Understanding excitonic dynamics in two-dimensional semiconducting transition metal dichalcogenides is important for developing their optoelectronic applications. Recently, transient absorption techniques based on resonant excitonic absorption have been used to study various aspects of excitonic dynamics in these materials. The transient absorption in such measurements originates from phase-space state filling, bandgap renormalization, or screening effects. Here we report a new method to probe excitonic dynamics based on exciton intraband absorption. In this Drude-like process, probe photons are absorbed by excitons in their intraband excitation to higher energy states, causing a transient absorption signal. Although the magnitude of the transient absorption is lower than that of the resonant techniques, the new method is less restrictive on the selection of probe wavelength, has a larger linear range, and can provide complementary information on photocarrier dynamics. Using the WS 2 monolayer and bulk samples as examples, we show that the new method can probe exciton–exciton annihilation at high densities and reveal exciton formation processes. We also found that the exciton intraband absorption cross section of the WS 2 monolayer is on the order of 10 −18 cm 2 .
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Controlling exciton transport in monolayer MoSe 2 by dielectric screening
Due to their atomic thinness with reduced dielectric screening, two-dimensional materials can possess a stable excitonic population at room temperature. This is attractive for future excitonic devices, where excitons are used to carry energy or information. In excitonic devices, controlling transport of the charge-neutral excitons is a key element. Here we show that exciton transport in a MoSe 2 monolayer semiconductor can be effectively controlled by dielectric screening. A MoSe 2 monolayer was partially covered with a hexagonal boron nitride flake. Photoluminescence measurements showed that the exciton energy in the covered region is about 12 meV higher than that in the uncovered region. Spatiotemporally resolved differential reflection measurements performed at the junction between the two regions revealed that this energy offset is sufficient to drive excitons across the junction for about 50 ps over a distance of about 200 nm. These results illustrate the feasibility of using van der Waals dielectric engineering to control exciton transport and contribute to understanding the effects of the dielectric environment on the electronic and optical properties of two-dimensional semiconductors.
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- Award ID(s):
- 1505852
- NSF-PAR ID:
- 10180860
- Date Published:
- Journal Name:
- Nanoscale Horizons
- Volume:
- 5
- Issue:
- 1
- ISSN:
- 2055-6756
- Page Range / eLocation ID:
- 139 to 143
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9NH00462A
