Interface engineering of charge-transfer excitons in 2D lateral heterostructures – Publication by B9 (Malic) in Nature Communications

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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

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

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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

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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