Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect

Geiregat et al. Light: Science & Applications (2021)10:112 https://doi.org/10.1038/s41377-021-00548-z ARTICLE Official journal of the CIOMP 2047-7538 www.nature.com/lsa Open Access Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Pieter Geiregat 1,2, Carmelita Rodá 1,2, Ivo Tanghe1,2,3, Shalini Singh 4, Alessio Di Giacomo1, Delphine Lebrun1, Gianluca Grimaldi5, Jorick Maes1,2, Dries Van Thourhout 2,3, Iwan Moreels 1,2, Arjan J. Houtepen6 and Zeger Hens1,2 Abstract 2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm2. Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems. Introduction Colloidal quantum wells of CdSe1,2 have attracted much attention in the past years due to narrow, exciton-related absorption features, an increased light-matter interaction, strong light amplification3–7 and exciton-polariton formation8,9. As two-dimensional (2D) materials, these socalled nanoplatelets fall in between atomically thin 2D materials, such as transition metal di-chalcogenides10,11, and the usually much thicker epitaxially grown quantum wells. Moreover, being capped by organic ligands, Correspondence: Pieter Geiregat () 1 Physics and Chemistry of Nanostructures, Department of Chemistry, Ghent University, Gent, Belgium 2 Center for Nano and Biophotonics, Ghent University, Gent, Belgium Full list of author information is available at the end of the article nanoplatelets are intrinsically embedded within a low permittivity environment. This dielectric confinement substantially enhances the exciton binding energy12. While the exciton binding energy of 15 meV in bulk CdSe should increase to 60 meV in a 2D CdSe quantum well13, typical estimates amount to ~190 meV for 4.5 monolayer (1.21 nm) thick CdSe nanoplatelets3,7,14,15. With such binding energies, excitons in nanoplatelets are stable quasi-particles at room temperature, and exciton-related transitions have been used to develop room temperature nanoplatelet-based light emitting diodes16 and lasers3. At cryogenic temperatures, the heavy-hole bright exciton in CdSe nanoplatelets was found to exhibit a radiative decay rate of ~1 ps−1, a rate that also determined the exciton dephasing14. Similar observations were made in © The Author(s) 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Geiregat et al. Light: Science & Applications (2021)10:112 the case of epitaxial quantum wells17,18, and attributed to the large in-plane coherence area of the exciton center-ofmass motion in these systems. Intriguingly, recent reports based on state-filling models proposed that even at room temperature, this coherence might be close to 100 nm2, a number that seems incompatible with the nanosecond radiative lifetime reported by various authors19–21. In addition, several studies indicated the potential for strong coupling of excitonic transitions with the light field at room temperature using 4.5 monolayer CdSe nanoplatelets, a feat that requires narrow transition lines with large oscillator strength8,9. Using an elaborate fitting procedure of exciton-polariton dispersion curves, heavy hole transition dipole moments of 575 Debye (D) at room temperature were extracted. Although promising, such dipole moments seem disruptively large as compared to literature reports on comparable material systems, such as epitaxial quantum wells (6 D)22, three and twodimensional perovskites (46 and 15 D, respectively)23,24, carbon nanotubes (12 D)25, and transition metaldichalcogenides (7 D for WSe226, 51 D for WS227, and 9 D for MoSe2 at 77K)28. In studies, the optical Stark effect (OSE) is used as a method to extract the desired dipole moment22,27. Using OSE spectroscopy, one pumps the material using a femtosecond pump pulse detuned relative to the exciton transition and measures the induced energy shift of the exciton using a broad, white-light probe pulse. This method alleviates the need for electrical contacting29 and does not rely on real charge carriers, thereby eliminating any spurious effects of defect trapping and assumptions on state-filling or electron-hole overlap19,20. Recent work by Diroll showed that also CdSe nanoplatelets display such a Stark effect and dipole moments in the range 15–23 D were extracted, numbers which are very much in line with other 2D materials30. However, translating such dipole moments into dimensionless oscillator strengths leads to numbers of around one. Since oscillator strengths of 5–15 are routinely found for 0D colloidal quantum dots, such a result questions (...truncated)