Complex molecules on surfaces – Sebastian Scherb receives the prize for the best master’s thesis in nanosciences

Sebastian Scherb investigated a new method for the surface deposition of individual complex molecules and will be awarded the prize for the best master’s thesis in nanosciences. He has started his PhD project at the Department of Physics at the University of Basel.

At the first master’s degree ceremony for nanosciences this fall, Sebastian Scherb will be awarded the prize for the best master’s thesis of 2018. As part of the team led by Professor Ernst Meyer, he investigated a new method for the surface deposition of individual complex molecules.

Seeking a method for large molecules
The self-assembly of molecules on surfaces can be used to create atomically precise functional surfaces with applications in many different fields, such as electronics. For smaller molecules, thermal evaporation is an established method of creating atomic coatings on surfaces such as gold or potassium bromide, but large, complex molecules generally do not survive being heated to boiling point, which is a key part of the process.

Now, in his prizewinning master’s thesis, Sebastian Scherb has investigated whether electrospray deposition can be used as an alternative. In this method, the molecules are converted from the liquid to gas state using a charge-transfer process. Then, in an ultrahigh vacuum, a differential pumping system sprays the individual molecules onto the surface, where they self-assemble into molecular layers.

A focus on graphene-like compounds
The focus here was on two graphene-like compounds that promise a wide range of applications thanks to their special electronic properties. The first, graphylene-1, is a hexagonal molecule with outer alkane chains that resembles a wagon wheel. Into this wheel, it is then possible to incorporate functional groups that give the molecule a specific set of properties. For the other compound, Sebastian studied graphene ribbons with lengths of up to 600 nanometers (graphene nanoribbons).

“It was hugely exciting for me to investigate how these complex molecules behave on different surfaces,” he says when asked about which part of the work fascinated him the most. Sebastian not only studied graphylene-1 molecules at room temperature on gold and potassium bromide surfaces using a non-contact atomic force microscope, but also prepared them for atomic force and scanning tunneling microscope studies at low temperatures (5 Kelvin).

Sebastian Scherb imaged graphylene-1 molecules on a gold surface (Image: Sebastian Scherb, Department of Physics, University of Basel)

Unexpected results
When the images were compared, they revealed an interesting and unexpected side effect: at low temperatures, the individual graphylene molecules were much closer together than at room temperature. Simulations performed after the master’s thesis suggest that the molecules are forced apart at higher temperatures due to increased mobility of their side chains. The extremely weak van der Waals forces responsible for holding the molecular layer together permit these different variations depending on the temperature.

Professor Ernst Meyer, who leads the group in which Sebastian completed his thesis, was very impressed with the results: “Using the electrospray method, Sebastian succeeded in preparing large molecules shaped like a wagon wheel under ultrahigh vacuum conditions. He was then able to image these molecules using high-resolution atomic force microscopy – and, interestingly, he discovered that the materials have an exceptionally high coefficient of thermal expansion.”

Sebastian found this work so exciting that he stayed on in Ernst Meyer’s team after finishing his master’s and will now complete his doctoral dissertation there, too. “I really enjoyed taking these measurements. Besides, the group works together very effectively and has a really great atmosphere,” he says in the interview.

Originally interested in chemistry
Seven years ago, when Sebastian attended an information day at the University of Basel shortly before completing his high school diploma, he couldn’t have imagined that he’d now be spending many of his working days in the basement of the Department of Physics, enthusiastically analyzing molecules deposited on surfaces. “I had actually wanted to study chemistry,” he recalls. “But when I attended the information day, I enjoyed the introduction to the nanosciences degree so much that I chose to do that instead.”

Sebastian never looked back, but he also went through some difficult times during his studies. As a pupil at a secondary school that teaches classical languages in Lörrach, he dropped biology at an early stage and therefore had some catching up to do at university. “Sometimes you just have to get through these things. But it was worth it in the end, because my degree provided me with insights into so many different areas, and I can now apply that knowledge in my work.”

During the block courses in particular, Sebastian familiarized himself with several different subject areas and working groups within the SNI network. He was especially interested in cold atoms and work involving the AFM – and these were also the topics of his two undergraduate projects. However, the practical applications of the AFM ultimately proved more fascinating to him than cold atoms, setting the stage for this excellent master’s thesis.

We would like to congratulate him on receiving this prize – and we look forward to more amazing images of complex molecules deposited on surfaces.

News from the group of Prof. Ernst Meyer, where Sebastian completed his master’s thesis and now works on his PhD thesis. Website of Ernst Meyer’s group − Nanolino