Felix Widdascheck, Daniel Bischof and Gregor Witte developed a robust method to prepare air-stable molecular contact primer layers allowing to reduce hole injection barriers of printable silver electrodes  into organic semiconductors.

Contact engineering is an important issue for organic electronics as it allows to reduce charge carrier injection barriers. While the use of molecular contact primer layers was demonstrated in many concept studies for single crystalline model substrates, the processability of electrodes and their robustness in real devices must also be considered. Although silver electrodes can be printed using silver ink, their low work function and sensitivity to oxidation severely limits their use for printable organic electronics.

In this study Prof. Witte and his coworkers demonstrate that F₆TCNNQ monolayers provide a reliable approach to engineer high work function silver electrodes, which is examined for Ag(111) as well as polycrystalline and silver ink substrates. Notably, upon multilayer growth, a pronounced intercalation of silver into the molecular adlayer occurs, yielding thermally stabilized organometallic interphases extending over the entire adlayer. It is shown that heating allows their controlled desorption leaving behind a well-defined monolayer that is further stabilized by additional charge transfer. Such primer layers enhance the work function to 5.5-5.6 eV and can even withstand air exposure but show no interdiffusion into subsequently deposited p-type organic semiconductor, hence validating their use for organic electronic devices.

Preparation scheme of F₆TCNNQ layers on silver electrodes yielding well-defined, charge-transfer stabilized contact primer monolayers. Adapted from Widdascheck et al. (full citation see below) licensed by CC BY-NC-ND 4.0 

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

Publication

F. Widdascheck, D. Bischof, G. Witte
Engineering of Printable and Air-Stable Silver Electrodes with High Work Function using Contact Primer Layer: From Organometallic Interphases to Sharp Interfaces
Adv. Funct. Mater. (2021) DOI:10.1002/adfm.202106687

Contact

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

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 3^rd 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 2^nd 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 2^nd 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 2^nd 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.

Present spokesman Ulrich Höfer (left) and future spokeswoman Kerstin Volz in front of a poster introducing SFB 1083, Foto: Stefan Kachel.

See also Press Release of Philipps-Universität Marburg (in German) and the German Research Foundation (DFG) for more detail.

Contact

Prof. Dr. Kerstin Volz
Department of Physics and Materials Science Center
Philipps-Universität Marburg
Tel: + 49 6421 28-22297
Email: kerstin.volz@physik.uni-marburg.de

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.

On the surface of a topological insulator (colored surface), electrons (small blue sphere) move with quasi-relativistic speed. When a strong light wave accelerates electrons through the so-called Dirac point (apex of the cone), their velocity abruptly flips. The instantaneous change of velocity leads to the emission of broadband electromagnetic radiation (flash in the middle of the colored surface). Reprinted with permission from Nature (link see below).

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.

Informational Material

– Press release of the university of Regensburg (in English and German).

– Press release of the university of Marburg (in German).

– News of Physics Department, Philipps-Universität Marburg (in German).

– Homepage of the Huber group in Regensburg.

Publication

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

Contact

Prof. Dr. Ulrich Höfer
Philipps-Universität Marburg
SFB 1083 project B6, B11
Tel.: 06421 28 24215
EMAIL

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.

 

Excitation scheme for time-resolved photoemission orbital tomography. (b) Measured LUMO momentum maps for three selected delay times between pump and probe pulse. (c) Scheme of intramolecular and substrate-to-molecule excitation pathways. The LUMO pattern of the 0° molecule in (b) is seen to light up faster due to resonant HOMO-LUMO excitation than that of the 90° populated across the CuO Interface. From Wallauer et al, Science 371 (2021) 1056. Reprinted with permission from AAAS.

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

 

Informational Material

– Joint press release of the universities of Marburg and Graz and the FZ Jülich (available in English and German).
– News, Philipps-Universität Marburg (in German).
– News, Universität Graz, Österreich (in German).

 

Publication

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
Science 371 (2021) 1056 DOI:10.1126/science.abf3286

 

Contact

Prof. Dr. Ulrich Höfer Philipps-Universität Marburg SFB 1083, project B6 Tel.: +49 6421 28-24215 ulrich.hoefer@physik.uni-marburg.de

Prof. Dr. Stefan Tautz FZ Jülich PGI, Experimental Physics, project A12 Tel.: +49 (0)2461 61-4561 s.tautz@fz-juelich.de

Peter Jakob and Sebastian Thussing derived ultrafast charge-transfer times at molecule-metal interfaces using the vibrational oscillation period as an internal clock reference

Interfacial dynamical charge transfer at the molecule-metal interface, associated with non-adiabatic electron-vibron coupling leads to vibrational bands displaying characteristic asymmetric line shapes. Copyright by SFB1083.

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.

Publication

P. Jakob, S. Thussing
Vibrational Frequency used as Internal Clock Reference to access Molecule — Metal Charge Transfer Times
Phys. Rev. Lett. 126 (2021) 116801 DOI:10.1103/PhysRevLett.126.116801