Explore the vast range of titles available.

Semiconductor quantum dots have long been considered artificial atoms, but despite the overarching analogies in the strong energy-level quantization and the single-photon emission capability, their emission spectrum is far broader than typical atomic emission lines. Here, by using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr 3 quantum dots, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35−65 meV (vs. initial values of 70–120 meV), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20−60 meV). Ultra-narrow emission at room-temperature is desired for conventional light-emitting devices and paramount for emerging quantum light sources. Narrow emission is desired for light-emitting devices. Here, Kovalenko et al. demonstrate that the emission line-broadening in perovskite quantum dots is dominated by the coupling between excitons and surface phonon modes which can be controlled by minimal surface modifications.

The halide perovskite family has, arguably, become today’s most promising emerging materials sets for optoelectronic applications. Here, we discuss the underperformance to date of the colloidal nanocrystal forms of these materials when employed in electroluminescent lighting devices relative to their counterparts, in which the emitter layer is in the form of polycrystalline films. However, we highlight the bright future of halide perovskite colloidal nanocrystals in light-emission technologies such as LCD displays, quantum light sources and even alternative LED configurations, as well as key guidelines for their further development to get there.

The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength times the local density of photonic states. As the oscillator strength is an intrinsic material property, the quest for ever brighter emission has relied on the local density of photonic states engineering, using dielectric or plasmonic resonators 1 , 2 . By contrast, a much less explored avenue is to boost the oscillator strength, and hence the emission rate, using a collective behaviour termed superradiance. Recently, it was proposed 3 that the latter can be realized using the giant oscillator-strength transitions of a weakly confined exciton in a quantum well when its coherent motion extends over many unit cells. Here we demonstrate single-photon superradiance in perovskite quantum dots with a sub-100 picosecond radiative decay time, almost as short as the reported exciton coherence time 4 . The characteristic dependence of radiative rates on the size, composition and temperature of the quantum dot suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations. The results aid in the development of ultrabright, coherent quantum light sources and attest that quantum effects, for example, single-photon emission, persist in nanoparticles ten times larger than the exciton Bohr radius. Excitonic single-photon superradiance is reported in individual perovskite quantum dots with a sub-100 ps radiative decay time, almost as short as the reported exciton coherence time.

The lowest-energy exciton state in caesium lead halide perovskite nanocrystals is shown to be a bright triplet state, contrary to expectations that lowest-energy excitons should always be dark. A bright future for semiconductors Lead halide perovskite semiconductor nanocrystals are attracting considerable interest as materials for solar cells and light-emitting diodes because of their excellent photophysical properties. But what makes them so special? Excitons are the electronic excitations that are ultimately responsible for the emissive properties of nanostructured semiconductors, and prevailing wisdom is that the lowest-energy excitonic state will be long-lived and hence poorly emitting (or 'dark'). Michael Becker et al . now show that caesium lead halide perovskites disobey this rule: the lowest-energy excitons are instead unusually 'bright', emitting much faster than any other semiconductor nanocrystal. Furthermore, they identify the structural and electronic factors responsible for this anomalous behaviour, providing vital clues for the identification of other semiconducting materials that might behave similarly. Nanostructured semiconductors emit light from electronic states known as excitons 1 . For organic materials, Hund’s rules 2 state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules 3 predict an analogue of this triplet state known as the ‘dark exciton’ 4 . Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX 3 , with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect 5 , 6 , 7 , 8 , 9 , 10 . We then apply our model to CsPbX 3 nanocrystals 11 , and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster 12 than any other semiconductor nanocrystal at room 13 , 14 , 15 , 16 and cryogenic 4 temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting 17 , lasers 18 and displays 19 , these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Gabriele Rainò, Lukas Novotny and Martin Frimmer discuss the approach they are pursuing at ETH Zürich to provide students with an education in quantum engineering.

An ensemble of emitters can behave very differently from its individual constituents when they interact coherently via a common light field. After excitation of such an ensemble, collective coupling can give rise to a many-body quantum phenomenon that results in short, intense bursts of light—so-called superfluorescence 1 . Because this phenomenon requires a fine balance of interactions between the emitters and their decoupling from the environment, together with close identity of the individual emitters, superfluorescence has thus far been observed only in a limited number of systems, such as certain atomic and molecular gases and a few solid-state systems 2 – 7 . The generation of superfluorescent light in colloidal nanocrystals (which are bright photonic sources practically suited for optoelectronics 8 , 9 ) has been precluded by inhomogeneous emission broadening, low oscillator strength, and fast exciton dephasing. Here we show that caesium lead halide (CsPbX 3 , X = Cl, Br) perovskite nanocrystals 10 – 13 that are self-organized into highly ordered three-dimensional superlattices exhibit key signatures of superfluorescence. These are dynamically red-shifted emission with more than 20-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham–Chiao ringing behaviour 14 at high excitation density. These mesoscopically extended coherent states could be used to boost the performance of opto-electronic devices 15 and enable entangled multi-photon quantum light sources 16 , 17 . Cooperative quantum effects in superlattices of quantum dots made of caesium lead halide perovskite give rise to superfluorescence, with the individual emitters interacting coherently to give intense bursts of light.

Atomically defined assemblies of dye molecules (such as H and J aggregates) have been of interest for more than 80 years because of the emergence of collective phenomena in their optical spectra 1 – 3 , their coherent long-range energy transport, their conceptual similarity to natural light-harvesting complexes 4 , 5 , and their potential use as light sources and in photovoltaics. Another way of creating versatile and controlled aggregates that exhibit collective phenomena involves the organization of colloidal semiconductor nanocrystals into long-range-ordered superlattices 6 . Caesium lead halide perovskite nanocrystals 7 – 9 are promising building blocks for such superlattices, owing to the high oscillator strength of bright triplet excitons 10 , slow dephasing (coherence times of up to 80 picoseconds) and minimal inhomogeneous broadening of emission lines 11 , 12 . So far, only single-component superlattices with simple cubic packing have been devised from these nanocrystals 13 . Here we present perovskite-type (ABO 3 ) binary and ternary nanocrystal superlattices, created via the shape-directed co-assembly of steric-stabilized, highly luminescent cubic CsPbBr 3 nanocrystals (which occupy the B and/or O lattice sites), spherical Fe 3 O 4 or NaGdF 4 nanocrystals (A sites) and truncated-cuboid PbS nanocrystals (B sites). These ABO 3 superlattices, as well as the binary NaCl and AlB 2 superlattice structures that we demonstrate, exhibit a high degree of orientational ordering of the CsPbBr 3 nanocubes. They also exhibit superfluorescence—a collective emission that results in a burst of photons with ultrafast radiative decay (22 picoseconds) that could be tailored for use in ultrabright (quantum) light sources. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures. Through precise structural engineering, perovskite nanocrystals are co-assembled with other nanocrystal materials to form a range of binary and ternary perovskite-type superlattices that exhibit superfluorescence.

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 CsPbBr 3 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. Colloidal perovskite quantum dots hold promise for polaritonic devices with strong excitonic confinement. Here the authors report the observation of room-temperature cavity exciton-polariton condensation in a perovskite-based quantum dot solid, opening the door towards quantum and photonics applications.

Lead halide perovskites (LHPs) in the form of nanometre-sized colloidal crystals, or nanocrystals (NCs), have attracted the attention of diverse materials scientists due to their unique optical versatility, high photoluminescence quantum yields and facile synthesis. LHP NCs have a ‘soft’ and predominantly ionic lattice, and their optical and electronic properties are highly tolerant to structural defects and surface states. Therefore, they cannot be approached with the same experimental mindset and theoretical framework as conventional semiconductor NCs. In this Review, we discuss LHP NCs historical and current research pursuits, challenges in applications, and the related present and future mitigation strategies explored.

Understanding the origin of electron–phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead–halide–lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump–electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr 3 . Our results indicate a stronger coupling in FAPbBr 3 than CsPbBr 3 . We attribute the enhanced coupling in FAPbBr 3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites. Time-resolved measurements show that coupling between electrons and phonons in lead halide perovskites can mediate attractive interactions between excitons, although the interaction strength depends on the specific material.