As the academic year comes to a close (and with Anna’s maternity leave imminent) we took the opportunity to take a new group photograph. This is the “group approved” version of the photograph – we’ll keep the outtakes to ourselves 🙂
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In April a team from the group including Curran, Aysha, Maria and Anna spent several days at beamline I09 at the Diamond Light Source for a combined SXPS/HAXPES experiment on a number of oxide materials, all relevant for electronic device applications.
We were hunting for both chemical state information and final state effects from core level spectra as well as for signatures of the electronic structure of the materials, including some 2D electron gases at buried interfaces. In order to avoid beam induced changes to the samples we used the defocussed setup at I09 and were rewarded with some intensely bright and large beam illumination on the samples (see image below). The electronic structure experiments needed extended acquisition times to obtain the needed signal quality, so we even had time to catch some fresh air and walk to the top of ISIS hill providing a great view of Harwell campus and the surrounding greenery.
This was also Maria’s first synchrotron experience. She is spending six months in the group as a visiting PhD student as part of her PhD, which she is undertaking at the University of Padua, Italy, under the supervision of Prof Alessandro Martucci. She works on crystallisation of solution-based metal oxides on temperature-sensitive substrates.
Where is my titanium going? Testing the stability of TiW barriers
Hot off the press! Check out our latest collaboration with colleagues from Infineon Technologies Austria, KAI and HarwellXPS, exploring the interface stability of TiW/Cu heterojunctions using SXPS and HAXPES. This work marks the second publication in a series by Curran Kalha on TiW diffusion barriers and continues a long and fruitful collaboration with beamline I09 at the Diamond Light Source.
Diffusion barriers are essential components in power semiconductor devices and are designed to isolate metallisation schemes from the semiconductor devices. The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier for copper metallisation schemes. However, little has been established regarding the chemical state of the TiW/Cu interface or the possible degradation mechanisms of the barrier during annealing.
In our recent paper in Journal of Applied Physics (the preprint is also on arxiv), we show that the TiW alloy is an excellent barrier for copper metallisation schemes, successfully isolating the copper after annealing for as long as 5 h at 400°C using both synchrotron-based SXPS and HAXPES. Under thermal stress the barrier starts to degrade via the out-diffusion of Ti, but using laboratory-based SXPS at HarwellXPS it is clear that the Ti quantity lost in the diffusion barrier does not significantly impact the performance of the barrier.
Stay tuned for more TiW research and the completion of Curran’s TiW trilogy (and maybe a prequel or origin story too).
SPIE Photonex 2021 – Glasgow
Nathalie, Curran and Anna ventured to Glasgow this week for the group’s first in-person conference since March 2020. Nathalie and Curran presented part of their PhD work in excellent contributed talks. Curran got people excited about studying the behaviour of TiW diffusion barriers in metallisation schemes for power electronics (see a recent paper) and Nathalie clearly showcased the importance of radiation dose when comparing the effects of X-rays on samples during diffraction and spectroscopy (see a recent paper and Diamond Light Source Science Highlight). Anna presented an invited talk on the influence of polymorphism in Ga2O3 trying to convince the audience that core level spectra are sensitive to local coordination environments (read all about it here).
It was great to get back to in-person talks and be inspired by an excellent line-up of speakers in the Photoemission Spectroscopy for Materials Analysis section organised by Robert Palgrave, Rosa Arrigo and Phil King.
Work on X-ray induced changes gets a shout-out in Diamond Light Source Science Highlights
Our recent work led by Nathalie Fernando on the effects X-ray irradiation has on small molecular crystals during X-ray diffraction and X-ray photoelectron spectroscopy has been chosen as a Diamond Light Source Science Highlight.
Nathalie and Anna had a great experience talking to the Diamond comms team, who put together an awesome article for the Science Highlights series, which has showcased work conducted at the synchrotron since 2014.
X-ray induced changes in small molecular crystals
Our most recent work, led by Nathalie Fernando, focuses on a subject matter typically considered a nuisance to many experimentalists who use X-rays. That is, the X-ray induced changes to the sample being probed. X-rays feature in many characterisation techniques today but are often (incorrectly!) regarded as non-destructive. In recent years, these unwanted X-ray-matter effects have been worsened by X-ray sources, both lab and synchrotron based, with ever increasing brightness. These X-ray induced phenomena have been widely studied in macromolecular crystallography, but unfortunately, the same cannot be said for small molecular crystals.
Recently published in the Journal of Physical Chemistry A, (available also as a preprint on ChemRxiV) our paper is the culmination of a huge collaborative effort comprising of crystallographers, spectroscopists, and theorists.
Here, we explore the effects of X-ray irradiation on two industrially important catalysts [Ir(COD)Cl]2 and [Rh(COD)Cl]2, using synchrotron-based X-ray diffraction and laboratory-based X-ray photoelectron spectroscopy. Both single crystal and powder X-ray diffraction were used to obtain a detailed understanding of the changes in structure and atomic positions upon irradiation. This was only possible through the close collaboration with an amazing team of crystallographers, including Dr Andrew Cairns (Imperial College London), Dr Claire Murray (Diamond Light Source), and Dr Amber Thompson (University of Oxford). Crystallography and spectroscopy, with density functional theory calculations, led by Dr. Laura Ratcliff (Imperial College London) and with the support of Nayera Ahmed, an MSci student in the group, provide insights into the structural, chemical, and electronic changes taking place within the samples upon X-ray irradiation. Crucially, these changes are studied with respect to X-ray dose, using the RADDOSE-3D software, with the support of Dr. Joshua Dickerson and Prof. Elspeth Garman at the University of Oxford.
This work presents an important first step towards understanding the changes in small molecular systems due to X-ray irradiation, and we hope that the combination of techniques outlined, will form the basis of many future systematic X-ray damage studies on a wide range of important small molecular materials.
How to stop diffusion (and how to peak fit tungsten core levels)
Check out our latest collaboration with colleagues from Infineon Technologies Austria and Kai, diving further into the world of power semiconductor devices. Here, we move away from the groovy science of front-end SiC/SiO2 device technologies, in favour of back-end metallisation architectures. In order to unlock the many advantages of copper metallisation schemes in power electronics, diffusion barriers are required to isolate the metallisation from the underlying silicon substructure. In recent years, the binary alloy of titanium-tungsten (TiW) has been proposed as an effective barrier but in order to have confidence in the material, extensive characterisation of its potential degradation mechanisms need to be conducted.
Using both laboratory SXPS and synchrotron HAXPES we explore the oxidation and thermal stability of TiW, providing insight into two prominent modes of degradation, from multiple depth perspectives. Additionally, we have paired experiment with theory, working with collaborators at Imperial College London and the Barcelona Supercomputing Center to aid with better understanding the observed systematic changes to the electronic structure of the alloy in response to thermal treatments.
This marks the first first author research paper of Curran’s PhD journey, so one down and hopefully many more to come. The paper can be found here in JApplPhys (the preprint is also on arxiv) and we’d love to hear your thoughts.
P.S For all the peak fitting XPS enthusiasts, be sure to check out the SI for a detailed explanation of the W 4f procedure in all its glory 🙂
HAXPES in 2020
Ever thought about wanting to do HAXPES but not sure where to start? Well hopefully now you have a useful resource to help you decide where and what you could measure with this exciting technique!
Our review on the state-of-the-art of HAXPES in 2020 is now out in the Journal of Physics: Condensed Matter and it is fully open access for your reading pleasure. This gargantuan effort was spear headed by two fantastic PhD students in the group, Curran Kalha and Nathalie Fernando, who took on a leading role in coordinating efforts and ensuring consistency throughout the paper, beyond contributing their own individual sections to the review paper. We are also particularly proud of Prajna Bhatt, who did her undergraduate literature review in the group and which has become the starting point for one of the sections of the review.
The review would not have been possible without the gargantuan effort by a large team of collaborators and friends including Fredrik Johansson, Andreas Lindblad, Håkan Rensmo, León Zendejas Medina, Rebecka Lindblad, Sebastian Siol, Lars Jeurgens, Claudia Cancellieri, Kai Rossnagel, Katerina Medjanik, Gerd Schönhense, Marc Simon, Alexander Gray, Slavomír Nemšák, Patrick Lömker, and Christoph Schlueter. Our breadth of different experiences across the HAXPES remit has hopefully enabled us to give a comprehensive and balanced overview of machines, systems and applications of HAXPES today.
We are also incredibly grateful for all the beamline scientists and instrument staff at both synchrotrons and in laboratories across the globe who have provided information on existing instrumentation through completing surveys and answering our many questions via email. We hope that the overview we were able to include in the review acts as a selection guide for users to choose the most suitable experimental setup to perform their experiments. There truly is a HAXPES system for everyone! We also hope that this review will act as a point of comparison in the future to benchmark how the technique develops and we hope we can look back in 10 years time (when we might have to write a follow up review) and marvel at all the progress we have made.
Aromatic Acids and ALL the molecules
Just before the holidays the second paper on our work on amino acids was accepted for publication in Electronic Structure (you can find it here). This is the second part of our exploration of amino acids in collaboration with Dr Laura Ratcliff at Imperial College London, who is the theory mastermind of the operation. Marta Wolinska, a talented Masters student at Imperial, laid the groundwork for the theoretical work and Nathalie Fernando, a PhD student in the AXS group, performed much of the experimental work.
Our interest in amino acids came from the search for a systematic group of molecules, which were readily available and which we could use to study changes in XPS core level binding energies with both theory and experiment. We started off exploring the simple amino acids glycine (Gly), alanine (Ala) and serine (Ser) and managed to show that using our theoretical approach, using ∆SCF implemented in a systematic basis set, we could reliably predict relative core level binding energies of amino acids both in the gas (multiwavelets) and solid, crystalline phase (plane waves). Due to the radiation sensitivity of amino acids we also had to employ a rastering approach to collect experimental spectra not influenced by radiation induced artefacts. This work was published in J. Phys. Chem. Lett. previously.
Encouraged by these initial results, we then decided to test the approach further by tackling the aromatic amino acids phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and histidine (His). It turned out to be a rather formidable challenge. Computationally the increase in unit cell sizes going from the simple to aromatic amino acids also meant an increase in computational cost, particularly for hybrid functionals. However, the true challenge was how to interpret the complex spectra and how to disentangle differences in binding energies.
It turns out that the chemical intuition and the experience of a spectroscopist is not able to fully explain and justify observed changes in binding energies and usually reflects the results given by calculations based on Koopmans’ theorem. The ∆SCF calculations, which describe the observed spectra much better, are harder to rationalise. To help us disentangle the web of varying contributions and energy changes we ended up calculating more than 20 related molecular species to systematically follow differences in core level binding energies.
The most important take-away message is that it is not only the nearest, but also next-nearest and even further removed neighbouring atoms that influence the final binding energy observed for a specific atom within the amino acids. This work will hopefully aid future interpretation of molecular systems and demonstrates the importance of combining theory and experiment to fully understand a material.
Next Great Impossible
In early March, before the pandemic fully hit the UK, we spent two days filming a clip for Merck/Sigma-Aldrich’s new campaign Next Great Impossible at both UCL and Diamond Light Source. The clip tries to convey some of the work we do and the excitement during synchrotron experiments and when you discover something new.
The campaign aims to showcase scientists and other inspirational people who don’t hesitate to take on the impossible within their work. The video below is the first one in what promises to become a great series of mini-documentaries showcasing a range of outstanding scientists.
If you know someone, who should be portrayed in this way, you can now nominate them through the Next Great Impossible website.