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.
Highlights
Introduction Colloidal quantum wells ofCdSe1,2 have attracted much attention in the past years due to narrow, exciton-related absorption features, an increased light-matter interaction, strong light amplification[3,4,5,6,7] and exciton-polariton formation[8,9]
Using an elaborate fitting procedure of exciton-polariton dispersion curves, heavy hole transition dipole moments of 575 Debye (D) at room temperature were extracted. 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]
Since a nanoplatelet can host multiple localized excitons, center-of-mass localization can strongly reduce the oscillator strength of a single exciton transition—as measured by the Stark-effect—without affecting the overall oscillator strength of the exciton absorption. Translating this interpretation into a quantum mechanical description, we show that the ratio between the oscillator strengths yields the coherence area of the heavyhole bright exciton at room temperature, resulting in a value of ≈6.1 nm[2]
Summary
Introduction Colloidal quantum wells ofCdSe1,2 have attracted much attention in the past years due to narrow, exciton-related absorption features, an increased light-matter interaction, strong light amplification[3,4,5,6,7] and exciton-polariton formation[8,9]. Using an elaborate fitting procedure of exciton-polariton dispersion curves, heavy hole transition dipole moments of 575 Debye (D) at room temperature were extracted 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]
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