Applied X-ray spectroscopy


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.


Welcome Veronica and Yujiang!

The PhD team in September 2020:
Yujiang Zhu, Veronica Nacci, Curran Kalha, and Nathalie Fernando (from left).

We are delighted to welcome Veronica Nacci and Yujiang Zhu to the group! They are both embarking on their PhD journeys at UCL Chemistry and in the group. You can check out their bios on the group tab of the website.

Veronica will be working on developing low-temperature sol-gel based routes for metal oxide thin films and Yujiang will be looking into metal/metal oxide nanoparticle formation and their performance in sensors.


ChemVoices

This month Anna took part in the first ChemVoices webinar on ‘Resilience: a key in overcoming challenges‘. The ChemVoices series is organised by the International Younger Chemists Network (IYCN) the International Union of Pure and Applied Chemistry (IUPAC).

The webinar, which took place on the 15th of September, was organised and hosted by the brilliant Torsten John and Lori Ferrins. Together with Dr. ‪César A. Urbina-Blanco, a Postdoctoral Researcher at Ghent University, and Dr. Jesús Esteban Serrano, a Lecturer in Chemical Engineering at the University of Manchester, we discussed the ins and outs of resilience and what it means to early career researchers in Chemistry.

Make sure to check out the ChemVoices webpage for more information on this awesome initiative. Future webinars will focus on a range of different aspects and will highlight the voices of younger chemists worldwide.

Do also check out the Periodic Table of Younger Chemists, which was created as a celebration of the 100th anniversary of IUPAC and the International Year of the Periodic Table, and in which Anna was awarded the element Praseodymium.


Gallium oxide – a triumph of experiment and theory

Our work on how structural differences in Ga2O3 polymorphs can be detected in X-ray photoelectron spectroscopy and what differences exist in the electronic structure of the different polymorphs has just been published in Chemistry of Materials. You can also find the paper on arxiv.

This one has been in our paper pipeline for a while – not because it’s not interesting, but because this was a rather complicated and multifaceted story to pull together. Taking a quick look at the author list (a whopping 24 co-authors) you can tell that we needed to involve several growth, characterisation, and theory experts to be able to systematically explore and understand the scientific questions we set out to answer. This work is a great example of how important collaboration and involvement of groups across different disciplines is. We are fortunate to be able to work with such a great team!

At the heart of the work are high quality single crystal and epitaxial thin film samples of alpha-, beta- and epsilon-Ga2O3, that we explored using a number of advanced characterisation techniques. In particular, we combined SXPS and HAXPES to compare surface and bulk spectra, from shallow to deep core levels and valence states. Combined with XAS we were able to provide an overview of both occupied and unoccupied states in the polymorphs. What is particularly exciting from a spectroscopy point of view is that we were able to explain the core level line shapes using high-level theory and to clearly show the influence of the local coordination of Ga atoms on the spectroscopic results.


Exfoliating MXenes – or not

The first paper from our collaboration with Dr Christina Birkel’s group at Arizona State University has just been published in Dalton Transactions.

The work focuses on experimental and theoretical investigation of the chemical exfoliation of Cr-based MAX phase particles. This publication seems particularly important as it reports on a negative result. By combining advanced experimental and theoretical approaches we could explain why it is not possible to exfoliate Cr-based MXene from Cr2GaC by HF-etching. Our contribution to the story was to use SXPS and HAXPES at beamline I09 at Diamond Light Source to understand the chemical states present after HF treatment.

The cherry on the cake so to say that this work also made it to the front cover of Dalton Transactions.


Anna wins the 2020 Joseph Black Award

Anna has been awarded the 2020 Joseph Black Award by the Royal Society of Chemistry “For outstanding contributions to the development and application of X-ray photoelectron spectroscopy in the area of electronic materials and devices.

You can find more information and an answer to why Anna loves Thallium on the awards pages of the RSC.

We also want to express special congratulations to our collaborators and friends who were awarded prizes yesterday including Dr Camille Petit, Imperial College London (Barrer Award) and Dr Thomas Bennett, University of Cambridge (Harrison-Meldola Memorial Prize) as well as colleagues at UCL, including Prof Richard Catlow (Faraday Lectureship Prize), Hayley Simon (Ronald Belcher Award), and Prof Angelos Michaelides (Surfaces and Interfaces Award).


More on SiC interfaces

We are continuing our work on understanding silicon carbide (SiC) and in particular its interface to its native dielectric SiO2. SiC is set to enable a new era in power electronics impacting a wide range of energy technologies, from electric vehicles to renewable energy. Following on from our previous work on the topic, where we used soft X-ray photoelectron spectroscopy (check out the paper in Journal of Materials Chemistry C – it’s open access), we have now moved to hard X-ray excitation sources (HAXPES) probing non-destructively SiC and SiO2 and their interface in device stacks treated in varying nitrogen-containing atmospheres.

We have worked together with colleagues from Infineon Technologies Austria and KAI to explore this buried interface using both laboratory and synchrotron HAXPES measurements to explore the local chemical states at and around the SiC/SiO2 interface. The synchrotron measurements were performed on beamline I09 at Diamond Light Source and the laboratory HAXPES experiments were done on one of the first prototypes of ScientaOmicron’s HAXPES Lab system.

Go check out the paper in JPhys Energy and let us know your thoughts. We’d love to hear about any other material systems that we could test this approach on. The work was published as part of the JPhys Energy Emerging Leaders 2020 collection. Do check out the collection as it contains a number of great papers from exciting young leaders in the field.