Sub-100 fs Formation of Dark Excitons in Monolayer WS₂

Summary, in English

Two-dimensional semiconducting transition metal dichalcogenides are promising materials for optoelectronic applications due to their strongly bound excitons. While bright excitons have been thoroughly scrutinized, dark excitons have been much less investigated, as they are not directly observable with far-field spectroscopy. However, with their nonzero momenta, dark excitons are significant for applications requiring long-range transport or coupling to external fields. We access such dark excitons in WS₂ monolayer using transient photoemission electron microscopy with subdiffraction limited spatial resolution (75 nm) and exceptionally high temporal resolution (13 fs). Image time series of the monolayer are recorded at several different fluences. We directly observe the ultrafast formation of dark K-Λ excitons occurring within 14-50 fs and follow their subsequent picosecond decay. We distinguish exciton dynamics between the monolayer’s interior and edges and conclude that the picosecond-scale evolution of dark excitations is defect-mediated while intervalley scattering is not affected by the defects.

• Author: Pavel V. Kolesnichenko; Lukas Wittenbecher; Qianhui Zhang; Run Yan Teh; Chandni Babu; Michael S. Fuhrer; Anders Mikkelsen; Donatas Zigmantas
• Department/s: Chemical Physics; NanoLund: Centre for Nanoscience; LTH Profile Area: Nanoscience and Semiconductor Technology; LTH Profile Area: Photon Science and Technology; Synchrotron Radiation Research
• Publishing year: 2024-11-20
• Language: English
• Pages: 14663-14670
• Publication/Series: Nano Letters
• Volume: 24
• Issue: 46
• Links: http://dx.doi.org/10.1021/acs.nanolett.4c03807
• Document type: Journal article
• Publisher: The American Chemical Society (ACS)
• Topic: Condensed Matter Physics (including Material Physics, Nano Physics)
• Keywords: 2D materials; excitons; femtosecond; microscopy; monolayers; photoemission
• Status: Published
• ISBN/ISSN/Other: ISSN: 1530-6984