Maryna I. Bodnarchuk

Controlling excitation transport is central to the operation of light-harvesting and light-emitting devices based on nanocrystal films. Exciton diffusion is typically described as hopping through a homogeneous medium; however, even highly monodisperse nanocrystal ensembles exhibit energetic disorder arising from size variation. We demonstrate that such disorder couples exciton diffusivity in real space to energetic relaxation within the density of states. Using spectrally and temporally resolved photoluminescence reabsorption spectroscopy of CsPbBr3 nanocrystal films, we separately track exciton diffusion and energetic relaxation across a range of temperatures and nanocrystal sizes. We find that at low temperature, excitons preferentially migrate downhill toward larger, lower-energy nanocrystals, causing a transient collapse of exciton diffusivity that is particularly pronounced for smaller nanocrystals exhibiting higher energetic disorder. At room temperature, these effects are effectively countered by thermally activated uphill transfer, restoring normal diffusion. Nanocrystal solids thus offer tunable transport properties governed by the interplay between disorder and thermal energy.

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.

This study introduces a structurally diverse library of long-chain trialkylsulfonium ligands designed to provide surface stabilization of lead halide perovskite nanocrystals (LHP NCs). We evaluate the influence of ligand structure on binding affinity and demonstrate that sulfonium-based ligands offer effective passivation compared to traditional ammonium ligands. Notably, the resulting NCs exhibit exceptional stability at high dilutions and maintain integrity in industrially relevant manufacturing solvents for down-conversion layer production. This work highlights the potential of sulfonium-based ligands to stabilize and produce robust, high-performance perovskite NCs for photonics and optoelectronics.

Lead halide perovskite nanoplatelets (LHP NPLs) are of immense interest in the materials science and optoelectronics communities owing to their strong quantum confinement leading to narrow emission peaks, thickness-dependent tunable photoluminescence, and large exciton binding energies. Thus far, their further development and photophysical investigations at ensemble and single-particle levels have been impeded by suboptimal ligand passivation, inferior environmental and colloidal stability compared to their 3D nanocrystal counterparts, and limited compositional diversity. Here, we report highly monodisperse methylammonium lead bromide (MAPbBr₃) NPLs (11.3 ± 2.3 × 1.7 ± 0.5 nm) with tunable ligand chemistry and enhanced photoluminescence quantum yields of up to 80%. NPLs capped with zwitterionic ligands exhibit improved stability upon air exposure and sequential purification. Nuclear magnetic resonance spectroscopy on ligand-exchanged NPLs, capped with either phosphocholine- or phosphoethanolamine-type ligands, confirms the partial replacement of the pristine ligands. Owing to the size and shape uniformity of synthesized MAPbBr₃ NPLs, they readily form assemblies of stacked face-to-face NPLs, as observed in both colloidal dispersions and films, giving rise to concentration-dependent multicolor emission. The temperature dependence of the NPL emission exhibits a nonmonotonous trend, attributed to the highly anisotropic confinement and the consequent exciton–phonon coupling. We also observed photoluminescence from single MAPbBr₃ NPLs at both room and cryogenic temperatures, revealing highly polarized fine-structure emission lines with single-photon purity exceeding 80%. The high uniformity and optical transparency of MAPbBr₃ NPL films enabled their integration into optical cavities, where they exhibited strong light-matter coupling with a substantial Rabi splitting of 200 meV.

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.

Colloidal self-assembly is one strategy for engineering anisotropic properties into otherwise isotropic materials. In this work, we demonstrate anisotropic exciton transport in an A₂B-type superlattice containing columns of 5.3 nm CsPbBr₃ nanocubes assembled into a hexagonal lattice around 6.5 nm LaF₃ nanodisks. Using transient photoluminescence microscopy, we determined that the exciton diffusivity along the fast axis of the superlattice is more than twice as large as that along the slow axis at T = 5 K, but that anisotropy is greatly suppressed at room temperature. Calculations of the diffusivity anisotropy ratio based on Förster theory overestimate the measured values, highlighting the limitations of this theory in completely describing exciton transport. Overall, our results demonstrate how self-assembly of colloidal nanocrystals can be used to engineer directional energy transport, and raise more questions about the microscopic nature of dipole coupling in CsPbBr₃ NC superlattices.

Single photons in Laguerre-Gaussian (LG) beams, which carry orbital angular momentum (OAM), could enable more robust and efficient photonic quantum communication and information processing, as well as enhanced sensitivity in quantum metrology and imaging. However, as most implementations are indirect or require additional mode-shaping elements, the direct generation of single photons with OAM has received growing interest. Colloidal lead halide perovskite quantum dots (QDs) have recently emerged as a versatile material that can produce indistinguishable single photons quasi-deterministically at a high rate. Here, we integrate single CsPbBr₃ QDs into an open Fabry-Perot microcavity with a nanofabricated Gaussian-shaped deformation, demonstrating Purcell-enhanced single-photon generation into individual cavity modes with up to 18.1 ± 0.2 times accelerated decay, down to tens of picoseconds. By in situ tuning of the cavity resonance, we can selectively couple a single QD to different LG modes and observe the spatial patterns of the generated single-photon beams emitted from the cavity. Our findings may guide the development of high-photon-rate sources that directly generate single-photon LG beams for advanced quantum photonic applications.

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.

Lead halide perovskite nanocrystals have emerged as promising candidates for classical light-emitting devices and single-photon sources, owing to their high photoluminescence quantum yield, narrow emission line width and tunable emission. Judicious choice of ligands to passivate nanocrystal surfaces has proven to be critical to the structural stability and optoelectronic performance of such nanocrystals. While many ligands have been deployed, the resulting quality of the nanocrystal surface can be difficult to assess directly. Here, we demonstrate ultralow frequency Raman spectroscopy as a powerful tool to resolve surface-sensitive changes in size and ligand choice in perovskite nanocrystals. By investigating a size series of CsPbBr₃ nanocrystals from the strong (5 nm) to the weak (28 nm) confinement range, we show that the line width of Raman-active modes provides a highly selective metric for surface disorder and quality. We further examine a series of 28 nm diameter nanocrystals with four different zwitterionic ligands, unravelling clear links between varying steric effects and surface quality evident from Raman analysis. Photoluminescence and THz photoconductivity probes reveal an evident correlation of charge-carrier dynamics and radiative emission yields with ligand chemistry and surface quality inferred from phonon broadening. We further show that surface defects preferentially trap hot charge carriers, which affects exciton stability and radiative emission yields. Overall, our approach offers powerful insights into optimizing nanocrystal-ligand boundaries to enhance the performance of nanoscale quantum light sources and optoelectronic devices.

An original but simple instrument is developed, capable of performing simultaneous Dynamic Light Scattering (DLS), Absorbance (ABS), and Photoluminescence (PL) measurements on suspensions of colloidal nanoparticles. The instrument is designed as a standard laboratory tool that, by using only commercial components, enables rapid analysis of the sample under investigation. The setup offers two significant benefits: first, it allows for the simultaneous analysis of the very same sample, overcoming limitations arising from the measurements being performed on distinct specimens, under diverse experimental settings, at different times; second, it enables to monitor the kinetic evolution of nanocrystals (Ncs) colloidal stability in time, with a time resolution of ≈ 100 s, which is considerably shorter than typical timescales related to aggregation kinetics or other colloidal instability‐related phenomena. The suitability of the instrument for characterizing time‐resolved processes is assessed through the case study of the long‐term instability of oleic acid/oleylamine‐capped colloidal CsPbBr ₃ NCs suspensions driven by dilution. It is found that, as ligands gradually desorb from the surface, the suspension becomes unstable, displaying two rather unexpected processes: the formation of transient randomly arranged NCs aggregates that reach micron‐sized dimensions, followed by the coalescence of the original NCs into larger crystalline domains.