Polarons play a role in a range of important technologies

Martin Setvín has had a great stretch over the last few years. In 2023, he received an ERC CZ grant for research on polarons in solids. Then, in the autumn, the journal Science Advances published the latest discoveries of his scientific group. And 2024 concluded with a prestigious ERC Consolidator Grant! “Polarons are electrons that 'get trapped' in a crystalline lattice, affecting properties like electrical conductivity, catalytic features of materials, or even enabling high-temperature superconductivity,” explain members of the research team, (from right) Martin Setvín, Jesus Redondo, and Pavel Kocán from the Faculty of Mathematics and Physics. To study these specific electrons, they use a unique method and microscope at the Department of Surface Physics and Plasma Science, Faculty of Mathematics and Physics at Charles University, which allows them to measure with accuracy down to a hundredth of an atom.

The research, the latest results of which were published in Science Advances, was made possible by last year's ERC CZ grant. How does all this relate to the current ERC Consolidator Grant?

Martin Setvín: There are various ways to answer that (laughs). The results published in the journal began to come together about five or six years ago at my previous position at the Technical University of Vienna. The breakthrough experiments were carried out within the GAČR EXPRO project, which allowed me to return to Prague and set up a laboratory. These results were so promising that they enabled me to secure the large ERC CZ project focused specifically on polarons. And now, of course, we have also secured the ERC Consolidator Grant. (The ERC CZ program is a national initiative by the Czech Republic's Ministry of Education, Youth, and Sports designed to support excellent research within the country. It specifically targets projects that were submitted to the European Research Council calls, evaluated in the second round, and categorized as A or B, but did not receive ERC funding due to budget constraints – editor’s note).

Pavel Kocán: Research that uncovers entirely new and unexpected physics is usually funded by whatever resources you have at that moment. Once you have promising results, securing a new grant for this specific discovery becomes much easier.

What exactly are you working on now?

PK: We're trying to image individual polarons in real space and track their movement, which will allow us to obtain complete information about their properties. We use a very sensitive microscope that lets us observe the arrangement of individual atoms.

Jesus Redondo: We can also track the behaviour of electrons, and we do this in a completely different way from anyone else. We select materials in which we transfer and localise an electron using the microscope's tip. If we want to move it elsewhere, we apply thermal or optical activation. We're interested in how the electron behaves within the material.

Why is it that you are among the few scientists in the world using this method?

MS: There are only about one to two hundred instruments in the world that allow for this kind of work, but they are mainly used for studying organic molecules. Very few people use them to study materials like we do. Simply put, it's a relatively new method, and understanding the intricacies of this issue isn't easy—it takes time.

Why did you choose to study polarons?

PK: Because of their very specific properties. A polaron is an electron that stops in the material, instead of spreading like a wave. In recent years, it has become clear that polarons play a crucial role in a range of important technologies such as photocatalysis, the electrical properties of materials, and optical properties.

Because you're among the few scientists studying polarons, you're also bringing completely new insights about them, correct?

PK: Polarons have been known to exist for nearly 100 years, but all the experimental data so far has been indirect. We can now look at individual polarons with a microscope and gather entirely new information.

MS: For example, we don’t know how localised their charge is. Whether it resides on a single atom, or whether it’s larger... And how the polaron moves impacts the material's properties. The current theories aren't yet advanced enough for us to predict individual phenomena, or even explain those that already exist.

What is your next step?

JR: We want to try to improve the resolution of the method. In our latest publication, we were able to work with an "electron cloud," and now we're focusing on obtaining local information on how individual electrons move.

Does this depend on new equipment or the measurement duration?

PK: It's everything you just mentioned (laughs).

JR: There are various types of interactions (electrostatic, van der Waals, ionic, covalent, and hydrogen bonds), and we measure the sum of them all. We want to isolate the tiny force corresponding to the polaron.

Do you feel like you are pioneers?

MS: In this part of the research, yes. The Vienna group, where I was previously based (we previously wrote about Martin Setvín’s work in Professor Ulrike Diebold’s group at the Technical University of Vienna – editor’s note), is also far advanced in this field, but they focus on something slightly different.

Can your results be applied practically?

MS: Our research focuses on surface chemistry of materials used in catalysis, photocatalysis, or electrocatalysis, where charge movement occurs. The extent to which the movement of this charge is restricted determines the effectiveness of the device. Our findings could be used, for example, in the transition to renewable energy sources, so they have considerable potential.

Do you feel like you're just at the beginning of something big, with many more discoveries ahead?

MS: We have a plan (laughs). At the beginning of October, I defended an ERC grant proposal, for the second time, on polaron research.

We now know your ERC Consolidator Grant was approved. What led up to it?

MS: Panels consisting of experts from various fields (from biology to chemistry and "our" physics) had to write reports justifying their decisions. All of these decisions were then approved by the ERC committee. Only after that were the final decisions announced to the applicants.

How will the ERC grant help you? What's your biggest concern?

MS: In recent years, the biggest bottleneck or sticking point has been equipment. We currently only have one effective microscope. Thanks to the ERC grant, we can acquire another high-quality device and secure five years of research funding.

Did you feel during the autumn interview, when you made it to the second round of the selection process, that this time the ERC Consolidator might work out?

MS: I had neutral feelings about it. My experience from last year, when I also applied for an ERC grant, tells me that my feelings don't correlate with the outcome. We could keep working and publishing without the ERC. We're part of the large OP JAK project, which gives us confidence. The research conditions at the faculty are very good. But of course, getting an ERC grant is rewarding and prestigious – it opens doors to interesting contacts and collaborations.

Since you're among the first to conduct polaron research, you probably don’t need to worry about someone beating you to the punch.

JR: That's true. On the other hand, precisely because the properties of polarons are still poorly understood, many things could surprise us during our work. You never know in advance what you'll measure – particles always behave slightly differently.

PK: That's why it's good to work on multiple projects simultaneously and spread them according to their risks. It's not possible to focus only on the most potentially interesting ones—they might not pan out.

M. Setvín: Also, as I mentioned earlier, truly groundbreaking work takes years.