Congratulations to Dr. Johannes Reimann, doctoral student in SFB-project B6 (Höfer), for being awarded a prize by Philipps- Universität Marburg for his excellent dissertation presented in 2018.

In his thesis entitled “Charge carried dynamics and photocurrents in the Dirac cone of topological insulators” Johannes Reimann investigated a novel class of materials, topological insulators. These materials, discovered only a decade ago, are insulating in the volume, but conductive at their surfaces and at their interfaces with conventional materials.

In the framework of his thesis, Johannes Reimann advanced the development of time- and angle-resolved photoelectron spectroscopy within the group of Prof. Höfer. In particular, his work is the first to combine this powerful technique with Terahertz excitation and to achieve subcycle time resolution. In collaboration with the group of Prof. Rupert Huber in Regensburg, he succeeded in taking band structure movies of electrical currents carried by Dirac electrons as they are driven by an intense THz wave. First results were published in Nature in September 2018 (see also SFB news, university press release.

The results of Johannes Reimann’s work hold great promise to realize new lightwave-driven electronics, a concept to increase the clock rates of conventional semiconductor devices by a factor of 1000 and more. Moreover, the successful demonstration of the combination of intense THz pulses as pump and angle-resolved photoelectron spectroscopy (ARPES) as probe, has triggered worldwide experimental efforts to take advantage of THz-APRES for time-resolved investigation of a variety of solids, surfaces and interfaces.

See here for details of the event.

In their review paper, Andreas Beyer and Kerstin Volz describe in detail their investigation of novel composite materials, which may eventually replace today’s silicon-based electronic devices. As the latter increasingly reach their performance limit, one option to overcome these largely physics-based limitations is to cover silicon with a different material layer.

However, covering silicon with different material layers like, for example, well-suited III/V semiconductors (containing elements of the 3^rd and 5^th group of the periodic system) is challenging. In joining different materials with their individual physico-chemical properties their interface may be marked by defects. Here, for example, “erroneous” attachments may lead to unwished-for local charges – rendering the combined material as unsuitable for application in devices.

The group of Kerstin Volz closely studied galliumphosphide on silicon as a model system of III/V semiconductors on silicon. In their invited review the authors now describe the various electron-microscopy-based approaches employed in the study of the internal interface between the two materials and its defects. By means of transmission-electron-microscopy the researchers were able to show that the interface between the two materials is far from smooth; in fact, it more resembles a pyramidal structure affecting several atomic layers. In addition, it was also possible to “see” the erroneous atomic attachments which cause the unwished-for charge effect and link the phenomenon directly to changes in preparatory procedures.

The insights gained will be applied to perfecting preparation methods in order to reduce the number of defects and to fine-tuning the interface with a focus on raising the efficiency of existing devices and encouraging the development of novel applications.

Publication

A. Beyer and K. Volz,
Quantitative Electron Microscopy for III/V on Silicon Integration
Adv. Mater. Interfaces (2019) DOI: 10.1002/admi.201801951

SFB 1083’s Ö-project closely cooperates with Chemikum Marburg e.V. by installing at its premises several new experiments and workshops providing hands-on insights into the SFB’s research objective and the study methods employed. In 2019, for a second time, the partnership supported Girls’ Day activities with dedicated offerings.

Girls’ Day 2019 with its 50 young (agegroup 10-16) female participants benefitted from these offerings, which included the application of acid-base-reactions, measuring thermal signatures of chemical reactions and electrical conductivity in various substances. The experiments provide insight into the central research aspect of SFB 1083: “What are the reactions taking place at the interface, that is the contact between two materials?”

“Girls’ Day allows us to showcase how researchers work. In interesting experiments and workshops we can entice enthusiasm for MINT-disciplines in female pupils”, state Prof. Stefanie Dehnen and Dr. Christof Wegscheid-Gerlach, director and co-director of Chemikum Marburg and principal investigators of SFB 1083’s Ö-project.

A special informatics-focused workshop, in cooperation with Michael Szabo (Fachdidaktik Informatik PUM/MLS), showcased how disciplines work together and how modern research needs detailed programming for optimal analytical results. The SFB’s Atomic Force Microscope (AFM), for example, needs a complex range of operational programming to realize its full potential. This was demonstrated using the Lego-model and it rapidly became obvious to the riveted audience how programming controls the instrument’s operation. 18 girls were then guided in developing their own little programs using the language scratch. At the end of the 3-hour workshop the young participants had all succeeded in letting their “dog run around the lake” and teasing out the impact of minor changes to their code.

See also a press release in German.

In a joint effort, the groups of SFB-projects A8 and B5 used scanning tunneling microscopy for controlling the final products of a textbook-type reaction of organic molecules on silicon surfaces. A detailed understanding of the driving forces of these reactions are obtained by means of energy decomposition analysis as developed in SFB-project A6.

Ether cleavage on silicon is the surface analogue of an SN2 reaction; SN2 reactions represent the textbook example for how to control solution-based chemical reactions by means of steric hinderance or the choice of solvent. In an STM study published in Angewandte Chemie, the team around Gerson Mette and Michael Dürr has now shown that tip-induced ether cleavage on Si(001) leads to additional final products which are not obtained by thermal activation. Moreover, different final products can be selectively addressed by different excitation channels, either direct excitation by electron transfer or multiple excitation of vibrational modes. As the two channels can be selectively addressed by the tunneling bias, a new way of reaction control was achieved.

In parallel, the advances in the theoretical description of these systems are illustrated in a review article by Lisa Pecher and Ralf Tonner. Within the framework of density functional theory, the chemists of A8 successfully applied energy decomposition analysis to extended systems in order to derive bonding concepts for molecules on surfaces. This allows to interpret experimental results and predict new reaction schemes.

Publications

G. Mette, A. Adamkiewicz, M. Reutzel, U. Koert, M. Dürr, and U. Höfer,
Controlling an S_N2 reaction by electronic and vibrational excitation ‐ tip‐induced ether cleavage on Si(001)
Angew. Chemie Int. Ed. 58/11 (2019) 3417-3420 DOI: 10.1002/anie.201806777
L. Pecher and R. Tonner,
Deriving bonding concepts for molecules, surfaces, and solids with energy decomposition analysis for extended systems
WIREs Comput. Mol. Sci. (2018) (21pp) DOI:10.1002/wcms.1401

See also joint press release by the universities of Gießen and Marburg under the auspices of the Forschungscampus Mittelhessen ( in German).

As part of a special event organized for the last day of November, Prof. Dr. Stefanie Dehnen, PI of SFB-project A9 and Professor of Inorganic Chemistry, and Prof. Dr. Carmen Birkle, Professor for American Studies, jointly received the “Frauenförderpreis” of the Philipps-Universität Marburg.

The “Frauenförderpreis” of Philipps-Universität Marburg is awarded every two years since 1998 and worth 2500 EUR. It recognizes Prof. Dr. Stefanie Dehnen and Prof. Dr. Carmen Birkle for their strong mentorship and in the case of Professor Dehnen in particular for her participation as a mentor for early-career female academic staff in the Hessen-wide project “SciMento” and her enduring engagement for a family-friendly research environment.

In balancing a professorship and family-life with four young children, Stefanie Dehnen is living proof that having both, a research career and a family is possible.