Jialu Li

Delayed emission, a long-lived nonexponential photoluminescence tail, is widely observed in quantum dots (QDs), but its microscopic origin and relationship with blinking and the quantum-confined Stark effect (QCSE) remain unclear. Here, we combine single-QD spectroscopy, time-gated photon analysis, and Monte Carlo simulations across CsPbX3 perovskite, CdSe-based, and InP-based QDs to identify a potential mechanism. Delayed emission correlates with band-edge carrier-trapping blinking, but is weak in QDs dominated by trion-mediated Auger blinking, suggesting delayed emission is not universally tied to charge-separated states. Delayed photons originate from nanosecond-scale carrier trapping and detrapping at nonradiative recombination centers: transiently trapped carriers return to the band edge and recombine radiatively. Time-gated analysis distinguishes this process from QCSE-driven intensity-lifetime fluctuations on slower blinking time scales. Furthermore, we demonstrate that delayed photons compromise the single-photon purity at a fixed excitation repetition rate. These findings establish a general framework of delayed emission in QDs.

Colloidal quantum dots (QDs) are promising optical gain materials that require a reduction in the threshold to reach their full potential. While QD charging theoretically reduces the threshold to zero, its effectiveness has been limited by strong Auger recombination and unstable charging. In this study, we theoretically determine the optimal combination of charging number and Auger recombination to minimize the lasing threshold. Experimentally, we develop stable, self-charged perovskite quantum rods (QRs) as an alternative to QDs via the state engineering and Mn (manganese) doping strategy. A two-order-of-magnitude reduction in nonradiative Auger recombination enables QRs to support a charging number of up to 6. We then achieve QR liquid lasing with a sub- ¹ / ₁₀ exciton threshold (an average of 0.098 excitons per QR) using 5-nanosecond pulse pumping. This threshold is exceptionally low among all reported QD lasers. These achievements demonstrate the potential of specially engineered QRs as excellent gain media and pave the way for their applications.

InP/ZnSe/ZnS quantum dots (QDs) are promising candidates for advancing optoelectronic devices. However, their applications are limited by their low emission efficiency caused by photo-oxidation. In this study, we investigate the impact of photo-oxidation on the emission of InP/ZnSe/ZnS QDs at the ensemble and single-particle levels. Transient absorption spectroscopy reveals that photo-oxidation-induced ultrafast exciton trapping exhibits complex, multitime scale decay dynamics ranging from sub-nanoseconds to picoseconds, indicating that photo-oxidation-induced surface defects form high-density trap states with a broad, continuous energetic distribution. Single-QD spectroscopy shows that these trap states act as multiple nonradiative recombination centers that trigger band-edge carrier blinking. The extensive formation of photo-oxidation-induced defects results in photoluminescence (PL) quenching of single QDs. Monte Carlo simulations reproduce ultrafast exciton trapping-induced PL blinking and quenching and quantify the nonradiative recombination rates involved in these processes. These findings provide new insights into the photodegradation of InP QD materials and devices due to photo-oxidation and contribute to the design of novel antioxidant materials.

Biexcitons play an important role in colloidal quantum dot (QD) applications. However, quantitative measurements of absolute biexciton quantum yields (QYs) have long been limited to extremely weak excitation conditions, which has hindered the study of biexciton dynamics and related applications. Here, we present a significant improvement to the time-resolved photon statistics method for measuring absolute biexciton QY. This improvement takes into account the contribution of biexciton emission to the rate of single-photon events, which enables the method to break the limitation of weak excitation conditions and measure the absolute biexciton QYs of single QDs under high excitation conditions. Using the measured QYs of both charged and neutral biexcitons at high excitation conditions, we can calculate the ratios of the radiative and Auger recombination rates of the two types of biexciton. These ratios are in agreement with the theoretical predictions of the asymmetric band structures of CdSe-based QDs, thereby verifying the accuracy and reliability of the method. Accurate measurement of absolute biexciton QYs under high excitation conditions enables the study of QD biexciton dynamics and applications based on biexcitons.

Colloidal quantum dots (QDs) are promising solid-state single-photon emitters for quantum information processing due to their facile integration with nanophotonic components and cost-effective production. However, research on near-infrared QDs-based single-photon emitters for telecommunications remains limited. Here, we present bright near-infrared InAs/InP/ZnSe/ZnS QDs that achieve high-purity single-photon emission through strong exciton confinement within the core. Single-QD measurements reveal that this confinement enables ultrafast biexciton Auger recombination, suppressing multiphoton generation and resulting in g⁽²⁾(0) values as low as 0.032 at room temperature. Unlike extensively studied Cd-based QDs with quasi-type II band alignment, which suffer from a trade-off between single-photon purity and blinking suppression, our QDs leverage a thick insulating ZnSe shell to minimize photoluminescence blinking by shielding photogenerated excitons from surface interactions without compromising single-photon purity. This study offers a viable strategy for achieving stable, high-purity single-photon emission in near-infrared QDs, highlighting their potential as heavy-metal-free quantum light sources for telecommunications applications.

Investigating exciton formation in semiconductor materials is essential for efficient light-to-energy conversion applications. Exciton formation has been extensively studied in two-dimensional materials but is rarely observed in single quantum systems. Here, we construct one-dimensional CsPbBr₃ quantum rods (QRs) with two confined dimensions and one unconfined dimension. We observe subpicosecond ultrafast exciton formation in such a single quantum system. Exciton formation is found to occur during hot carrier cooling instead of at the band edge, in agreement with theoretical expectations. The coexistence of Mahan excitons and abundant free carriers is found in QRs when the carrier density is higher than the Mott density. Furthermore, by comparing QRs with weak-confined zero-dimensional quantum dots, the presence of the unconfined dimension of QRs is found to be the key factor for the observation of exciton formation. These results facilitate the understanding of many-body phenomena and open up new application platforms for single quantum systems.

The achievement of size uniformity and monodispersity in perovskite quantum dots (QDs) requires the implementation of precise temperature control and the establishment of optimal reaction conditions. Nevertheless, the accurate control of a range of reaction variables represents a considerable challenge. This study addresses the aforementioned challenge by employing manganese (Mn) doping to achieve size uniformity in CsPbBr₃ perovskite QDs without the necessity for the precise control of the reaction conditions. By optimizing the Mn:Pb ratio, it is possible to successfully dope CsPbBr₃ QDs with the appropriate concentrations of Mn²⁺ and achieve a uniform size distribution. The spectroscopic measurements on single QDs indicate that the appropriate Mn²⁺ concentrations can result in a narrower spectral linewidth, a longer photoluminescence (PL) lifetime, and a reduced biexciton Auger recombination rate, thus positively affecting the PL properties. This study not only simplifies the size control of perovskite QDs but also demonstrates the potential of Mn-doped CsPbBr₃ QDs for narrow-linewidth light-emitting diode applications.

The achievement of size uniformity and monodispersity in perovskite quantum dots (QDs) requires the implementation of precise temperature control and the establishment of optimal reaction conditions. Nevertheless, the accurate control of a range of reaction variables represents a considerable challenge. This study addresses the aforementioned challenge by employing manganese (Mn) doping to achieve size uniformity in CsPbBr₃ perovskite QDs without the necessity for precise control of the reaction conditions. By optimizing the Mn:Pb ratio, it is possible to successfully dope CsPbBr₃ QDs with appropriate concentrations of Mn²⁺ and achieve a uniform size distribution. The spectroscopic measurements on single QDs indicate that the appropriate Mn²⁺ concentrations can result in a narrower spectral linewidth, a longer photoluminescence (PL) lifetime, and a reduced biexciton Auger recombination rate, thus positively affecting the PL properties. This study not only simplifies the size control of perovskite QDs, but also demonstrates the potential of Mn-doped CsPbBr₃ QDs for narrow linewidth light-emitting diode applications.

Determining the correlation between the size of a single quantum dot (QD) and its photoluminescence (PL) properties is a challenging task. In the study, we determine the size of each QD by measuring its absorption cross section, which allows for accurate investigation of size-dependent PL blinking mechanisms and volume scaling of the biexciton Auger recombination at the single-particle level. A significant correlation between the blinking mechanism and QD size is observed under low excitation conditions. When the QD size is smaller than their Bohr diameter, single CsPbI3 perovskite QDs tend to exhibit BC-blinking, whereas they tend to exhibit Auger-blinking when the QD size exceeds their Bohr diameter. In addition, by extracting bright-state photons from the PL intensity trajectories, the effects of QD charging and surface defects on the biexcitons are effectively reduced. This allows for a more accurate measurement of the volume scaling of biexciton Auger recombination in weakly confined CsPbI3 perovskite QDs at the single-dot level, revealing a superlinear volume scaling (τXX,Auger ∝ σ1.96).