Vibrational Frequency Used as Internal Clock Reference to Access Molecule-Metal Charge-Transfer Times – Publication by A3 (Jakob)

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

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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.

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

Contact

Prof. Dr. Peter Jakob
Philipps-Universität Marburg
SFB project A3
Tel.: 06421 28-24328
EMAIL

Quantitative Characterization of Nanometer-Scale Electric Fields via Momentum-Resolved STEM– Publication by A5 (Volz)

Andreas Beyer and coworkers achieved the determination and spatial resolution of electric fields at interfaces with the transmission electron microscope.

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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.

Publication

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
Nano Lett. (2021) DOI:10.1021/acs.nanolett.0c04544

Contact

Prof. Dr. Kerstin Volz
Philipps-Universität Marburg
SFB 1083 project A5
Tel.: 06421 28 22297
EMAIL

Tracing orbital images on ultrafast time scales – Publication by B6 (Höfer/Wallauer) and A12 (Tautz/Bocquet/Kumpf) in Science

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.

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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.

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.

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.

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