Electrical control of hybrid exciton transport in a van-der-Waals heterostructure – Publication by B9 (Malic) in Nature Photonics

/in /by

A research team including the experimental group of Andras Kis from the EPFL and the theoretical group of Ermin Malic demonstrated electrical control of the hybrid exciton transport in 2D material heterostructures.

PL spectra as a function of the applied vertical electric field. Low and high field regions are related to predominant KΛ/K′Λ′ and KΛ′/K′Λ transitions, respectively. (Copyright CC-BY 4.0)

Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. However, the lack of direct control over collective dipolar properties has hitherto limited the degrees of tunability and the microscopic understanding of exciton transport.

In this work, the authors modulated the layer hybridization and interplay between many-body interactions of excitons in a van-der-Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, the dipole-dependent properties and transport of excitons with different degrees of hybridization were characterized. Moreover, it was found that constant emission quantum yields of the transporting species as a function of excitation power with dominating radiative decay mechanisms over nonradiative ones, a fundamental requirement for efficient excitonic devices.

The findings provide a complete picture of the many-body effects in the transport of dilute exciton gases and have crucial implications for the study of emerging states of matter, such as Bose-Einstein condensation, as well as for optoelectronic applications based on exciton propagation.

Publication

F. Tagarelli, E. Lopriore, D. Erkensten, R. Perea-Causín, S. Brem, J. Hagel, Z. Sun, G. Pasquale, K. Watanabe, T. Taniguchi, E. Malic, A. Kis
Electrical control of hybrid exciton transport in a van der Waals heterostructure
Nat. Photon. (2023) DOI:10.1038/s41566-023-01198-w

Contact

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

When electrons dress up in light – Publication by B11 (Güdde/Höfer) in Nature

/in /by

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

Shape control in 2D molecular nanosheets by tuning anisotropic intermolecular interactions and assembly kinetics – Publication by A2 (Witte) and A8 (Koert/Dürr)

/in /by

In a new publication in Nature Communications, the groups of Gregor Witte (A2) and Ulrich Koert (A8) introduce a new concept that allows to control the mesoscopic shape of 2D molecular islands grown on weakly interacting substrates like MoS2 without affecting their nanoscopic packing motif.

Schematic representation of the growth and desorption kinetics of partially fluorinated pentacene nanosheets (Image: Maximilian Dreher, Copyright CC-BY 4.0)

Hybrid heterostructures of transition metal dichalcogenides (TMDCs) and molecular materials combine the excellent charge carrier transport properties of TMDCs with the possibility to tailor optoelectronic properties of organic semiconductors. Since molecular materials often decompose upon exposure to radiation, lithographic patterning techniques established for inorganic materials are usually not applicable for the fabrication of organic nanostructures. Compared to metallic substrates, where molecule-substrate interactions dominate the mutual intermolecular interactions, the latter becomes decisive for adlayers grown on weakly interacting substrates such as TMDCs. This fact can be used to employ electrostatic Coulomb interactions between specifically designed, partially fluorinated pentacene derivatives to tailor the intermolecular interactions.

Using scanning tunneling microscopy, Maximilian Dreher and Pierre Dombrowski found that while the anisotropic attractive Coulomb forces between partially fluorinated pentacenes determine the molecular packing motif, distinctly elongated nanosheets are formed at submonolayer coverage, where the direction of elongation is different between directly grown nanosheets and those prepared by partial desorption of a complete molecular monolayer. Using kinetic Monte Carlo (MC) simulations, it could be shown that the nanosheet formation is not driven by an energy minimization of the intermolecular interactions. Instead, the sheet shape is determined by the evolution of individual molecules either attaching to or detaching from the nanosheets. Further MC simulations demonstrate statistically, that their formation is determined i) by the geometrical anisotropy of the intermolecular interactions and ii) by the kinetics during growth and desorption. By comparison of the behavior of differently fluorinated molecules, both experimentally and computationally, important design rules for molecules could be derived.

Publication

M. Dreher, P.M. Dombrowski, M.W. Tripp, N. Münster, U. Koert, G. Witte
Shape control in 2D molecular nanosheets by tuning anisotropic intermolecular interactions and assembly kinetics
Nat. Comm. 14 (2023) 1554 DOI:10.1038/s41467-023-37203-7

Contact

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