Non-equilibrium diffusion of dark excitons in atomically thin semiconductors. Issue 47 (25th November 2021)

• Record Type: Journal Article
• Authors: Rosati, Roberto; Wagner, Koloman; Brem, Samuel; Perea-Causín, Raül; Ziegler, Jonas D.; Zipfel, Jonas; Taniguchi, Takashi; Watanabe, Kenji; Chernikov, Alexey; Malic, Ermin
• Is Part Of: Nanoscale. Volume 13:Issue 47(2021)
• Page Start: 19966
• Page End: 19972
• Publication Date: 2021-11-25
• Subjects: Nanoscience -- Periodicals; Nanotechnology -- Periodicals; 620.505
• DOI: 10.1039/d1nr06230a
• Languages: English
• ISSNs: 2040-3364

Abstract

Abstract : Combining microscopic theory and spatiotemporal photoluminescence experiments we reveal an unconventional, time-dependent exciton diffusion in atomically thin semiconductors. This behavior originates from hot dark excitons with large excess energies. 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.