Diamond Light Source - Annual Review 2022/23

28 29 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 2 / 2 3 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 2 / 2 3 More than the sumof its parts: The electronic properties in a stack of crystals Physics – Hard Condensed Matter – Structures – Materials Science Traditional solid state physics focuses on crystals with a well-defined periodicity. However, in recent years it has become increasingly clear that stacks of thin crystals with non-compatible periodicities can bring about new properties that are different from those of the constituent parent crystals. Researchers from Aarhus University in Denmark wanted to develop a better understanding of this phenomenon. Most stacks of crystals with non-compatible periodicities are artificially made by bringing two-dimensional materials into contact with each other. Rather than using such an artificial crystal, the research team chose to investigate a so-called misfit compound. A misfit compound is a naturally occurring infinite stack of two-dimensional materials with incompatible periodicities and hence an easily accessible material to test the effect of stacking layers in incompatible periodicities. Their chosen misfit compound was a stack of square and hexagonal layers with no common periodicity. Using Diamond’s I05 nanoARPES branch allowed them to study the surface with a very high spatial resolution. This is necessary because the misfit crystal is a stack of two types of layers. It can have two terminations at the surface with either one or the other layer. The areas of these different terminations are quite small, so the beam spot used for probing must be strongly focused to probe only one of them. As the angle-resolved photoemission technique they used is very surface-sensitive, the underlying layers do not contribute strongly to the measured signal. The measurement is thus dominated by the surface layer. Their results showed that the properties of each layer strongly resemble those expected for a free-standing version of that layer, without the influence of the other layer. However, there were some new properties arising in the form of one-dimensional electronic states. The findings provide a new way to create one-dimensional electronic states. Such states have interesting fundamental properties that could potentially be used for next-generation electronic devices in the future. Related publication: Chikina, A. et al. One-dimensional electronic states in a natural misfit structure. Physical ReviewMaterials 6 ,(2022). DOI: 10.1103/ PhysRevMaterials.6.L092001 Funding acknowledgement: VILLUM FONDEN via the Centre of Excellence for Dirac Materials (Grant No. 11744) Corresponding authors: Philip Hofmann, Aarhus University, [email protected] Structures and Surfaces Group Beamline I05 Crystal structure of the misfit compound used in this study. The compound is a stack of square and hexagonal layers and the lattice constants of the two layers do not quite fit in length. A surface of the misfit crystal shows areas terminated by one or the other type of layer and the beam from Diamond is sufficiently focused to probe just one area of termination. The bottom row shows experimental data from the two terminations in different colours. In the image on the right hand side, line-like features appear that are not expected for any of the constituent layers. They emerge due to the interaction of the two materials. BiSe NbSe 2 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 -1.0 0.0 1.0 k ( -1 ) -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 -1.0 0.0 1.0 hv e NbSe 2 -termination BiSe-termination E - E F (eV) E - E F (eV) k ( -1 ) Г C’ S misfit direction C 2 1 -1 0 0 1 2 k x’ (Å -1 ) k y’ (Å -1 ) BiSe-termination Mixing it up: can sulfobetaines helpwith drug delivery? Surfaces – Drug Delivery – Physics – Physical Chemistry – Health &Wellbeing – Chemistry – Interfaces and thin surfaces – Life Sciences & Biotech The cell membranes of almost all organisms are formed of two layers of lipid molecules that prevent ions, proteins and other molecules from entering or leaving the cell. Vesicles are small, membrane-bound structures used to transport substances within a cell, and scientists can engineer vesicles to carry drugs to where they are needed. Engineered vesicles typically use lipids to form a membrane around the drugmolecules. As phosphocholine (PC) phospholipids are the main component of natural lipid bilayers, they are often used as simple models to study whether drug compounds are likely to be able to enter the cell. Phospholipids are non-toxic to cells and potentially useful for drug delivery and other medical applications. However, they are neither cheap nor easy to synthesise, unlike sulfobetaine (SB) based lipids. SB lipids are non-toxic and used in medical applications such as commercial eye drops, but their structure is less well studied. Researchers from the University of Bath conducted experiments to investigate the structure of mixed monolayers of sulfobetaines and phosphocholine phospholipids. Using a monolayer simplifies the system and makes it 2D rather than 3D. They wanted to explore the interactions between the two types of lipids, which have opposite charge distributions in their head groups, and how they affect the properties of the mixed monolayers. They created lipid mixtures with different ratios of sulfobetaines and phospholipids and used a combination of X-ray Reflectometry (XRR) studies on Diamond’s I07 beamline and Neutron Reflectometry (NR) experiments at ISIS Neutron and Muon Source to investigate the structure of the mixed monolayers. During the experiments, the research team also manipulated the surface pressure, developing an understanding of the nature of the structure and how it changes with pressure. The team found that adding the sulfobetaine to the mixture did not significantly affect the monolayer structure. However, the SB tails are longer than the PC tails, making the layer surface appear rougher. At higher SB concentrations, the data showed the phospholipids behaved normally, but that the sulfobetaine molecules were closer to perpendicular to the water surface. This suggests that the two molecules have different configurations at these higher concentrations, making successful preparation of vesicles unlikely, as they won’t pack well together. A very high concentration of sulfobetaines would probably also interact badly with cells, making the mixture toxic. Their results confirm that the interactions between sulfobetaines and phospholipids are favourable and that these combinations have potential applications in future drug delivery methods. With their opposite charge distribution, sulfobetaines may also interact differently with drug components, facilitating the delivery of drugs that wouldn’t be possible with phospholipids alone. Related publication: Elstone N et al . Structural investigation of sulfobetaines and phospholipid monolayers at the air-water interface. Physical Chemistry Chemical Physics 24 , 22679-22690 (2022) DOI:10.1039/D2CP02695C Funding acknowledgement: EPSRC Centre for Doctoral Training in Sustainable & Circular Technologies: EP/ L016354/1 Corresponding author: Dr Naomi Elstone, University of Bath, [email protected] Structures and Surfaces Group Beamline I07 Artistic representation of the possible structure of the 1 : 1 DMPC : SB3-18 mixtures monolayer at the air–water interface from the results obtained from fitting NR and XRR data.