Alessandro Surrente, Wroclaw University of Science and Technology, Paulina Plochocka, Wroclaw University of Science and Technology and LNCMI Toulouse.
The synthesis of colloidal nanocrystals with near-unity photoluminescence (PL) quantum yields has vastly extended the potential of metal-halide perovskites for solid-state lighting and display applications. It is possible to template the growth of nanocrystals to form planar, ultrathin perovskite sheets embedded between long organic molecules, which stabilize the colloids, referred to as nanoplatelets, shown schematically in the Figure (a). These colloidal quantum wells are of interest as highly efficient emitters in the blue spectral region. In the context of light emitters, the splitting between optically dark and optically bright excitons is of paramount importance. After photogeneration, excitons usually relax to the lowest-lying dark state, which is detrimental for the device efficiency. We performed optical-spectroscopy measurements with an applied in-plane magnetic field to mix the bright and dark excitonic states of CsPbBr3 -based nanoplatelets with a different thickness of the lead-halide slab, ranging from two to four layers of lead-halide octahedral plane. The induced brightening of the dark state allows us to directly observe an enhancement of the PL signal on the low-energy side of the spectrum, see Figure (b), which we explain as the magnetic-field induced brightening of the dark state. In-plane magnetic fields allow us to extract accurately the energy splitting between the dark and bright excitons directly, without resorting to further measurements or modelling, see Figure (c). We also evaluate the ratio between the intensities of the magnetic-field-brightened dark state and of the bright state. This ratio increases quadratically, as expected, but the experimental data can be described only by assuming a temperature of the excitons considerably larger than the lattice temperature, as shown in Figure (d). Thus, the evolution of the PL signal in magnetic field suggests that at low temperatures the exciton population is not fully thermalized, which is indicative of the existence of a phonon bottleneck.
Figure: (a) Top: schematic of the crystal structure of lead-halide perovskite nanoplatelet. Bottom: spatial dependence of the band gap and the dielectric constant. (b) Magneto-PL spectra of nanoplatelets. BX: bright exciton. DX: dark exciton. (c) Measured bright-dark splitting as a function of nanoplatelet thickness. (d) PL intensity ratio between dark and bright exciton states for the three nanoplatelet thicknesses investigated as a function of the applied magnetic field. Full circles represent experimental points. The curves are calculated using the temperature indicated in the legend.
Thickness-dependent dark-bright exciton splitting and phonon bottleneck in CsPbBr3-based nanoplatelets revealed via magneto-optical spectroscopy, S. Wang, M. Dyksik, C. Lampe, M. Gramlich, D. K. Maude, M. Baranowski, A. S Urban, P. Plochocka, and A. Surrente, Nano Letters 22, 7011 (2022).
https://pubs.acs.org/doi/10.1021/acs.nanolett.2c01826
Contact: alessandro.surrente@pwr.edu.pl, paulina.plochocka@lncmi.cnrs.fr
