In a joined publication, the groups of Ermin Malic, Bernhard Urbaszek and Andrey Turchanin explained the behavior of quasiparticles in composite semiconductor nanosheets

In a lateral heterostructure, an electron-hole pair spans the interface between two mated TMD semiconductor surfaces. (Figure: Giuseppe Meneghini, Copyright CC-BY 4.0 )

The presence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been a topic of debate in the literature. However, unlike interlayer excitons in vertical heterostructures, their confirmation through observation is still pending.

In this work, a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe2-WSe2 heterostructures is presented. The fully microscopic and material-specific theory illustrates the many-particle processes behind the formation of CT excitons and details their potential manipulation through interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius the appearance of a low-energy CT exciton is predicted. This theory is further compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. It is observed that CT excitons in hBN-encapsulated heterostructures possess small binding energies of just a few tens meV while exhibiting significant dipole moments. These properties make them ideal materials for optoelectronics applications that take advantage of efficient exciton dissociation and fast dipole-driven exciton propagation.

The joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.

Informational Material

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

Publication

R. Rosati, I. Paradisanos, L. Huang, Z. Gan, A. George, K. Watanabe, T. Taniguchi, L. Lombez, P. Renucci, A. Turchanin, B. Urbaszek, E. Malic
Interface engineering of charge-transfer excitons in 2D lateral heterostructures
Nat Commun 14 (2023) 2438 DOI:10.1038/s41467-023-37889-9

Contact

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28-22640
EMAIL

Suguru Ito, Jens Güdde and Ulrich Höfer, together with the group of Rupert Huber in Regensburg and physicists from across Europe, observed the ultrafast birth, rise, and collapse of a Floquet-Bloch band structure.

When electrons (spheres) are accelerated by strong light waves in the linearly dispersing surface state of Bi₂Te₃ (lowest cone), Floquet-Bloch replicas (higher cones) of the original band structure are formed. Videos of the band structure with sub-cycle time resolution reveal for the first time the formation dynamics (cones in the background). Photo: © Brad Baxley (parttowhole.com)

New material properties, at lightning speed and on demand – this vision moves a step closer thanks to the team’s findings. They used time- and angle-resolved photoemission spectroscopy to study the electrons in the Dirac surface state of the topological insulator Bi₂Te₃ that were previously shown to be effectively protected from scattering [Reimann et al., Nature 562, 396 (2018)]. By employing intense 25 THz light pulses, Ito and coworkers could drive these electrons back and forth periodically and watch how hybrid states between electrons and light, known as Floquet states or Floquet-Bloch bands, form at electric field strengths of ~ 1 MV/cm.

In Floquet states, the electrons don’t have just one fixed energy, but many energy states evenly spaced apart by the driving photon energies. The original eigenstate of the electron surrounds itself as if it was dressed with several envelopes of light. In terms of the dynamics of these exotic states, the team’s measurements went far beyond the limit of what could be achieved to date. They managed to take actual videos of the moving electrons with a time resolution better than a single oscillation cycle of the electromagnetic carrier wave of light. As a result, they made an unforeseen discovery, namely that Floquet-Bloch bands form already after a single optical cycle. This surprising finding paves the way to tailored quantum functionalities and ultrafast electronics. It is supported by theoretical modeling contributed by Michael Schüler of the Paul Scherrer Institute in Villigen, Switzerland, and Michael Sentef of the Max Planck Institute for Structure and Dynamics of Matter in Hamburg.

Dr. Suguru Ito, postdoc of SFB Project B11, conducts time-resolved ARPES measurements using a hemispherical electron analyzer. Photo: Jens Güdde

Having established the fundamental time limit for light-induced material engineering, the breakthrough discovery of Ito and coworkers could lead to a new age of physics, enabling the creation of new functionalities on demand. Following this avenue, Ulrich Höfer, Rupert Huber, Peter Puschnig (Graz), and Stefan Tautz (Jülich) were recently jointly awarded an ERC Synergy Grant from the European Research Council for their “Orbital Cinema” research project. This project aims to take slow-motion videos of electronic motion in quantum mechanical orbitals of adsorbed molecules and to push the research of SFB 1083 on organic/solid interfaces into the high-field regime.

Informational Material

– Joint Press release of the Universities of Marburg and Regensburg (in German).

– Press release of the Max Planck Institute for Structure and Dynamics of Matter in Hamburg (in English).

– Homepage of the Huber Group in Regensburg.

– News & Views Article by David Abergel (Chief Editor, Nature Physics).

Publication

S. Ito, M. Schüler, M. Meierhofer, S. Schlauderer, J. Freudenstein, J. Reimann, D. Afanasiev, K.A. Kokh, O.E. Tereshchenko, J. Güdde, M.A. Sentef, U. Höfer, R. Huber
Build-up and dephasing of Floquet–Bloch bands on subcycle timescales
Nature (2023) DOI:10.1038/s41586-023-05850-x

Contact

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

On the LernortLabor conference, the Chemikum and the Oe project of SFB 1083 was awarded with the LeLa 2^nd price for the outstanding experiments and educational offer in Marburg.

Award ceremony – Lernort Labor Meeting, Göttingen (v.l. Dr. Andreas Paetz (BMBF), Erich Weber (LK MR-BID), Luise Cleres, Dr. Marion Enßle, Dr. Ina Budde, Dr. Christof Wegscheid-Gerlach (alle Chemikum Marburg), Dr. Andreas Kratzer (LeLa) (Foto: Christoph Mischke, Goettingen).

Pupil labs are central facilities in many educations and STEM (Science, Technology, Engineering and Mathematics) regions. They are constantly offering new experiments, trying out new methods of teacher education and using creative methods. This year, the educational offer of the Chemikum Marburg and the SFB 1083 was awarded the LernortLabor (LeLa) 2^nd Prize in the category “Experiment of the Year”. The jury judged the format to be an innovative and admirable approach. The LeLa Prize recognizes outstanding achievements by school laboratories and school laboratory networks.

The awarded experiment is a two-hour experimental workshop for young people from grade 9 and upwards and the general public, which focuses on the properties of hydrogen, its production, storage and ultimately the conversion, e.g. in fuel cells. SFB 1083-related experiments, like using solar cells for hydrogen production or the detection of hydrogen by means of suitable detectors, rounded off the overall very good impression of the Chemikum and the Oe project.

Poster presentation (Foto: Chemikum Marburg).

At the conference, the activities of the Oe project were also presented in detail with a poster contribution and within the lecture session “Science Communication, Public Relations and Research: School Labs in DFG-funded Projects”.

LernortLabor – Bundesverband der Schülerlabore e.V. represents extracurricular learning venues that focus on independent work, discovery, research and development in an authentic environment, such as a laboratory or workshop. LernortLabor is the leading lobby group for school laboratories in German-speaking countries and an expert contact for its members, stakeholders in the education scene as well as for politics, business and interested society.

Contact

Dr. Christof Wegscheid-Gerlach
Philipps-Universität Marburg
SFB 1083 project Oe
Tel.: 06421 28-25843
EMAIL

In their combined experimental and theoretical study published in the Journal of Physical Chemistry Letters, the groups of Holger Bettinger (Uni Tübingen), Ulrich Koert (A8) and Gregor Witte (A2) investigated the impact of fluorination on the C1s core level energies in fluorinated acene derivates.

Fluorination affects the inner potential and thus the energy levels, as symbolized by the water level. The electron binding energies are probed via X-ray photoelectron spectroscopy (XPS). (Image: Y. Radiev, Reprinted with permission from ACS J. Phys. Chem. Lett. 2023. Copyright 2023 American Chemical Society.).

Fluorination is commonly used to tailor the frontier energy levels. In the present study, the authors utilized the recent achievements in the synthesis of regio-selectively fluorinated acenes and systematically investigated the core level binding energies by means of X-ray photoelectron spectroscopy (XPS) complemented by density functional theory (DFT) calculations.

These investigations reveal that fluorination leads to core level shifts, which are not limited to directly fluorinated carbon atoms, but also affect more distant carbon atoms. These shifts depend on the degree of fluorination, indicating that local fluorination affects the electron density of the entire aromatic system similar to doping. Since commonly core level shifts are used as fingerprint signatures for the identification of molecular entities, the new results challenge this method for fluorinated π-conjugated molecules. Moreover, as these shifts do not only influence the core levels, but also the molecular valence orbital energies, transitions from the former into the latter are hardly affected by fluorination, as verified by X-ray absorption spectroscopy (NEXAFS) measurements.

Overall, the results show that not only the energies of frontier orbitals but also of all core levels in fully π-conjugated systems are affected by local fluorination, hence limiting a chemical identification based on supposedly characteristic core level energies since they instead depend also on the degree of fluorination.

Publication

D. Bischof, Y. Radiev, M.W. Tripp, P.E. Hofmann, T. Geiger, H.F. Bettinger, U. Koert, G. Witte
Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems
J. Phys. Chem. Lett. (2023) DOI:10.1021/acs.jpclett.3c00287

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083 project A2
Tel.: 06421 28-21384
EMAIL

We congratulate Dr. Philip Klement on receiving the 2021/22 dissertation prize of the Justus-Liebig-Universität Gießen in the category natural sciences for his excellent dissertation.

Prof. Martin Kramer (r.) and Prof. Volker Wissemann (m.) congratulate Dr. Philip Klement (l.). Photo: JLU / Roland Duss

Philip Klement advanced the understanding of interfaces by his studies on the influence between materials and their environment. Interfaces ‒ junctions between materials ‒ are crucial for the design and performance of modern electronic devices. The impact of interfaces on the materials properties increases continuously as structures become smaller.

In his dissertation “Interface Phenomena in Two-Dimensional Materials” Philip Klement combined the dynamic and highly competitive research areas of two-dimensional materials and organic-inorganic perovskites to gain innovative insights. In his central work, he discovered free-standing, single layers of an organic-inorganic mixed crystal ‒ something not deemed feasible before. Further, he discovered the thickness dependence of the emission wavelength of this material ‒ an effect which was to date unknown. These results may enable the facile color tuning in next-generation efficient and sustainable lighting and display technologies.

Philip Klement continues to explore these new developments and opportunities as a member of the SFB projects B2 (Chatterjee) & B13 (Chatterjee/Volz), and in close collaboration with A15 (Heine).

Please see the coverage of the award ceremony for more impressions and a recent audio interview of Dr. Klement and Prof. Chatterjee for more details of their research (both in German).