Last News Items
- Momentum-forbidden dark excitons in WS2 – Publication by B6 (Höfer/Wallauer) and B9 (Malic)25. June 2021 - 11:19
- SFB 1083 extended by four more years25. May 2021 - 09:30
- Biphenylene Network: A Nonbenzenoid Carbon Allotrope – Publication by A4 (Gottfried) and A8 (Koert/Dürr) in Science21. May 2021 - 08:00
- Turning at top speed – Publication by B11 (Güdde/Höfer) in Nature19. May 2021 - 17:00
- Quantitative Characterization of Nanometer-Scale Electric Fields via Momentum-Resolved STEM– Publication by A5 (Volz)8. March 2021 - 12:50
- Tracing orbital images on ultrafast time scales – Publication by B6 (Höfer/Wallauer) and A12 (Tautz/Bocquet/Kumpf) in Science19. February 2021 - 12:41
- New Two-Dimensional Materials by Design – Publication by A11 (Heine) and B2 (Chatterjee)18. February 2021 - 15:26
- Vibrational Frequency Used as Internal Clock Reference to Access Molecule-Metal Charge-Transfer Times – Publication by A3 (Jakob)18. February 2021 - 12:31
- Engineering of TMDC-OSC Hybrid Interfaces: The Thermodynamics of Unitary and Mixed Acene Monolayers on MoS2 – Publication by A2 (Witte) and A4 (Gottfried)21. January 2021 - 12:39
- Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes – Publication by A2 (Witte)13. January 2021 - 12:36
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At „Falling Walls – The World Science Summit“ during the Berlin Science Week, Prof. Dr. F.S. Tautz, PI of SFB project A12 (Tautz/Bocquet/Kumpf), was declared the winner in the category „Engineering and Technology“ with his contribution „Breakting the Wall of Building with Molecules“.
„Falling Walls“ is an event that brings together some of the best researchers of the world, discussing and celebrating the latest breakthroughs in science and society since over 10 years. The breakthroughs represent significant advances, groundbreaking developments and innovative ideas stretching over ten different categories.
Prof. Tautz (SFB project A12) submitted his 5-min long nomination film (link see below) in the category “Engineering and Technology” titled “Breaking the Wall of Building with Molecules”. In this video, Prof. Tautz gives a short insight into his research. Although manipulating atoms on surfaces is already performed for several years, researchers are struggling with moving and arranging molecules. He explains how an artificial intelligence was trained by reinforcement learning in reality and in model systems at the same time. Consequently, the agent got highly adaptive and become able to successfully manipulate molecules.
With this technique, targeted fabrication of molecular machines can be achieved giving rise to new promising technologies with high-level functionalities. Winning this price is not only a great honor for Prof. Tautz, but also represents the high importance and appreciation of the SFB’s research.
For further details, see the.
Prof. Dr. Stefan Tautz
FZ Jülich, SFB project A12
PGI, Experimental Physics
Tel.: +49 (0)2461 61-4561
The researchers developed a method, which was published in Science, to reconstruct the band structure of quantum materials with very high precision
Stacking of two-dimensional (2D) materials yield promising properties for the development of new devices with outstanding functionality. However, detailed knowledge of the electronic structure is necessary to tailor their properties. Within the cooperation of Prof. Huber (Uni Regensburg), Prof. Kira (Uni Michigan) and Prof. Koch (Uni Marburg, project B4), a new method was developed enabling the reconstruction of the band structure of 2D materials.
The quantum material WSe2 was simultaneously excited by two laser pulses in the visible range and in the THz spectral range. The resonant excitation of the weak optical pulse and simultaneous irradiation with a strong THz pulse result in harmonic sideband generation (HSG) in the transmitted spectrum. Due to the wave-particle dualism of electrons, electronic interference combs evolve in momentum space (cf. Figure on the left). By analyzing multiple sideband spectra at different THz frequencies and intensities, it is possible to reconstruct the band structure with super-resolution.
This concept offers an all-optical and practical approach for a three-dimensional tomography of the electronic structure of small quantum materials as shown for WSe2. This work is a great extend to the research in the SFB as it provides the possibility to examine the electronic band structure of 2D materials, such as TMDCs, even under ambient conditions.
Super-resolution lightwave tomography of electronic bands in quantum materials
Prof. Dr. Stephan W. Koch
Department of Physics, Theoretical Semiconductor Physics
Renthof 5, 35032 Marburg
Tel.: 06421 28-21336
Congratulation to Dr. Lars Bannow and Dr. Benedikt P. Klein, PhD-students of the GRK 1782 (SW Koch) and the SFB project A4 (Gottfried), respectively, for being awarded a prize by Philipps-Universität Marburg for their excellent dissertations presented in 2019.
In the thesis of Dr. Bannow with the title “Optical and Electronic Properties of Semiconductor Materials”, he investigates optical and electronic properties of novel semiconductor materials such as Ga(AsBi), In(AsBi) and the methylammonium (MA) perovskite MAPbI3. Using a combination of density functional theory calculation and semiconductor Bloch equations, a precise prediction of opto-electronic properties was possible at a minimum of experimental data. The examined materials are promising options for the fabrication of more efficient laser diodes and more economic solar cells.
In his thesis “The Surface Chemical Bond of Non-alternant Aromatic Molecules on Metal Surfaces”, Dr. Klein explores interfaces between model organic semiconductors and metals. He compares π-electron systems with alternant and non-alternant topologies and finds that the non-alternant topology leads to much stronger interfacial interactions. These studies pave the way to novel organic semiconductors with tailored properties and provide important insight into the bonding of non-alternant defect structures in graphene with metals.
See the news release of the Philipps-Universität Marburg for details of the event.
In a new publication in Nanoscale Horizons, Zimmermann and coworkers introduce time-resolved SHG imaging microscopy as a new experimental method for investigating ultrafast charge-transfer processes in heterostructures of transition metal dichalcogenides.
Heterostructures of transition metal dichalcogenides (TMD) feature a type-II band alignment which can separate photoexcited electrons and holes into different layers through ultrafast charge transfer. While this charge transfer is essential for potential applications, the underlying mechanisms still remain elusive. Main drawbacks of previous experiments are insufficient time-resolution of the employed microscopy setups and deficiencies of linear optical spectroscopies to address individual layers of the heterostructure selectively.In their new approach, Zimmermann and coworkers have combined the advantages of time-resolved optical second-harmonic generation (SHG) with an optical microscopy setup. On the one hand, their method allows for pump-probe experiments in µm small structures with a superior time-resolution. On the other hand, the tensorial nature of the second-order nonlinear susceptibility allows them to distinguish the response from differently oriented layers to elucidate directional interlayer charge transfer as demonstrated for a rotationally mismatched WSe2/MoSe2 heterostructure. As their results show, the new approach is particularly suited to perform systematic investigations of the charge transfer in dependence of the rotational layer mismatch in TMD heterostructures.
J. E. Zimmermann, Y. D. Kim, J. C. Hone, U. Höfer, G. Mette
Directional ultrafast charge transfer in a WSe2/MoSe2 heterostructure selectively probed by time-resolved SHG imaging microscopy
Dr. Gerson Mette
SFB 1083 subproject B5
Tel.: +49 6421 28-24123
In a new publication in Chemistry of Materials project A2 (Witte) reports on the epitaxial alignment of crystalline perfluoropentacene (PFP) films on various transition metal dichalcogenides (TMDCs). This van der Waals epitaxy results in characteristic twist angles between substrate and film lattices, which are of particular interest for the optoelectronic coupling at the interface.
Two-dimensional (2D) materials are a subject of current research, because their different electronic properties as well as the ability to prepare films as thin as one monolayer opens up the prospect of producing new nanoscale heterostructures and devices. Of particular interest is the stacking of such films with controlled twist angle as it critically affects the electronic interface properties. A promising extension is the combination of TMDCs with organic semiconductors (OSC), as it allows to combine the high charge carrier mobility of the TMDCs with the OSC’s large photo-absorption cross section, which is beneficial for photovoltaic applications.
Using the example of the prototypical OSC PFP, Maximilian Dreher and coworkers analyzed in the present study the epitaxial alignment of the crystalline molecular adlayers on the basal plane of different TMDCs (MoSe2, WSe2, MoS2 MoTe2). By utilizing the optical anisotropy of PFP films, their azimuthal alignment was analyzed by means of polarization resolved reflection anisotropy. This sensitive and non-invasive method allows to characterize the epitaxial alignment even for thin films of few nanometers. The analysis yielded specific twist angles of the crystalline adlayer domains with respect to the substrate lattice, which are characteristic for the individual material combinations. Notably, the observed epitaxial order is not caused by any higher-order commensurability between substrate and adlayer, where individual molecules are bound to locally favorable adsorption sites. Instead, it results from an energetically favored alignment of the entire crystalline adlayer on the substrate surface and can be rationalized as an on-line coincidence. This peculiar epitaxy could also be theoretically modelled using a modified scheme of projection of real-space adlayer lattice points onto the substrate unit cell. In addition, the extreme sensitivity of this van der Waals epitaxy on small lattice distortions was demonstrated by films grown at slightly higher substrate temperature. Although raising the growth temperature by about 30 K yields only a small increase of the lattice constants of the PFP film due to thermal expansion in the order of a few hundredths of Angstrom, while the more rigid TMDC surface lattice is hardly affected, it causes a distinct change of the twist angle of more than 20°. The achieved epitaxial alignment and control of twist angles is an important mile stone and will be used in future studies on the optoelectronic adlayer-substrate coupling in OSC/TMDC hybrid systems within the SFB 1083.
Maximilian Dreher, Darius Günder, Steffen Zörb, and Gregor Witte
Van der Waals Bound Organic Semiconductor/2D-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment of Perfluoropentacene Films on Transition Metal Dichalcogenides
Prof. Dr. Gregor Witte
SFB 1083 project A2
Tel.: 06421 28-21384
David Peter Krug, PhD-student in SFB-project A5 of Prof. Dr. Kerstin Volz, was awarded with the poster prize at the virtual Microscopy & Microanalysis Meeting 2020.
The Microscopy & Microanalysis Meeting (M&M) is an annual meeting in the USA covering the research fields of microscopy, imaging, and compositional analysis. Due to the current Corona pandemic, the meeting was held online.
Poster “Formation mechanisms for the dominant kinks in GaP nanowires in an in-situ (S)TEM gas cell holder” by D. Krug, M. Widemann, F. Gruber, A. Beyer, and K. Volz (Materials Sciences Center and Faculty of Physics, Philipps-Universität Marburg) – Microscopy & Microanalysis Meeting 2020, August 04 – 07, 2020, virtual meeting.
How fast is the charge transfer between two layers of transition metal dichalcogenides (TMDCs) and where does it take place in momentum space? Two-photon photoemission using high-harmonic probe pulses can answer these questions as Wallauer and coworkers demonstrate for the topmost layers of MoS2.
The experiment of Wallauer and coworkers exploits both the high surface sensititivity of photoelectron spectroscopy and the fact, that the bandgap of the topmost layer of TMDCs is enlarged due to reduced screening. By tuning pump pulses below the top-layer gap at K, it is thus possible to excite electrons in deeper layers and probe only the topmost layer. The experiment then images the population dynamics of initially unoccupied electronic states and the charge transfer directly in momentum space with femtosecond time resolution. The results show that the electron transfer between the topmost layers of a 2H-MoS2-crystal, takes place at Σ and proceeds on a timescale of less than 20 fs.
GW-based tight binding calculations by Marauhn and Rohlfing support the experimental findings and explain why the electron transfer takes place at Σ. The GW-based tight-binding calculations not only confirm that the band gap in the surface layer is indeed considerably larger than in deeper layers. They reveal that the coupling between surface and deeper layers is strongly momentum-dependent throughout the Brillouin zone. The coupling is found to be particularly strong at at the conduction-band minimum at Σ, which explains the ultrafast interlayer charge transfer observed in the experiment at this location.
The publication is an “Editor’s Suggestion” in the September 2020 issue of Physical Preview B.
Momentum-resolved observation of ultrafast interlayer charge transfer between the topmost layers of MoS2
Dr. Robert Wallauer
Prof. Dr. Michael Rohlfing
Westfälische Wilhelms-Universität Münster
SFB 1083 subproject A13
Phone: +49 251 83-36340
We congratulate Prof. Dr. Ralf Tonner, prinicple investigator of SFB-Project A6 “Unified density functional description of bonding and interaction at inorganic/organic interfaces” on his new postion as Chair for Theoretical Chemistry at the University of Leipzig.
In their study published in Angewandte Chemie International Edition, the authors from two SFB-projects present their research into unilaterally fluorine-substituted pentacenes. This new reaction path will make it possible to synthesize functional materials and to create molecular nanostructures, the properties of which can be used in future organic components.
Organic semiconductor materials consist of polycyclic aromatic hydrocarbons (PAH), which serve as molecular building blocks for the realization of functional materials and thin-film devices. The electronic properties of these often structurally simple materials can be changed by specific chemical modifications and thus tailored to the respective application. For example, perfluorination of such p-conjugated molecular materials affects the polarity of the charge carrier thus allowing a change from p-type to n-type semiconductors. For the prototypical organic semiconductors of acenes, it has so far only been possible to implement substitution patterns in which the molecules have been substituted symmetrically with respect to their long axis or entire ring units have been modified. In a recent collaboration between synthetic chemistry and molecular solid-state physics at the Philipps-University in Marburg, a novel synthetic strategy has been introduced that enables a regioselective functionalization of acenes.
This new concept was demonstrated using the example of unilaterally substituted fluoroacenes, whose electronic structure is a hybrid of the parental non-fluorinated and perfluorinated pentacenes. An important milestone in the synthesis strategy developed in the group of Prof. Dr. Ulrich Koert is the transition metal-catalyzed C-C bond formation, which makes this synthesis controllable. This novel material was crystallized and characterized with respect to its optical and electronic molecular- and solid state properties in the group of Prof. Dr. Gregor Witte. It was shown that the unilateral fluorination causes a distinctive dipole moment in contrast to the symmetrical substituted molecules. In addition, the molecules in crystals show a novel packing motif and also their optical solid states (excitons) are significantly altered. The identified new molecular packing motif and the additional electrostatic interactions open up new possibilities for the controlled fabrication of functional thin films and molecular heterostructures with special molecular interface structures and thus enable a tailoring of the electronic interface coupling.
P.E. Hofmann, M.W. Tripp, D. Bischof, Y. Grell, A.L.C. Schiller, T. Breuer, S.I. Ivlev, G. Witte, U. Koert,
Unilaterally Fluorinated Acenes: Synthesis and Solid State Properties,
Angewandte Chemie 59, 16501 (2020)
Prof. Dr. Ulrich Koert
SFB 1083 project A8
Tel.: +49 6421 28-26970
Prof. Dr. Gregor Witte
SFB 1083 project A2
Tel.: +49 6421 28-21384
SFB 1083 published an activity report covering its scientific achievements in the first six years from 2013-2019.
The activity report of the SFB 1083 gives a scientific introduction into the research done from 2013 until 2019. It comprises the motivation of the SFB 1083 as well as several publications as highlights. Furthermore, it provides an overview over the projects within the SFB 1083. Another part of it features the scientific communications in the form of workshops and conferences, e.g., the ICII-2016 and the ASOMEA-IX, in the relevant period. Besides these activities, the principal investigators are introduced and an overview over the staff, including numerous PhD-students, guest scientists and visitors. In the last part, interesting statistics of the SFB 1083 are presented.
The activity report can be downloaded here for further details.
Complementary to the activity report that addresses mainly other researchers, the SFB 1083 image brochure in German aims at interested students and the public. It gives a general introduction to the research on internal interfaces and portraits the participating researchers.
The image brochure (German) can be downloaded here for further details.
A printed version of both documents is available upon request.
Tel.: 06421 28-24223