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Obituary for Prof. Dr. Stephan W. Koch – Nachruf für Prof. Dr. Stephan W. Koch

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Die Mitglieder des Sonderforschungsbereichs 1083 „Struktur und Dynamik innerer Grenzflächen“ trauern um Prof. Dr. Stephan W. Koch, der im September 2022 im Alter von 69 Jahren verstorben ist.

Foto: Tim van de Bovenkamp; Copyright: SFB 1083

Stephan W. Koch promovierte 1979 in Frankfurt und war von 1977 bis 1984 wissenschaftlicher Mitarbeiter am Institut für Theoretische Physik der Universität Frankfurt, an dem er auch bereits 1983 habilitierte. Nach drei Jahren als Stipendiat der F. Thyssen-Stiftung und der DFG im Rahmen eines Heisenberg-Stipendiums ging er 1986 als Associate Professor an das Physics Department and Optical Sciences Center der University of Arizona in Tucson, wo er 1989 zum Full Professor ernannt wurde. 1993 kam er dann nach Marburg und übernahm in Nachfolge von Otfried Madelung und Stefan Schmitt-Rink den Lehrstuhl für Theoretische Festkörperphysik an der Philipps-Universität Marburg, wobei er weiterhin Adjunct Professor an der University of Arizona blieb. Diesen beiden Stationen blieb er bis zu seiner Pensionierung treu.

Seine Forschungsschwerpunkte lagen auf dem Gebiet der theoretischen Festkörperphysik. Besonders interessierten ihn hier die theoretischen Grundlagen der Wechselwirkung von Licht mit Materie in Halbleitermaterialien sowie in Laserstrukturen und Mikro-Resonatoren.

Stephan W. Koch war ein herausragender, international hoch angesehener Wissenschaftler. Durch seine Arbeiten zur Vielteilchenphysik und den optoelektronischen Eigenschaften von Halbleitern trug er maßgeblich zum heutigen Verständnis von Festkörpern und insbesondere Halbleiternanostrukturen bei. Für seine Leistungen wurde er 1997 mit dem Leibniz-Preis der Deutschen Forschungsgemeinschaft (DFG) sowie 1999 mit dem Max-Planck-Forschungspreis der Alexander-von-Humboldt-Stiftung und der Max-Planck-Gesellschaft ausgezeichnet.

Stephan Koch hat die Forschungslandschaft der Marburger Physik entscheidend mitgeprägt. Von 1995 bis 2001 leitete er den erfolgreichen SFB 383 „Disorder on mesoscopic scales“ und war seit 2013 Mitglied des SFB 1083 „Structure and dynamics of internal interfaces“.

Stephan Koch verstand es, seine vielen Schülerinnen und Schüler für die Halbleiterphysik zu begeistern. Viele von Ihnen sind heute an Hochschulen und Forschungseinrichtungen tätig.

Mit großer Dankbarkeit und Anerkennung werden wir uns an ihn als einen stets aufgeschlossenen Kollegen erinnern, der in seinem gesamten Schaffen immer wieder neue Impulse gegeben und Zeichen gesetzt hat.

Unser tiefes Mitgefühl und unsere Anteilnahme gelten seiner Ehefrau und seinen Angehörigen.

Wir werden ihn vermissen.

 

Formation of Moiré Interlayer Excitons in Space and Time – Publication by B9 (Malic) in Nature

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A large research team including Ermin Malic and coworkers observed the formation of a “dark” moiré interlayer exciton for the first time

A large number of so-called optically dark excitons form between two twisted layers of tungsten diselenide (top) and molybdenum disulfide (bottom) after optical excitation. (Reprinted with permission from Nature, link see below)

Atomically thin structures made of two-dimensional semiconductor materials are promising candidates for future devices in electronics, optoelectronics and photovoltaics. The properties of these semiconductors can be controlled by stacking atomically thin layers on top of each other. However, the angle of rotation in the structure of the semiconductors can be adjusted as desired, and this angle of rotation is of interest for the production of novel solar cells. Typical experimental approaches have only indirect access to the moiré interlayer excitons and are blind to the ‘dark’ excitons.

An international research team including Ermin Malic and coworkers from the SFB succeeded in directly visualizing so-called dark moiré interlayer excitons by using time-resolved ARPES measurements combined with microscopic many-particle theory. The researchers show how the time-resolved momentum microscopy provides deepest microscopic insights into these technologically relevant questions.

These results not only provide a fundamental insight into the formation of dark moiré interlayer excitons, but also open up a new perspective to study the optoelectronic properties of these new and fascinating materials, e.g., the signature of the moiré potential and the influence of the combined properties of the two twisted semiconductor layers.

For further information, please see the press release by the university of Göttingen (in German).

Publication

D. Schmitt, J.P. Bange, W. Bennecke, A.A. Al Mutairi, G. Meneghini, K. Watanabe, T. Taniguchi, D. Steil, D.R. Luke, R.T. Weitz, S. Steil, G.S.M. Jansen, S. Brem, E. Malic, S. Hofmann, M. Reutzel, S. Mathias
Formation of moiré interlayer excitons in space and time
Nature 608 (2022) 499 DOI:10.1038/s41586-022-04977-7

Contact

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

Topological Stone–Wales Defects Enhance Bonding and Electronic Coupling at the Graphene/Metal Interface – Publication by A4 (Gottfried) and A6 (Tonner) in ACS Nano

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Benedikt Klein and coworkers of SFB 1083, together with external collaborators, have gained new insight into interfacial interactions of Stone-Wales graphene defects by using molecular models.

Graphene is an astonishing two-dimensional material with diverse and technologically important properties. However, these properties are heavily dependent on topological defects, which have a direct impact on the graphene/metal interface. A common defect is the Stone-Wales (SW) defect, consisting of two five- and two seven-membered rings resulting in a non-alternating bonding situation.  Researchers of the SFB 1083 projects A4 (Gottfried) and A6 (Tonner) investigated the interface between a SW defect and a metal by mimicking the defect with the molecule azupyrene. Pyrene was used as a model for defect-free graphene of the same size as azupyrene. The experiments were complemented by extensive modelling of the graphene-embedded defects.

Figure: Graphene/metal interface with typical topological defect. The local interaction of a topological S–W graphene defect with a metal surface is mimicked by azupyrene, which allow the application of a wide range of experimental techniques. Copyright 2022 American Chemical Society.

In the present work, it was shown by a multi-technique approach (XPS/UPS, NIXSW, NEXAFS, TPD, LT-AFM, DFT) that the embedded defects, modelled by azupyrene, undergo enhanced bonding and electron transfer with a Cu(111) surface. This indicated by increased bond energies of 68 kJ/mol, by 0.9 Å reduced bond distances and enhanced charge transfer. The consistent experimental results were corroborated by DFT calculations.

The defect-induced enhanced electronic coupling at the graphene/metal interface is expected to have significant impact on the performance of (opto-)electronics, e.g., by increasing charge injection rates. Tailoring the topological structure of graphene layer may result in the development of new or imprived devices.

Publication

B.P. Klein, A. Ihle, S.R. Kachel, L. Ruppenthal, S.J. Hall, L. Sattler, S.M. Weber, J. Herritsch, A. Jaegermann, D. Ebeling, R.J. Maurer, G. Hilt, R. Tonner-Zech, A. Schirmeisen, J.M. Gottfried
Topological Stone–Wales Defects Enhance Bonding and Electronic Coupling at the Graphene/Metal Interface
ACS Nano (2022) DOI:10.1021/acsnano.2c01952

Contact

Prof. Dr. J. Michael Gottfried
Philipps-Universität Marburg
SFB 1083 project A4
Tel.: 06421 28-22541
EMAIL

On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat-Lying on Various Metals and Hetero-Structures – Publication by A2 (Witte) and A8 (Koert)

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In a new publication in Advanced Materials Interfaces, the groups of Gregor Witte (A2), Ulrich Koert (A8) as well as Jérôme Cornil from the University of Mons report on the formation of an internal interface dipole at a metal/2D metal/organic hetero-interface, which can be modified by the outer organic monolayer.

Schematic representation of the outer and internal interface dipoles at the FxDNTT/cesium/copper hetero-interface (Image: Maximilian Dreher, CC BY-NC-ND 4.0).

The use of organic contact layers is a versatile tool to control the work function of metal electrodes. While partial fluorination of robust organic molecules leads to a significant shift of their frontier energy levels in the isolated molecules, this effect can be wiped out for organic films adsorbed on high work function metal substrates leading to an equalization of the corresponding HOMO levels. Consequently, also the work function shift is equalized in the condensed phase, which is often referred to as HOMO (resp. LUMO) level pinning. Especially the LUMO level pinning has been reported only on a theoretical level in literature yet.

By using partially fluorinated DNTTs, which were synthesized by project A8 and exhibit such a HOMO level pinning on high work function noble metals, the group of Jérôme Cornil (Mons, Belgium) demonstrated that a LUMO level pinning also exists on low work function Cs(110) surfaces on a theoretical level. To face low work function surfaces experimentally, Maximilian Dreher and coworkers used atomically thin cesium layers that grow epitaxial on Cu(100) single crystals and provide more inert, low work function surfaces. In contrast to the expectation, this copper/cesium/organic hetero-stack reveals no LUMO pinning effect. Complementary DFT calculations demonstrate, that the contributions prevailing on the work function shift can be separated into (i) an outer interface dipole between the organic layer and the 2D cesium layer and (ii) an inner, buried interface dipole at the metal/cesium interface. While the outer interface dipole is again equalized for the different FxDNTT species, the buried interface dipole is modulated dependent on the degree of fluorination of the molecules.

Such a sandwich hetero-interface provides new possibilities to effectively tailor contact layers between metal electrodes and active organic layers improving their energy level alignment and emphasizes the importance of internal interfaces.

Publication

M. Dreher, D. Cornil, M. W. Tripp, U. Koert, J. Cornil, G. Witte
On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat-Lying on Various Metals and Hetero-Structures
Adv. Mater. Interfaces (2022) DOI:10.1002/admi.202200361

Contact

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

German Science and Humanities Council recommends research building for materials sciences

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A research building together with a modern transmission electron microscope will be established on the Lahnberge campus of Philipps-University Marburg

Dr. Andreas Beyer, a researcher in SFB 1083, operates a Transmission Electron Microscope, which provides important insights in the development of new materials.

On the Lahnberge Campus, a new research building for a transmission electron microscope for the investigation of novel materials will be established. The German Science and Humanities Council gave its recommendation for the project, which is called ATEMMA (Advanced Transmission Electron Microscopy, Marburg). ATEMMA comprises a volume of 10 Mio €. This is divided into 4 Mio € for the building itself as well as 6 Mio € for the new (S)TEM.

ATEMMA strengthens the focus on material sciences and especially on interfaces at the Philipps-University Marburg and paves the way for high-quality research, e.g., on new materials used for communication and energy technologies, as these represent extremely important topics in our today’s society. The new research lab combines structural characterization with the development of new methods. This combination will boost the research on novel materials also with respect to device applications.

ATEMMA will be used jointly by different groups from physics, chemistry and material sciences distributed over the Philipps-University Marburg as well as Justus-Liebig-University Giessen and the Forschungscampus Mittelhessen. Several of the groups are also part of the SFB 1083, highlighting the importance of interface-related research for ATEMMA.

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

Update (12.07.2022): ATEMMA was now officially granted and is scheduled to go into operation in 2026. Again, please see the press release by the Philipps-Universität Marburg for further infromation (in German).

Contact

Prof. Dr. Kerstin Volz
Department of Physics and Materials Science Center
Philipps-Universität Marburg
Tel.: 06421 28-22297
EMAIL

Dr. Gerson Mette (B5) completed his habilitation at the Philipps-University Marburg

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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.

Terahertz Fingerprint of Monolayer Wigner Crystals – Publication by B9 (Malic) in Nano Letters

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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

Sketch of the 2D Wigner crystal with a honeycomb lattice and alternating spin polarization. The colored curves underneath the particles illustrate their wave functions. Reprinted with permission from Brem et al. Copyright 2022 American Chemical Society.

Wigner crystals are solid, crystalline phases of electrons, formed at low temperatures in order to minimize their repulsive energy. This formation is one of the most intriguing quantum phase transitions and their experimental realization remains challenging since their theoretical prediction. However, the strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields.

In this work, the group of Ermin Malic predicts that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. The density matrix formalism was used to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multipeak shape of the obtained terahertz spectrum. Moreover, a characteristic shift of the peak position as a function of charge density for different atomically thin materials was predicted and showed how the results can be generalized to an arbitrary two-dimensional system.

The results will guide future experiments toward the detection of Wigner crystallization and help to study the interaction dynamics in pure and generalized Wigner crystals in twisted bilayers.

Publication

S. Brem, E. Malic
Terahertz Fingerprint of Monolayer Wigner Crystals
Nano Lett. (2022) DOI:10.1021/acs.nanolett.1c04620

Contact

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

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

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