Dark exciton anti-funneling in atomically thin semiconductors – Publication by B9 (Malic) in Nature Communication

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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

Publication of new SFB 1083 Image Brochure

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SFB 1083 published a new image brochure introducing the projects and the principle investigators in the third funding period.

Cover of the image brochure of the third funding period. Design by Bosse&Meinhard.

In October 2021, the SFB 1083 updated its image brochure to feature the goals and the focus of the research center in the third funding period. The image brochure gives a general introduction to the research on internal interfaces and portraits the participating researchers mainly for interested students and for the general public.  The numbers on the SFB for the past two as well as the current funding period can also be found in the booklet.

The image brochure (German) can be downloaded here.

A printed version of the image brochure is available upon request.

Contact

Sonderforschungsbereich 1083
Philipps-Universität Marburg
Hans-Meerwein-Str. 6
35043 Marburg
Tel.: 06421 28-24223
EMAIL

 

Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Cocrystalline Films: Limits of Nonperiodic Modeling– Publication by A2 (Witte)

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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