Kenta Kuroda receives Young Scientist Award of the Physical Society of Japan

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Kenta Kuroda receives Young Scientist Award of the Physical Society of Japan (Photo: Univ. of Tokyo).

We congratulate Dr. Kenta Kuroda, former JSPS fellow and guest scientist of project B6 (Höfer) on receiving the prestigious “Young Scientist Award” of the Physical Society of Japan for his “Research on spin-polarized surface electrons and their light control in topological insulators using photoelectron spectroscopy”.

The prize is awarded annually in recognition of an outstanding young researcher who promises to make a lasting contribution to the future of physical research in Japan. It was given to Dr. Kuroda at the Annual Meeting of the Physical Society of Japan held at the Tokyo University of Science on March 22, 2018.

In work conducted in Marburg in 2014-15 [1] and continued at the Institute for Solid State Physics of the University of Tokyo [2], he demonstrated by precise measurements of topological insulators using photoelectron spectroscopy that light is able to control the spin-polarized Dirac surface electrons. In previous research conducted as a PhD student at Hiroshima University Kuroda had already characterized the electronic structure of these special surface electrons as Dirac particles [3].

Publications

  1. K. Kuroda, J. Reimann, J. Güdde, and U. Höfer
    Generation of Transient Photocurrents in the Topological Surface State of Sb2Te3 by Direct Optical Excitation with Midinfrared Pulses
    Phys. Rev. Lett. 116, 076801 (2016).
  2. K. Kuroda, K. Yaji, M. Nakayama, A. Harasawa, Y. Ishida, S. Watanabe, C.-T. Chen, T. Kondo, F. Komori, and S. Shin
    Coherent control over three-dimensional spin polarization for the spin-orbit coupled surface state of Bi2Se3
    Phys. Rev. B. 94, 165162 (2016).
  3. K. Kuroda, M. Arita, K. Miyamoto, M. Ye, J. Jiang, A. Kimura, E. E. Krasovskii, E. V. Chulkov, H. Iwasawa, T. Okuda, K. Shimada, Y. Ueda, H. Namatame and M. Taniguchi
    Hexagonally Deformed Fermi Surface of 3D topological Insulator Bi2Se3
    Phys. Rev. Lett. 105, 076802 (2010).

Student Poster Day

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SFB’s student speaker organized a “Meet and Greet” Poster Event for all the new – and old – PhD-students and young postdocs of the second funding period and in particular the new members from Gießen, Jülich and Münster to meet the others from Marburg.

With snacks and beer to break the ice, it was a lively 3 hours of dicussion and scientific exchange across project-boundaries and disciplinary constraints. More opportunities for inner-SFB networking will arise at the 2nd SFB/GRK-Seminar in September and the 3rd Winter Student Seminar in February 2019.

Dynamics of charge transfer excitons at PFP/PEN interfaces – Publication by B2 (Chatterjee) and A2 (Witte)

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Members from projects B2 and A2 explored the dynamics of charge-transfer excitons at the internal interface in heterostructures of the organic donor-acceptor molecules pentacene and perfluoropentacene.

Reprinted with permission from A. Rinn et al, 2017, 9, 48, 42020–42028. Copyright 2017 American Chemical Society.

Organic photovoltaics is a vivid research field as it promises the fabrication of large scale and thin film devices on flexible supports. One of the key challenges is the separation of optically excited bound electron-hole pairs, so-called excitons. Of particular relevance are charge-transfer (CT) excitons at donor-acceptor interfaces with the electron in the acceptor and the hole in the donor molecule as they are prime candidates as intermediaries for charge separation. Studying these CT excitons in state-of-the-art devices is challenging as these consist of blends of molecular donors and acceptors in order to provide maximum internal interface area. The resulting, complex interface geometry structure hampers microscopic characterization of such CT-excitons and, in particular, denies a well-defined correlation of the electronic properties with the molecular packing at the interface. Hence, detailed studies on the underlying physical mechanism of charge-separation in organic photovoltaics rely on highly-ordered model heterojunctions.

In the present study, the researchers combine their expertise in the fabrication of such highly ordered crystalline molecular heterostructures with precise, time-resolved optical microspectroscopy to study the energetics and dynamics of CT-excitons at donor-acceptor interfaces for selected pentacene / perfluoropentacene heterostructures. Based on previous work, various molecular heterostructures with different molecular orientation are realized and further compared with the dynamics of blends, i.e., completely intermixed heterostructures, as well as the respective pure materials.

The spatial separation of electrons and holes in the CT-excitons leads to extended lifetimes compared to the pristine species. Intriguingly, the energetics reveal that the common description of such excitons based on a straight-forward orbital picture as a mere linear combination of the involved individual constituents is insufficient and a more intricate description of CT-excitons is needed. The present study provides the first experimental data systematically investigating such excitations in highly-ordered crystalline molecular heterostructures, which will enable further theoretical calculations on the involved electronic effects.

Publication

Andre Rinn, Tobias Breuer, Julia Wiegand, Michael Beck, Jens Hübner, Robin C. Döring, Michael Oestreich, Wolfram Heimbrodt, Gregor Witte, Sangam Chatterjee, Interfacial Molecular Packing Determines Exciton Dynamics in Molecular Heterostructures: the Case of Pentacene – Perfluoropentacene,
ACS Applied Materials & Interfaces, (2017) DOI: 10.1021/acsami.7b11118

Contact
Prof. Dr. Sangam Chatterjee
Institute of Experimental Physics I, Justus-Liebig-University Giessen
SFB 1083 Project B2 (Chatterjee)
https://www.uni-giessen.de/ipi
Tel. +49 (0)641 99-33100
Email: Sangam.Chatterjee@physik.uni-giessen.de

Prof. Dr. Gregor Witte
AG Molekulare Festkörperphysik, Philipps-Universität Marburg
SFB 1083 Project A2 (Witte)
https://www.uni-marburg.de/sfb1083/projects/A2
Tel. +49 (0)6421 28-21384
Email: Gregor.Witte@physik.uni-marburg.de