Diamond Annual Review 2023/24

26 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 The focus of new beamline developments has been to consolidate the existing ambient-pressure sample environments and to develop new cells for solid-liquid and high-pressure experiments. The beamline’s electrochemical flow cell, which was developed as part of the EU-funded project HySolChem, has proven to be very popular with users. It enables Total Electron Yield (TEY) and Total Fluorescence Yield (TFY) Near Edge Extended X-ray Absorption Fine Structure (NEXAFS) experiments on electrochemical interfaces in ES2 of Branch B using catalyst-coated SiNx windows, as well as operando XPS in Branch C using coated membranes in a water-vapour environment. In addition, a high- pressure cell, which was developed in collaboration with the group of Dr Paul Webb from the University of St Andrews, has been rolled out for standard user operation. Using SiNx windows, it enables TEY NEXAFS at pressures up to around 2 bar and temperatures up to 400° C on ES2 of Branch B. Furthermore, a new manipulator for the endstation of Branch C provides extended and more reproducible motion and thus contributed to a significantly improved productivity of user experiments. The beamline’s cluster source, a collaboration with Prof. Richard Palmer’s group at the University of Swansea, has seen an upgrade that dramatically improved the size selectivity of clusters. It has been used in the last year by a small number of user groups who have been supplied with clusters for their experiments. In 2023, Branch B became associated with the EPSRC-funded national facility Harwell XPS. As a consequence, rapid access to ES1 and ES2 is now managed through the Harwell XPS website. VerSoX will undergo a major upgrade as part of the Diamond-II project. This will mostly affect Branch C, which will see an increase in flux by up to 100x and an expansion of its energy range to 125 – 4000 eV. This upgrade will enable measurements at higher pressures and with much improved time resolution compared to the current beamline. The Technical Design Report has been accepted and design work and procurement will start in 2024. Gratings for a novel multilayer PGMdesign, which is led by the Optics Group of Diamond, will be tested at the beamline already before the dark period to ensure as little downtime as possible. Beamline I09 Beamline I09 has maintained strong user support for materials research. Due to the possibility of accessing both soft and hard X-rays, photoelectron spectroscopy has been exploited with variable probing depth to elucidate the degradation mechanism at the cathode-electrolyte interfaces in lithium- ion batteries. It was also employed to determine the band edge profiles across oxide/semiconductor interfaces and to unveil gap states near the oxide surfaces. Hard X-ray photoelectron spectroscopy (HAXPES) proved to be a unique technique for investigating bulk electronic properties of novel materials such as (layered, hexagonal carbides and nitrides) MAX phases and transition-metal dihydrides. Heterostructures of two-dimensional materials, either transferred flakes or UHV-prepared epitaxial layers, have attracted more interest in the I09 user community. In addition to measurements probing the chemical compositions and band alignments with the substrates, the X-ray standing wave (XSW) method was also used to extract key structural information closely linked to the electronic properties of the heterostructures. The upgrade of the sample manipulator enabled operando HAXPES analysis incorporating in situ sample biasing being applied reliably to improve studies of lithium plating/stripping in solid-state batteries and degradation of perovskite-based solar cells and light emitting diodes. The continued commissioning of the newly installed soft X-ray ARPES end- station has been the main focus of the I09 team in the past 12 months. This new facility is equipped with a momentum microscope comprising entrance and exit optics, a single hemispherical analyser, a time-of-flight (ToF) analyser and a fast delay-line detector. A monochromatic UV lamp that delivered a sub-100 µm beam spot for the He I and He II energies was used to facilitate the initial commissioning. At 30 K, a gold Fermi edge was measured to be 10 meV wide using a 10 eV pass energy of the hemispherical analyser, and a Rashba splitting of the gold surface state was observed with an instrumental momentum resolution estimated to be 0.01 Å -1. Using the synchrotron light, stacks of constant binding energy ( kx , ky ) maps were recorded, each over a binding energy or photon energy (i.e., kz ) range, from UHV-prepared and cleaved single crystals and epitaxial thin films. The sample manipulator is being modified to improve the temperature control, and the commissioning of the ToF to work with the hemispherical analyser is expected to take place in the second half of 2024. It is envisaged that limited commissioning experiments with users will commence towards the end of 2024. Over the years , most of the studies at I09 have utilised the end-station in Experimental Hutch 2, where users can investigate the same spot on a sample with either soft or hard X-rays. To better control the sample environment, a new version of the in-house developed manipulator stick was installed during the October 2023 shutdown to improve surface preparation and X-ray measurements. This upgrade offered higher repeatability and reduced parasitic motions of the sample azimuth and polar rotations coupled with faster cooling and lower liquid He consumption. It also allowed the manipulator to accommodate more samples for in situ or operando biasing with more contacts and improved electrical isolation. To better support offline sample surface preparation for beamtimes, a Diamond-developed UHV system is being assembled for the group. This facility consists of multiple chambers designed for surface preparation and characterisation. The characterisation chamber is fitted with a mass spectrometer and a variable-temperature sample manipulator (from below 70 K to above 800 K) to enable temperature programmed (TP) desorption experiments. The chamber is also equipped with an electron analyser and a non-monochromatic X-ray source for XPS andTP-XPS. The preparation chamber permits standard sputtering and annealing procedures and deposition of atomically thin layers from various sources. The vacuum commissioning of the system is expected to start in June 2024, and will be followed by the commissioning of the surface science instruments. The addition of an Aarhus scanning tunnelling microscope (STM) to this system is planned as a future upgrade. In preparation for the Diamond-II upgrade, the Diamond insertion device team reviewed the specification of the two current I09 undulators and confirmed that the two devices would be able to cover the required energy ranges after the upgrade. This was made possible by ensuring that specific minimum gap values could be reached by the two devices with the new electron optics and narrow gap vessel in the I09 straight. The list of critical beamline components is also being finalised. It includes replacing the first mirror of the soft X-ray branch to account for the new source position and upgrading the second crystal cage of the hard X-ray monochromator to improve the stability and repeatability of the mechanics. The Structures and Surfaces group continues to be a vibrant and innovative group, based on true collaborations across the team and with our growing user community. We aim to deliver the best possible instrumentation and support for all of the experiments and to continue developing the capabilities as we prepare for the Diamond-II upgrade. We are always keen to hear the views of our users, to explore novel techniques or to improve our delivery. Please contact any of the beamline teams to discuss your requirements and ideas.