Applied X-ray spectroscopy


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.


Looking for cross sections?

Over the past few weeks the group set off on a joint adventure to digitise the famous photoionisation cross section dataset from Yeh and Lindau. Initally published in 1985 in Atomic Data and Nuclear Data Tables under the riveting title Atomic Subshell Photoioization Cross Sections and Asymmetry Parameters, this work has become one of the staple references for practitioners of spectroscopy far and wide. The only slight complication: the available online version is on the blurrier end of digitised PDF documents and it’s hard to search and copy out data points by hand, when you need them (automatic digitisation software doesn’t work very well due to the pixelation of the PDF).

This is why the group, under the expert leadership and organisational talent of Curran Kalha, embarked on a Covid-19 lock down activity of going through the tables of 103 elements, entering them one by one into a rather large excel file and then cross checking all entries. After an encouraging email exchange with Prof. Lindau we are now able to share the files with everyone who can make use of them. Go to the research subsection of our website to download the dataset. You can also find the dataset on figshare. Feel free to share widely and we hope it will help many of you!

A big THANK YOU to Curran, Nathalie, Carolina, Ebru, Yun and Jiebin who all contributed to this group effort!

PS: Our main motivation to do this was to now use the dataset to incorporate it into the Galore software package. This is happening as we speak so for all you pDOS lovers out there you should soon be able to use the full set of Yeh/Lindau cross sections in Galore.


PhD studentship available

We currently have a fully-funded PhD studentship available in the group.

The project is on metal oxide thin films for electronic devices to start in September 2020.

Metal oxides are one of the top candidates to help us move from the silicon age into a new era of more powerful, energy efficient, and flexible electronics. They show the widest range of physical characteristics of any material family and in devices are often used in the form of thin films. High-quality oxide films are necessary to develop advanced device generations and in this project you will explore wet chemistry processes, like sol-gel synthesis, to prepare such films. The sol-gel process is fast, inexpensive, technologically simple, and can be executed at low temperatures enabling the use of flexible substrates. Through adjustment of the process parameters, including precursor type and concentration, use of stabilisers and catalysts, reaction temperature, and many more, the film characteristics can be engineered and optimised. This approach allows the comparatively easy fabrication of high-quality new oxide thin film materials, which can subsequently be tested for their fundamental chemical and physical characteristics. You will investigate structure, electronic structure, and chemical state of the thin films using a combination of characterisation techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM) and atomic force microscopy (AFM). The detailed knowledge of the characteristics and behaviour of new materials then enables their implementation in applications, such as new generations of electronic and optical devices. This project will combine elements of thin film deposition, solid state chemistry, and electronic devices. It is best suited for students with a keen interest in multidisciplinary work at the interface of fundamental materials chemistry and device applications.

Interested candidates should initially contact the supervisor DI Dr Anna Regoutz () with a degree transcript and a motivation letter expressing interest in this project. Informal inquiries are encouraged. Suitable candidates will be required to complete an electronic application form at http://www.ucl.ac.uk/prospective-students/graduate/apply. Any admissions queries should be directed to Dr Jadranka Butorac ().

Applications will be accepted until 30th April 2020 but the position will be filled as soon as a suitable candidate has been identified.


Welcome Curran!

Curran_I09

Curran Kalha joined the group last week and will be undertaking a PhD in the X-ray spectroscopy of multilayer structures particularly concerning materials for power electronics. He is starting his PhD with his first beamtime at beamline I09 at Diamond Light Source studying metallisation systems for power electronics this week. His PhD project will focus on developing and establishing measurement protocols for these important multilayer systems and he will make extensive use of XPS, HAXPES, AR-HAXPES, and XAS.

Curran graduated with an Integrated Master’s degree in Chemical and Process Engineering from the University of Leeds in 2019. During his third year of study, Curran worked on a project in association with Proctorand Gamble, to design a new processing route for the manufacture of high bulkdensity detergent powder. During this project, Curran gained experience in the field of powder research, both theoretically and experimentally. This experience led him to an industrial placement at the Ministry of Defence, wherehe switched working with detergent powder for energetic formulations. Here he developed the process route and characterisation protocol for a new PBX formulation. Curran’s work was selected for presentation at the 49th International Annual Conference of the Fraunhofer ICT on Energetic Materials. After completing the year placement, Curran developed an interest in developing and characterising new materials. Upon returning to university, Curran decided to focus his master’s research on investigating the synthesis of garnet oxide materials for all-solid-state Li-ion batteries using various sol-gel processing techniques.


Nathalie Fernando presents poster at MC14

Nathalie Fernando, PhD student in the group who is part of the CDT in Advanced Materials Characterisation, presented her first poster at MC14 (14th International conference on materials chemistry) in Birmingham this week.

Nathalie is working on photon-matter interactions and in particular the effects of X-rays on the structural and electronic structure of catalyst materials. She combines X-ray spectroscopy and X-ray diffraction, and compares results to DFT outputs, to gain an in-depth understanding of the processes involved. Nathalie is supervised by Anna and co-supervised by Rob Palgrave (UCL) and Andrew Cairns (ICL). The work includes a number of awesome collaborators, including Claire Murray (Diamond Light Source), Amber Thompson (University of Oxford), and David Scanlon (UCL).

nathalie_poster


It’s all about osmium (and parties)

OsO2

OsO2 is one of those forgotten and ingnored binary oxides in the periodic table. The main reason for this lack of interest is it’s unwanted tendency to form highly toxic and volatile OsO4, which makes the synthesis and characterisation of samples somewhat challenging. It is fair to say that to publish this paper a number of H&S officers had to be convinced that we would not kill an entire beamline team with an ill adviced heating or sputtering attempt.

Although studying OsO2 has it’s challenges, there are a number of clear motivations for why we want to know more about it’s characteristics, beyond sheer scientific curiosity. OsO2 is a transition metal dioxide and it is part of an illustrious group of rutile metallic oxides, including IrO2, RuO2, PtO2, TcO2, and ReO2. It is also the parent oxide of the family of osmates, which similar to their cousins the iridates, are starting to show a range of interesting physics, including metal-insulator transitions and exotic magnetic behaviour.

In this work we used a combination of theory and experiment to gain some understanding of the electronic structure using hard and soft X-ray spectroscopy and density functional and many-body perturbation theory. Beyond providing an understanding of all occupied states of OsO2 we also identified a low-energy plasmon within the valence states. And if you ever wanted to see an intimidating peak fit look no further than the Os 4f/5p core level.

If you’d like to read more about this work, check out the full manuscript in Physical Review Materials.

Now at this point you might rightly ask what all of this has to do with parties. Well, this paper presents a personal milestone for Anna, as it is her 50th peer-reviewed manuscript to be published.