Colloidal quantum dots (QDs) have become firmly established in the display market, yet many relevant QD compositions contain heavy metals whose concentrations are regulated in consumer products. Accurate characterization of hazardous metal content in QD displays is therefore essential but challenging due to their complex multilayer structure. Here, a commercially available QD display film is analyzed using complementary techniques, with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as the central tool. The film contains CdSe-based core-shell QDs as the green component and, for the first time in a commercial product, lead halide perovskite (LHP) QDs as the red component. Principal component analysis combined with k-means clustering enabled the extraction of bead-like pixel populations corresponding to CdSe and LHP QDs, with classification accuracies of 84% and 95%, respectively. This approach establishes a pathway toward advanced analytical strategies for assessing the regulatory compliance of commercial QD displays.

Tin halide perovskites represent an emerging alternative to lead halide perovskites, offering comparably advantageous electronic structures, lower toxicity, and bandgaps that extend into the near-infrared region. However, the synthesis of tin halide perovskite quantum dots (QDs) remains challenging due to the tendency toward lower-dimensional phase formation, limited control over QD size and composition, and high sensitivity to intrinsic defects. Here, we introduce a room-temperature synthesis protocol employing cesium oleate and tin halide adducts with trioctylphosphine oxide in the absence of oleylamine. This non-templating route effectively suppresses 2D impurities and yields monodisperse CsSnI3 QDs, finely tunable between 4 and 22 nm, providing access to size-dependent optical bandgap energies across the strong-to-weak confinement regimes. Increasing the availability of Sn(II) through an additional tin carboxylate source reduces defect densities and leads to the emergence of pronounced and spectrally well-resolved excitonic absorption in strongly confined QDs. Variation of the tin halide and A-cation yields formamidinium, methylammonium, and cesium tin bromide and iodide, all with narrow size distributions. This synthetic route sets the stage for further defect engineering to yield bright tin halide perovskite emitters.

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.

Electronic doping underpins many semiconductor applications, but it has remained elusive for lead halide perovskite nanocrystals because their high density and low formation energy of defects easily compensate introduced dopants. Here we use electrochemical doping as a strategy to avoid doping compensation. Electrochemical injection of electrons or holes into the nanocrystal is compensated by surface-adsorbed electrolyte counterions, therefore not compromising crystal structure. We accomplished ambipolar (n- and p-type) doped CsPbBr3 nanocrystals with charge carrier concentrations up to the heavy doping regime (4.8x10 17 cm −3 , corresponding to 0.25 charges per nanocrystal). We show that electrochemical doping is reversible and stable at these high free carrier concentrations. The high extent of doping and stability is enabled by the use of lecithin, a zwitterionic ligand that strongly binds to the surface of the CsPbBr 3 nanocrystals while still allowing ion and electron transfer. Lecithin stabilizes the surface atoms by shifting the potentials for Pb2+ reduction and Br- oxidation outside of the bandgap. The inferred surface-chemistry design rules for stable electrochemical doping can also serve as a guideline for developing CsPbBr 3 nanocrystals stable under high free-carrier concentrations in optoelectronic devices, such as light-emitting diodes or electronically triggered quantum light sources.

Electronic excitations in solids are commonly described within a hierarchy in which the excitonic Hamiltonian is defined first and the lattice acts later through renormalization, relaxation, and dephasing. This picture assumes that the optically accessible excitonic manifold is already present at the moment of photoexcitation. Here we show that this assumption fails in a soft polar semiconductor. Using femtosecond coherent multidimensional spectroscopy on lead-halide perovskite nanocrystals, we observe quantum back-action between an electronic excitation and a collective lattice-polarization field that expands the excitonic Hilbert space in real time. The optical pulse first prepares an excitonic polarization, X1. A second configuration, X2, emerges only after the polaron field develops, while coherent X1-X2 coupling appears at later times. State formation and coherence formation are therefore resolved as distinct stages of quasiparticle formation. In contrast, CdSe quantum dots exhibit the conventional limit in which excitonic states and couplings are present at time zero and are only weakly perturbed by phonons. The observed diagonal and anti-diagonal splittings increase with nanocrystal size and correlate with radiative oscillator strength, opposite to expectations from simple quantum confinement. A dynamical polaron-field model describes the lattice polarization as an order parameter that expands the optically accessible manifold and generates time-dependent coherent coupling. These results show that strong system-bath coupling can actively create excitonic states and the coherent manifold in which they evolve.

The simplicity in the fabrication of photoconductors makes them a valuable choice to investigate optoelectronic properties of colloidal quantum dot (CQD) films. Lateral photoconductors generally require a large size, in the mm2, and are limited in operation speed due to the presence of trapping sites. In contrast, hybrid phototransistors are fabricated in the mu m2 scale and benefit from such trapping sites, allowing the measurement of low light levels in the nW/cm2. The question, however, arises whether high responsivity values are required for the detection of low light levels or the compatible detectivity of photoconductors is sufficient. Here, we directly compare photoconductors and hybrid phototransistors with an identical EDT-treated PbS CQD film. We highlight that a comparable D* is not enough for the purpose of measuring low light levels, as the resulting photocurrents need to be readily accessible. Furthermore, we also showcase temperature-activated photocurrent dynamics resulting in a negative photocurrent (NPC) effect. This NPC simultaneously improves the frequency bandwidth and photocurrent, enabling operation speeds up to 100 kHz.

Understanding the fine structure of excitons is crucial for optoelectronic and quantum photonic applications of lead halide perovskites. It is demonstrated that polarization-sensitive photon echo spectroscopy in magnetic field provides a powerful method to access coherent exciton dynamics and reveal their energy level structure, which is hidden by inhomogeneous broadening. Exciton quantum beats observed in both Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1s exciton states, enabling determination of the fine structure and bright-dark splittings. Application of this technique to bulk mixed halide perovskite crystals FA_0.9Cs_0.1PbI_2.8Br_0.2 reveals a bright-dark exciton splitting of Δ_X=0.46meV, along with electron and hole Landé g factors g_e=3.38 and g_h=-1.14, respectively. The quantum beats persist on timescales of 20–50ps, demonstrating remarkably robust spin and optical coherences at cryogenic temperature of 2K. The decay of the quantum beats of the outer doublet is governed by dephasing due to dispersion of the bright-dark splitting of ∼0.06meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of ∼0.035meV due to anisotropic potentials.

Control over the electron density and conductivity is a cornerstone of semiconductor technology. Here, we report electrochemical control over electron density and conductivity in films of InAs colloidal quantum dots (QDs) capped with ethanedithiol ligands. The quantum-confined twofold degenerate 1S1/2(e) electron state can be reversibly and completely filled. Increasing the electron population yields four bleach features in the optical-absorption spectrum associated transitions to the 1S1/2(e) state, and state-resolved electronic conductivity which follows the 1S1/2(e) density of states, reaching a maximum of 0.45 S/m at 0.5 electrons per QD. The absence of 1P(e) bleach features and state-resolved conductivity imply a wide separation between the 1S1/2(e) and 1P(e) states resulting in electronic transport between 1S(e) states exclusively. The reversible electrochemistry of InAs QDs films allows determination of the absolute energy levels. InAs QDs of 4.2 nm in edge length and capped with ethanedithiol ligands are natively n-doped with the Fermi level at -4.6 eV, the 1S1/2(e) state at -4.28 eV and the 1S3/2(h) state at -5.54 eV vs vacuum. This work establishes a way to precisely control the charge carrier density and conductivity and gives insights into the charge transport properties and electronic structure of InAs QD films, opening the possibility of making devices with InAs QDs in which the charge carrier density is precisely controlled electrochemically.

Electron-hole plasma (EHP), a high-density correlated state, plays a pivotal role in influencing the optical properties of semiconductors, which has recently observed in various materials such as group II-VI semiconductors, transition metal dichalcogenides (TMDCs) and halide perovskites. This macroscopic population of charge carriers presents potential applications for high-temperature optoelectronic and photonic devices. However, the collective emission phenomena such as amplified spontaneous emission (ASE) and superfluorescence (SF) induced by the EHP state in perovskites nanocrystals not investigated thoroughly, without using any external stimuli such as external magnetic field or complex micro/nano structures. In this study, we presented that this EHP state can produce broadband and sizable optical gain in metal halide perovskites nanocrystals.

The exploitation of the strong light-matter coupling regime and exciton-polariton condensates has emerged as a compelling approach to introduce strong interactions and nonlinearities into numerous photonic applications. The use of colloidal semiconductor quantum dots with strong three-dimensional confinement as the active material in optical microcavities would be highly advantageous due to their versatile structural and compositional tunability and wet-chemical processability, as well as potentially enhanced, confinement-induced polaritonic interactions. Yet, to date, exciton-polariton condensation in a microcavity has neither been achieved with epitaxial nor with colloidal quantum dots. Here, we demonstrate room-temperature polariton condensation in a thin film of monodisperse, colloidal CsPbBr3 quantum dots, placed in a tunable optical resonator with a Gaussian-shaped deformation serving as wavelength-scale potential well for polaritons. The onset of polariton condensation under pulsed optical excitation is manifested in emission by its characteristic superlinear intensity dependence, reduced linewidth, blueshift, and extended temporal coherence.

Optical orientation of exciton and carrier spins by circularly polarized light is the basic phenomenon in the spin physics of semiconductors. Here, we investigate spin orientation in MAPbI3 lead halide perovskite crystals at the cryogenic temperature of 1.6 K, where the material has an orthorhombic crystal structure. The recombination and spin dynamics of excitons and carriers are measured by time-resolved photoluminescence after circularly polarized excitation. The optical orientation of excitons reaches 85%, which persists within their lifetime of 15-80 ps. This high orientation is maintained for excitation laser detunings from the exciton resonance to higher energies by up to 0.3 eV, then decreases and vanishes above 1.5 eV detuning. This indicates that the Dyakonov-Perel spin relaxation mechanism based on inversion symmetry breaking is inactive in MAPbI3 crystals with orthorhombic symmetry. The optical orientation of localized and spatially-separated electrons and holes results in 40% circular polarization of their emission. Their contributions can be identified from the complex spin beats dynamics in transverse magnetic field. The dynamics analysis gives values of the Landé g-factors of 2.83 for electrons and 0.54 for holes. Also, the magnetic-field-induced polarization of excitons and carriers is analyzed in magnetic fields up to 6 T, showing that their spin relaxation times are longer than their lifetimes. Namely, for the excitons, the spin relaxation time exceeds the lifetime by a factor of 6. We model the dynamics of optical orientation degree for cumulative contributions of excitons and carriers and show that the exciton recombination dynamics can control these dynamics. The polarized emission of excitons and localized carriers, produced by their polarization on Zeeman-split levels in magnetic fields, is modeled.

Lead halide perovskite quantum dots (LHP QDs) CsPbX3 generate immense interest as narrow-band emitters for displays, lasers, and quantum light sources. All QD applications rely on suited engineering of surface capping ligands. The first generation of LHP QDs employed oleic acid/oleyl amine capping and have found only a limited use in photoredox catalysis. These catalysts have been reported to be unstable and decompose over the course of the reaction, thus reducing turnover numbers (TONs) and limiting their synthetic ability. Herein, the impact of eight distinct surface ligands on monodisperse CsPbBr3 QDs is reported, affording a thorough comprehension of their performance in photocatalytic C-H brominations. These efforts yielded QDs operating at extremely low catalyst loadings (<100 ppb) with TONs over 9,000,000 per LHP QD. We emphasize that the optimal catalytic performance requires increased QD surface accessibility without compromising the QD structural and colloidal integrity. Control experiments indicated that well-known photoredox catalysts such as Ir(ppy)3, Ru(bpy)3Cl2, or 4CzlPN are ineffective in the same reaction. Mechanistic studies reveal that the C-Br bond reduction in CH2Br2 is the rate-limiting step and is likely facilitated through interaction with the CsPbBr3 QD surface. This work outlines a holistic approach toward the design of practically useful photocatalysts out of QDs comprising structurally soft QD cores and dynamically bound capping ligands.

Optical orientation of exciton and carrier spins by circularly polarized light is the basic phenomenon in the spin physics of semiconductors. Here, we investigate spin orientation in methylammonium lead iodide (MAPbI3) perovskite crystals at the cryogenic temperature of 1.6 K, where the material has an orthorhombic crystal structure. The recombination and spin dynamics of excitons and carriers are measured by time-resolved photoluminescence after circularly polarized excitation. The optical orientation of excitons reaches 85%, which persists within their lifetime of 15-80 ps. This high orientation is maintained for excitation laser detunings from the exciton resonance to higher energies by up to 0.3 eV, then decreases and vanishes above 1.5 eV detuning. This indicates that the Dyakonov-Perel spin-relaxation mechanism based on inversion symmetry breaking is inactive in MAPbI3 crystals with orthorhombic symmetry. The optical orientation of localized and spatially separated electrons and holes results in 50% circular polarization of their emission. Their contributions can be identified from the complex spin beats dynamics in a transverse magnetic field. The dynamics analysis gives values of the Land & eacute; g factors of |gV,e| = 2.83 for electrons and |gV,h| = 0.54 for holes. Also, the magnetic-field-induced polarization of excitons and carriers is analyzed in magnetic fields up to 6 T, showing that their spin-relaxation times are longer than their lifetimes. Namely, for the excitons, the spin-relaxation time exceeds the lifetime by a factor of 6. We model the dynamics of optical orientation degree for cumulative contributions of excitons and carriers and show that the exciton recombination dynamics can control these dynamics. The polarized emission of excitons and localized carriers, produced by their polarization on Zeeman-split levels in magnetic fields, is modeled. Their dependence on the magnetic field is identical for very short spin-relaxation times, but becomes qualitatively different as the spin-relaxation time approaches the carrier lifetime.

Superfluorescence, the cooperative burst of spontaneous emission from an ensemble of dipoles, arises when microscopic oscillators spontaneously synchronize their phases. Here we show that this process can be reversed in time within quantum materials. Coherent multidimensional spectroscopy of halide perovskite quantum dots reveals a delayed cooperative absorption burst, the mirror image of superfluorescent emission, driven by transient polaron fields that phase-lock unit-cell dipoles within 100 fs. The effect scales systematically with quantum-dot size and halide composition, reaching near-unity coherence fidelity even at 300 K. A microscopic exciton-polaron model reproduces the buildup and decay of the coherent state, identifying lattice polarons as the mediators of synchronization. These results demonstrate that many-body temporal coherence can self-organize and persist at room temperature, opening routes toward engineered collective optical states and superabsorbing quantum devices.

Superfluorescence, a coherent burst of light from an excited ensemble of emitters, is a crucial quantum optical phenomenon with far-reaching implications in nanophotonics and many-body optical processes. Despite its observation in various systems, realizing superfluorescence in an electron-hole plasma (EHP) at room temperature has remained a formidable challenge, hindering the development of continuous-wave and electrically excited superfluorescence devices. Herein, we address this challenge by condensing the high-density EHP into an electron-hole liquid (EHL) at room temperature, thereby preserving quantum coherence. Using a model system of nanocrystal thin films, we demonstrate the first experimental observation of room temperature superfluorescence from an EHL. Key attributes heralding superfluorescence include a redshift of ∼94  meV from uncorrelated exciton emission, a fluence-dependent delayed growth of macroscopic coherence with abrupt radiative decay ∼1250 times faster than spontaneous emission, a distinct quadratic fluence dependence with a clear threshold, and Burnham-Chiao ringing. These findings open up exciting possibilities for developing electrically pumped colloidal nanocrystals lasers and quantum technologies operating at room temperature.

Lead halide perovskite (LHP) quantum dots (QD) are an attractive material for light emissive applications due to their lattice, which is both dynamically disordered and defect tolerant. The development of LHP QD for quantum emitters critically depends on the characterization of their homogeneous lineshapes. Homogeneous lineshapes are measured by single dot photoluminescence spectroscopy only at cryogenic temperatures for which their lattice dynamics are frozen out. There are presently no experiments which reveal the lineshapes at 300 K, let alone their dynamics due to lattice fluctuations. Here, we perform coherent multi-dimensional spectroscopy (CMDS) on a broad size range of CsPbBr3 QD at 300 K to observe homogeneous lineshapes and their dynamics on the femtosecond time scale of lattice motions. In contrast to linear spectroscopies, these nonlinear CMDS experiments unambiguously reveal that the room temperature homogeneous line widths are both narrow and size independent at 300 K. The ensuing femtosecond line shape dynamics is governed by size-dependent exciton-lattice interactions. These results demonstrate the potential of LHP QD for high-temperature quantum light sources.

Semiconductor nanomaterials offer a promising platform to produce optically addressable spins for use in quantum technologies. Here, we examine CsPbBr3 nanospheres, cubes, and rods spanning the zero-dimensional (0D) to three-dimensional (3D) transition to investigate the influence of dimensionality and shape on exciton spin decoherence. Using circularly polarized transient absorption spectroscopy, we find that the spin relaxation rate is independent of the surface to volume ratio and instead follows a dependence on the length of the shortest dimension. Additionally, differences in surface quality and termination appear to have no effect on the spin relaxation rate for measured materials, and instead the spin relaxation rate is most clearly correlated with the exciton binding energy. Finally, decreasing the dimensionality of the nanomaterials decreases the influence of multiexciton interactions on the spin relaxation rate.

Hot exciton relaxation dynamics is one of the main processes in quantum dots (QD), conferring their functions in optoelectronic devices spanning photovoltaics and solar fuel generation to light emitting diodes, lasers, and quantum light sources. The challenge has been to monitor energy relaxation dynamics in parallel with resolution of excitation or excess energy. Here, we exploit the unique capacity of Coherent Multi-Dimensional Spectroscopy (CMDS) to provide the first observation of the hot exciton cooling landscape of a large size range of CsPbBr3 lead halide perovskite QD, notable for their impact on optoelectronic devices, as well as their strong and unique exciton-lattice coupling. The CMDS data reveal that the hot exciton relaxation landscape is a complex function of the energy. Ab initio quantum dynamics simulations rationalize the observed behavior through energy dependent nonadiabatic exciton-phonon coupling. This first observation of cooling landscapes in QD suggests that materials science that either accelerates or slows hot exciton cooling can better be understood as a landscape to optimize for applications.

Electronic coherence is central to numerous areas of science, from quantum biology to quantum materials. In quantum materials, lead-halide perovskite (LHP) quantum dots (QDs) have been shown to support electronic coherence through observation of coherent single-photon emission and superfluorescence arising from spatial coherence at low temperatures. In contrast, direct measurement of temporal coherence between exciton states has been lacking. Here, we employ coherent multi-dimensional spectroscopy to observe an electronic coherence between exciton states in CsPbBr3 QDs that is long-lived at room temperature, surviving nearly three times longer than the electronic dephasing time. This observation of a long-lived electronic coherence at room temperature points to nearly perfectly correlated lattice fluctuations for each excitonic state in the superposition. These experiments reveal that the properties of LHP QDs extend to lattice dynamics that give rise to correlated fluctuations in the basis exciton states, a process that may next be optimized by design.