Non-equilibrium diffusion of dark excitons in atomically thin semiconductors

Roberto Rosati; Koloman Wagner; Samuel Brem; Raül Perea-Causín; Jonas D. Ziegler; Jonas Zipfel; Takashi Taniguchi; Kenji Watanabe; Alexey Chernikov; Ermin Malic

doi:10.1039/D1NR06230A

Non-equilibrium diffusion of dark excitons in atomically thin semiconductors†

Roberto Rosati,

*^a Koloman Wagner,

^bf Samuel Brem,

^a Raül Perea-Causín,

^c Jonas D. Ziegler,

^bf Jonas Zipfel,

^bg Takashi Taniguchi,

^d Kenji Watanabe,

^e Alexey Chernikov

^bf and Ermin Malic

^ac

Author affiliations

* Corresponding authors

^a Department of Physics, Philipps-Universität Marburg, Renthof 7, D-35032 Marburg, Germany
E-mail: roberto.rosati@physik.uni-marburg.de

^b Department of Physics, University of Regensburg, Regensburg D-93053, Germany

^c Chalmers University of Technology, Department of Physics, 412 96 Gothenburg, Sweden

^d International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-004, Japan

^e Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-004, Japan

^f Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden, 01062 Dresden, Germany

^g Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Abstract

Atomically thin semiconductors provide an excellent platform to study intriguing many-particle physics of tightly-bound excitons. In particular, the properties of tungsten-based transition metal dichalcogenides are determined by a complex manifold of bright and dark exciton states. While dark excitons are known to dominate the relaxation dynamics and low-temperature photoluminescence, their impact on the spatial propagation of excitons has remained elusive. In our joint theory-experiment study, we address this intriguing regime of dark state transport by resolving the spatio-temporal exciton dynamics in hBN-encapsulated WSe₂ monolayers after resonant excitation. We find clear evidence of an unconventional, time-dependent diffusion during the first tens of picoseconds, exhibiting strong deviation from the steady-state propagation. Dark exciton states are initially populated by phonon emission from the bright states, resulting in creation of hot (unequilibrated) excitons whose rapid expansion leads to a transient increase of the diffusion coefficient by more than one order of magnitude. These findings are relevant for both fundamental understanding of the spatio-temporal exciton dynamics in atomically thin materials as well as their technological application by enabling rapid diffusion.

Nanoscale

Non-equilibrium diffusion of dark excitons in atomically thin semiconductors†

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