Last News Items
- Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure – Publication by B9 (Malic) in Science Advances7. February 2024 - 12:00
- Heteroepitaxy in Organic/TMD Hybrids and Challenge to Achieve it for TMD Monolayers: The Case of Pentacene on WS2 and WSe2 – Publication by A2 and B58. January 2024 - 10:06
- Layer-by-layer deposition of organic molecules controlled by selective click reactions – Publication by A8 (Koert/Dürr) in Chemistry of Materials 23. December 2023 - 14:48
- Enhanced Circular Dichroism and Polarized Emission in an Achiral, Low Band Gap Bismuth Iodide Perovskite Derivative5. October 2023 - 11:25
- 34. Erfinderlabor: Scientific curiosity of the next Generation1. August 2023 - 14:34
- Measuring spatially-resolved potential drops at semiconductor hetero-interfaces using 4D-STEM– Publication by A5 (Volz) in Small Methods28. May 2023 - 14:07
- Interface engineering of charge-transfer excitons in 2D lateral heterostructures – Publication by B9 (Malic) in Nature Communications9. May 2023 - 09:27
- Electrical control of hybrid exciton transport in a van-der-Waals heterostructure – Publication by B9 (Malic) in Nature Photonics20. April 2023 - 09:46
- When electrons dress up in light – Publication by B11 (Güdde/Höfer) in Nature12. April 2023 - 16:21
- Shape control in 2D molecular nanosheets by tuning anisotropic intermolecular interactions and assembly kinetics – Publication by A2 (Witte) and A8 (Koert/Dürr)22. March 2023 - 12:58
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In a publication in Nano Letters, Robert Wallauer and co-workers trace the early-stage exciton dynamics in a two-dimensional semiconductor and report first results on the ultrafast formation of momentum-forbidden dark excitons.
Excitons that form out of electrons and holes at different locations of the Brillouin zone, so-called dark excitons, play a key role for the optical properties of TMDC monolayers in general and for the formation of interlayer excitons in TMDC heterostructures, in particular. Whereas dark excitations are difficult to access by purely optical experiments they can be imaged directly in momentum space by time- and angle-resolved photoelectron spectroscopy [Madéo et al., Science 370, 1199 (2020)]. With the superior time-resolution of the momentum microscope operated by B6 (Höfer/Wallauer), the dynamics of formation of a dark KΣ exciton could now be resolved for the first time.
The formation process occurs on timescales where coherence between valence and conduction band play a major role. The short pump pulses of the experiment induce an optical polarisation in the K valley. This polarisation was found to decay and form bright and dark excitons in a few tens of femtoseconds after optical excitation. A fully microscopic theory by B9 (Malic) revealed the influence of the coherence on the formation process of the excitons, in excellent agreement with the experiment that could tune the excitation energy. The high quality WS2-samples for the measurements were provided by the Huber group in Regensburg, who earlier succeeded in probing momentum-indirect excitons via the intraexcitonic 1s-2p transition [Poellmann et al., Nat. Mater. 14, 889 (2015)].
Future experiments of this kind will address the role of dark excitons in the formation process of interlayer excitons. The excellent agreement between experiment and theory in this work holds great promise to investigate such charge transfer processes on a microscopic level.
R. Wallauer, R. Perea-Causin, L. Münster, S. Zajusch, S. Brem, J. Güdde, K. Tanimura, K.-Q. Lin, R. Huber, E. Malic, U. Höfer
Momentum-resolved observation of exciton formation dynamics in monolayer WS2
Nano Lett. (2021) DOI:10.1021/acs.nanolett.1c01839
Dr. Robert Wallauer
SFB 1083 project B6
Tel.: 06421 28 21406
Prof. Dr. Ermin Malic
SFB 1083 project B9
Tel.: 06421 28 22640
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 3rd 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 2nd 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 2nd 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 2nd 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.
See also Press Release of Philipps-Universität Marburg (in German) and the German Research Foundation (DFG) for more detail.
Prof. Dr. Kerstin Volz
Department of Physics and Materials Science Center
Tel: + 49 6421 28-22297
Not graphene: Dr. Qitang Fan and coworkers of SFB 1083 discover new type of atomically thin carbon material
Carbon exists in various forms, of which graphene is one of the most astonishing. In this atomically thin material, each carbon atom is linked to three neighbors, forming hexagons arranged in a honeycomb network. Researchers in the SFB 1083 projects A4 (Gottfried) and A8 (Koert/Duerr) have now discovered a new carbon network, which is planar like graphene, but is made up of squares, hexagons, and octagons forming an ordered lattice. In collaboration with physicists from Aalto University in Finland, the unique structure was confirmed using high-resolution scanning probe microscopy methods. In addition, it was found that the electronic properties of the new material are very different from those of graphene.
Biphenylene network, as the new material is named, is made from organic molecules on an atomically smooth gold surface. These molecules first form polymer chains, which consist of linked hexagons. A subsequent reaction connects these chains and forms the squares and octagons. An important feature of the chains is that they are chiral. Chains of the same type aggregate on the gold surface forming well-ordered assemblies, before they connect. This is critical for the formation of the new carbon material, because reaction between two different types of chains leads to the well-known graphene.
In contrast to graphene and other forms of carbon, the new material has metallic properties. Therefore, it can be used as conducting wires in future carbon-based electronic devices. The authors of the study are confident that their synthesis method will contribute to the discovery of further novel carbon networks. For now, their goal is to prepare larger sheets of the material and to study its interface-related properties.
For further information, please see the press release of the university of Marburg.
Q.T. Fan, L.H. Yan, M.W. Tripp, O. Krejči, S. Dimosthenous, S.R. Kachel, M.Y. Chen, A.S. Foster, U. Koert, P. Liljeroth, J.M. Gottfried
Biphenylene Network: A Nonbenzenoid Carbon Allotrope
Science 372 (2021) 852 DOI:10.1126/science.abg4509
Prof. Dr. Michael Gottfried
SFB 1083 project A4
Tel.: 06421 28 22541
In collaboration with the group of Rupert Huber in Regensburg, Suguru Ito, Jens Güdde and Ulrich Höfer from the new SFB project B11 “Ultrafast dynamics of interface currents” demonstrate efficient high-order harmonic generation at the surface of a topological insulator by driving ballistic electron currents at THz frequencies.
In the conducting surface states of topological insulators electrons behave like massless particles, characterized by a linear dispersion (Dirac cone). Under the influence of an electric field, the electrons rapidly switch their direction of motion when their trajectories in momentum come close to the minimum of the cone, the Dirac point. At frequencies in the range of 25 to 35 THz, this is the case for electric field strengths of several MV/cm. It results in the emission of an ultrafast flash of light containing a broadband spectrum up to frequencies of 800 THz that can be analyzed with optical detectors.
This novel mechanism of high-harmonic generation is only possible because the spin-momentum-locking in the Dirac cone prevents efficient electron scattering. The resulting long scattering times of ~1 ps allow to drive strong ballistic electron currents as demonstrated previously with THz-ARPES (Reimann et al. Nature 2018). The emitted high-harmonic radiation has a characteristic polarization dependence related to the Berry curvature of the Dirac cone. Moreover, it can be shifted to arbitrary non-integer multiples of the driving frequency by varying the carrier-envelope phase of the driving field. These specific properties set it apart from HHG processes in bulk semiconductors (Hohenleutner et al. Nature 2015).
In the upcoming third funding period, SFB 1083 will utilize THz high-harmonic radiation generated in this way and exploit its characteristic properties to investigate electron currents at interfaces of topological insulators buried under protecting capping layers. For future device applications, it will be crucial to screen the unusual movement of Dirac electrons from the environment.
– Homepage of thein Regensburg.
C.P. Schmid, L. Weigl, P. Grössing, V. Junk, C. Gorini, S. Schlauderer, S. Ito, M. Meierhofer, N. Hofmann, D. Afanasiev, J. Crewse, K.A. Kokh, O.E. Tereshchenko, J. Güdde, F. Evers, J. Wilhelm, K. Richter, U. Höfer, R. Huber
Tuneable non-integer high-harmonic generation in a topological insulator
Nature (2021) DOI:10.1038/s41586-021-03466-7
Prof. Dr. Ulrich Höfer
SFB 1083 project B6, B11
Tel.: 06421 28
Andreas Beyer and coworkers achieved the determination and spatial resolution of electric fields at interfaces with the transmission electron microscope.
Nanometer-scale built-in electric field are the basis of many modern (opto)electronic devices, such as solar cells, lasers or batteries. Optimization of these devices requires precise characterization of such fields at small length scales. With a fast pixelated-detector, A. Beyer and coworkers in SFB project A5 (Volz) acquire a 2D diffraction pattern for every real-space position of the impinging electron beam. In doing so, the momentum transfer of an electric field (or a charge) on the electron beam can be measured, and the electric field, which is invisible in “normal high angle annular dark field images”, can be calculated from the 4D data-set.
In this work, key characteristics, like doping concentration or polarity, of GaAs-based p-n junctions were quantitatively obtained by 4D scanning transmission electron microscopy (4DSTEM). The values are in excellent quantitative agreement with results from other techniques, which – of course – lack lateral resolution.
A. Beyer, M.S. Munde, S. Firoozabadi, D. Heimes, T. Grieb, A. Rosenauer, K. Müller-Caspary, K. Volz
Quantitative Characterization of Nanometer-Scale Electric Fields via Momentum-Resolved STEM
Prof. Dr. Kerstin Volz
SFB 1083 project A5
Tel.: 06421 28 22297
Robert Wallauer and coworkers combined a high harmonic laser source with an electron momentum microscope to record orbital images of the charge transfer at an organic/metal interface with femtosecond time resolution.
The microscopic charge-transfer dynamics across molecular interfaces is reflected in the population of electronic orbitals. These were, for the first time, directly monitored with ultrafast time resolution in a joint experimental effort of B6 (Höfer/Wallauer) in Marburg and A12 (Tautz/Bocquet/Kumpf) in Jülich. The experiment records the full two-dimensional intensity distribution of photoemitted electrons in momentum space in a femtosecond pump-probe scheme. Real-space electron distributions and photoemission momentum maps, called orbital tomographs, are related by a Fourier transform.
The model interface PTCDA/CuO/Cu(100) exhibits two distinct excitation pathways for the PTCDA molecule. The parallel component of the electric field of the pump pulse makes a direct HOMO-LUMO transition, while the perpendicular component transfers an electron from the metal across the atomically thin CuO spacer into the molecular LUMO. Once excited, the LUMO decays with a lifetime of 250 fs, independent of the excitation pathway. Real-space electron distributions and photoemission momentum maps, called orbital tomographs, are related by a Fourier transform (Photoemission Orbital Tomography, Wikipedia).
In the future, the new experimental capability is expected to facilitate the microscopic understanding of charge-transfer and exciton-formation processes at several other classes of organic heterointerfaces with unprecedented detail, including interfaces between 2D semiconductors and layered organic molecular structures.
Video: Ultrafast orbital tomography PTCDA molecules on a copper oxide surface are used as a probe. An electron of a molecule is excited by a laserpulse into another orbital and changes its spatial allocation. The electron in the excited orbital has a limited lifetime, which can be measured by ultrafast orbital tomography. Copyright: Sonderforschungsbereich 1083 / Till Schürmann
R. Wallauer, M. Raths, K. Stallberg, L. Münster, D. Brandstetter, X. Yang, J. Güdde, P. Puschnig, S. Soubatch, C. Kumpf, F.C. Bocquet, F.S. Tautz, U. Höfer
Tracing orbital images on ultrafast time scales
Johanna Heine and Sangam Chatterjee break boundaries in two-dimensional materials’ design towards enhanced light-harvesting and emitting capabilities of hybrid perovskites
Low-dimensional organic−inorganic perovskites synergize the virtues of organic perovskites and inorganic two-dimensional (2D) materials featuring intriguing possibilities for next-generation optoelectronics: they offer tailorable building blocks for atomically thin, layered materials while providing the enhanced light-harvesting and emitting capabilities. However, the quest for new materials is limited by the generally-accepted paradigm that atomically thin materials require covalent in-plane bonding.
The groups of Dr. Heine (A11) and Prof. Chatterjee (B2) within the SFB 1083 lift this apparent paradigm and report single layers of the 1D organic–inorganic perovskite [C7H10N]3[BiCl5]Cl. Its unique 1D–2D interface structure enables single layers and the formation of self-trapped excitons, which show white-light emission. The thickness dependence of the emission energy may enable facile color tuning for next-generation lighting and display technologies.
This class of materials enables interface-controlled device integration of brightly luminescent 1D and 0D hybrid perovskites and offers a promising pathway for the non-covalent functionalization of classical 2D materials through heterostructures.
For further information, please see the press release by the Philipps-Universität Marburg (in German).
P. Klement, N. Dehnhardt, C.‐D. Dong, F. Dobener, S. Bayliff, J. Winkler, D.M. Hofmann, P.J. Klar, S. Schumacher, S. Chatterjee, J. Heine
Atomically Thin Sheets of Lead‐Free 1D Hybrid Perovskites Feature Tunable White‐Light Emission from Self‐Trapped Excitons
Dr. Johanna Heine
SFB project A11
Tel.: 06421 28-22425
Prof. Dr. Sangam Chatterjee
SFB project B2
Tel.: 0641 99-33100
Peter Jakob and Sebastian Thussing derived ultrafast charge-transfer times at molecule-metal interfaces using the vibrational oscillation period as an internal clock reference
Dynamical charge-transfer processes at molecule-metal interfaces proceed in the few fs timescale that renders them highly relevant to electronic excitations in optoelectronic devices. This is particularly true when electronic ground state situations are considered that implicate charge transfer directly at the fermi energy.
Prof. Jakob and Dr. Thussing showed that such processes can be accessed by means of vibrational excitations, with nonadiabatic electron-vibron coupling leading to distinct asymmetric line shapes. Thereby the characteristic timescale of this interfacial dynamical charge transfer can be derived by using the vibrational oscillation period as an internal clock reference.
P. Jakob, S. Thussing
Vibrational Frequency used as Internal Clock Reference to access Molecule — Metal Charge Transfer Times
Prof. Dr. Peter Jakob
SFB project A3
Tel.: 06421 28-24328
In a new publication in Chemical Science, projects A2 (Witte) and A4 (Gottfried) report on the intricate desorption characteristics of pentacene (PEN) and perfluoropentacene (PFP) monolayers on the MoS2 surface. Unitary molecular monolayers are thermally stabilized by entropy due to their high mobility rather than the organic/inorganic interface bond, which hampers the formation of close-packed and well-ordered monolayers.
Van der Waals (vdW) bound hybrid heterosystems of inorganic two-dimensional materials (2DMs) and OSCs are currently receiving great attention due to their promising characteristics for the fabrication of flexible electronics and ultra-thin devices. While prototypical devices with 2DM-OSC hybrid heterostructures have been realized, the fundamental understanding of the 2DM-OSC interface is lacking. In their new study, the authors were able to thoroughly unravel the interplay of interface and intermolecular interactions and their effect on the structure and thermal stability of molecular monolayers.
Through temperature-programmed desorption (TPD) experiments, it was shown that the first molecular layers of PEN and PFP on MoS2 are thermally more stable than subsequent molecular layers in spite of an interface bond that is weaker than the molecular interlayer bond. This is possible due to an entropic stabilization that can only occur if the molecular adlayer is highly mobile. Thus, if the first molecular layer is only stabilized by its mobility, it cannot be well-ordered and close-packed even at low temperatures as low as 100 K. The high molecular diffusivity in the gas-like unitary PEN and PFP monolayers was identified by Monte-Carlo simulations: Intermolecular repulsion of intrinsic molecular electrostatic quadrupole moments, in combination with a weak substrate interaction, prohibits the formation of a condensed molecular monolayer.
By introducing attractive intermolecular interactions, condensation of the molecular films is favored. This was achieved in mixed monolayers of PEN and PFP that adopt a well-ordered stoichiometric 1:1 intermixture. In spite of a reduced mobility, the mixed monolayer is thermally stabilized with respect to the bulk substance due to the attractive intermolecular forces and can therefore be fabricated by selective multilayer desorption. This provides a promising prospect for the fabrication and subsequent study of well-defined 2DM-OSC interfaces for future studies within SFB 1083.
S.R. Kachel, P.-M. Dombrowski T. Breuer, J.M. Gottfried, G. Witte
Engineering of TMDC–OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2
Prof. Dr. Gregor Witte
SFB 1083, project A2
Tel.: +49 6421 28-21 384
In a new publication in Organic Electronics, project A2 (Witte) reports on the establishment of a complete high vacuum-based manufacturing and electronic characterization of organic field effect transistors (OFET).
The electronic coupling between OSC and metallic electrodes is of key importance for the efficiency of charge carrier injection in organic electronic devices, such as OFETs or photovoltaic cells, as it determines their idle power. Surface science-based model studies have mainly focused on the energy level alignment at such metal-organic interfaces without measuring real contact resistances, while device studies are typically performed without any microscopic structural and electronic interface characterization. In the present work, the authors introduced a high vacuum-based manufacturing of bottom contact OFETs that enables cleaning and controlled modification of metal contacts before the organic film deposition. This approach not only excludes any exposure to air, it also allows to examine the influence of controlled exposure to air on the device characteristics.
Using the example of the prototypical OSC pentacene it is demonstrated that FET structures with thoroughly cleaned gold electrodes reveal a remarkably low contact resistance. This can be further improved if the electrodes are O2 plasma treated before the pentacene film growth, which results in a thin gold oxide layer and yields one of the lowest contact resistances ever reported for this system. It is shown that this not only causes an improved energy level alignment at the metal-organic interface but also suppresses a pronounced dewetting. In addition, it was demonstrated that controlled exposure to air – even for a short time – significantly affects the device performance.
The present study is an important milestone as it enables detailed electronic transport measurements through metal-OSC interfaces with poly- and single crystal organic semiconductors. This work paves the way for a knowledge transfer about the properties of idealized model interfaces to real electronic devices applications.
Yurii Radiev, Felix Widdascheck, Michael Göbel, Alrun Aline Hauke and Gregor Witte,
Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes
Prof. Dr. Gregor Witte
SFB 1083, project A2
Tel.: 06421 28-21384