Near Infrared Light-Emitting Diodes Based on Colloidal InAs/ZnSe Core/Thick-Shell Quantum Dots.

Heavy-metal-free III-V colloidal quantum dots (QDs) exhibit promising attributes for application in optoelectronics. Among them, InAs QDs are demonstrating excellent optical performance with respect to absorption and emission in the near-infrared spectral domain. Recently, InAs QDs attained a substantial improvement in photoluminescence quantum yield, achieving 70% at a wavelength of 900 nm through the strategic overgrowth of a thick ZnSe shell atop the InAs core. In the present study, light-emitting diodes (LEDs) based on this type of InAs/ZnSe QDs are fabricated, reaching an external quantum efficiency (EQE) of 13.3%, a turn-on voltage of 1.5V, and a maximum radiance of 12 Wsr-1m-2. Importantly, the LEDs exhibit an extensive emission dynamic range, characterized by a nearly linear correlation between emission intensity and current density, which can be attributed to the efficient passivation provided by the thick ZnSe shell. The obtained results are comparable to state-of-the-art PbS QD LEDs. Furthermore, it should be stressed not only that the fabricated LEDs are fully RoHS-compliant but also that the emitting InAs QDs are prepared via a synthetic route based on a non-pyrophoric, cheap, and commercially available as precursor, namely tris(dimethylamino)-arsine.

Partial Ligand Stripping from CsPbBr3 Nanocrystals Improves Their Performance in Light-Emitting Diodes.

Halide perovskite nanocrystals (NCs), specifically CsPbBr3, have attracted considerable interest due to their remarkable optical properties for optoelectronic devices. To achieve high-efficiency light-emitting diodes (LEDs) based on CsPbBr3 nanocrystals (NCs), it is crucial to optimize both their photoluminescence quantum yield (PLQY) and carrier transport properties when they are deposited to form films on substrates. While the exchange of native ligands with didodecyl dimethylammonium bromide (DDAB) ligand pairs has been successful in boosting their PLQY, dense DDAB coverage on the surface of NCs should impede carrier transport and limit device efficiency. Following our previous work, here, we use oleyl phosphonic acid (OLPA) as a selective stripping agent to remove a fraction of DDAB from the NC surface and demonstrate that such stripping enhances carrier transport while maintaining a high PLQY. Through systematic optimization of OLPA dosage, we significantly improve the performance of CsPbBr3 LEDs, achieving a maximum external quantum efficiency (EQE) of 15.1% at 516 nm and a maximum brightness of 5931 cd m-2. These findings underscore the potential of controlled ligand stripping to enhance the performance of CsPbBr3 NC-based optoelectronic devices.

Controlled Growth of Large SiO2 Shells onto Semiconductor Colloidal Nanocrystals: A Pathway Toward Photonic Integration.

The growth of SiO2 shells on semiconductor nanocrystals is an established procedure and it is widely employed to provide dispersibility in polar solvents, and increased stability or biocompatibility. However, to exploit this shell to integrate photonic components on semiconductor nanocrystals, the growth procedure must be finely tunable and able to reach large particle sizes (around 100 nm or above). Here, we demonstrate that these goals are achievable through a design of experiment approach. Indeed, the use of a sequential full-factorial design allows us to carefully tune the growth of SiO2 shells to large values while maintaining a reduced size dispersion. Moreover, we show that the growth of a dielectric shell alone can be beneficial in terms of emission efficiency for the nanocrystal. We also demonstrate that, according to our modeling, the subsequent growth of two shells with increasing refractive index leads to an improved emission efficiency already at a reduced SiO2 sphere radius.

Near-Infrared Light Emitting Metal Halides: Materials, Mechanisms, and Applications.

Near-Infrared (NIR) light emitting metal halides are emerging as a new generation of optical materials owing to their appealing features, which include low-cost synthesis, solution processability, and adjustable optical properties. NIR-emitting perovskite-based light-emitting diodes (LEDs) have reached an external quantum efficiency (EQE) of over 20% and a device stability of over 10,000 h. Such results have sparked an interest in exploring new NIR metal halide emitters. In this review, several different types of NIR-emitting metal halides, including lead/tin bromide/iodide perovskites, lanthanide ions doped/based metal halides, double perovskites, low dimensional hybrid and Bi3+/Sb3+/Cr3+ doped metal halides, are summarized, and their recent advancement is assessed. The characteristics and mechanisms of narrow-band or broadband NIR luminescence in all these materials are discussed in detail. Also, the various applications of NIR-emitting metal halides are highlighted and an outlook for the field is provided.

Plasmon-enhanced circular dichroism spectroscopy of chiral drug solutions.

We investigate the potential of surface plasmon polaritons at noble metal interfaces for surface-enhanced chiroptical sensing of dilute chiral drug solutions with nl volume. The high quality factor of surface plasmon resonances in both Otto and Kretschmann configurations enables the enhancement of circular dichroism differenatial absorption thanks to the large near-field intensity of such plasmonic excitations. Furthermore, the subwavelength confinement of surface plasmon polaritons is key to attain chiroptical sensitivity to small amounts of drug volumes placed around ≃100 nm by the metal surface. Our calculations focus on reparixin, a pharmaceutical molecule currently used in clinical studies for patients with community-acquired pneumonia, including COVID-19 and acute respiratory distress syndrome. Considering realistic dilute solutions of reparixin dissolved in water with concentration ≤5 mg/ml and nl volume, we find a circular-dichroism differential absorption enhancement factor of the order ≃20 and chirality-induced polarization distortion upon surface plasmon polariton excitation. Our results are relevant for the development of innovative chiroptical sensors capable of measuring the enantiomeric imbalance of chiral drug solutions with nl volume.

High Quality Factor in Solution-Processed Inorganic Microcavities Embedding CsPbBr3 Perovskite Nanocrystals.

Optical microcavities grant manipulation over light-matter interactions and light propagation, enabling the fabrication of foundational optical and optoelectronic components. However, the materials used for high-performing systems, mostly bulk inorganics, are typically costly, and their processing is hardly scalable. In this work, we present an alternative way to fabricate planar optical resonators via solely solution processing while approaching the performances of conventional systems. Here, we couple fully solution-processed high dielectric contrast inorganic Bragg mirrors obtained by sol-gel deposition with the remarkable photoluminescence properties of CsPbBr3 perovskite nanocrystals. The approach yields microcavities with a quality factor of ∼220, which is a record value for solution-processed inorganic structures, and a strong emission redistribution resulting in a 3-fold directional intensity enhancement.

Boosting the Photoluminescence Efficiency of InAs Nanocrystals Synthesized with Aminoarsine via a ZnSe Thick-Shell Overgrowth.

InAs-based nanocrystals can enable restriction of hazardous substances (RoHS) compliant optoelectronic devices, but their photoluminescence efficiency needs improvement. We report an optimized synthesis of InAs@ZnSe core@shell nanocrystals allowing to tune the ZnSe shell thickness up to seven mono-layers (ML) and to boost the emission, reaching a quantum yield of ≈70% at ≈900 nm. It is demonstrated that a high quantum yield can be attained when the shell thickness is at least ≈3ML. Notably, the photoluminescence lifetimeshows only a minor variation as a function of shell thickness, whereas the Auger recombination time (a limiting aspect in technological applications when fast) slows down from 11 to 38 ps when increasing the shell thickness from 1.5 to 7MLs. Chemical and structural analyses evidence that InAs@ZnSe nanocrystals do not exhibit any strain at the core-shell interface, likely due to the formation of an InZnSe interlayer. This is supported by atomistic modeling, which indicates the interlayer as being composed of In, Zn, Se and cation vacancies, alike to the In2 ZnSe4 crystal structure. The simulations reveal an electronic structure consistent with that of type-I heterostructures, in which localized trap states can be passivated by a thick shell (>3ML) and excitons are confined in the core.

Single-Photon Emitting Arrays by Capillary Assembly of Colloidal Semiconductor CdSe/CdS/SiO2 Nanocrystals.

The controlled placement of colloidal semiconductor nanocrystals (NCs) onto planar surfaces is crucial for scalable fabrication of single-photon emitters on-chip, which are critical elements of optical quantum computing, communication, and encryption. The positioning of colloidal semiconductor NCs such as metal chalcogenides or perovskites is still challenging, as it requires a nonaggressive fabrication process to preserve the optical properties of the NCs. In this work, periodic arrays of 2500 nanoholes are patterned by electron beam lithography in a poly(methyl methacrylate) (PMMA) thin film on indium tin oxide/glass substrates. Colloidal core/shell CdSe/CdS NCs, functionalized with a SiO2 capping layer to increase their size and facilitate deposition into 100 nm holes, are trapped with a close to optimal Poisson distribution into the PMMA nanoholes via a capillary assembly method. The resulting arrays of NCs contain hundreds of single-photon emitters each. We believe this work paves the way to an affordable, fast, and practical method for the fabrication of nanodevices, such as single-photon-emitting light-emitting diodes based on colloidal semiconductor NCs.

Near-Infrared Light-Emitting Diodes Based on RoHS-Compliant InAs/ZnSe Colloidal Quantum Dots.

We demonstrate efficient, stable, and fully RoHS-compliant near-infrared (NIR) light-emitting diodes (LEDs) based on InAs/ZnSe quantum dots (QDs) synthesized by employing a commercially available amino-As precursor. They have a record external quantum efficiency of 5.5% at 947 nm and an operational lifetime of ∼32 h before reaching 50% of their initial luminance. Our findings offer a new solution for developing RoHS-compliant light-emitting technologies based on Pb-free colloidal QDs.

Indium arsenide quantum dots: an alternative to lead-based infrared emitting nanomaterials.

Colloidal quantum dots (QDs) emitting in the infrared (IR) are promising building blocks for numerous photonic, optoelectronic and biomedical applications owing to their low-cost solution-processability and tunable emission. Among them, lead- and mercury-based QDs are currently the most developed materials. Yet, due to toxicity issues, the scientific community is focusing on safer alternatives. In this regard, indium arsenide (InAs) QDs are one of the best candidates as they can absorb and emit light in the whole near infrared spectral range and they are RoHS-compliant, with recent trends suggesting that there is a renewed interest in this class of materials. This review focuses on colloidal InAs QDs and aims to provide an up-to-date overview spanning from their synthesis and surface chemistry to post-synthesis modifications. We provide a comprehensive overview from initial synthetic methods to the most recent developments on the ability to control the size, size distribution, electronic properties and carrier dynamics. Then, we describe doping and alloying strategies applied to InAs QDs as well as InAs based heterostructures. Furthermore, we present the state-of-the-art applications of InAs QDs, with a particular focus on bioimaging and field effect transistors. Finally, we discuss open challenges and future perspectives.

Layer-by-layer assembly of CsPbX3 nanocrystals into large-scale homostructures.

Advances in surface chemistry of CsPbX3 (where X = Cl, Br or I) nanocrystals (NCs) enabled the replacement of native chain ligands in solution. However, there are few reports on ligand exchange carried out on CsPbX3 NC thin films. Solid-state ligand exchange can improve the photoluminescence quantum yield (PLQY) of the film and promote a change in solubility of the solid surface, thus enabling multiple depositions of subsequent nanocrystal layers. Fine control of nanocrystal film thickness is of importance for light-emitting diodes (LEDs), solar cells and lasers alike. The thickness of the emissive material film is crucial to assure the copious recombination of charges injected into a LED, resulting in bright electroluminescence. Similarly, solar cell performance is determined by the amount of absorbed light, and hence the light absorber content in the device. In this study, we demonstrate a layer-by-layer (LbL) assembly method that results in high quality films, whose thicknesses can be finely controlled. In the solid state, we replaced oleic acid and oleylamine ligands with didodecyldimethylammonium bromide or ammonium thiocyanate that enhance the PLQY of the film. The exchange is carried out through a spin-coating technique, using solvents with strategic polarity to avoid NC dissolution or damage. Exploiting this technique, the deposition of various layers results in considerable thickening of films as proven by atomic force microscope measurements. The ease of handling of our combined process (i.e. ligand exchange and layer-by-layer deposition) enables thickness control over CsPbX3 NC films with applicability to other perovskite nanomaterials paving the way for a large variety of layer permutations.

ZnCl2 Mediated Synthesis of InAs Nanocrystals with Aminoarsine.

The most developed approaches for the synthesis of InAs nanocrystals (NCs) rely on pyrophoric, toxic, and not readily available tris-trimethylsilyl (or tris-trimethylgermil) arsine precursors. Less toxic and commercially available chemicals, such as tris(dimethylamino)arsine, have recently emerged as alternative As precursors. Nevertheless, InAs NCs made with such compounds need to be further optimized in terms of size distribution and optical properties in order to meet the standard reached with tris-trimethylsilyl arsine. To this aim, in this work we investigated the role of ZnCl2 used as an additive in the synthesis of InAs NCs with tris(dimethylamino)arsine and alane N,N-dimethylethylamine as the reducing agent. We discovered that ZnCl2 helps not only to improve the size distribution of InAs NCs but also to passivate their surface acting as a Z-type ligand. The presence of ZnCl2 on the surface of the NCs and the excess of Zn precursor used in the synthesis enable the subsequent in situ growth of a ZnSe shell, which is realized by simply adding the Se precursor to the crude reaction mixture. The resulting InAs@ZnSe core@shell NCs exhibit photoluminescence emission at ∼860 nm with a quantum yield as high as 42±4%, which is a record for such heterostructures, given the relatively high mismatch (6%) between InAs and ZnSe. Such bright emission was ascribed to the formation, under our peculiar reaction conditions, of an In-Zn-Se intermediate layer between the core and the shell, as indicated by X-ray photoelectron spectroscopy and elemental analyses, which helps to release the strain between the two materials.

Cesium Manganese Bromide Nanocrystal Sensitizers for Broadband Vis-to-NIR Downshifting.

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible-to-NIR spectral conversion. However, lead-based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr3 and Cs3MnBr5 nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr3. The correlation of the lanthanide emission to the CsMnBr3 visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr3 matrix to the lanthanides was understood by density functional theory calculations. This study shows that lead-free metal halides with an appropriate phase are effective sensitizers for lanthanides and offer a route to efficient downshifting applications.

Mild Sol-Gel Conditions and High Dielectric Contrast: A Facile Processing toward Large-Scale Hybrid Photonic Crystals for Sensing and Photocatalysis.

Solution processing of highly performing photonic crystals has been a towering ambition for making them technologically relevant in applications requiring mass and large-area production. It would indeed represent a paradigm changer for the fabrication of sensors and for light management nanostructures meant for photonics and advanced photocatalytic systems. On the other hand, solution-processed structures often suffer from low dielectric contrast and poor optical quality or require complex deposition procedures due to the intrinsic properties of components treatable from solution. This work reports on a low-temperature sol-gel route between the alkoxides of Si and Ti and poly(acrylic acid), leading to stable polymer-inorganic hybrid materials with tunable refractive index and, in the case of titania hybrid, photoactive properties. Alternating thin films of the two hybrids allows planar photonic crystals with high optical quality and dielectric contrast as large as 0.64. Moreover, low-temperature treatments also allow coupling the titania hybrids with several temperature-sensitive materials including dielectric and semiconducting polymers to fabricate photonic structures. These findings open new perspectives in several fields; preliminary results demonstrate that the hybrid structures are suitable for sensing and the enhancement of the catalytic activity of photoactive media and light emission control.

Mixed Dimethylammonium/Methylammonium Lead Halide Perovskite Crystals for Improved Structural Stability and Enhanced Photodetection.

The solvent acidolysis crystallization technique is utilized to grow mixed dimethylammonium/methylammonium lead tribromide (DMA/MAPbBr3 ) crystals reaching the highest dimethylammonium incorporation of 44% while maintaining the 3D cubic perovskite phase. These mixed perovskite crystals show suppression of the orthorhombic phase and a lower tetragonal-to-cubic phase-transition temperature compared to MAPbBr3 . A distinct behavior is observed in the temperature-dependent photoluminescence properties of MAPbBr3 and mixed DMA/MAPbBr3 crystals due to the different organic cation dynamics governing the phase transition(s). Furthermore, lateral photodetectors based on these crystals show that, at room temperature, the mixed crystals possess higher detectivity compared to MAPbBr3 crystals caused by structural compression and reduced surface trap density. Remarkably, the mixed-crystal devices exhibit large enhancement in their detectivity below the phase-transition temperature (at 200 K), while for the MAPbBr3 devices only insignificant changes are observed. The high detectivity of the mixed crystals makes them attractive for visible-light communication and for space applications. The results highlight the importance of the synthetic technique for compositional engineering of halide perovskites that governs their structural and optoelectronic properties.

Switchable Anion Exchange in Polymer-Encapsulated APbX3 Nanocrystals Delivers Stable All-Perovskite White Emitters.

We report a one-step synthesis of halide perovskite nanocrystals embedded in amphiphilic polymer (poly(acrylic acid)-block-poly(styrene), PAA-b-PS) micelles, based on injecting a dimethylformamide solution of PAA-b-PS, PbBr2, ABr (A = Cs, formamidinium, or both) and "additive" molecules in toluene. These bifunctional or trifunctional short chain organic molecules improve the nanocrystal-polymer compatibility, increasing the nanocrystal stability against polar solvents and high flux irradiation (the nanocrystals retain almost 80% of their photoluminescence after 1 h of 3.2 w/cm2 irradiation). If the nanocrystals are suspended in toluene, the coil state of the polymer allows the nanocrystals to undergo halide exchange, enabling emission color tunability. If the nanocrystals are suspended in methanol, or dried as powders, the polymer is in the globule state, and they are inert to halide exchange. By mixing three primary colors we could prepare stable, multicolor emissive samples (for example, white emitting powders) and a UV-to-white color converting layer for light-emitting diodes entirely made of perovskite nanocrystals.

Mechanical switching of orientation-related photoluminescence in deep-blue 2D layered perovskite ensembles.

The synergy between the organic component of two-dimensional (2D) metal halide layered perovskites and flexible polymers offers an unexplored window to tune their optical properties at low mechanical stress. Thus, there is a significant interest in exploiting their PL anisotropy by controlling their orientation and elucidating their interactions. Here, we apply this principle to platelet structures of micrometre lateral size that are synthesized in situ into free-standing polymer films. We study the photoluminescence of the resulting films under cyclic mechanical stress and observe an enhancement in the emission intensity up to ∼2.5 times along with a switch in the emission profile when stretching the films from 0% to 70% elongation. All the films recovered their initial emission intensity when releasing the stress throughout ca. 15 mechanical cycles. We hypothesize a combined contribution from reduced reabsorption, changes on in-plane and out-of-plane dipole moments that stem from different orientation of the platelets inside the film, and relative sliding of platelets within oriented stacks while stretching the films. Our results reveal how low-mechanical stress affects 2D layered perovskite aggregation and orientation, an open pathway toward the design of strain-controlled emission.

Halide Perovskite-Lead Chalcohalide Nanocrystal Heterostructures.

We report the synthesis of colloidal CsPbX3-Pb4S3Br2 (X = Cl, Br, I) nanocrystal heterostructures, providing an example of a sharp and atomically resolved epitaxial interface between a metal halide perovskite and a non-perovskite lattice. The CsPbBr3-Pb4S3Br2 nanocrystals are prepared by a two-step direct synthesis using preformed subnanometer CsPbBr3 clusters. Density functional theory calculations indicate the creation of a quasi-type II alignment at the heterointerface as well as the formation of localized trap states, promoting ultrafast separation of photogenerated excitons and carrier trapping, as confirmed by spectroscopic experiments. Postsynthesis reaction with either Cl- or I- ions delivers the corresponding CsPbCl3-Pb4S3Br2 and CsPbI3-Pb4S3Br2 heterostructures, thus enabling anion exchange only in the perovskite domain. An increased structural rigidity is conferred to the perovskite lattice when it is interfaced with the chalcohalide lattice. This is attested by the improved stability of the metastable γ phase (or "black" phase) of CsPbI3 in the CsPbI3-Pb4S3Br2 heterostructure.

Core-shell silica-rhodamine B nanosphere for synthetic opals: from fluorescence spectral redistribution to sensing.

Photonic crystals are a unique tool to modify the photoluminescence of light-emitting materials. A variety of optical effects have been demonstrated by infiltrating opaline structures with photoactive media. On the other hand, the fabrication of such structures includes complex infiltration steps, that often affect the opal lattice and decrease the efficiency of light emission control. In this work, silica nanospheres were directly functionalized with rhodamine B to create an emitting shell around the dielectric core. Simple tuning of the microsphere preparation conditions allows selecting the appropriate sphere diameter and polydispersity index approaching 5%. These characteristics allow facile self-assembling of the nanospheres into three-dimensional photonic crystals whose peculiar density of photonic states at the band-gap edges induces spectral redistribution of the rhodamine B photoluminescence. The possibility to employ the new stable structure as sensor is also investigated. As a proof of principle, we report the variation of light emission obtained by exposure of the opal to vapor of chlorobenzene.

Robustness to High Temperatures of Al2O3-Coated CsPbBr3 Nanocrystal Thin Films with High-Photoluminescence Quantum Yield for Light Emission.

Lead-halide perovskite nanocrystals are a promising material in optical devices due to their high photoluminescence (PL) quantum yield, excellent color purity, and low stimulated emission threshold. However, one problem is the stability of the nanocrystal films under different environmental conditions and under high temperatures. The latter is particularly relevant for device fabrication if further processes that require elevated temperatures are needed after the deposition of the nanocrystal film. In this work, we study the impact of a thin oxide layer of Al2O3 on the light emission properties of thin nanocrystal films. We find that nanocrystals passivated with quaternary ammonium bromide ligands maintain their advantageous optical properties in alumina-coated films and do not suffer from degradation at temperatures up to 100 °C. This is manifested by conservation of the PL peak position and line width, PL decay dynamics, and low threshold for amplified spontaneous emission. The PL remains stable for up to 100 h at a temperature of 80 °C, and the ASE intensity decreases by less than 30% under constant pumping at high fluence for 1 h. Our approach outlines that the combination of tailored surface chemistry with additional protective coating of the nanocrystal film is a feasible approach to obtain stable emission at elevated temperatures and under extended operational time scales.