We congratulate Dr. Gerson Mette, former PI of SFB project B5, on completing his habilitation in experimental physics at the Philipps-University Marburg.

Dr. Gerson Mette studied physics at the Philipps-University Marburg and finished his PhD in the group of Prof. Höfer in 2012. After working as a postdoc at the University of Zurich for two years, he went back to Marburg and became a research associate in 2015 while simultaneously joining the SFB 1083 as a young researcher and co-PI of project B5.

With his broad background in surface science and laser spectroscopy, he has set up new SHG imaging microscopy for pump-probe experiments of van der Waals heterostructures and explored the dynamics of charge-transfer processes across interfaces of 2D materials in well-defined environments. Furthermore, he explored the influence of electronic interface states on the ultrafast charge-transfer at buried GaP/Si interfaces.

In February 2022 he gave his habilitation talk on “How big is the proton? The proton radius puzzle” and completed his habilitation in experimental physics. The members of the SFB thank Dr. Mette for his work and commitment for the SFB 1083 and wish him all the best on his future career path.

The Ultrafast Quantum Dynamics group of Ermin Malic (Project B9) together with Rudolf Bratschitsch from the University of Münster revealed unexpected transport behavior of excitons in ultrathin semiconductors

Adapted from Rosati et al. (full citation see below) licensed by CC BY 4.0.

Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by applying external electric fields. However, the optoelectronic properties of the emerging material class of atomically thin semiconductors are governed by tightly bound excitons. These are neutral Coulomb-bound electron-hole pairs and as such their propagation cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate the propagation of excitons in these technologically promising materials. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Excitons have been observed to propagate towards spatial regions with the strongest strain gradient, where the energy is minimal. However, the transport of dark excitons, which govern the optoelectronic response of these materials, has remained literally in the dark up till now.

In this joint theory-experiment work, the research groups of Ermin Malic and Rudolf Bratschitsch combined spatiotemporal photoluminescence measurements with microscopic many-particle theory to track the way of excitons in time, space and energy. They found that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be traced back to the dominating role of propagating dark excitons, which possess an opposite strain-induced energy variation compared to bright excitons. The findings open new possibilities to control the transport in materials dominated by excitons.

See also the press release by Philipps-University Marburg (in German).

Publication

R. Rosati, R. Schmidt, S. Brem, R. Perea-Causín, I. Niehues, J. Kern, J.A. Preuß, R. Schneider, S.M. de Vasconcellos, R. Bratschitsch, E. Malic
Dark exciton anti-funneling in atomically thin semiconductors
Nat. Commun. 12 (2021) 7221 DOI:10.1038/s41467-021-27425-y

Contact

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28-22640
EMAIL

In their combined experimental and theoretical study published in The Journal of Physical Chemistry Letters, the groups of Caterina Cocchi and Gregor Witte investigated the nature of charge transfer excitons in crystalline PEN:PFP heterostructures.

Absorption and schematic representation of CTX that are only formed in crystalline solids and not in dimers (Image: D. Günder, Reprinted with permission from J. Phys. Chem. Lett. 2021, 12, 40, 9899–9905. Copyright 2021 American Chemical Society.)

Charge-transfer excitons (CTX) at organic donor/acceptor interfaces are considered important intermediates for charge separation in photovoltaic devices. While typically blends are used in real solar cells, their mostly amorphous arrangement prevents microscopic insights into the nature of such CTX states. In contrast, crystalline model systems allow to derive structure-property interrelations and also enable detailed theoretical modeling based on the known molecular arrangement.

In this study Prof. Witte and coworkers characterized the CTX of the prototypical molecular donor/acceptor system pentacene:perfluoropentacene (PEN:PFP). Using template controlled co-crystalline films of different orientation, allowed to precisely determine the polarization of the CTX state from angular-resolved UV/Vis absorption spectroscopy. Complementary, this co-crystalline system was analyzed theoretically in the group of Prof. Cocchi (Oldenburg) by first-principles many-body calculations and solving the Bethe-Salpeter equation, which confirms that the lowest-energy excitation is a true CTX state with a polarization along the molecular stacking direction. In addition, it was shown that analogous simulations performed on bimolecular clusters are unable to reproduce this state, which is ascribed to the lack of long-range interactions and wave-function periodicity in these calculations and represents an important finding for the description of molecular donor/acceptor systems.

Publication

D. Günder, A.M. Valencia, M. Guerrini, T. Breuer, C. Cocchi, G. Witte
Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Cocrystalline Films: Limits of Nonperiodic Modeling
J. Phys. Chem. Lett. 12 (2021) 9899 DOI:10.1021/acs.jpclett.1c02761

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083 project A2
Tel.: 06421 28-21384
EMAIL

Felix Widdascheck, Daniel Bischof and Gregor Witte developed a robust method to prepare air-stable molecular contact primer layers allowing to reduce hole injection barriers of printable silver electrodes  into organic semiconductors.

Contact engineering is an important issue for organic electronics as it allows to reduce charge carrier injection barriers. While the use of molecular contact primer layers was demonstrated in many concept studies for single crystalline model substrates, the processability of electrodes and their robustness in real devices must also be considered. Although silver electrodes can be printed using silver ink, their low work function and sensitivity to oxidation severely limits their use for printable organic electronics.

In this study Prof. Witte and his coworkers demonstrate that F₆TCNNQ monolayers provide a reliable approach to engineer high work function silver electrodes, which is examined for Ag(111) as well as polycrystalline and silver ink substrates. Notably, upon multilayer growth, a pronounced intercalation of silver into the molecular adlayer occurs, yielding thermally stabilized organometallic interphases extending over the entire adlayer. It is shown that heating allows their controlled desorption leaving behind a well-defined monolayer that is further stabilized by additional charge transfer. Such primer layers enhance the work function to 5.5-5.6 eV and can even withstand air exposure but show no interdiffusion into subsequently deposited p-type organic semiconductor, hence validating their use for organic electronic devices.

Preparation scheme of F₆TCNNQ layers on silver electrodes yielding well-defined, charge-transfer stabilized contact primer monolayers. Adapted from Widdascheck et al. (full citation see below) licensed by CC BY-NC-ND 4.0 

For further information, please see the press release by the Philipps-Universität Marburg (in German).

Publication

F. Widdascheck, D. Bischof, G. Witte
Engineering of Printable and Air-Stable Silver Electrodes with High Work Function using Contact Primer Layer: From Organometallic Interphases to Sharp Interfaces
Adv. Funct. Mater. (2021) DOI:10.1002/adfm.202106687

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB project A2
Tel.: 06421 28-21384
EMAIL

The German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) has granted the Collaborative Research Center SFB 1083 „Structure and Dynamics of Internal Interfaces“ 12.3 Million Euros for a third funding period from July 2021 to June 2025.

SFB 1083 was established at Philipps-Universität Marburg in 2013. It included a guest project from the Donostia-International Physics Center in San Sebastián, Spain. Meanwhile groups from the universities of Gießen, Leipzig and Münster as well as the Forschungszentrum Jülich participate in the center. From October 2013 to June 2021, Marburg and the participating institutions received DFG funding that amounts to 20.4 Million Euros. Together the researchers have published more than 330 scientific papers. For a report on the scientific activities from 2013 to 2019 see the activity report. A video clip, also available in German, explains the relevance of research on internal interfaces to the general public and highlights selected contributions of SFB 1083. With the new grant, SFB 1083 will be supported for altogether 12 years, the maximum funding period for a DFG Collaborative Research Center.

The 3^rd SFB funding period will bring a number of changes. Kerstin Volz will become the new spokesperson and follow Ulrich Höfer, who initiated the SFB more than ten years ago and successfully guided it in the first and second funding period. Seven projects of the 2^nd funding period will end, either because their principle investigators (PIs) reached retirement age, or because of a shift of scientific focus. Instead, eight new projects will become part of the center. Three of these new projects will be led by new PIs, namely Marina Gerhard, Jens Güdde and Ermin Malic. Altogether, SFB 1083 will consist of 19 scientific and three service projects in its last funding period. The projects will be led by 21 professors, senior scientists or junior group leaders. They will involve a total of about 80 scientists working in physics, chemistry and materials sciences.

Scientifically, the SFB will focus on a couple of new aspects in the coming years, such as the influence of the interface on lateral charge-carrier transport and the tailored synthesis at interfaces to design desired structures bottom-up. Research on interfaces of 2D materials, which started with the 2^nd funding period in 2017, will be further extended. Last but not least, applications and devices will become more in to focus. The research on novel interface-dominated lasers will be intensified, including new material systems and emission wavelengths. Moreover, strong THz-emitters based on charge-carrier recombination across interfaces are included in the program due to promising results from the 2^nd funding period. More applications and devices are envisioned, particularly as a result of research on hybrid organic/inorganic materials. Another important focus of the SFB will be on development and usage of sophisticated experimental methods, which allow unprecedented insights into processes at the nanoscale across interfaces.

Present spokesman Ulrich Höfer (left) and future spokeswoman Kerstin Volz in front of a poster introducing SFB 1083, Foto: Stefan Kachel.

See also Press Release of Philipps-Universität Marburg (in German) and the German Research Foundation (DFG) for more detail.

Contact

Prof. Dr. Kerstin Volz
Department of Physics and Materials Science Center
Philipps-Universität Marburg
Tel: + 49 6421 28-22297
Email: kerstin.volz@physik.uni-marburg.de