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Interview: Dr. Andre Maier and Prof. Marcus Scheele

Tell us about your journeys as scientists up to now.

Marcus: I have been fascinated by the chemistry and physics of semiconductor nanocrystals for over 15 years now. In my early career, I was fortunate enough to work with and learn from leading scientists in the field such as Horst Weller, Dmitri Talapin and Paul Alivisatos. Some of the ideas we currently pursue in my group still find their origins in those early times. In my opinion, this is a perfect example of how innovations in science emerge from a frequent exchange of ideas and people among institutions.

Andre: I graduated in what was then the newly founded Nano-Science program at the University of Tübingen. This strongly interdisciplinary program sparked my interest in a wide variety of nanotechnology research areas, which was a reason why I opted for a research visit in the group of Prof. Ulrich Keyser at the University of Cambridge, where they focus on DNA nanotechnology and nanopore sensing. Back in Tübingen, I joined the research group of Prof. Marcus Scheele as a PhD student in 2018. I focused on the electronic and structural characterization of novel nanomaterials based on self-assembled nanocrystals and nanoclusters. In particular, I investigated the structure-transport properties that are of fundamental importance for new solution-processed semiconductor devices. After obtaining my PhD in 2021, I continued as a postdoc in the same group.

What are you working on at the moment?

Marcus: Having worked predominantly on the synthesis of novel semiconductor nanocrystals for most of my early career, I am convinced that the field is now mature enough to use these materials for actual applications with real market opportunities. The successful launch of the so-called Q-LED displays, which use semiconductor nanocrystals for bright colour pixels, is most likely just the beginning. Besides these LEDs, the research group currently looks into materials for fast optical switching to be used for optical transceivers. These devices are pivotal interconnects between optical fibers and computer end-stations: improving their speed of operation will help increase data transmission rates. This project is generously funded by the European Research Council with a Starting Grant that extends until 2024.

Andre: As a postdoc in the group of Marcus Scheele, I have developed a setup to characterize ultrafast photodetectors based on colloidal nanocrystals. Due to their size-tunable properties and their fabrication from solution, colloidal nanocrystals are promising candidates for photodetector materials, which are required in various technological applications including optical communications. We implemented a characterization method based on the pump-probe two-pulse coincidence photoresponse technique along with asynchronous optical sampling (ASOPS), where the UHFLI Lock-in Amplifier from Zurich Instruments plays an important role. With this technique, we can unravel the intrinsic temporal response of the developed photodetectors in the range of pico- to nanoseconds.

Andre, you’ve just referred to the UHFLI – can you say a bit more about the role of the instrument in this research?

Andre: We use the UHFLI to perform ultrafast pump-probe measurements that investigate the intrinsic response time of photodetectors conceived to be used as optical transceivers. Our devices are excited by two femtosecond laser pulses separated by a varying time delay. Usually, the delay between the pulses is generated by changing the optical path length stepwise through a mechanical delay line. In this way, it is possible to achieve typical time delays of up to a few hundred picoseconds; the pump-probe measurements are acquired point-by-point by gradually varying this delay and recording the corresponding photoresponse. This results in hours of data acquisition time for a single measurement.

With our ASOPS approach, we use two electronically coupled lasers with a slight offset in the repetition rate (frep + Δf) that causes an accumulating time delay. The UHFLI, with its periodic waveform analyzer (PWA) function, is locked on the detuning frequency Δf to perform pump-probe photoresponse measurements with ASOPS. This setup allows us to achieve delay windows of 10 ns with a scan resolution of 10 fs within only 10 ms of acquisition time, as the periodic signal can be recorded in real-time thanks to the PWA. This means that a measurement takes seconds instead of hours.

What is especially promising about this research area in your view?

Andre: Photodetectors convert light into electrical signals, and this is a key capability for optical communications. Indeed, optical transceivers ‘translate’ the arriving optical data from optical fibers into electrical data that can be processed by conventional computers. As this translation step is the bottleneck for faster communications, materials with a high-speed operation frequency are highly sought after. Next to 2D materials such as graphene and transition metal dichalcogenides, solution-processed low-cost nanocrystal materials stand as another valid option.

What do you like to do in your free time?

Marcus: I play basketball and Go. I spend a lot of time at my desk, so running down a floor in a group of five and trying to figure out how to get past a well-organized defence is a great way to balance that sitting time. Go is the most effective way to clear my mind, and it teaches me a lot about decision-making: in Go there is no point in regretting a previous debatable decision for every move that follows is full of possibilities. This kind of experience is also helpful for planning a career in science, although coincidence is certainly a bigger factor than in Go.

Andre: When I am not in the lab or at my desk, I like to spend my time hiking, mountain biking, snowboarding, hitting the gym, or enjoying a dinner with my friends – these are all activities that allow me to relax after a hectic day.

Andre Maier

Dr. Andre Maier, postdoctoral researcher in the Physical Chemistry of Nanocrystals group at the University of Tübingen, Germany

Marcus Scheele

Prof. Marcus Scheele, principal investigator of the Physical Chemistry of Nanocrystals group at the University of Tübingen, Germany

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