Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut

Zero-Dimensional Hybrid Organic-Inorganic Halide Perovskite Modeling: Insights from First Principles

We discuss the properties of zero dimensional (cluster) hybrid organic-inorganic halide perovskite in view of their possible applicability in photovoltaics, light-emitting, and lasing devices. To support the need of theoretical investigations of such systems and pave the way for future investigations of clusters with different orientations, terminations, and compositions, we have assembled and characterized some zero dimensional models of methylammonium lead iodide, MAPbI3, by "cutting" its bulk. Interesting properties of such clusters that have been here theoretically investigated include their charge distribution, bandgap, wave function localization, and reduced effective mass. The surface orientation/termination and the organic/inorganic cation ratios have been discussed together with the roles they play in determining the electronic properties of such clusters. Also in agreement with experiments, it emerges that surface termination is crucial in determining the structural and optoelectronic properties of this largely overlooked, dimensionally reduced class of materials. Analogies and differences between clusters and bulk are discussed.

Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells

A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly efficient low cost double-junction solar cells on crystalline Si. Solution-processable organic-inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and though it is possible to tune the bandgap of (CH3NH3)Pb(BrxI1-x)3 between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photoinduced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV.

Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single-Photon Emission, Reduced Blinking, and Exciton Fine Structure

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX3, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.

Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. (1)H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.

Scanning probe microscopy and spectroscopy of colloidal semiconductor nanocrystals and assembled structures

Colloidal semiconductor nanocrystals become increasingly important in materials science and technology, due to their optoelectronic properties that are tunable by size. The measurement and understanding of their energy levels is key to scientific and technological progress. Here we review how the confined electronic orbitals and related energy levels of individual semiconductor quantum dots have been measured by means of scanning tunneling microscopy and spectroscopy. These techniques were originally developed for flat conducting surfaces, but they have been adapted to investigate the atomic and electronic structure of semiconductor quantum dots. We compare the results obtained on colloidal quantum dots with those on comparable solid-state ones. We also compare the results obtained with scanning tunneling spectroscopy with those of optical spectroscopy. The first three sections provide an introduction to colloidal quantum dots, and a theoretical basis to be able to understand tunneling spectroscopy on dots attached to a conducting surface. In sections 4 and 5 , we review the work performed on lead-chalcogenide nanocrystals and on colloidal quantum dots and rods of II-VI compounds, respectively. In section 6 , we deal with colloidal III-V nanocrystals and compare the results with their self-assembled counter parts. In section 7 , we review the work on other types of semiconductor quantum dots, especially on Si and Ge nanocrystals.

Enhancing the Brightness of Cesium Lead Halide Perovskite Nanocrystal Based Green Light-Emitting Devices through the Interface Engineering with Perfluorinated Ionomer

High photoluminescence quantum yield, easily tuned emission colors, and high color purity of perovskite nanocrystals make this class of material attractive for light source or display applications. Here, green light-emitting devices (LEDs) were fabricated using inorganic cesium lead halide perovskite nanocrystals as emitters. By introducing a thin film of perfluorinated ionomer (PFI) sandwiched between the hole transporting layer and perovskite emissive layer, the device hole injection efficiency has been significantly enhanced. At the same time, PFI layer suppressed charging of the perovskite nanocrystal emitters thus preserving their superior emissive properties, which led to the three-fold increase in peak brightness reaching 1377 cd m(-2). The full width at half-maximum of the symmetric emission peak with color coordinates of (0.09, 0.76) was 18 nm, the narrowest value among perovskite based green LEDs.

Lasing in robust cesium lead halide perovskite nanowires

The rapidly growing field of nanoscale lasers can be advanced through the discovery of new, tunable light sources. The emission wavelength tunability demonstrated in perovskite materials is an attractive property for nanoscale lasers. Whereas organic-inorganic lead halide perovskite materials are known for their instability, cesium lead halides offer a robust alternative without sacrificing emission tunability or ease of synthesis. Here, we report the low-temperature, solution-phase growth of cesium lead halide nanowires exhibiting low-threshold lasing and high stability. The as-grown nanowires are single crystalline with well-formed facets, and act as high-quality laser cavities. The nanowires display excellent stability while stored and handled under ambient conditions over the course of weeks. Upon optical excitation, Fabry-Pérot lasing occurs in CsPbBr3 nanowires with an onset of 5 μJ cm(-2) with the nanowire cavity displaying a maximum quality factor of 1,009 ± 5. Lasing under constant, pulsed excitation can be maintained for over 1 h, the equivalent of 10(9) excitation cycles, and lasing persists upon exposure to ambient atmosphere. Wavelength tunability in the green and blue regions of the spectrum in conjunction with excellent stability makes these nanowire lasers attractive for device fabrication.

Critical Red Components for Next-Generation White LEDs

Warm white LEDs with a high color rendering index and a low correlated color temperature have undergone rapid development. In this regard, red-emitting materials-such as fluoride phosphors, namely, A2MF6:Mn(4+) (A = K, Na, and Cs; M = Si, Ge, Zr, Sn, and Ti) and XSiF6:Mn(4+) (X = Ba or Zn), nitridoaluminate phosphor (Sr[LiAl3N4]:Eu(2+)), and nanocrystals of cesium lead iodide perovskite (CsPbI3)-have been extensively investigated recently. These compounds generate narrow emissions in the visible red spectral region that are highly perceived by the human eye and lead to excellent chromatic saturation of the red spectra. This paper describes the structure, luminescence properties, morphologies, thermal features, and moisture resistance of critical red components, as well as their limitations for practical applications. This Perspective also provides a basis for future development and scientific challenges in optical research.

The surface science of nanocrystals

All nanomaterials share a common feature of large surface-to-volume ratio, making their surfaces the dominant player in many physical and chemical processes. Surface ligands - molecules that bind to the surface - are an essential component of nanomaterial synthesis, processing and application. Understanding the structure and properties of nanoscale interfaces requires an intricate mix of concepts and techniques borrowed from surface science and coordination chemistry. Our Review elaborates these connections and discusses the bonding, electronic structure and chemical transformations at nanomaterial surfaces. We specifically focus on the role of surface ligands in tuning and rationally designing properties of functional nanomaterials. Given their importance for biomedical (imaging, diagnostics and therapeutics) and optoelectronic (light-emitting devices, transistors, solar cells) applications, we end with an assessment of application-targeted surface engineering.

X-ray Lithography on Perovskite Nanocrystals Films: From Patterning with Anion-Exchange Reactions to Enhanced Stability in Air and Water

Films of colloidal CsPbX3 (X = I, Br or Cl) nanocrystals, prepared by solution drop-casting or spin-coating on a silicon substrate, were exposed to a low flux of X-rays from an X-ray photoelectron spectrometer source, causing intermolecular C═C bonding of the organic ligands that coat the surface of the nanocrystals. This transformation of the ligand shell resulted in a greater stability of the film, which translated into the following features: (i) Insolubility of the exposed regions in organic solvents which caused instead complete dissolution of the unexposed regions. This enabled the fabrication of stable and strongly fluorescent patterns over millimeter scale areas. (ii) Inhibition of the irradiated regions toward halide anion exchange reactions, when the films were exposed either to halide anions in solution or to hydrohalic vapors. This feature was exploited to create patterned regions of different CsPbIxBryClz compositions, starting from a film with homogeneous CsPbX3 composition. (iii) Resistance of the films to degradation caused by exposure to air and moisture, which represents one of the major drawbacks for the integration of these materials in devices. (iv) Stability of the film in water and biological buffer, which can open interesting perspectives for applications of halide perovskite nanocrystals in aqueous environments.

Organic-inorganic and all-inorganic lead halide nanoparticles [Invited]

Organic-inorganic (hybrid) and all-inorganic lead halide perovskites, in particular APbX(3) where A is an organic cation (methylammonium or formamidinium) or cesium cation and X = Cl, Br, I, respectively, are of great interest in photovoltaic devices and as luminescent materials for light-emitting devices. It has recently been demonstrated that they can be prepared not only as nanoparticulate material by using the pores of mesoporous films, but also as colloidal nanoparticles, which exhibit enhanced optical properties with respect to the bulk material. We summarize here the methods reported for their preparation as well as their optical features. Experimental and theoretical studies on this class of materials are ongoing and there is still a demand for enhancing their emissive properties, stability in polar solvents, dispersibility in different media and/or photostability.

Fluorescence Blinking and Photoactivation of All-Inorganic Perovskite Nanocrystals CsPbBr3 and CsPbBr2I

Study of the emission behavior of all-inorganic perovskite nanocrystals CsPbBr3 and CsPbBr2I as a function of the excitation power employing fluorescence correlation spectroscopy and conventional techniques reveals fluorescence blinking in the microsecond time scale and photoinduced emission enhancement. The observation provides insight into the radiative and nonradiative deactivation pathways of these promising substances. Because both blinking and photoactivation processes are intimately linked to the charge separation efficiency and dynamics of the nanocrystals, these key findings are likely to be helpful in realizing the true potential of these substances in photovoltaic and optoelectronic applications.

Structure-Tuned Lead Halide Perovskite Nanocrystals

Colloidally stable suspensions of lead halide perovskite nanocrystals are prepared from high-quality lead halide nanocrystal seeds. Perovskite nanocrystals with different layered crystal structures are reported. These systems are well suited for investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide perovskites.

Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

Nonlinear Absorption and Low-Threshold Multiphoton Pumped Stimulated Emission from All-Inorganic Perovskite Nanocrystals

Halide perovskite materials have attracted intense research interest due to the striking performance in photoharvesting photovoltaics as well as photoemitting applications. Very recently, the emerging CsPbX3 (X = Cl, Br, I) perovskite nanocrystals have been demonstrated to be efficient emitters with photoluminescence quantum yield as high as ∼90%, room temperature single photon sources, and favorable lasing materials. Herein, the nonlinear optical properties, in particular, the multiphoton absorption and resultant photoluminescence of the CsPbBr3 nanocrystals, were investigated. Notably, a large two-photon absorption cross-section of up to ∼1.2 × 10(5) GM is determined for 9 nm sized CsPbBr3 nanocrystals. Moreover, low-threshold frequency-upconverted stimulated emission by two-photon absorption was observed from the thin film of close-packed CsPbBr3 nanocrystals. The stimulated emission is found to be photostable and wavelength-tunable. We further realize the three-photon pumped stimulated emission in green spectra range from colloidal nanocrystals for the first time. Our results reveal the strong nonlinear absorption in the emerging CsPbX3 perovskite nanocrystals and suggest these nanocrystals as attractive multiphoton pumped optical gain media, which would offer new opportunities in nonlinear photonics and revive the nonlinear optical devices.

Spray reaction prepared FA1−xCsxPbI3 solid solution as a light harvester for perovskite solar cells with improved humidity stability

High efficiency and humidity stability perovskite solar cells based on FA_1−xCs_xPbI₃ solid solution are prepared by spray reaction technology.

Phase transformation, morphology control, and luminescence evolution of cesium lead halide nanocrystals in the anion exchange process

CsPbX₃ NCs with different photoluminescence properties were synthesized by anion exchange. A mechanism was supposed by exploring luminescence evolution.

Room-temperature and gram-scale synthesis of CsPbX3 (X = Cl, Br, I) perovskite nanocrystals with 50–85% photoluminescence quantum yields

CsPbX₃ perovskite quantum dots with 50–85% photoluminescence quantum yields have been successfully synthesized at room temperature in open air.

Synthesis, properties, and optical applications of low-dimensional perovskites

This feature article provides an overview of synthesis, properties and applications of low-dimensional perovskites.

Highly stable, luminescent core–shell type methylammonium–octylammonium lead bromide layered perovskite nanoparticles

Perovskite–perovskite core–shell type nanoparticles with a high photoluminescence quantum yield and long term ambient stability.

Inorganic red perovskite quantum dot integrated blue chip: a promising candidate for high color-rendering in w-LEDs

Schematic representation of the generation of warm-white lightviaa red perovskite quantum dot (R-PQD) assisted YAG:Ce phosphor blend on a blue chip.

Temperature-dependent photoluminescence of inorganic perovskite nanocrystal films

The photoluminescence stability of all-inorganic perovskite nanocrystals (CsPbBr₃) with different size is studied.

Efficient photoluminescent thin films consisting of anchored hybrid perovskite nanoparticles

Methylammonium lead bromide nanoparticles are synthetized with a new bifunctional ligand which allows anchoring of the nanoparticles on a variety of conducting and semiconducting surfaces, showing bright photoluminescence with a quantum yield exceeding 50%.

Shape and phase evolution from CsPbBr3 perovskite nanocubes to tetragonal CsPb2Br5 nanosheets with an indirect bandgap

Tetragonal CsPb₂Br₅ nanosheets, involving lateral shape evolution, were obtained via oriented attachment of orthorhombic CsPbBr₃ nanocubes.

Composition dependent optical, structural and photoluminescence characteristics of cesium tin halide perovskites

Compositionally tunable photoluminescence from environment friendly tin based inorganic perovskites covering the visible to near-infrared regions of electromagnetic spectrum.

High performance hybrid graphene–CsPbBr3−xIx perovskite nanocrystal photodetector

We demonstrate a highly sensitive hybrid photodetector based on graphene–CsPbBr_3−xI_x perovskite nanocrystals.

Highly luminescent nanoscale quasi-2D layered lead bromide perovskites with tunable emissions

Herein, we report a new color tuning approach for highly luminescent nanoscale lead(ii) bromide perovskites with a quasi-2D layered structure.

On the crystal structure of colloidally prepared CsPbBr3 quantum dots

Colloidally synthesized quantum dots of CsPbBr₃ are orthorhombic in crystal structure not cubic.

Superior Optical Properties of Perovskite Nanocrystals as Single Photon Emitters

The power conversion efficiency of photovoltaic devices based on semiconductor perovskites has reached ∼20% after just several years of research efforts. With concomitant discoveries of other promising applications in lasers, light-emitting diodes, and photodetectors, it is natural to anticipate what further excitement these exotic perovskites could bring about. Here we report on the observation of single photon emission from single CsPbBr3 perovskite nanocrystals (NCs) synthesized from a facile colloidal approach. Compared with traditional metal-chalcogenide NCs, these CsPbBr3 NCs exhibit nearly 2 orders of magnitude increase in their absorption cross sections at similar emission colors. Moreover, the radiative lifetime of CsPbBr3 NCs is greatly shortened at both room and cryogenic temperatures to favor an extremely fast output of single photons. The above superior optical properties have paved the way toward quantum-light applications of perovskite NCs in various quantum information processing schemes.

Assessment of Hybrid Organic-Inorganic Antimony Sulfides for Earth-Abundant Photovoltaic Applications

Hybrid organic-inorganic solar absorbers are currently the subject of intense interest; however, the highest-performing materials contain Pb. Here we assess the potential of three Sb-based semiconductors: (i) Sb2S3, (ii) Cs2Sb8S13, and (iii) (CH3NH3)2Sb8S13. While the crystal structure of Sb2S3 is composed of 1D chains, 2D layers are formed in the ternary cesium and hybrid methylammonium antimony sulfide compounds. In each case, a stereochemically active Sb 5s(2) lone pair is found, resulting in a distorted coordination environment for the Sb cations. The bandgap of the binary sulfide is found to increase, while the ionization potential also changes, upon transition to the more complex compounds. Based on the predicted electronic structure, device configurations are suggested to be suitable for photovoltaic applications.

Air-Stable Surface-Passivated Perovskite Quantum Dots for Ultra-Robust, Single- and Two-Photon-Induced Amplified Spontaneous Emission

We demonstrate ultra-air- and photostable CsPbBr3 quantum dots (QDs) by using an inorganic-organic hybrid ion pair as the capping ligand. This passivation approach to perovskite QDs yields high photoluminescence quantum yield with unprecedented operational stability in ambient conditions (60 ± 5% lab humidity) and high pump fluences, thus overcoming one of the greatest challenges impeding the development of perovskite-based applications. Due to the robustness of passivated perovskite QDs, we were able to induce ultrastable amplified spontaneous emission (ASE) in solution processed QD films not only through one photon but also through two-photon absorption processes. The latter has not been observed before in the family of perovskite materials. More importantly, passivated perovskite QD films showed remarkable photostability under continuous pulsed laser excitation in ambient conditions for at least 34 h (corresponds to 1.2 × 10(8) laser shots), substantially exceeding the stability of other colloidal QD systems in which ASE has been observed.

Quantum Dot Light-Emitting Diodes Based on Inorganic Perovskite Cesium Lead Halides (CsPbX3 )

Novel quantum-dot light-emitting diodes based on all-inorganic perovskite CsPbX3 (X = Cl, Br, I) nanocrystals are reported. The well-dispersed, single-crystal quantum dots (QDs) exhibit high quantum yields, and tunable light emission wavelength. The demonstration of these novel perovskite QDs opens a new avenue toward designing optoelectronic devices, such as displays, photodetectors, solar cells, and lasers.

All-Inorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics

All-inorganic colloidal cesium lead halide perovskite quantum dots (CsPbX3 , X = Cl, Br, I) are revealed to be a new class of favorable optical-gain materials, which show -combined merits of both colloidal quantum dots and halide perovskites. Low-threshold and -ultrastable stimulated emission is -demonstrated under atmospheric conditions with wavelength tunability across the whole -visible spectrum via either size or composition control.

Strain tuning of ferroelectric polarization in hybrid organic inorganic perovskite compounds

Metal-organic frameworks (MOFs) are hybrid crystalline compounds comprised of an extended ordered network made up of organic molecules, organic linkers and metal cations. In particular, MOFs with the same topology as inorganic perovskites have been shown to possess interesting properties, e.g., coexistence of ferroelectric and magnetic ordering. Using first-principles density functional theory, we have investigated the effect of strain on the compounds C(NH2)3Cr(HCOO)3 and (CH3CH2NH3)Mn(HCOO)3. Here, we show that compressive strain can substantially increase the ferroelectric polarization by more than 300%, and we discuss the mechanism involved in the strain enhancement of polarization. Our study highlights the complex interplay between strain and organic cations' dipoles and put forward the possibility of tuning of ferroelectric polarization through appropriate thin film growing.

Efficient Luminescence from Perovskite Quantum Dot Solids

Nanocrystals of CsPbX3 perovskites are promising materials for light-emitting optoelectronics because of their colloidal stability, optically tunable bandgap, bright photoluminescence, and excellent photoluminescence quantum yield. Despite their promise, nanocrystal-only films of CsPbX3 perovskites have not yet been fabricated; instead, highly insulating polymers have been relied upon to compensate for nanocrystals' unstable surfaces. We develop solution chemistry that enables single-step casting of perovskite nanocrystal films and overcomes problems in both perovskite quantum dot purification and film fabrication. Centrifugally cast films retain bright photoluminescence and achieve dense and homogeneous morphologies. The new materials offer a platform for optoelectronic applications of perovskite quantum dot solids.

Colloidal CsPbBr3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots

Traditional CdSe-based colloidal quantum dots (cQDs) have interesting photoluminescence (PL) properties. Herein we highlight the advantages in both ensemble and single-nanocrystal PL of colloidal CsPbBr3 nanocrystals (NCs) over the traditional cQDs. An ensemble of colloidal CsPbBr3 NCs (11 nm) exhibits ca. 90 % PL quantum yield with narrow (FWHM=86 meV) spectral width. Interestingly, the spectral width of a single-NC and an ensemble are almost identical, ruling out the problem of size-distribution in PL broadening. Eliminating this problem leads to a negligible influence of self-absorption and Förster resonance energy transfer, along with batch-to-batch reproducibility of NCs exhibiting PL peaks within ±1 nm. Also, PL peak positions do not alter with measurement temperature in the range of 25 to 100 °C. Importantly, CsPbBr3 NCs exhibit suppressed PL blinking with ca. 90 % of the individual NCs remain mostly emissive (on-time >85 %), without much influence of excitation power.

Inorganic Halide Perovskites for Efficient Light-Emitting Diodes

Lead-halide perovskites have transcended photovoltaics. Perovskite light-emitting diodes (PeLEDs) emerge as a new field to leverage on these fascinating semiconductors. Here, we report the first use of completely inorganic CsPbBr3 thin films for enhanced light emission through controlled modulation of the trap density by varying the CsBr-PbBr2 precursor concentration. Although pure CsPbBr3 films can be deposited from equimolar CsBr-PbBr2 and CsBr-rich solutions, strikingly narrow emission line (17 nm), accompanied by elongated radiative lifetimes (3.9 ns) and increased photoluminescence quantum yield (16%), was achieved with the latter. This is translated into the enhanced performance of the resulting PeLED devices, with lower turn-on voltage (3 V), narrow electroluminescence spectra (18 nm) and higher electroluminescence intensity (407 Cd/m(2)) achieved from the CsBr-rich solutions.

Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots

Lead-halide-based perovskites have been the subject of numerous recent studies largely motivated by their exceptional performance in solar cells. Electronic and optical properties of these materials have been commonly controlled by varying the composition (e.g., the halide component) and/or crystal structure. Use of nanostructured forms of perovskites can provide additional means for tailoring their functionalities via effects of quantum confinement and wave function engineering. Furthermore, it may enable applications that explicitly rely on the quantum nature of electronic excitations. Here, we demonstrate that CsPbX3 quantum dots (X = I, Br) can serve as room-temperature sources of quantum light, as indicated by strong photon antibunching detected in single-dot photoluminescence measurements. We explain this observation by the presence of fast nonradiative Auger recombination, which renders multiexciton states virtually nonemissive and limits the fraction of photon coincidence events to ∼6% on average. We analyze limitations of these quantum dots associated with irreversible photodegradation and fluctuations ("blinking") of the photoluminescence intensity. On the basis of emission intensity-lifetime correlations, we assign the "blinking" behavior to random charging/discharging of the quantum dot driven by photoassisted ionization. This study suggests that perovskite quantum dots hold significant promise for applications such as quantum emitters; however, to realize this goal, one must resolve the problems of photochemical stability and photocharging. These problems are largely similar to those of more traditional quantum dots and, hopefully, can be successfully resolved using advanced methodologies developed over the years in the field of colloidal nanostructures.

Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets

Organometal halide perovskites have recently emerged displaying a huge potential for not only photovoltaic, but also light emitting applications. Exploiting the optical properties of specifically tailored perovskite nanocrystals could greatly enhance the efficiency and functionality of applications based on this material. In this study, we investigate the quantum size effect in colloidal organometal halide perovskite nanoplatelets. By tuning the ratio of the organic cations used, we can control the thickness and consequently the photoluminescence emission of the platelets. Quantum mechanical calculations match well with the experimental values. We find that not only do the properties of the perovskite, but also those of the organic ligands play an important role. Stacking of nanoplatelets leads to the formation of minibands, further shifting the bandgap energies. In addition, we find a large exciton binding energy of up to several hundreds of meV for nanoplatelets thinner than three unit cells, partially counteracting the blueshift induced by quantum confinement. Understanding of the quantum size effects in perovskite nanoplatelets and the ability to tune them provide an additional method with which to manipulate the optical properties of organometal halide perovskites.

A sustainable future for photonic colloidal nanocrystals

Colloidal nanocrystals - produced in a growing variety of shapes, sizes and compositions - are rapidly developing into a new generation of photonic materials, spanning light emitting as well as energy harvesting applications. Precise tailoring of their optoelectronic properties enables them to satisfy disparate application-specific requirements. However, the presence of toxic heavy metals such as cadmium and lead in some of the most mature nanocrystals is a serious drawback which may ultimately preclude their use in consumer applications. Although the pursuit of non-toxic alternatives has occurred in parallel to the well-developed Cd- and Pb-based nanocrystals, synthetic challenges have, until recently, curbed progress. In this review, we highlight recent advances in the development of heavy-metal-free nanocrystals within the context of specific photonic applications. We also describe strategies to transfer some of the advantageous nanocrystal features such as shape control to non-toxic materials. Finally, we present recent developments that have the potential to make substantial impacts on the quest to attain a balance between performance and sustainability in photonics.

Ultrafast Interfacial Electron and Hole Transfer from CsPbBr3 Perovskite Quantum Dots

Recently reported colloidal lead halide perovskite quantum dots (QDs) with tunable photoluminescence (PL) wavelengths covering the whole visible spectrum and exceptionally high PL quantum yields (QYs, 50-90%) constitute a new family of functional materials with potential applications in light-harvesting and -emitting devices. By transient absorption spectroscopy, we show that the high PL QYs (∼79%) can be attributed to negligible electron or hole trapping pathways in CsPbBr3 QDs: ∼94% of lowest excitonic states decayed with a single-exponential time constant of 4.5 ± 0.2 ns. Furthermore, excitons in CsPbBr3 QDs can be efficiently dissociated in the presence of electron or hole acceptors. The half-lives of electron transfer (ET) to benzoquinone and subsequent charge recombination are 65 ± 5 ps and 2.6 ± 0.4 ns, respectively. The half-lives for hole transfer (HT) to phenothiazine and the subsequent charge recombination are 49 ± 6 ps and 1.0 ± 0.2 ns, respectively. The lack of electron and hole traps and fast interfacial ET and HT rates are key properties that may enable the development of efficient lead halide perovskite QDs-based light-harvesting and -emitting devices.

Control of Emission Color of High Quantum Yield CH3NH3PbBr3 Perovskite Quantum Dots by Precipitation Temperature

Emission color controlled, high quantum yield CH3NH3PbBr3 perovskite quantum dots are obtained by changing the temperature of a bad solvent during synthesis. The products for temperatures between 0 and 60 °C have good spectral purity with narrow emission line widths of 28-36 nm, high absolute emission quantum yields of 74% to 93%, and short radiative lifetimes of 13-27 ns.