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

Abstract

Abstract2D 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.

Stimulated Emission through an Electron–Hole Plasma in Colloidal CdSe Quantum Rings

“…Clearly, the reduced confinement in the out-of-plane direction compared to thin nanoplatelets, combined with strong carrier–carrier interactions evidenced by strong band gap renormalization and comparatively fast radiative bimolecular recombination, strongly reduces the excitonic nature of the fundamental excitations in QRs. We did not observe any correlation between the QR’s unique g = 1 topology and the ultrafast phenomena discussed here which could be due to limited rotational symmetry, as observed similarly by Hartmann et al, or carrier localization at room temperature . Overall, CdSe QRs at room temperature behave more as classical quantum wells, providing a unique platform for studying unbound charge carriers in colloidal materials.…”

“…We did not observe any correlation between the QR’s unique g = 1 topology and the ultrafast phenomena discussed here which could be due to limited rotational symmetry, as observed similarly by Hartmann et al, 10 or carrier localization at room temperature. 44 Overall, CdSe QRs at room temperature behave more as classical quantum wells, providing a unique platform for studying unbound charge carriers in colloidal materials.…”

Dark and Bright Excitons in Halide Perovskite Nanoplatelets

“…iv) Finally, at low temperatures, the exciton giant oscillator strength connected with the coherent exciton motion in NPLs leads to a further increase of the radiative recombination rate. [ 14 ] In CdSe NPLs, the shortening of the radiative decay time [ 3 , 15 ] and giant oscillator transition strength [ 16 , 17 ] have been reported already, suggesting that all the above‐discussed phenomena could bring the radiative decay time of various NPLs down to the range of tens to a few hundreds of ps.…”

Ultralow Threshold Room Temperature Polariton Condensation in Colloidal CdSe/CdS Core/Shell Nanoplatelets

“…In addition, the ground state exciton has a giant oscillator strength (15–23 Debye) due to the exciton center of mass coherent motion. [ 14 ] Therefore, nanoplatelets are beneficial for studying strong light‐matter coupling in microcavities. Lucas et al.…”

“…In addition, the ground state exciton has a giant oscillator strength (15-23 Debye) due to the exciton center of mass coherent motion. [14] Therefore, nanoplatelets are beneficial for studying strong light-matter coupling in microcavities. Lucas et al [15] observed the anti-crossing behavior of polariton using CdSe nanoplatelets and a tunable planar microcavity with which the coupling constant was estimated to be 33 meV.…”