She uses light to modify matter

Part chemist, part physicist and 100% researcher, Niéli Daffé is interested in materials that change colour or magnetism when illuminated. She studies them using X-rays in her SNSF-supported research.

From the very first questions, Niéli Daffé’s frank laughter echoes in the small café corner of the library at the Paul Scherrer Institute (PSI) in Aargau. “It’s true that being a physics researcher sometimes scares people,” she laughs. However, for this expert in magnetic nanosized materials, her job is just like any other. “With perhaps the difference that the concept is to explore what we don’t know.” The PSI is the site of the Swiss Light Source, an electron accelerator (synchrotron) that resembles a giant doughnut with a circumference of almost 300 metres and is Daffé’s second home. Thanks to the beam of X-rays generated by this accelerator, she can study molecules at the level of their atomic structure. In particular, she is investigating molecules that are capable of changing colour as well as electronic and magnetic properties when illuminated. These new materials could be used as light-activated switches in molecular electronic devices.

Making the most of every minute of beam time

“My research is still at a fundamental stage, so these applications are not for tomorrow or any time soon,” insists Daffé. Which is not to say that her day-to-day routine is abstract – quite the opposite, in fact. Among her favourite activities are the “synchrotron campaigns” on the facility’s X-ray beamlines. These data acquisition sessions last up to a week, and she can do an average of three or four a year. “Every minute is counted. Projects are submitted to committees, and if they seem feasible and interesting, we get beam time, which we have to make the most of.” She recounts the handovers between the day and night teams, the exchanges with other scientists during the day and the slightly surreal calm during the night. They have to come with preliminary results and hypotheses, but also a whole battery of samples and pre-programmed experiments. “In the first few hours, we analyse the results live so that we can react according to what we observe and adapt the protocols as we go along.” This may involve, for example, identifying at what experimental temperature interesting phenomena are visible, or which batches of samples have the most promising properties for further testing. The aim is to collect data that will provide new insights.

Inspired by Prussian blue

Niéli Daffé works with tiny cubes made up of eight alternating atoms of iron and cobalt. These molecular units are miniature versions of derivatives of Prussian blue, a pigment used in the famous print Under the Wave off Kanagawa. More than two centuries after a colour merchant accidentally discovered the pigment in the early 18th century, scientists realised that the dye often used by artists had other interesting properties. During experiments with polymers derived from this pigment, researchers discovered that they could change their colour or magnetic properties by irradiating them at very cold temperatures. Chemists then developed smaller molecules that maintained these properties. The resulting materials are tiny, but they have the photomagnetic properties of the Prussian blue derivatives on which they are based. Daffé is now working with cubes that measure just two nanometres per side. More specifically, she is developing a method for depositing these molecules on surfaces while seeking to preserve their photomagnetic characteristics. This is a necessary step if they are ever to be used in the manufacture of electronic compounds.

Materials science instead of perfumery

This mix of applied chemistry and physics, analysis, modelling and mechanics appeals to Daffé. “There’s the part where you’re playing with your brain, but at the same time you’re active, you’re experimenting, you’re reacting, you’re tinkering. That’s what makes me love experimental research.”

That said, the Franco-Senegalese scientist had “not at all planned” on pursuing an academic career. “My father became an economics professor thanks to scholarships and studies. It was in my DNA that I was going to go to university. But afterwards, I was thinking of going into industry.” At 18, she wanted to become a perfumer. She left Dakar to study chemistry in Montpellier, France. She ended up finishing her degree with a master’s in material chemistry at the Université Pierre et Marie Curie in Paris. “I didn’t realise until the end of the master’s that it was a course designed for a career in research,” she laughs. During her internship in a laboratory at the end of her studies, she was offered the opportunity to continue her project in the form of a PhD. “That’s how I started my doctorate and how I first set foot in a synchrotron, to characterise the magnetic materials I was working on.”

A few years later, in 2017, she arrived in the German-speaking part of Switzerland to work at the PSI particle accelerator. After more than two years there, as well as a stint at the synchrotron in Grenoble, she was awarded an Ambizione grant from the Swiss National Science Foundation (SNSF) in 2021. These geographical changes have not been particularly unsettling for her. “Since I left Dakar, I haven’t stayed in one place for long. I’m open, I seize opportunities, and I tell myself that, as long as I like what I’m doing, it’s the right strategy.” She likes living in Switzerland, especially thanks to its lakes and rivers. “They remind me of the sea, I like that,” she smiles. “I’m a very normal and simple person: I like exercising, seeing my friends, and enjoying nature with my husband. And sometimes, just doing nothing.”

A first step towards scientific Independence

Ambizione grants are aimed at early-career researchers who wish to conduct, manage and lead an independent project at a Swiss higher education institution. The scheme supports researchers from both Switzerland and abroad. Afterwards, grantees have the opportunity to either continue on their academic career path or to transfer their skills outside academia.

The text of this press release, a download image and further information are available on the website of the Swiss National Science Foundation.

Wissenschaftliche Ansprechpartner:

Niéli Daffé
Paul Scherrer Institute PSI
Forschungsstrasse 111
5232 Villigen PSI
Switzerland
Phone: +41 56 310 47 60
Email: nieli.daffe@psi.ch

Weitere Informationen:

https://www.snf.ch/en/ldHdpySpZS6Sm8ih/news/she-uses-light-to-modify-matter