Memories in the photoluminescence intermittency of single cesium lead bromide nanocrystals

Exploiting the Photo-Physical Properties of Metal Halide Perovskite Nanocrystals for Bioimaging

Perovskite nanomaterials have recently been exploited for bioimaging applications due to their unique photo-physical properties, including high absorbance, good photostability, narrow emissions, and nonlinear optical properties. These attributes outperform conventional fluorescent materials such as organic dyes and metal chalcogenide quantum dots and endow them with the potential to reshape a wide array of bioimaging modalities. Yet, their full potential necessitates a deep grasp of their structure-attribute relationship and strategies for enhancing water stability through surface engineering for meeting the stringent and unique requirements of each individual imaging modality. This review delves into this evolving frontier, highlighting how their distinctive photo-physical properties can be leveraged and optimized for various bioimaging modalities, including visible light imaging, near-infrared imaging, and super-resolution imaging.

Charge and energy transfer dynamics in single colloidal quantum dots/monolayer MoS2 heterostructures

The charge and energy transfer dynamics in colloidal CdSeTe/ZnS quantum dots (QDs)/monolayer molybdenum disulfide (MoS₂) heterostructures have been investigated by time-resolved single-dot photoluminescence (PL) spectroscopy. A time-gated method is used to separate the PL photons of single QDs from the PL photons of monolayer MoS₂, which are impossible to be separated by the spectral filter due to their spectral overlap. It is found that the energy transfer from MoS₂ to single QDs increases the exciton generation of the QDs by 37.5% and the energy transfer from single QDs to MoS₂ decreases the PL quantum yield of the QDs by 66.9%. In addition, it is found that MoS₂ increases the discharging rate of single QDs by 59%, while the charging rate remains unchanged. This investigation not only provides valuable insight into the exciton generation and recombination at the single-dot level across such hybrid 0D-2D interfaces but also promotes the application of the hybrid system in various optoelectronic devices.

Memory in quantum dot blinking

The photoluminescence intermittency (blinking) of quantum dots is interesting because it is an easily measured quantum process whose transition statistics cannot be explained by Fermi's golden rule. Commonly, the transition statistics are power-law distributed, implying that quantum dots possess at least trivial memories. By investigating the temporal correlations in the blinking data, we demonstrate with high statistical confidence that there is nontrivial memory between the on and off brightness duration data of blinking quantum dots. We define nontrivial memory to be statistical complexity greater than one. We show that this memory cannot be discovered using the transition distribution. We show by simulation that this memory does not arise from standard data manipulations. Finally, we conclude that at least three physical mechanisms can explain the measured nontrivial memory: (1) storage of state information in the chemical structure of a quantum dot; (2) the existence of more than two intensity levels in a quantum dot; and (3) the overlap in the intensity distributions of the quantum dot states, which arises from fundamental photon statistics.

Water-dispersed CsPbBr3 nanocrystals for single molecule localization microscopy with high location accuracy for targeted bioimaging

Single-molecule localization microscopy (SMLM) is one of the most promising super-resolution imaging techniques for visualizing ultrasmall cellular structures. Here, water-dispersed perovskite CsPbBr₃ nanocrystals (CsPbBr₃ NCs) fabricated by a one-step mechanochemical method are explored as a SMLM fluorophore for bioimaging. Due to their ultrahigh photoluminescence quantum yield (PLQY), inherent frequent fluorescence blinking, proper duty cycle and long-term photostability, an extremely high location precision of ∼3 nm was achieved, a sixfold enhancement than those reported previously. In addition, the spatial resolution of a SMLM image depends on the size of CsPbBr₃ NCs, which is approximately 23 nm. Two closely spaced CsPbBr₃ NCs with a gap of 40 nm can be clearly distinguished in the SMLM image. More importantly, unlike most perovskite quantum dots (QDs), one-step mechanochemically prepared CsPbBr₃ NCs can retain their excellent fluorescence characteristics even after surface biofunctionalization, greatly reducing the current limitations of perovskite QDs on bioimaging. As an example, cell-derived exosomes (30-150 nm in diameter) labeled with CsPbBr₃ NCs were easily identified by SMLM. In addition, after being functionalized with biotin, targeted SMLM imaging of the nuclear lamina or cell membranes of cells was achieved with an enhanced resolution. This work may open up a promising avenue to expand the field of perovskite QD-based SMLM to bioimaging with a high location accuracy.

Shape-Tuned Multiphoton-Emitting InP Nanotetrapods

As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape-tuning of single-crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape-effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum-confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.

Nanomaterials for Quantum Information Science and Engineering

Quantum information science and engineering (QISE)-which entails the use of quantum mechanical states for information processing, communications, and sensing-and the area of nanoscience and nanotechnology have dominated condensed matter physics and materials science research in the 21st century. Solid-state devices for QISE have, to this point, predominantly been designed with bulk materials as their constituents. This review considers how nanomaterials (i.e., materials with intrinsic quantum confinement) may offer inherent advantages over conventional materials for QISE. The materials challenges for specific types of qubits, along with how emerging nanomaterials may overcome these challenges, are identified. Challenges for and progress toward nanomaterials-based quantum devices are condidered. The overall aim of the review is to help close the gap between the nanotechnology and quantum information communities and inspire research that will lead to next-generation quantum devices for scalable and practical quantum applications.

A Python Toolbox for Unbiased Statistical Analysis of Fluorescence Intermittency of Multilevel Emitters

We report on a Python toolbox for unbiased statistical analysis of fluorescence intermittency properties of single emitters. Intermittency, that is, step-wise temporal variations in the instantaneous emission intensity and fluorescence decay rate properties, is common to organic fluorophores, II-VI quantum dots, and perovskite quantum dots alike. Unbiased statistical analysis of intermittency switching time distributions, involved levels, and lifetimes are important to avoid interpretation artifacts. This work provides an implementation of Bayesian changepoint analysis and level clustering applicable to time-tagged single-photon detection data of single emitters that can be applied to real experimental data and as a tool to verify the ramifications of hypothesized mechanistic intermittency models. We provide a detailed Monte Carlo analysis to illustrate these statistics tools and to benchmark the extent to which conclusions can be drawn on the photophysics of highly complex systems, such as perovskite quantum dots that switch between a plethora of states instead of just two.

Intermittency of CsPbBr3 Perovskite Quantum Dots Analyzed by an Unbiased Statistical Analysis

We analyze intermittency in intensity and fluorescence lifetime of CsPbBr3 perovskite quantum dots by applying unbiased Bayesian inference analysis methods. We apply change-point analysis (CPA) and a Bayesian state clustering algorithm to determine the timing of switching events and the number of states between which switching occurs in a statistically unbiased manner, which we have benchmarked particularly to apply to highly multistate emitters. We conclude that perovskite quantum dots display a plethora of gray states in which brightness, broadly speaking, correlates inversely with decay rate, confirming the multiple recombination centers model. We leverage the CPA partitioning analysis to examine aging and memory effects. We find that dots tend to return to the bright state before jumping to a dim state and that when choosing a dim state, they tend to explore the entire set of states available.

Unravelling halide-dependent charge carrier dynamics in CsPb(Br/Cl)3 perovskite nanocrystals

With an increasing bromide content in CsPb(Br/Cl)₃ perovskite nanocrystals (PNCs), the steady state photoluminescence quantum yield value increases from 28% to 50% to 76%. Ultrafast transient absorption analyses reveal that the normalized band edge population increases more than two-fold on excitation at the band edge with increasing bromide content, and the hot exciton trapping time increases from 450 fs to 520 fs to 700 fs with increasing bromide content. Ultrasensitive single particle spectroscopic analyses reveal that the peak of the ON fraction distribution increases from 0.65 to 0.75 to 0.85 with increasing bromide content. More specifically, the percentage of PNCs with the ON fraction >75% increases four fold from 24% to 50% to 98% with increasing bromide content. Moreover, the ratio of the detrapping rate and trapping rate increases more than 25 fold with an increase in bromide content, signifying the excitons remaining in the trap state for a smaller time with increasing bromide content. In order to standardize the measurement and analyses, all these three PNCs have the same size and shape, and all the excitations have been made at the same energy above the band edge for all three PNCs and for both ultrafast transient absorption and ultrasensitive single particle measurements.

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time-Resolved Microspectroscopy

Halide perovskites are promising candidate materials for the next generation high-efficiency optoelectronic devices. Since perovskites are electronic-ionic mixed conductors, ion dynamics have a critical impact on the performance and stability of perovskite-based applications. However, comprehensively understanding ionic dynamics is challenging, particularly on nanoscale imaging of ionic dynamics in perovskites. In this review, mobile ion dynamics in halide perovskites investigated via luminescence spectroscopy combined with confocal microscopy are discussed, including mobile ion induced fluorescence quenching, phase segregation in mixed halide hybrid perovskite, and mobile ion accumulation at the interface in perovskite devices. Steady-state and time-resolved luminescence imaging techniques, combined with confocal microscopy, are unique tools for probing ionic dynamics in perovskites, providing invaluable insights on ionic dynamics in nanoscale resolution, along with a wide temporal range from picoseconds to hours. The works in this review are not only for understanding mobile ions to improve the design of perovskite-based devices but also foster the development of microspectroscopic methodologies in a broader solid-state physics context of investigating ionic transports in polycrystalline materials.