14 December 2022 / SNI INSight, Preise

Ending her doctoral dissertation on a high: Alexina Ollier receives an award for her presentation

Former SNI doctoral student Alexina Ollier has been presented with the Young Researcher Award at the International Vacuum Congress 2022 in Sapporo (Japan). She received the award, which is sponsored by Canon Anelva Corporation, for her presentation on the electronic properties of single-layer free graphene sheets. As part of her SNI-supported doctoral dissertation, Alexina worked with atomically thin compounds of this kind and investigated energy losses in various two-dimensional materials.

2D is unlike 3D
Two-dimensional materials such as graphene or molybdenum disulfide are the subject of worldwide research — including within the SNI network, where various groups are investigating the properties of these crystalline structures. Consisting of single atomic layers, the 2D materials have completely different physical properties from those of a three-dimensional crystal of the same material and are therefore of great interest for a wide range of applications — for example, in electronics or computer technology.

Over the last four years, Alexina Ollier has been working intensively with two-dimensional materials as part of her doctoral dissertation at the SNI PhD School, where she investigated energy losses and quantum effects in 2D materials of this kind.

Pendulum mode
Alexina carried out these investigations using a special atomic force microscope (AFM) that was developed in her working group, which is supervised by Professor Ernst Meyer at the Department of Physics of the University of Basel.

The AFM features a scanning cantilever probe that swings back and forth like a tiny pendulum. In order to carry out low-temperature measurements, the researchers position this oscillating pendulum perpendicular to the sample and then bring the sample and pendulum very close to one another. When an electric field is applied to the sample, energy losses lead to an interaction between the sample and cantilever probe, such that the oscillation of the probe decreases. Inside the AFM, this reduced oscillation can be offset by excitation to a higher energy level. The amount of excitation required is correlated with the energy loss, which can therefore be calculated.

In a short video, Alexina explains how she studied energy loss in two-dimensional materials during her doctoral thesis at the SNI PhD School.

Energy loss and quantum effects
This method is so sensitive that it can be used to investigate energy losses — also known as dissipation — in various two-dimensional samples. Researchers can also image quantum effects in order to obtain vital information and improve their understanding of 2D materials.In the prize-winning presentation that Alexina Ollier delivered at the International Vacuum Congress 2022 in Sapporo (Japan), she specifically presented the properties of single-layer free graphene.

As part of her doctoral dissertation, Alexina also investigated molybdenum disulfide and a special double layer of graphene. This “twisted bilayer graphene” is made up of two graphene layers that are twisted relative to one another through the “magic angle” of 1.1°. What makes this material so special is that it exhibits various quantum mechanical phenomena — such as superconduction, for example, in which electric current can flow with no loss of energy.

Through her investigations with the AFM, Alexina was able to show that the three different 2D materials are very different from one another both in terms of energy loss and when it comes to quantum mechanical effects.

The free-floating single-layer graphene behaves in a similar manner to quantum dots. In other words, the movement of the electrons is highly restricted, and their energy cannot adopt continuous — but rather only discrete — values. In twisted bilayer graphene, Alexina observed that the energy losses oscillate as a function of the external magnetic field, probably as a result of quantum mechanical interference effects.

Within an atomic layer of molybdenum disulfide, she then also identified a phase transition between ferromagnetism and paramagnetism. In paramagnetism, magnetization occurs only in the presence of an external magnetic field, whereas in ferromagnetism, magnetization persists for some time even without external magnets.

Deeper knowledge and greater confidence
In the meantime, Alexina has successfully defended her doctoral dissertation. The young researcher is keen to continue her research activity as a postdoc in another laboratory, and she is also excited to get to know another country and culture.

“When I came to Switzerland from France after my master’s, I noticed a lot of differences,” says Alexina. “In Basel, I enjoyed not only the freedom I had in my work but also the fact that my opinion really mattered.”

All in all, Alexina thoroughly enjoyed her time in Ernst Meyer’s team and at the SNI PhD School. She valued the numerous opportunities to present and discuss her work at the “Nanoscience in the Snow” Winter School and the Annual Event, and says she also learned a great deal at workshops such as the rhetoric course. “Because everyone got along so well, courses like these were also an opportunity to give each other open and honest feedback, which I found very helpful. As a result, I’m now able to present my work with certainty and self-confidence.”

The recent award in Sapporo serves to confirm Alexina’s appraisal. Not only has she produced some fascinating scientific insights in her doctoral dissertation, but she can now also present these findings in a clear and entertaining manner.

“Using pendulum AFM, Alexina Ollier was able to explore exciting questions relating to two-dimensional materials — and she discovered new physical phenomena in the process. It was amazing to see how she established numerous contacts within the SNI network, which was ultimately the key to her success.”

Professor Ernst Meyer, Department of Physics of the University of Basel

Further information:

Research group Ernst Meyer
Video with Alexina Ollier