Exciton Fine-Structure in Halide Perovskite Nanoplatelets

Halide perovskite nanocrystals have emerged as excellent light sources for light-emitting devices and lasers due to quantum yields approaching unity and an emission wavelength tunable throughout the visible range. As recently shown, it is the only material currently known that exhibit Rashba effect-induced bright-dark exciton inversion, greatly enhancing radiative efficiency. Nanoplatelets, colloidal semiconductor quantum wells, have additional advantages for light emission, as quantum confinement and dielectric confinement substantially enhance their radiative rates and outcoupling efficiencies.<br/>Here, we investigate the excitonic fine structure of thickness tunable halide perovskite nanoplatelets by merging time- and temperature-resolved photoluminescence spectroscopy with an effective mass model. We find a thickness-dependent bright-dark exciton splitting reaching up to 33 meV, the largest ever reported value. Additionally, we find a splitting of the bright exciton into in-plane and out-of-plane polarized states by up to 16 meV. Due to these immense splitting energies, the radiative properties of the nanoplatelets will also be effected at room temperature. The experimental results obtained in this paper are in excellent agreement with the novel theory, which takes quantum confinement and dielectric confinement anisotropy into account. Notably, the model we develop is applicable to any semiconductor nanoplatelet and generalized for any nanostructure.<br/>We further confirm the model through single NPL PL spectroscopy at low temperatures. Additionally, we investigate a giant oscillator strength transition (GOST) of the dark exciton state and quantify multiexcitonic effects (trions, biexcitons, exciton-exciton annihilation).