties. Their results show good agreement between experimental measurements and theoretical simulations, confirming that the models can be used to explore the electronic band structure and emergent transport and optical properties of twisted-few-layer graphenes. Nunn, J.E. et al. DOI:10.1021/acs.nanolett.3c01173 Figure: Electronic structures of twisted double bilayer graphene at large (left) and “magic” (centre) twist angles, showing the emergence of a flat band at the top, which is at the heart of the various phenomena that emerge in this system. Data measured at I05 at Diamond and reported in Nunn et al. Image credit: MatthewWatson. Understanding how the structure of boron oxynitride affects its photocatalytic properties CO 2 is a greenhouse gas, released during the combustion of fossil fuels. Reducing our CO 2 emissions is critical to a sustainable future. CO 2 is also a by- product of many industrial processes. On the other hand, many industries need a regular supply of CO 2 , and shortages have caused problems in recent years. It makes sense, therefore, to find ways to recycle some of the waste CO 2 we produce into useful products. However, CO 2 conversion reactions are energy- intensive, and new catalysts are needed to make the reactions more efficient. Photocatalysts absorb light energy, creating a charge separation that can then drive a chemical reaction. A team of researchers from Imperial College London are researching CO 2 conversion using boron nitride as a photocatalyst. Boron nitride has a similar structure to graphite, with layers of hexagonally arranged atoms that can slide over each other. However, unlike graphite, boron nitride is an electrical insulator. As it also has high thermal conductivity and can withstand high temperatures without breaking down. Their group has already shown that it can photocatalyse CO 2 conversion reactions. But they realised that it is still not efficient enough. They used the B07 Beamline at Diamond Light Source to understand how oxygen doping affects the photocatalytic and optoelectronic properties of boron nitride. By clarifying the importance of paramagnetism in BNO semiconductors and providing fundamental insight into their photophysics, this study paves the way to tailoring its properties for CO 2 conversion photocatalysis. The group has also recently used a similar methodology to investigate phosphorus doping of boron nitride, which they will explore in a future publication. ​ Mistry E.D.R. et al. DOI: 10.1021/acs.chemmater.2c01646 ​ Figure: Image via Chem. Mater. 2023, 35, 5, 1858-1867.