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

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

Adapted from Beyer et al. (full citation see below) licensed by CC BY-NC-ND 4.0.

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

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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

New Two-Dimensional Materials by Design – Publication by A11 (Heine) and B2 (Chatterjee)

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Johanna Heine and Sangam Chatterjee break boundaries in two-dimensional materials’ design towards enhanced light-harvesting and emitting capabilities of hybrid perovskites

Adapted from Klement et al. (full citation see below) licensed by CC BY-NC-ND 4.0.

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).

Publication

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
Adv. Mater. (2021) DOI:10.1002/adma.202100518

Contact

Dr. Johanna Heine
Philipps-Universität Marburg
SFB project A11
Tel.: 06421 28-22425
EMAIL

Prof. Dr. Sangam Chatterjee
Justus-Liebig-Universität Gießen
SFB project B2
Tel.: 0641 99-33100
EMAIL