Understanding the Blinking Mechanism in Strongly Confined Lead Bromide Perovskite Quantum Dots

Strongly confined lead bromide perovskite quantum dots (PQDs), benefit from their high photoluminescence (PL) quantum yield, defect tolerance and fast radiative emission rate, possess the potential of making high purity single photon emission sources that are “bright and blue” (high emission rate with high energy of emitted photon), which can be used directly in photonics for quantum information technologies, or as a quantum excitation light source to trigger quantum photon emission from traditional emitters.<br/>The phenomenon that the quantum dots (QDs) have PL intermittency greatly exceeds their radiative lifetime, commonly known as “blinking”, hinders their potential applications. Decades of effort has been put on studying the blinking mechanism of traditional QDs and currently three types of mechanisms are widely acknowledged: 1. Auger recombination from trions formed by exciting a charged QD (with the opposite charge carrier being deep-trapped or ejected), namely “Auger-blinking”; 2. photoactivation of interband states causing the band-edge charge carriers to non-radiatively recombine, namely “BCNR-blinking”; 3. photo- or electrical activation of intraband states causing the interception of hot-carriers, namely “HC-blinking”. However, few studies have focused on strongly confined colloidal PQDs, of which the materials are just recently made available by advances in syntheses. Compared to traditional II-VI or III-V QDs, PQDs are more ionic with friable surface, and are believed free from deep hole traps that commonly exist in II-VI QDs to cause Auger-blinking.<br/>In this work, the blinking mechanism of a model type of single CsPbBr₃ PQDs were studied in comparison to commercial CdSe/ZnS core-shell QDs. The latter is considered standard samples to make lateral comparison with previous studies. Mono-dispersed colloidal PQDs with &gt;80% PL quantum yield (75% for single PQDs, estimated with PL lifetime) are prepared. The single PQD emission was measured, and the blinking statistics associated with fluorescence lifetime-intensity distribution (FLID) analyses were performed to study the PL “on”/“off” states distribution and corresponding lifetimes.<br/>The PL quantum yield of single CdSe/ZnS QDs varies from 90% to 50%, and there coexist Auger-blinking and BCNR-blinking. FLID analyses show that the BCNR-blinking is dominating at low excitation power, while Auger-blinking become more significant at increased power, where the “off” state also increases. We have not observed HC-blinking in CdSe QDs. The results are consistent with previous reports.<br/>In CsPbBr₃ PQDs, higher excitation power also leads to higher “off” time. However, the blinking mechanism is different in almost all other aspects. At low power, the FLID analyses suggest Auger blinking is the major contributor. Surprisingly, despite the trion Auger recombination is expected to be fast due to the small size, the “off” probability of PQDs is rather insignificant compared to CdSe QDs with similar “on” brightness that show Auger-blinking. The result indicates that the Auger-like behavior in PQDs stems from a “weak trion”, which consists of a charge in PQD is partially stabilized by the distortion of the lattice (a polaron) with an exciton. The latter weakly couples with the polaron and yields a slow Auger pathway compared to radiative recombination. With increased power the PQDs show BCNR-blinking which is attributed to the friability of the un-shelled PQDs surface.<br/>In conclusion, our strongly confined PQDs exhibit suppressed blinking, despite Auger process is expected to be fast as a result of their small size. The blinking suppression is achieved possibly by formation of polaron that weakens the Auger effect. These results illustrate the direction for rational design and optimization of lead halide PQDs towards this application.