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Colloidal CdSe nanoplatelets (NPLs) are unique systems to study two-dimensional excitons and excitonic complexes. However, while absorption and emission of photons through exciton formation and recombination have been extensively quantified, few studies have addressed the exciton-biexciton transition. Here, we use cross-polarized pump- probe spectroscopy to measure the absorption coefficient spectrum of this transition and determine the biexciton oscillator strength (fBX). We show that fBX is independent of the NPL area and that the concomitant biexciton area (SBX) agrees with predictions of a short-range interaction model. Moreover, we show that fBX is comparable to the oscillator strength of forming localized excitons at room temperature while being unaffected itself by center-of-mass localization. These results confirm the relevance of biexcitons for light-matter interaction in NPLs. Moreover, the quantification of the exciton-biexciton transition introduced here will enable researchers to rank 2D materials by the strength of this transition and to compare experimental results with theoretical predictions.

CdSe nanoplatelets emitting in the blue region of the visible spectrum are promising candidates for light-amplification and light-emitting diode applications. For this reason, recently an improved synthesis protocol for 3.5 monolayer CdSe NPLs was put forward, leading to photoluminescence (PL) quantum yields up to 30%. [1] However, due to the high surface-to-volume ratio, blue emitting core-only nanoplatelets still suffer from charge trapping that results in intra-gap radiative emission from the defect states. As such, it remains an open question to which extent these defects affect their ultrafast properties as well, including the development of net stimulated emission. Here, we first show that optimized 3.5 ML CdSe nanoplatelets show optical gain between 480-520 nm due to stimulated emission along the biexciton-to-exciton transition.[2] Next, we compare the gain characteristic of core-only CdSe with core/crown CdSe/CdS 3.5 ML NPLs of increasing crown volume. The crown procedure results in both a faster exciton radiative recombination rate and an improvement of the PL quantum yield up to 60%. Our results show that crowned samples exhibit overall a lower gain threshold with compared to core-only CdSe nanoplatelets. On the other hand, we observe comparable gain lifetime regardless of the crowning procedure due to residual ultrafast charge trapping not alleviated by the crown growth. Our results pave the way towards accurate design of ultra-thin quasi two-dimensional systems for blue spectrum light amplifiers and lasers based on. [1] Di Giacomo, A.; Rodà, C.; Khan, A. H.; Moreels, I. Colloidal Synthesis of Laterally Confined Blue-Emitting 3.5 Monolayer CdSe Nanoplatelets. Chem. Mater. 2020, 32, 9260-9267. [2] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644.

Radiation detection is of utmost importance in fundamental scientific research, as well as medical diagnostics, homeland security, environmental monitoring and industrial control. Lead halide perovskites (LHPs) are attracting growing attention as high-atomic-number materials for next-generation scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional materials, it is necessary for LHPs to conjugate high scintillation yields with emission stability under high doses of ionizing radiation. To date, no definitive solution has been devised to optimize the scintillation efficiency and kinetics of LHPs and nothing is known of their radiation hardness for doses above a few kilograys, to the best of our knowledge. Here we demonstrate that CsPbBr3 nanocrystals exhibit exceptional radiation hardness for γ-radiation doses as high as 1 MGy. Spectroscopic and radiometric experiments highlight that despite their defect tolerance, standard CsPbBr3 nanocrystals suffer from electron trapping in dense surface defects that are eliminated by post-synthesis fluorination. This results in >500% enhancement in scintillation efficiency, which becomes comparable to commercial scintillators, and still retaining exceptional levels of radiation hardness. These results have important implications for the widespread use of LHPs in ultrastable and efficient radiation detectors.