DOI: https://doi.org/10.29363/nanoge.incnc.2021.013
Publication date: 8th June 2021
Halide perovskite nanocrystals (NCs) are a promising platform for light-emitting devices, including LEDs and single-photon emitters. Excitonic properties can be precisely tuned via the NC shape for improved device performance. Control of dimensionality is particularly powerful since excitons show dramatically different properties in lower-dimensional structures than 3D NCs. We explore the thickness-dependent fine structure of excitons in square Csn−1PbnBr3n+1 nanoplatelets through time- and temperature-resolved photoluminescence measurements. In parallel, we build a two-dimensional effective-mass model of these excitons, which are weakly confined in-plane but strongly confined in the out-of-plane direction. The model introduces the critical effect of shape anisotropy on the band-edge Bloch functions and presents a new numerical approach for calculating long-range exchange interaction. The predicted fine structure is very different from that observed in 3D NCs and is in good agreement with observed spectral shifts. Notably, the bright excitons show a shape anisotropy-induced splitting of several meVs, meaning they cannot be considered a single degenerate state. Taking this into account, we expand the traditional two-level decay model to include both the in-plane-polarized and out-of-plane-polarized bright exciton states. Using the three-level model, we show that the experimentally determined decay rates are consistent with the calculated fine structure. The results of this work may be readily extended to other materials and NC shapes, and will help to unleash the full potential of dimensionality control in halide perovskite optoelectronics.
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