Quantum Dot/Siloxane Composite Film Exceptionally Stable against Oxidation under Heat and Moisture

Long-Term Comparisons of Photoluminescence Affected by Organic Cations of Formamidinium and Methylammonium in Monophasic Lead Iodide Perovskite Quantum Dots

This study compared the photoluminescence (PL) stabilities of formamidinium (FA) and methylammonium (MA) in lead iodide perovskite quantum dots (QDs). To exclude other factors, such as size and purity, that may affect stability, MAPbI3 and FAPbI3 QDs with nearly identical sizes (~10.0 nm) were synthesized by controlling the ligand concentration and synthesis temperature. Transmission electron microscopy images and X-ray diffraction patterns confirmed homogeneous single-phase perovskite structures. Additionally, the bandgaps and sizes of the synthesized QDs closely matched those of the infinite quantum well model, which guaranteed that the photostability was solely caused by the different organic molecules in the two QDs. We analyzed the PL peak centers and full-width at half maximum of the QDs for 32 days. The enhanced stability of FAPbI3 was found to be caused by the nearly zero redshift (1.615 eV) of its PL peak, in contrast to the redshift (1.685→1.670 eV) of MAPbI3.

Preparation and performance of alumina/epoxy-siloxane composites: A comparative study on thermal- and photo-curing process

Although epoxy-based composites that consist of inorganic fillers and matrixes are widely used in "conventional" electronic packaging applications due to their excellent insulation and robust properties, they limit their uses in "advanced electronic packaging" which requires enhanced thermal conductivity. However, conventional thermal curing methods for fabrication of epoxy-based composites do not fulfill sufficient thermal conductivity. Herein, we apply photo-induced curing strategy for fabricating alumina-incorporated epoxy-siloxane composites that consist of sol-gel derived siloxane matrix and bimodal sized alumina particles as a thermally conductive filler. We investigate how curing mechanism (thermal- or UV-curing) and varying the ratios of the alumina particles of two different sizes affect the various physical properties. It is found that photo-curing process makes greatly enhanced thermal conductivity, low thermal expansion, and high mechanical robustness compared to thermally-cured composites. As the results, we can achieve significantly enhanced thermal conductivity (>11 W/m K) with high thermal stability and mechanical robustness.

Direct Optical Patterning of Quantum Dots: One Strategy, Different Chemical Processes

Patterning, stability, and dispersion of the semiconductor quantum dots (scQDs) are three issues strictly interconnected for successful device manufacturing. Recently, several authors adopted direct optical patterning (DOP) as a step forward in photolithography to position the scQDs in a selected area. However, the chemistry behind the stability, dispersion, and patterning has to be carefully integrated to obtain a functional commercial device. This review describes different chemical strategies suitable to stabilize the scQDs both at a single level and as an ensemble. Special attention is paid to those strategies compatible with direct optical patterning (DOP). With the same purpose, the scQDs' dispersion in a matrix was described in terms of the scQD surface ligands' interactions with the matrix itself. The chemical processes behind the DOP are illustrated and discussed for five different approaches, all together considering stability, dispersion, and the patterning itself of the scQDs.

Luminescent Quantum Dots: Synthesis, Optical Properties, Bioimaging and Toxicity

Luminescent nanomaterials such as semiconductor nanocrystals (NCs) and quantum dots (QDs) attract much attention to optical detectors, LEDs, photovoltaics, displays, biosensing, and bioimaging. These materials include metal chalcogenide QDs and metal halide perovskite NCs. Since the introduction of cadmium chalcogenide QDs to biolabeling and bioimaging, various metal nanoparticles (NPs), atomically precise metal nanoclusters, carbon QDs, graphene QDs, silicon QDs, and other chalcogenide QDs have been infiltrating the nano-bio interface as imaging and therapeutic agents. Nanobioconjugates prepared from luminescent QDs form a new class of imaging probes for cellular and in vivo imaging with single-molecule, super-resolution, and 3D resolutions. Surface modified and bioconjugated core-only and core-shell QDs of metal chalcogenides (MX; M = Cd/Pb/Hg/Ag, and X = S/Se/Te,), binary metal chalcogenides (MInX₂; M = Cu/Ag, and X = S/Se/Te), indium compounds (InAs and InP), metal NPs (Ag, Au, and Pt), pure or mixed precision nanoclusters (Ag, Au, Pt), carbon nanomaterials (graphene QDs, graphene nanosheets, carbon NPs, and nanodiamond), silica NPs, silicon QDs, etc. have become prevalent in biosensing, bioimaging, and phototherapy. While heavy metal-based QDs are limited to in vitro bioanalysis or clinical testing due to their potential metal ion-induced toxicity, carbon (nanodiamond and graphene) and silicon QDs, gold and silica nanoparticles, and metal nanoclusters continue their in vivo voyage towards clinical imaging and therapeutic applications. This review summarizes the synthesis, chemical modifications, optical properties, and bioimaging applications of semiconductor QDs with particular references to metal chalcogenide QDs and bimetallic chalcogenide QDs. Also, this review highlights the toxicity and pharmacokinetics of QD bioconjugates.

Encapsulation of Cadmium-Free InP/ZnSe/ZnS Quantum Dots in Poly(LMA-co-EGDMA) Microparticles via Co-flow Droplet Microfluidics

Quantum dots (QDs) are semiconductor nanocrystals that are used in optoelectronic applications. Most modern QDs are based on toxic metals, for example Cd, and do not comply with the European Restriction of Hazardous Substances regulation of the European Union. Latest promising developments focus on safer QD alternatives based on elements from the III-V group. However, the InP-based QDs lack an overall photostability under environmental influences. One design path of achieving stability is through encapsulation in cross-linked polymer matrices with the possibility to covalently link the matrix to surface ligands of modified core-shell QDs. The work focuses on the formation of polymer microbeads suitable for InP-based QD encapsulation, allowing for an individual protection of QDs and an improved processibility via this particle-based approach. For this, a microfluidic based method in the co-flow regime is used that consists of an oil-in-water droplet system in a glass capillary environment. The generated monomer droplets are polymerized in-flow into poly(LMA-co-EGDMA) microparticles with embedded InP/ZnSe/ZnS QDs using a UV initiation. They demonstrate how a successful polymer microparticle formation via droplet microfluidics produces optimized matrix structures leading to a distinct photostability improvement of InP-based QDs compared to nonprotected QDs.

Full-color micro-LED display with photo-patternable and highly ambient-stable perovskite quantum dot/siloxane composite as color conversion layers

In this paper, we successfully fabricated color conversion layers (CCLs) for full-color-mico-LED display using a perovskite quantum dot (PQD)/siloxane composite by ligand exchanged PQD with silane composite followed by surface activation by an addition of halide-anion containing salt. Due to this surface activation, it was possible to construct the PQD surface with a silane ligand using a non-polar organic solvent that does not damage the PQD. As a result, the ligand-exchanged PQD with a silane compound exhibited high dispersibility in the siloxane matrix and excellent atmospheric stability due to sol-gel condensation. Based on highly ambient stable PQD/siloxane composite based CCLs, full-color micro-LED display has a 1 mm pixel pitch, about 25.4 pixels per inch (PPI) resolution was achieved. In addition, due to the thin thickness of the black matrix to prevent blue light interference, the possibility of a flexible display that can be operated without damage even with a bending radius of 5 mm was demonstrated.

Achieving High-Efficiency Large-Area Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are semitransparent windows that are able to generate electricity from sunlight absorption. LSCs have shown huge promise for realizing building-integrated photovoltaics (BIPV). Unfortunately, to date, the power conversion efficiency (PCE) of LSCs is still very low which dramatically hampers their practical applications. In this Perspective, We summarize and review the latest developments of LSCs by looking at different structures. Among others, we focus more on the next developments in the field of LSCs, i.e., the possibility of high PCE, large area, mass production, and durability needed for future industrial development. We hope to promote the application of uniform testing standards and to draw attention to industrial development, toxicity, and durability. Then, we will provide a critical assessment of the field of LSCs. Finally, the challenge and solution will be discussed.

Thermally Processed Quantum-Dot Polypropylene Composite Color Converter Film for Displays

Quantum dots (QDs) have attracted much attention as one of the most promising candidates for next-generation display materials. However, stability is still a big challenge for QDs. Herein, we encapsulated QDs in a thermoplastic polypropylene (PP) matrix by thermal processing technology to prepare a stabler color conversion film for the first time. Thermal processing technology expands the packaging materials of QDs from traditional soluble polymers to thermoplastic polymers such as PP with easy processing and a low cost. We showed that the QDs in the PP film exhibited longer-lasting stability than the traditional PMMA film. After 216 h of blue light accelerated aging test, the QDs maintained more than 90% of the initial performance in the PP film but dropped to less than 25% in the PMMA film. Moreover, the reasons for the improved stability have been further discussed. It was found that the PP-H film not only possessed better barriers to moisture and oxygen, but the absence of ester groups also led to a milder environment around the QDs. The results show that ester groups have stronger electronegativity and easily cause the ligands on the surface of QDs to fall off, which lead to performance degradation.

Siloxane Hybrid Material-Encapsulated Highly Robust Flexible μLEDs for Biocompatible Lighting Applications

Flexible micro-light-emitting diodes (f-μLEDs) have been regarded as an attractive light source for the next-generation human-machine interfaces, thanks to their noticeable optoelectronic performances. However, when it comes to their practical utilizations fulfilling industrial standards, there have been unsolved reliability and durability issues of the f-μLEDs, despite previous developments in the high-performance f-μLEDs for various applications. Herein, highly robust flexible μLEDs (f-HμLEDs) with 20 × 20 arrays, which are realized by a siloxane-based organic-inorganic hybrid material (SHM), are reported. The f-HμLEDs are created by combining the f-μLED fabrication process with SHM synthesis procedures (i.e., sol-gel reaction and successive photocuring). The outstanding mechanical, thermal, and environmental stabilities of our f-HμLEDs are confirmed by a host of experimental and theoretical examinations, including a bending fatigue test (10⁵ bending/unbending cycles), a lifetime accelerated stress test (85 °C and 85% relative humidity), and finite element method simulations. Eventually, to demonstrate the potential of our f-HμLEDs for practical applications of flexible displays and/or biomedical devices, their white light emission due to quantum dot-based color conversion of blue light emitted by GaN-based f-HμLEDs is demonstrated, and the biocompatibility of our f-HμLEDs is confirmed via cytotoxicity and cell proliferation tests with muscle, bone, and neuron cell lines. As far as we can tell, this work is the first demonstration of the flexible μLED encapsulation platform based on the SHM, which proved its mechanical, thermal, and environmental stabilities and biocompatibility, enabling us to envisage biomedical and/or flexible display applications using our f-HμLEDs.

Superior hydrophobic silica-coated quantum dot for stable optical performance in humid environments

Quantum dot (QD) features many exceptional optical performances but is also vulnerable to moisture which results in structural damage and luminescent decrease. This work provided and fabricated a novel superior hydrophobic methylated core/shell silica-coated QD (MSQ) for high water stability. QD was coated with a silica shell and then surface-methylated by trimethyl silane. Mercaptopropyl trimethoxy silane, tetraethyl orthosilicate, and ethoxy trimethyl silane were utilized as the ligand exchanger, the raw material of silica, and the surface modification, respectively. Characterization results illustrated the core/shell structure of MSQ. In addition, its water contact angle was up to 159.6°. QD-, silica-coated QD(SQ)-, and MSQ-silicone were made and displayed similar absorption, emission, and excitation spectra but different water stabilities. The photoluminescence intensity and photoluminescence quantum yield of MSQ-silicone hardly changed during 15 d of water immersion, in contrast to the dramatical decrease of other two kinds of composite silicone. Specifically, the photoluminescence quantum yield decreases of MSQ-, SQ-, and QD-silicone were 1%, 40%, and 43%, respectively. Therefore, MSQ had a much better water stability. The superior hydrophobic methylated silica-coated QD has a great potential to realize the long-term working stability in a humid environment and the wider application in diverse fields.

Optical Performance and Moisture Stability Enhancement of Flexible Luminescent Films Based on Quantum-Dot/Epoxy Composite Particles

Quantum dots (QDs) have been widely applied in luminescent sources due to their strong optical characteristics. However, a moisture environment causes their quenching, leading to an inferior optical performance in commercial applications. In this study, based on the high moisture resistance of epoxy resin, a novel epoxy/QDs composite particle structure was proposed to solve this issue. Flexible luminescent films could be obtained by packaging composite particles in silicone resin, combining the hydrophobicity of epoxy resin and the flexibility of PDMS simultaneously. The photoluminescence and light extraction were improved due to the scattering properties of the structure of composite particles, which was caused by the refractive index mismatch between the epoxy and silicone resin. Compared to the QD/silicone film under similar lighting conditions, the proposed flexible film demonstrated increased light efficiency as well as high moisture stability. The results revealed that a light-emitting diode (LED) device using the composite particle flexible (CPF) structure obtained a 34.2% performance enhancement in luminous efficiency as well as a 32% improvement in color conversion efficiency compared to those of devices with QD/silicone film (QSF) structure. Furthermore, the CPF structure exhibited strong thermal and moisture stability against extreme ambient conditions of 85 °C and 85% relative humidity simultaneously. The normalized luminous flux degradation of devices embedded in CPF and QSF structures after aging for 118 h were ~20.2% and ~43.8%, respectively. The satisfactory performance of the CPF structure in terms of optical and moisture stability shows its great potential value in flexible commercial QD-based LED displays and lighting applications.

Extremely Stable Luminescent Crosslinked Perovskite Nanoparticles under Harsh Environments over 1.5 Years

Organic-inorganic hybrid perovskite nanoparticles (NPs) are a very strong candidate emitter that can meet the high luminescence efficiency and high color standard of Rec.2020. However, the instability of perovskite NPs is the most critical unsolved problem that limits their practical application. Here, an extremely stable crosslinked perovskite NP (CPN) is reported that maintains high photoluminescence quantum yield for 1.5 years (>600 d) in air and in harsher liquid environments (e.g., in water, acid, or base solutions, and in various polar solvents), and for more than 100 d under 85 °C and 85% relative humidity without additional encapsulation. Unsaturated hydrocarbons in both the acid and base ligands of NPs are chemically crosslinked with a methacrylate-functionalized matrix, which prevents decomposition of the perovskite crystals. Counterintuitively, water vapor permeating through the crosslinked matrix chemically passivates surface defects in the NPs and reduces nonradiative recombination. Green-emitting and white-emitting flexible large-area displays are demonstrated, which are stable for >400 d in air and in water. The high stability of the CPN in water enables biocompatible cell proliferation which is usually impossible when toxic Pb elements are present. The stable materials design strategies provide a breakthrough toward commercialization of perovskite NPs in displays and bio-related applications.

Desolvation-Triggered Versatile Transfer-Printing of Pure BN Films with Thermal-Optical Dual Functionality

Although hexagonal boron nitride (BN) nanostructures have recently received significant attention due to their unique physical and chemical properties, their applications have been limited by a lack of processability and poor film quality. In this study, a versatile method to transfer-print high-quality BN films composed of densely stacked BN nanosheets based on a desolvation-induced adhesion switching (DIAS) mechanism is developed. It is shown that edge functionalization of BN sheets and rational selection of membrane surface energy combined with systematic control of solvation and desolvation status enable extensive tunability of interfacial interactions at BN-BN, BN-membrane, and BN-substrate boundaries. Therefore, without incorporating any additives in the BN film and applying any surface treatment on target substrates, DIAS achieves a near 100% transfer yield of pure BN films on diverse substrates, including substrates containing significant surface irregularities. The printed BNs demonstrate high optical transparency (>90%) and excellent thermal conductivity (>167 W m^-1 K^-1) for few-micrometer-thick films due to their dense and well-ordered microstructures. In addition to outstanding heat dissipation capability, substantial optical enhancement effects are confirmed for light-emitting, photoluminescent, and photovoltaic devices, demonstrating their remarkable promise for next-generation optoelectronic device platforms.

Kinetics and Thermodynamics of Killing a Quantum Dot

Light-emitting nanocrystal quantum dots (QDs) are of high interest for use as down-conversion phosphors and direct emission sources in bulk solid-state devices and as reliable sources of single photons in quantum information science. However, these materials are prone to photooxidation that reduces the emission quantum yield over time. Current commercial applications use device architectures to prevent oxidation without addressing the underlying degradation reactions at the nanocrystal level. To instead prevent loss of functionality by better synthetic engineering of the nanoscale emitters themselves, the underlying properties of these reactions must be understood and readily accessible. Here, we use solid-state spectroscopy to obtain kinetic and thermodynamic parameters of photothermal degradation in single QDs by systematically varying the ambient temperature and photon pump fluence. We describe the resulting degradation in emission with a modified form of the Arrhenius equation and show that this reaction proceeds via pseudo-zero-order reaction kinetics by a surface-assisted process with an activation energy of 60 kJ/mol. We note that the rate of degradation is ∼12 orders of magnitude slower than the rate of excitonic processes, indicating that the reaction rate is not determined by electron or hole trapping. In the search for new robust light-emitting nanocrystals, the reported analysis method will enable direct comparisons between differently engineered nanomaterials.

Highly transparent and luminescent gel glass based on reabsorption-free gold nanoclusters

Luminescent and transparent composites formed by embedding luminophores in a solid matrix are essential components for several photonic applications, such as luminescent solar concentrators (LSCs) and luminescent down-shifting/conversion layers. For these applications, the optical losses, including reabsorption and scattering need to be minimized, while the photoluminescence (PL) emission must be stable against outdoor environments. Here, highly transparent and luminescent aluminosilicate glass doped with surface-engineered gold nanoclusters (AuNCs) was prepared without involving toxic elements and hazardous solvents. Such an AuNC@glass composite with a high loading (∼14 wt%) exhibits a unique absorption profile; near-unity absorptance in the absorption range but near-zero reabsorption in the emission region, and thus generates bright PL emission with negligible reabsorption losses. Meanwhile, the PL quantum yield was enhanced (from ∼1% to ∼14%) without sacrificing the Stokes shift, while still maintaining high optical transparency. In addition, they have high stability due to the effective protection of rigid inorganic matrices, and thus would be eco-friendly candidates for further preparation of efficient and reabsorption-free LSCs.

Hybrid fluorescent liquid crystalline composites: directed assembly of quantum dots in liquid crystalline block copolymer matrices

Hybrid fluorescent liquid crystalline (LC) composites containing inorganic quantum dots (QDs) are promising materials for many applications in optics, nanophotonics and display technology, combining the superior emission capability of QDs with the externally controllable optical properties of LCs. In this work, we propose the hybrid LC composites that were obtained by embedding CdSe/ZnS QDs into a series of host LC block copolymers of different architectures by means of a two-stage ligand exchange procedure. The ABA/BAB triblock copolymers and AB diblock copolymers with different polymerization degrees are composed of nematogenic phenyl benzoate acrylic monomer units and poly(4-vinylpyridine) blocks, which are capable of binding to the QD surface. Our results clearly show that the spatial distribution of QDs within composite films as well as the formation of QD aggregates can be programed by varying the structure of the host block copolymer. The obtained composites form a nematic LC phase, with isotropization temperatures being close to those of the initial host block copolymers. In addition, the influence of the molecular architecture of the host block copolymers on fluorescence properties of the obtained composites is considered. The described strategy for the QD assembly should provide a robust and conventional route for the design of highly ordered hierarchical hybrid materials for many practical applications.

Efficient and Stable CdSe/CdS/ZnS Quantum Rods-in-Matrix Assembly for White LED Application

CdSe/CdS core-shell quantum rods (QRs) are a promising prospect in optoelectronic applications but usually have a relatively low quantum efficiency and stability. Here, we report on an efficient and stable CdSe/CdS/ZnS QRs-in-matrix assembly (QRAs) by growing and embedding CdSe/CdS QRs in ZnS matrices. Structural characterizations show that the CdSe/CdS QRs are encapsulated and interconnected by ZnS in the QRAs structure. The stable ZnS encapsulation renders the CdSe/CdS QRs high quantum efficiency (QE) up to 85%. The QRAs also present high photo- and thermal-stability and can preserve 93% of the initial QE at 100 °C. The QRAs powder presents a light degradation of only 2% under continuous excitation for 100 h, displaying profound potential in optoelectronic applications. White light-emitting diodes (WLEDs) are fabricated by packaging the QRAs powder as phosphor on top of blue GaN chip. The WLED shows high optical performance and light quality.

Photo-Patternable Quantum Dots/Siloxane Composite with Long-Term Stability for Quantum Dot Color Filters

Incorporation of quantum dots (QDs) into color filters (CFs) are desired for less energy loss and wider viewing angle compared to a conventional display. However, aggregation and vulnerability to heat, moisture, and chemicals in the photo-patternable matrix are critical issues of the QD-CFs with high QDs concentration. Herein, we fabricated red (10 wt %) and green (20 wt %) QD-CFs using photolithography of QD/siloxane ink containing secondary thiol monomer. Ligand-exchanged QDs were chemically incorporated in methacrylate oligosiloxane resin. QD/siloxane composite showed superior stability under harsh heat and moisture (85 °C/5% RH and 85 °C/85% RH) conditions and chemicals (EtOH, HCl, and NaOH) compared to conventional QD/PR (commercial negative photoresist). QD-CFs (10 μm thick) effectively converted blue light emitted from LED chip into red and green light, and the obtained white PL through QD-CF showed wide color gamut, which was 108% relative to NTSC. From these advantages, QD/siloxane composite will be beneficial as color-conversion photoresists are to be used as color filters in liquid crystal displays, micro light-emitting diodes, and organic light-emitting diodes.

Covalent Epitope Decoration of Carbon Electrodes using Solid Phase Peptide Synthesis

Long-term, minimally perturbative brain electrophysiology requires electrodes to seamlessly integrate into surrounding tissue. In this work, we demonstrate electrodes composed of covalently functionalized graphite, decorated with various functional affinity and epitope tags, and use them to detect changes in electrical potential on the surfaces of illuminated quantum dots and near fluorescing molecules. Affinity and epitope tagging of carbon was achieved using direct attachment of biotin and solid phase peptide synthesis (SPPS) of histidine (His)- and human influenza hemagglutinin (HA)-tags. Surface modification was confirmed with Auger, Energy-Dispersive X-ray (EDX), Raman, and fluorescence spectroscopy. Photoresponse was detected with compatible binding protein-surface tag combinations, confirming desired tag and electrode functionality. These results provide a path to organic, biofunctionalized, fully molecularly-defined electrodes for neuronal applications, and to a wide range of other secondary reactions and modifications of carbon; potential uses include affinity chromatography, DNA sequencing technologies, biomolecular sensors, and surfaces and scaffolds for targeted interfaces with biological tissues.

Exceptionally stable quantum dot/siloxane hybrid encapsulation material for white light-emitting diodes with a wide color gamut

Herein, we report a luminescent light-emitting diode (LED) encapsulating material using a thermally curable quantum dot (QD)/siloxane hybrid (TSE-QD) color converter, which has superior long-term stability even at elevated temperatures, in high humidity, and in various chemicals. The TSE-QD is cured by a thermal-induced hydrosilylation reaction of an in situ sol-gel synthesized QD dispersed siloxane resin (QD/siloxane resin) without additional ligand-exchange processes. QDs are successfully encapsulated by highly condensed and linear structured siloxane networks with additional chemical linkages between the surface ligands of the QDs and organic functional groups of the siloxane matrix. Moreover, QDs are uniformly distributed within the siloxane matrix retaining their optical properties during the fabrication processes of the TSE-QD. The result is that the stability, as evaluated by the photoluminescence (PL) quantum yield (QY), is greatly improved under harsh conditions, for example, 120 °C/5% relative humidity (RH), ethanol and acetone for 30 days. Based on the exceptionally stable TSE-QD, we demonstrate a white LED using a blue LED chip directly encapsulated by a yellow emitting TSE-QD that shows excellent spectral stability, outstanding reliability at 85 °C/85% RH and a wide color gamut (116% of NTSC).

Sacrificial oxidation of a self-metal source for the rapid growth of metal oxides on quantum dots towards improving photostability

Growth of metal oxide layers on quantum dots (QDs) has been regarded as a good way to improve the photostability of QDs. However, direct growth of metal oxides on individual QD remains a great challenge. Here we report a novel approach to rapidly anchor metal oxides on QD surfaces through a sacrificial oxidation of a self-metal source strategy. As typical core/shell QDs, CdSe/CdS or aluminum doped CdSe/CdS (CdSe/CdS:Al) QDs were chosen and treated with peroxide (benzoyl peroxide). Self-metal sources (cadmium or/and aluminum) can be easily sacrificially oxidized, leading to the quick growth of cadmium oxide (CdO) or aluminum/cadmium hybrid oxides (Al2O3/CdO) on the surface of individual QD for improved photostability. Compared with CdO, Al2O3 possesses excellent barrier properties against moisture and oxygen. Therefore, CdSe/CdS QDs with the protection of an Al2O3/CdO hybrid layer show much superior photostability. Under strong illumination with blue light, the QDs coated with the Al2O3/CdO hybrid layer retained 100% of the original photoluminescence intensity after 70 h, while that of the untreated CdSe/CdS:Al, the treated CdSe/CdS and the CdSe/CdS QDs dropped to 65%, 45%, and 5%, respectively. Furthermore, we demonstrate that this method can be extended to other metal-doped QD systems, even including some inactive metals difficult to be oxidized spontaneously in an ambient atmosphere, which provides a new way to stabilize QDs for diverse optoelectronic applications.

Two-Step-Enhanced Stability of Quantum Dots via Silica and Siloxane Encapsulation for the Long-Term Operation of Light-Emitting Diodes

Despite innovative optical properties of quantum dots (QDs) for QDs-converted light-emitting diodes (QD-LEDs), the vulnerability of the QDs, against heat and moisture, has been a critical issue for commercialization and long-term use. To overcome the instabilities, we fabricated a thermally and photostable QDs-embedded silica/siloxane (S-QD/siloxane) film by embedding QDs in silica and siloxane encapsulation through a two-step sol-gel reaction. S-QDs were stably dispersed in the oligo-siloxane resin with even a QD concentration of 5 wt % without aggregation. The two-step physical barriers of silica and siloxane acted to decrease the toxicity of QDs and improve the stability against heat and moisture [85 °C/5% relative humidity (RH), 85 °C/85% RH, and 120 °C/5% RH], light (50 and 100 mA), and chemicals (ethanol, HCl, and NaOH). Our S-QD/siloxane film was applied as a color-conversion material on a blue LED chip without additional solidification and encapsulation processes for red and white QD-LEDs, exhibiting a wider color gamut (107% in CIE 1931) compared to NTSC. These enhancements indicate that our S-QD/siloxane film is a suitable material for long-term operation of QD-enhanced films and QD-LEDs in next-generation displays.

Largely Enhancing Luminous Efficacy, Color-Conversion Efficiency, and Stability for Quantum-Dot White LEDs Using the Two-Dimensional Hexagonal Pore Structure of SBA-15 Mesoporous Particles

Quantum-dot (QD) white light-emitting diodes (LEDs) are promising for illumination and display applications due to their excellent color quality. Although they have a high quantum yield close to unity, the reabsorption of QD light leads to high conversion loss, significantly reducing the luminous efficacy and stability of QD white LEDs. In this report, SBA-15 mesoporous particles (MPs) with two-dimensional hexagonal pore structures (2D-HPS) are utilized to largely enhance the luminous efficacy and color-conversion efficiency of QD white LEDs in excess of 50%. The reduction in conversion loss also helps QD white LEDs to achieve a lifetime 1.9 times longer than that of LEDs using QD-only composites at harsh aging conditions. Simulation and testing results suggest that the waveguide effect of 2D-HPS helps in reducing the reabsorption loss by constraining the QD light inside the wall of 2D-HPS, decreasing the probability of being captured by QDs inside the hole of 2D-HPS. As such, materials and mechanisms like SBA-15 MPs with 2D-HPS could provide a new path to improve the photon management of QD light, comprehensively enhancing the performances of QD white LEDs.

Siloxane-Encapsulated Upconversion Nanoparticle Hybrid Composite with Highly Stable Photoluminescence against Heat and Moisture

Herein, we report a siloxane-encapsulated upconversion nanoparticle hybrid composite (SE-UCNP), which exhibits excellent photoluminescence (PL) stability for over 40 days even at an elevated temperature, in high humidity, and in harsh chemicals. The SE-UCNP is synthesized through UV-induced free-radical polymerization of a sol-gel-derived UCNP-containing oligosiloxane resin (UCNP-oligosiloxane). The siloxane matrix with a random network structure by Si-O-Si bonds successfully encapsulates the UCNPs with chemical linkages between the siloxane matrix and organic ligands on UCNPs. This encapsulation results in surface passivation retaining the intrinsic fluorescent properties of UCNPs under severe conditions (e.g., 85 °C/85% relative humidity) and a wide range of pH (from 1 to 14). As an application example, we fabricate a two-color binary microbarcode based on SE-UCNP via a low-cost transfer printing process. Under near-infrared irradiation, the binary sequences in our barcode are readable enough to identify objects using a mobile phone camera. The hybridization of UCNPs with a siloxane matrix provides the capacity for highly stable UCNP-based applications in real environments.

A generalizable method for the construction of MOF@polymer functional composites through surface-initiated atom transfer radical polymerization

A universal method to grow polymers on MOF surfaces with well-defined thickness, sequence and functionality.

We report a generalizable approach to construct MOF@polymer functional composites through surface-initiated atom transfer radical polymerization (SI-ATRP). Unlike conventional SI-ATRP that requires covalent pre-anchoring of the initiating group on substrate surfaces, in our approach, a rationally designed random copolymer (RCP) macroinitiator first self-assembles on MOF surfaces through inter-chain hydrogen bond crosslinking. Subsequent polymerization in the presence of a crosslinking monomer covalently threads these polymer chains into a robust network, physically confining the MOF particle inside the polymer shell. We demonstrated the universality of this approach by growing various polymers on five MOFs of different metals (Zr, Zn, Co, Al, and Cr) with complete control over shell thickness, functionality and layer sequence while still retaining the inherent porosity of the MOFs. Moreover, the wettability of UiO-66 can be continuously tuned from superhydrophilic to superhydrophobic simply through judicious monomer(s) selection. We also demonstrated that a 7 nm polystyrene shell can effectively shield UiO-66 particles against 1 M H₂SO₄ and 1 M NaOH at elevated temperature, enabling their potential application in demanding chemical environments.

Overcoming the Ambient Manufacturability-Scalability-Performance Bottleneck in Colloidal Quantum Dot Photovoltaics

Colloidal quantum dot (CQD) solar cells have risen rapidly in performance; however, their low-cost fabrication under realistic ambient conditions remains elusive. This study uncovers that humid environments curtail the power conversion efficiency (PCE) of solar cells by preventing the needed oxygen doping of the hole transporter during ambient fabrication. A simple oxygen-doping step enabling ambient manufacturing irrespective of seasonal humidity variations is devised. Solar cells with PCE > 10% are printed under high humidity at industrially viable speeds. The devices use a tiny fraction of the ink typically needed and are air stable over a year. The humidity-resilient fabrication of efficient CQD solar cells breaks a long-standing compromise, which should accelerate commercialization.

Photophysics of Thermally-Assisted Photobleaching in "Giant" Quantum Dots Revealed in Single Nanocrystals

Quantum dots (QDs) are steadily being implemented as down-conversion phosphors in market-ready display products to enhance color rendering, brightness, and energy efficiency. However, for adequate longevity, QDs must be encased in a protective barrier that separates them from ambient oxygen and humidity, and device architectures are designed to avoid significant heating of the QDs as well as direct contact between the QDs and the excitation source. In order to increase the utility of QDs in display technologies and to extend their usefulness to more demanding applications as, for example, alternative phosphors for solid-state lighting (SSL), QDs must retain their photoluminescence emission properties over extended periods of time under conditions of high temperature and high light flux. Doing so would simplify the fabrication costs for QD display technologies and enable QDs to be used as down-conversion materials in light-emitting diodes for SSL, where direct-on-chip configurations expose the emitters to temperatures approaching 100 °C and to photon fluxes from 0.1 W/mm² to potentially 10 W/mm². Here, we investigate the photobleaching processes of single QDs exposed to controlled temperature and photon flux. In particular, we investigate two types of room-temperature-stable core/thick-shell QDs, known as "giant" QDs for which shell growth is conducted using either a standard layer-by-layer technique or by a continuous injection method. We determine the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging. The findings presented here will assist in the further development of advanced QD heterostructures for maximum device lifetime stability.

Ultrastable Quantum Dot Composite Films under Severe Environments

Semiconductor quantum dots (QDs) have attracted extensive attention because of their remarkable optical and electrical characteristics. However, the practical application of QDs and further the QD composite films have greatly been hindered mainly owing to their essential drawbacks of extreme unstability under oxygen and water environments. Herein, one simple method has been employed to enhance enormously the stability of Cd _xZn_{1- x}Se _yS_{1- y} QD composite films by a combination of Cd _xZn_{1- x}Se _yS_{1- y} QDs and poly(vinylidene) fluoride (PVDF), which is characteristic of closely arranged molecular chains and strong hydrogen bonds. There are many particular advantages in using QD/PVDF composite films such as easy processing, low cost, large-area fabrication, and especially extreme stability even in the boiling water for more than 240 min. By employing K₂SiF₆:Mn⁴⁺ as a red phosphor, a prototype white light-emitting diode (WLED) with color coordinates of (0.3307, 0.3387), T_c of 5568 K, and color gamut 112.1NTSC(1931)% at 20 mA has been fabricated, and there is little variation under different excitation currents, indicating that the QD/PVDF composite films fabricated by this simple blade-coating process make them ideal candidates for liquid-crystal display backlight utilization via assembling a WLED on a large scale owing to its ultrahigh stability under severe environments.

A Building Brick Principle to Create Transparent Composite Films with Multicolor Emission and Self-Healing Function

A cellulose paper is used impregnated with light-emitting CdTe nanocrystals and carbon dots, and filled with a polyurethane to fabricate uniform transparent composite films with bright photoluminescence of red (R), green (G), and blue (B) (RGB) colors. A building brick-like assembly method is introduced to realize RGB multicolor emission patterns from this composite material. By sectioning out individual pixels from monochrome-emissive composite sheets, the advantage of the self-healing properties of polyurethane is taken to arrange and weld them into a RGB patterned fabric by brief exposure to ethanol. This provides an approach to form single layer RGB light-emitting pixels, such as potentially required in the display applications, without the use of any lithographic or etching processing. The method can utilize a wide range of different solution-based kinds of light-emitting materials.

Synthesis of highly stable quantum-dot silicone nanocomposites via in situ zinc-terminated polysiloxane passivation

Quantum dot (QD) silicone nanocomposites are promising luminescent materials for developing high performance light-emitting diodes (LEDs). However, their practical application still faces a critical issue of strong fluorescence quenching in commercial silicone, which is normally induced by the agglomeration of QDs and the impurities such as a Pt-catalyst and oxygen in the silicone matrices. This article reports the development of zinc-terminated polydimethylsiloxane (Zn-PDMS) to passivate CdSe/CdS/ZnS QDs via an in situ approach. The Zn-PDMS passivation protects the QDs from reacting with impurities and provides the mono-dispersion of QDs in silicone resin, leading to over 80% quantum efficiency as well as effective anti-quenching properties for the QD-silicone nanocomposite under an ambient atmosphere. A high performance warm-white LED prototype with direct on-chip packaging using the as-prepared QDs is developed.