Gabriele Rainò

Lead halide perovskites are widely reported as promising optical gain media for optoelectronic applications in the short‐pulse regime. However, their operation under continuous‐wave excitation remains severely limited by thermal accumulation and gain instability. Here, we present a systematic study of amplified spontaneous emission under long‐pulse and continuous‐wave excitation in thermally evaporated CsPbBr ₃ thin films, providing direct insight into the limits of sustained optical gain in perovskite materials. By correlating excitation pulse duration, substrate thermal conductivity, temperature‐dependent emission properties, and time‐resolved carrier dynamics, we establish a physical framework linking thermal dissipation to gain stability. We identify excitation‐time thresholds and define optimal operating regimes, demonstrating sustained amplified spontaneous emission for pulse widths up to 30 µs in films without encapsulation or additional post‐processing. Our results reveal that light‐induced thermal accumulation is the dominant mechanism limiting amplified spontaneous emission under extended excitation and highlight thermal management as a critical design parameter for steady‐state optical gain. These findings provide practical design rules for perovskite gain media and represent a key step toward electrically pumped, continuous‐wave‐performing perovskite superluminescent and laser diodes.

The functionality of colloidal quantum dots (QDs), whether in fundamental studies, optoelectronics, or emerging quantum technologies, critically depends on their surface chemistry, with organic ligands influencing nearly every aspect of their behavior. Whereas the inorganic core can be comprehensively characterized at atomic resolution, ligands remain difficult to probe owing to their low atomic contrast and soft, disordered structure. Here, we empower single‐particle photoluminescence (PL) spectroscopy to characterize the organic ligand shell. Tagging ligand tails with organic dyes enables measuring their distance from the QD via Förster resonance energy transfer (FRET) at sub‐nanometer resolution. Single‐particle, single‐molecule FRET experiments performed on lead halide perovskite QDs reveal nanometer‐scale variations in dye‐surface distances, both within individual particles (0.81 ± 0.22 nm on average) and between QDs with varied ligand chemistries (from 0.25 to 1.02 nm). Supported by atomistic modelling, our experiments tie diverse local ligand environments to these variations, validating a powerful yet accessible spectroscopic method for interrogating the complex surface chemistry of QDs, highly relevant to QD‐based optoelectronics, quantum applications, and photocatalysis.

Excitons and biexcitons emission traits, for example, quantum yield, lifetime, and coherence, govern device performance metrics including brightness, optical gain thresholds, and the efficiency of single‐ and entangled‐photon generation. Biexcitons are conventionally understood to radiatively decay more rapidly than excitons. This view stems largely from studies in strongly confined quantum dots (QDs), where inter‐exciton correlations are often neglected. In contrast, weakly confined systems such as large CsPbBr ₃ QDs exhibit significant Coulomb correlations, necessitating a revised framework for biexciton radiative dynamics, one that has remained incomplete due to limited experimental insights. Here, we combine single‐particle photoluminescence (PL) spectroscopy with effective‐mass variational quantum Monte Carlo (VQMC) simulations to directly probe the biexciton geometry, exciton–phonon interactions, and radiative decay in CsPbBr ₃ QDs across a range of sizes and temperatures. We reveal that the biexciton‐to‐exciton radiative decay lifetime ratio is strongly modulated by inter‐exciton correlations, with a striking inversion of the expected decay lifetime hierarchy in the weak‐confinement regime. Specifically, at cryogenic temperatures, biexcitons exhibit anomalously longer radiative lifetime (148 ± 30 ps) than excitons (106 ± 16 ps), challenging conventional models. These findings uncover a correlation‐driven regime of multiexciton physics and suggest new routes for tailoring light–matter interactions for classical and quantum photonic applications.

Interactions between emitters can create cooperative effects that alter light emission. In superfluorescence (SF), excited dipoles couple coherently and radiate collectively, requiring low energetic disorder and strong temporal coherence. Conversely, amplified spontaneous emission results from stimulated amplification and does not require temporal coherence but, unlike SF, sufficient propagation for optical gain. Caesium lead halide perovskite nanocrystals exhibit both amplified spontaneous emission (in disordered films) and SF (in ordered assemblies); however, the connections between these regimes remain unclear. Here we demonstrate that temperature and excitation density can drive the transition between both regimes in a thin film of giant CsPbBr ₃ perovskite nanocrystals. At temperatures below 45 K, excitonic SF was observed, whereas above a transition range between 45 K and 100 K, amplified spontaneous emission prevails but requires increased optical excitation and emitter density. Our results work out the different collective effects present in lead halide perovskites, providing fundamental insights into cooperative phenomena and guidance for the development of compact and bright (quantum) light sources.

Surface chemistry is central to the design of colloidal quantum dots (QDs) for photoredox catalysis, where charge transfer, access to active sites, and colloidal stability must be balanced. Using CsPbBr₃ QDs as a model system, we show that systematic variation of surface ligand coverage serves as a diagnostic tool to distinguish reactions proceeding via direct electron transfer through the ligand shell from those dependent on chemisorption. In the latter case, reducing surface coverage increases the rates of reactions requiring substrate adsorption, such as C(sp³)-H brominations, with yields measured at a constant time increasing by up to 3-fold without compromising QD stability. This improvement arises from the efficient activation of chemisorbed substrates, which scales with the number of available surface sites. Extending this approach, we demonstrate that substrate adsorption on CsPbBr₃ QDs enables the activation of more demanding chlorinated compounds, establishing surface coverage as a key parameter for controlling reactivity in QD-based photocatalysis.

In lead halide perovskites (APbX ₃ ), the effect of the A-site cation on optical and electronic properties has initially been thought to be marginal. Yet, evidence of beneficial effects on solar-cell performance and light emission is accumulating. Here, we report that the A-site cation in soft APbBr ₃ colloidal quantum dots (QDs) controls the phonon-induced localization of the exciton wavefunction. Insights from ab-initio molecular-dynamics simulations and single-particle fluorescence spectroscopy demonstrate that anharmonic crystal vibrations and the resulting disorder act as an additional confinement potential. Avoiding the trade-off between single-photon purity and optical stability faced by downsizing conventional QDs into the strong confinement regime, dynamical phonon-induced confinement in large organic-inorganic perovskite QDs enables bright (10 ⁶ photons/s), stable ( > 1 h), and pure (> 95%) single-photon emission tunable across a wide spectral range (495-745 nm). Strong electron-phonon interaction in soft perovskite QDs provides an unconventional route toward developing scalable room-temperature quantum-light sources.

Plasmonic cavities provide a powerful platform for nanoscale light confinement far beyond the diffraction limit, enabling exceptionally strong light-matter interactions. Recent advances in plasmonic architectures and active materials enable stable, scalable routes toward fully integrated on-chip photonics. In this work, we demonstrate the strong coupling of 2D layered perovskites with a transferable plasmonic cavity directly fabricated on top of hexagonal boron nitride (hBN). Under optical excitation, we observe clear interaction between the exciton and cavity modes leading to strong and even ultrastrong coupling, with coupling factors g up to 10% of the resonance energy. Combined with the polariton mode hybridization and a waveguiding effect, this results in pronounced line width narrowing of the polariton modes well below both the bare cavity and exciton line widths. Electrical excitation of the hybrid modes is achieved via electron tunneling and simultaneous energy transfer, making this design an attractive configuration for on-chip integration.

Nanocrystals (NCs) of various compositions have made important contributions to science and technology, with their impact recognized by the 2023 Nobel Prize in Chemistry for the discovery and synthesis of semiconductor quantum dots (QDs). Over four decades of research into NCs has led to numerous advancements in diverse fields, such as optoelectronics, catalysis, energy, medicine, and recently, quantum information and computing. The last 10 years since the predecessor perspective “Prospect of Nanoscience with Nanocrystals” was published in ACS Nano have seen NC research continuously evolve, yielding critical advances in fundamental understanding and practical applications. Mechanistic insights into NC formation have translated into precision control over NC size, shape, and composition. Emerging synthesis techniques have broadened the landscape of compounds obtainable in colloidal NC form. Sophistication in surface chemistry, jointly bolstered by theoretical models and experimental findings, has facilitated refined control over NC properties and represents a trusted gateway to enhanced NC stability and processability. The assembly of NCs into superlattices, along with two-dimensional (2D) photolithography and three-dimensional (3D) printing, has expanded their utility in creating materials with tailored properties. Applications of NCs are also flourishing, consolidating progress in fields targeted early on, such as optoelectronics and catalysis, and extending into areas ranging from quantum technology to phase-change memories. In this perspective, we review the extensive progress in research on NCs over the past decade and highlight key areas where future research may bring further breakthroughs.

Beyond single-photon emission, generating correlated N-photon bundles, e.g., a photon pair, is essential for various quantum technologies including quantum teleportation and metrology. A widely explored approach exploits the radiative biexciton cascade in individual (mainly epitaxially grown) quantum dots (QDs). Here, we investigate such a cascade in colloidal CsPbBr₃ QDs, a scalable and solution-processable quantum-light emitter. By matching their size-dependent biexciton binding energies to their size-independent phonon energies, we demonstrate the generation of time-correlated and energy-degenerate photon pairs in large (>15 nm) QDs. Under pulsed excitation at 4 K, we observe pronounced photon bunching, with a g⁽²⁾(0) of up to 7 in Hanbury Brown and Twiss measurements. The excitation-density-dependent bunching is quantitatively reproduced by multicolor numerical calculations, suggesting the cascade involving biexciton and phonon-mediated exciton decay as origin of the photon pair. Our findings provide new insights into energy-degenerate photon-pair generation in these highly engineerable quantum-light emitters, marking important steps toward their application in quantum-information technologies.