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At the synchrotron, I use X-ray spectroscopy to study surfaces and interfaces at the nanoscale [1] — where a material meets air, liquid, or another material, and where most of the interesting (and useful) chemistry happens.
What is a Synchrotron?
What is a Synchrotron?
A synchrotron is a type of particle accelerator that uses a combination of electric and magnetic fields to accelerate electrons to very high speeds. The electrons are then directed into a circular path (storage ring) giving the annular shape of the building. They can reach speeds close to the speed of light, and the resulting high-energy beams can be used to produce X-rays. For that, the electrons go through so called ondulators, composed of strong magnets, moving the electrons around, accelerating them and thus producing the X-ray photons. This is called Bremsstrahlung emission. These X-rays are then used for spectroscopy such as X-ray absorption, Photoemission, etc. (e.g. BESSY II, Berlin)
Where can I find a Synchrotron?
Where can I find a Synchrotron?
In Europe, there are 10 main synchrotrons in 8 different countries. Each of them have they specificities. For instance, DESY synchrotron, with the PETRA III storage ring, is one of the most brilliant storage ring based X-ray source worldwide. The accelerator produces a particle energy of 6 GeV and three experimental halls host up to 30 experiments simultaneously. Because of the large energy of the electrons accelerated, DESY is also one of the largest synchrotron with circumference of more than 2 km!
On the contrary, BESSY II, in Berlin, where I worked, opened in 1998, is 1.7 GeV for a circumference of 240 m. However, in contrary to other synchrotrons, BESSY produces low energy X-ray, called soft X-ray, perfect to study carbon materials and diamonds!
The biggest is not always the best.
What do I do in Synchrotrons?
What do I do in Synchrotrons?
I perform X-ray absorption spectroscopy (XAS), which reveals the surface chemistry of a material with great precision, and X-ray photoemission spectroscopy (XPS), which quantifies the different atoms and chemical bonds present.
The great advantage of using a synchrotron for XPS, compared to lab-based XPS, is that the energy resolution is much better, and by changing the excitation energy, I can probe the sample more or less deep. And believe me, it helps to understand the measurement!
This depth sensitivity is the basis of a technique I develop to model energy-dependent XPS [2,3]. By recording XPS spectra at a whole series of excitation energies and modelling how the signal from each chemical species evolves with probing depth, I can reconstruct where atoms and bonds sit within the first few nanometers of a surface — down to Angstrom resolution, without damaging the sample.
I also develop in situ XAS [4], measuring surfaces in contact with liquids rather than in vacuum. This is essential to understand what actually happens at an interface — for instance between a solid and water — and to design new materials for (photo)electrochemistry. It's challenging, because the X-rays we use only travel through vacuum and are easily absorbed by any material. Did you ever try to put water under vacuum?
In one project, I used it to track how a light-absorbing dye grafted onto a nanomaterial is electronically coupled to its surface — a key step in designing materials that use light to drive chemical reactions, such as CO₂ reduction in water. Watching electrons move between a molecule and a surface under real, working conditions, rather than in a simplified vacuum model, is exactly the kind of measurement only a synchrotron makes possible!
To measure at the synchrotron, we apply for a beamtime. If granted, we have one week, days, and nights to perform our experiments. It's quite an adventure! And it is a team work. We take shifts with colleagues to rest a bit, and we can count on the great support of the beamline scientists, working on the measurement station.
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