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.

The typical synthesis protocol for blue-emitting CdSe nanoplatelets (NPLs) yields particles with extended lateral dimensions and large surface areas, resulting in NPLs with poor photoluminescence quantum efficiency. We have developed a synthesis protocol that achieves an improved control over the lateral size, by exploiting a series of long-chained carboxylate precursors that vary from cadmium octanoate (C8) to cadmium stearate (C18). The length of this metallic precursor is key to tune the width and aspect ratio of the final NPLs, and for the shorter chain lengths, the synthesis yield is improved. NPLs prepared with our procedure possess significantly enhanced photoluminescence quantum efficiencies, up to 30%. This is likely due to their reduced lateral dimensions, which also grant them good colloidal stability. As the NPL width can be tuned below the bulk exciton Bohr radius, the band edge blue-shifts, and we constructed a sizing curve relating the NPL absorption position and width. Further adjusting the synthesis protocol, we were able to obtain even thinner NPLs, emitting in the near-UV region, with a band-edge quantum efficiency of up to 11%. Results pave the way to stable and efficient light sources for applications such as blue and UV light-emitting devices and lasers.

Colloidal semiconductor nanoplatelets exhibit strong quantum confinement for electrons and holes as well as excitons in one dimension, while their in-plane motion is free. Because of the large dielectric contrast between the semiconductor and its ligand environment, the Coulomb interaction between electrons and holes is strongly enhanced. By means of one- and two-photon photoluminescence excitation spectroscopy, we measure the energies of the 1S and 1P exciton states in CdSe nanoplatelets with thicknesses varied from 3 up to 7 monolayers. By comparison with calculations, performed in the effective mass approximation with account of the dielectric enhancement, we evaluate exciton binding energies of 195–315 meV, which is about 20 times greater than that in bulk CdSe. Our calculations of the effective Coulomb potential for very thin nanoplatelets are close to the Rytova-Keldysh model, and the exciton binding energies are comparable with the values reported for monolayer-thick transition metal dichalcogenides.

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