Single Particle Analysis (SPA) is a cornerstone technique that allows us to investigate the photophysical properties of nanomaterials without the averaging effects inherent in ensemble measurements. By probing individual nanocrystals, we reveal heterogeneity, resolve discrete excitonic states, and quantify quantum efficiencies with ultimate precision. Our recent work applies SPA to a range of materials, from perovskite nanocrystals to complex quantum shells, to drive innovations in light-emitting technologies.
Resolving Multi-Emitter Behavior in Zero-Dimensional Perovskites
Zero-dimensional (0D) perovskites, such as Cs₄Pb(Br₀.₂₅I₀.₇₅)₆, are promising single-photon sources, but their emission properties are dominated by defect states. Ensemble measurements suggest simple behavior, but SPA reveals a more complex reality. We conduct photon correlation spectroscopy using a Hanbury-Brown and Twiss (HBT) interferometer on individual nanocrystals at various temperatures. SPA uncovers that individual 0D perovskite nanocrystals can contain multiple, independent emission centers [1]. At low temperatures (77 K), these centers act as isolated single-photon emitters (g²(0) < 0.5). However, at room temperature, thermal activation enables interaction with charge “reservoir” states, leading to multi-photon emission behavior (g²(0) > 0.5) and long-lived “delayed” photoluminescence components. This temperature-dependent transition, only visible at the single-particle level, highlights the critical role of defects and provides a pathway to engineer the quantum optical properties of perovskite materials.
Schematic of Single-Particle Analysis optical set up
Quantifying Near-Unity Biexciton Quantum Yields in Quantum Shells
A major goal in nanocrystal science is to suppress non-radiative Auger recombination, which quenches light emission from multiple excitons. SPA is the only method capable of directly measuring the biexciton quantum yield (QY^BX) of individual nanostructures. We employ time-gated photon antibunching measurements. By analyzing the g²(0) value of the second-order correlation function and applying a temporal gate to exclude biexciton emission, we can isolate and quantify the quantum yields of exciton (X) and biexciton (XX) states separately [2]. This precise SPA method revealed that large-core CdS/CdSe/CdS quantum shells (QSs) achieve near-unity biexciton quantum yields, with individual particles reaching QY^BX = 100% and an ensemble average of ~82% [2]. Direct measurement via SPA confirmed that the QS geometry effectively suppresses Auger decay, validating this design for applications requiring high-efficiency multiexciton emission, such as lasers and high-brightness LEDs.
Visualizing Blinking Suppression via Advanced Surface Passivation
Photoluminescence (PL) blinking—random switching between bright (“ON”) and dim (“OFF”) states—is a universal phenomenon in nanocrystals linked to charging and surface traps. SPA allows us to visualize blinking and directly test strategies to suppress it. We record long-time-trajectory PL blinking traces (intensity vs. time) of individual nanocrystals. Comparing standard CdS/CdSe/CdS QSs with advanced CdS/CdSe/CdS/ZnS “quantum shell in a shell” structures, SPA demonstrates a dramatic suppression of blinking [3]. The ZnS-passivated samples exhibit stable, non-blinking emission, while control samples show pronounced ON/OFF switching. The elimination of blinking, directly observed through SPA, proves that the ZnS shell effectively passivates surface traps. This leads to higher ensemble PL quantum yields and greater photostability, which are essential for reliable device performance.
(a) A low-resolution TEM image of PbS/CdS QDs and (b)a typical wide-field PL image of single QDs. (c) A representative PLintensity time trace of a single PbS/CdS QD.Figure 2. (a) Representative PL emission spectra of four single PbS/CdS QDs at 4 K. (b) Scatter plot of spectral line width Γ and peakenergy of single QDs. The top and the right panels display thedistribution histograms of peak energy and Γ, respectively.Nano Letters LetterDOI: 10.1021/acs.nanolett.9b02937Nano Lett. 2019, 19, 8519−85258520
Characterizing Excitonic Complexes via Intensity-Level Lifetime Spectroscopy
Single nanocrystals can host various excitonic complexes, such as neutral excitons (X), charged excitons (trions, X±), and multi-excitons. SPA allows us to dissect this complexity by correlating intensity levels with lifetime dynamics. Using time-tagged, time-correlated single-photon counting (TCSPC), we extract PL decay curves from specific intensity levels identified within a single particle’s blinking trace. In high-performing quantum shells, we observe well-defined intensity plateaus corresponding to different excitonic states. The highest intensity level exhibits a long, single-exponential decay, signifying near-unity quantum yield from the neutral exciton state. Lower-intensity “gray” states have shorter lifetimes, indicative of charged excitons or non-radiative decay channels [2]. This ability to assign lifetime dynamics to specific excitonic populations provides insight into the recombination pathways and charge dynamics within a single nanocrystal, enabling rational material optimization.
Single Particle Analysis provides a level of detail that is masked in ensemble studies. Through SPA, we can identify multi-emitter behavior and defect interactions [1], quantify ultimate performance metrics like near-unity biexciton yields [2], visualize and engineer photostability by suppressing blinking [3], and dissect complex excitonic states within a single nanocrystal [2]. These capabilities allow us to understand and engineer nanomaterials for next-generation quantum and optoelectronic applications.
[1] Zhou, X. et al. Highly Emissive Zero-Dimensional Cesium Lead Iodide Perovskite Nanocrystals with Thermally Activated Delayed Photoluminescence. J. Phys. Chem. Lett. 2023, *14*, 2933–2939.
[2] Marder, A. A. et al. CdS/CdSe/CdS Spherical Quantum Wells with Near-Unity Biexciton Quantum Yield for Light-Emitting-Device Applications. ACS Materials Lett. 2023, *5*, 1411–1419.
[3] Harankahage, D. et al. Quantum Shell in a Shell: Engineering Colloidal Nanocrystals for a High-Intensity Excitation Regime. J. Am. Chem. Soc. 2023, *145*, 13326–13334.