Applied X-ray spectroscopy


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).

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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.


Interfaces in high power electronics

TOC

Silicon carbide (SiC) is one of the candidates for future metal-oxide-semiconductor (MOS) devices, in particular for high power applications. One main reason for the interest in SiC is that it comes with its own native dielectric, silicon dioxide (SiO2). However, devices made from SiC still struggle to achieve the high quality SiC/SiO2 interfaces necessary for optimum device performance and stability.

We have worked together with colleagues from Infineon Technologies Austria and KAI to explore this buried interface using X-ray photoelectron spectroscopy (XPS) to systematically study the local elemental distributions and chemical environment. We compared a range of device stacks after varying nitridation treatments, which can help lower interface defects and improve electrical device behaviour.

If you want to know more about this exciting exploration of an interface using X-ray spectroscopy head to the Journal of Materials Chemistry C to check out our recent paper. It was published as part of the 2018 Journal of Materials Chemistry C Emerging Investigators themed collection. Do check out the collection as it contains a number of great papers from exciting young materials chemists.


MSc and UROP student success

This summer the group hosted two MSc students, Shijia Liu and Ayse Ay, who worked on CuO nanostructures for glucose sensors, and four UROP students, Amy Tall, Zhuocheng Xu, Xiangqi Hu and Qiaochu Luo, who worked on transparent conducting oxides, radiation damage in amino acids, and copper oxide glucose sensors.

All of them contributed a great deal to the group. Excellent science happened, lots of fun was had (from nanogranola to nanocornflakes), and we conquered many realistic labexperiences (and struggles). It was such a pleasure to work with all of these excellent Materials students! We can’t wait to see where they go next.

 

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Group lunch: Amy Tall, Zhuocheng Xu, Xiangqi Hu, Shijia Lia, Ayse Ay, Qiaochu Luo, Anna Regoutz.

MSc Advanced Materials 2017-18

MSc Class 2017-18, including Ayse Ay (1st row, 4th from left) and Shijia Liu (1st row 4th from right) who worked on projects in the group (photo courtesy of Raj Adcock).


Hard X-ray Photoelectron Spectroscopy in the Laboratory

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Hard X-ray Photoelectron Spectroscopy (HAXPES) is becoming more and more popular as a characterisation technique for the bulk of materials as well as buried layers and interfaces. To date, most experiments are hosted as beamlines on synchrotrons and only a small number of such systems is available worldwide.

In order to open up the technique to a wider user base, new laboratory-based systems are being developed. Together with Scienta Omicron we have worked on such a system based on a monochromated, liquid Gallium X-ray source delivering a microfocused X-ray beam. This in combination with a state-of-the-art photoelectron analyser enables high resolution measurements of a range of samples.

If you want to know more about this exciting prototype and see some spectra collected on reference and applied materials, check out our paper in Review of Scientific Instruments. The paper is open access so freely available to everyone.


Galore – when experiment and theory come (closer) together

Bringing experiment and theory together can sometimes be a bit of a challenge. Lead by excellent colleagues at UCL, Adam Jackson, Alex Ganose, and David Scanlon, we have bridged one of the many exisiting gaps between the two.

Photoelectron spectroscopy generates valence band spectra, which are directly related to the electronic density of states of a material. Sounds simple, but although the density of states can nowadays be easily calculated using ab initio methods, a number of adjuments are necessary to make the pure theoretical results comparable to the measured spectra. The most crucial one is to apply weightings to the different orbitals based on the photoionisation cross sections. This is usually combined with the application of some level of Gaussian and/or Lorentzian broadening.

Galore is a software package that automates the corrections to the calculated density of states, which previously had to be done in often rather laberous ways. Galore is available on GitHub and any feedback is very welcome! We’ve also published a paper in The Journal of Open Source Software, where you can find more background and details about Galore.

galore