Size-Dependent Multiexciton Dynamics Governs Scintillation From Perovskite Quantum Dots

Over the last few years, nanocomposite scintillators have emerged in the attempt to address the drawbacks of inorganic scintillator crystals (expensive, generally slow and hardly scalable) and plastic scintillators (low density and efficiency) and at the same time capitalize on their strengths. Nanocomposite scintillators feature optical-grade plastic matrices as the waveguiding component, while high-Z semiconductor nanocrystal (NCs) synthesized using scalable chemical techniques provide scintillation. Importantly, using NCs as nanoscintillators in polymeric waveguides overcome the scalability limitations of conventional materials and also possibly enhance the scintillation performance. This is due to the unique photophysics of quantum-confined materials, providing size- tunable emission spectra matching with the spectral sensitivity of light detectors and ultrafast sub- nanosecond scintillation kinetics resulting from recombination of multi-exciton generated upon interaction with ionizing radiation, as demonstrated recently across various classes of NCs. In this context, lead halide perovskite LHP-NCs gained increasing attention, with CsPbBr3 emerging as the dominant player. Lead halides NCs feature an effective high-Z, remarkable resistance to radiation, extensive scalability facilitated by low-temperature methods, and efficient scintillation owing to the unique tolerance of their luminescence to structural defects. However, despite its potential, the multiexciton regime suffers from detrimental losses via Auger recombination (AR), which in NCs is not constrained by momentum conservation as it is in bulk materials and its rate increases with the inverse of the particle volume, posing a significant challenge in NC-based technologies such as lasers, light emitting diodes or solar cells. Although the effects of AR on such technologies have been extensively studied over the years, its impact on the scintillation of NCs remains an open question. Here we aim to fill this gap by investigating the effect of particle size on the scintillation efficiency and kinetics of CsPbBr3 NCs ranging in size from 3 nm to 15 nm, with tunable emission from 470 nm to 520 nm and AR rates spanning nearly two orders of magnitude. The dependence of scintillation efficiency and timing on particle size is first theoretically analysed and then experimentally validated, yielding a complicated parametrical space where the initial exciton population per NC and the AR rate are the key elements. We have evaluated all the parameters necessary to describe the recombination mechanisms using a combination of optical spectroscopy and scintillation experiments on two sets of CsPbBr3 NCs synthesised independently by two laboratories, in order to generalise the observed trends. AR was found to be efficient under ionising excitation. The scintillation efficiency was largely dominated by the single exciton PL efficiency in all samples, with an essentially negligible effect of AR. This is important as it highlights the importance of optimising the emission process in the single exciton regime to maximise the scintillation yield, while NC engineering to suppress AR plays a minor role. It is also noteworthy that the acceleration of the ultrafast sub-ns biexciton contribution to scintillation decay by AR results in an increasingly faster effective scintillation lifetime, which together with the invariant light output leads to faster estimated coincidence time resolution (CTR) values for small NCs, suggesting a possible strategy for fast timing technologies.