Magnetically brightened dark excitons in two-dimensional metal halide perovskites

The synthesis of colloidal nanocrystals and nanoplatelets with near-unity photoluminescence (PL) quantum yields, shown schematically in Fig. 1(a),  has vastly extended the potential of metal halide perovskites for solid-state lighting and display applications. In the context of light emitters, the splitting between optically dark and optically bright excitons is of paramount importance. 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 planes. 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 Fig. 1(b). 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 Fig. 1(c)]. We also evaluate the ratio between the intensities of the magnetic field-brightened dark state and of the bright state, which can be fitted only by assuming a temperature of the excitons considerably larger than the lattice temperature, as shown in Fig. 1(d). Thus, the exciton population is not fully thermalized, which is indicative of the existence of a phonon bottleneck.

Reference – Shuli Wang, Mateusz Dyksik, Carola Lampe, Moritz Gramlich, Duncan K Maude, Michał Baranowski, Alexander S Urban, Paulina Plochocka, Alessandro Surrente, Nano Letters 22 7011 (2022)

Figure 1. (a) Top: schematic of 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 inset.