Rational Design of Tetrahedral Derivatives as Efficient Light-Emitting Materials Based on "Super Atom" Perspective.

Traditional semiconductor quantum dots of groups II-VI are key ingredients of next-generation display technology. Yet, the majority of them contain toxic heavy-metal elements, thus calling for alternative light-emitting materials. Herein, we have explored three novel categories of multicomponent compounds, namely, tetragonal II-III2-VI4 porous ternary compounds, cubic I2-II3-VI4 ternary compounds, and cubic I-II-III3-V4 quaternary compounds. This is achieved by judicious introduction of a "super atom" perspective and concurrently varying the solid-state lattice packing of involved super atoms or the population of surrounding counter cations. Based on first-principles calculations of 392 candidate materials with designed crystal structures, 53 highly stable materials have been screened. Strikingly, 34 of them are direct-bandgap semiconductors with emitting wavelengths covering the near-infrared and visible-light regions. This work provides a comprehensive database of highly efficient light-emitting materials, which may be of interest for a broad field of optoelectronic applications.

Understanding the Growth Mechanism of HgTe Colloidal Quantum Dots through Bilateral Injection.

As potential low-cost alternatives of traditional bulk HgCdTe crystals, HgTe colloidal quantum dots (CQDs) synthesized through reactions between HgCl2 and trioctylphosphine-telluride in hot oleylamine have shown promising performances in mid-wave infrared photodetectors. Tetrapodic or tetrahedral HgTe CQDs have been obtained by tuning the reaction conditions such as temperature, reaction time, concentrations, and ratios of the two precursors. However, the principles governing the growth dynamics and the mechanism behind the transitions between tetrapodic and tetrahedral HgTe CQDs have not been sufficiently understood. In this work, synthesis of HgTe CQDs through bilateral injection is introduced to study the growth mechanism. It suggests that tetrahedral HgTe CQDs usually result from the breaks of tetrapodic HgTe CQDs after their legs grow thick enough. The fundamental factor determining whether the growth makes their legs longer or thicker is the effective concentration of the Te precursor during the growth, rather than temperature, Hg-rich environment, or reactivity of precursors. A chemical model is proposed to illustrate the principles governing the growth dynamics, which provides valuable guidelines for tuning the material properties of HgTe CQDs according to the needs of applications.

Development of a Dual Fluorescence Signal-Enhancement Immunosensor Based on Substrate Modification for Simultaneous Detection of Interleukin-6 and Procalcitonin.

High-sensitivity detection of biomarkers is of great significance to improve the accuracy of disease diagnosis and the rate of occult disease diagnosis. Using a substrate modification and two-color quantum dot (QD) nanobeads (QBs), we have developed a dual fluorescence signal-enhancement immunosensor for sensitive, simultaneous detection of interleukin 6 (IL-6) and procalcitonin (PCT) at low volumes (∼20 µL). First, the QBs compatible with QDs with different surface ligands were prepared by optimizing surfactants based on the microemulsion method. Through the use of a fluorescence-linked immunosorbent assay (FLISA), the feasibility of a dual signal-enhancement immunosensor was verified, and a 5-fold enhancement of fluorescence intensity was achieved after the directional coating of the antibodies on sulfhydryl functionalization (-SH) substrates and the preparation of QBs by using a polymer and silica double-protection method. Next, a simple polydimethylsiloxane (HS-PDMS) immunosensor with a low volume consumption was prepared. Under optimal conditions, we achieved the simultaneous detection of IL-6 and PCT with a linear range of 0.05-50 ng/mL, and the limit of detection (LOD) was 24 and 32 pg/mL, respectively. The result is comparable to two-color QBs-FLISA with a sulfhydryl microplate, even though only 20% of its volume was used. Thus, the dual fluorescence signal-enhancement HS-PDMS immunosensor offers the capability of early microvolume diagnosis of diseases, while the detection of inflammatory factors is clinically important for assisting disease diagnosis and determining disease progression.

Highly Sensitive, Stable InP Quantum Dot Fluorescent Probes for Quantitative Immunoassay Through Nanostructure Tailoring and Biotin-Streptavidin Coupling.

Nontoxic, highly sensitive InP quantum dot (QD) fluorescent immunoassay probes are promising biomedical detection modalities due to their unique properties. However, InP-based QDs are prone to surface oxidation, and the stability of InP QD-based probes in biocompatible environments remains a crucial challenge. Although the thick shell can provide some protection during the phase transfer process of hydrophobic QDs, the photoluminescence quantum yield (PLQY) is generally decreased because of the contradiction between lattice stress relaxation and thick shell growth. Herein, we developed thick-shell InP-based core/shell QDs by inserting a ZnSeS alloy layer. The ternary ZnSeS intermediate shell could effectively facilitate lattice stress relaxation and passivate the defect states. The synthesized InP/ZnSe/ZnSeS/ZnS core/alloy shell/shell QDs (CAS-InP QDs) with nanostructure tailoring revealed a larger size, high PLQY (90%), and high optical stability. After amphiphilic polymer encapsulation, the aqueous CAS-InP QDs presented almost constant fluorescence attenuation and stable PL intensity under different temperatures, UV radiation, and pH solutions. The CAS-InP QDs were excellent labels of the fluorescence-linked immunosorbent assay (FLISA) for detecting C-reactive protein (CRP). The biotin-streptavidin (Bio-SA) system was first introduced in the FLISA to further improve the sensitivity, and the CAS-InP QDs-based SA-Bio sandwich FLISA realized the detection of CRP with an impressive limit of detection (LOD) of 0.83 ng/mL. It is believed that the stable and sensitive InP QD fluorescent probes will drive the rapid development of future eco-friendly, cost-effective, and sensitive in vitro diagnostic kits.

The Narrow Synthetic Window for Highly Homogenous InP Quantum Dots toward Narrow Red Emission.

Low-toxicity InP-based quantum dots (QDs) exhibit potential for replacing Cd/Pb-containing QDs in the visible and near-infrared regions. Despite advancements, further improvement relies on synthesizing homogeneous InP QDs to achieve a high color purity. In a commonly employed two-step "seed-mediated" synthetic approach, we demonstrate the high sensitivity of InP seed sizes and size distribution to the quantities of trioctylphosphine (TOP) and tris(trimethylsilyl)phosphine [(TMS)3P], attributed to the process of "self-focusing of size distribution" and enhanced reactivity of In-oleate through coordination with TOP. During growth, the processes of size focusing and defocusing are modulated by the accumulation of oleic acid and TOP molecules, as well as the amount of (TMS)3P in the growth precursor, which may relate to the dissolution process of InP magic size clusters. Through precise control, the best valley/depth ratio of InP QDs was 0.52 at the first absorption peak at 571 nm, resulting in luminescence with a full width at half-maximum of 35 at 620 nm with an absolute photoluminescence quantum yield around 90% after heteroepitaxial growth with ZnSe and ZnS shells.

Stable and efficient pure blue quantum-dot LEDs enabled by inserting an anti-oxidation layer.

The efficiency and stability of red and green quantum-dot light-emitting diodes have already met the requirements for commercialization in displays. However, the poor stability of the blue ones, particularly pure blue color, is hindering the commercialization of full-color quantum-dot light-emitting diode technology. Severe hole accumulation at the blue quantum-dot/hole-transport layer interface makes the hole-transport layer prone to oxidation, limiting the device operational lifetime. Here, we propose inserting an anti-oxidation layer (poly(p-phenylene benzobisoxazole)) between this interface to take in some holes from the hole-transport layer, which mitigates the oxidation-induced device degradation, enabling a T50 (time for the luminance decreasing by 50%) of more than 41,000 h with an initial brightness of 100 cd m-2 in pure blue devices. Meanwhile, the inserted transition layer facilitates hole injection and helps reduce electron leakage, leading to a peak external quantum efficiency of 23%.

Correction: Elaborating the interplay between the detecting unit and emitting unit in infrared quantum dot up-conversion photodetectors.

Correction for 'Elaborating the interplay between the detecting unit and emitting unit in infrared quantum dot up-conversion photodetectors' by Qiulei Xu et al., Nanoscale, 2023, 15, 8197-8203, https://doi.org/10.1039/D3NR01237A.

Tailored three-color quantum dots nanobeads for multiplexed detection with tunable detection range and multilevel sensitivity of signal-amplified immunosensor.

The excellent optical properties of quantum dots (QDs) make them as an ideal fluorescent probe for multiplexed detection, however, the interference between different emission spectra, the dependence of excitation wavelengths, and the sharp decrease of quantum yield (QY) during surface modification are issues that cannot be ignored. Herein, a dual protection scheme of polymer and silica was proposed to prepare high-quality three-color QDs nanobeads using QDs with different ligands. In comparison with single-core QDs, the fluorescence signal of the prepared QD nanobeads (QBs) is increased by more than 1,000 times and has better stability. Considering the excitation efficiency of QDs, we tailor three-color QBs as fluorescent probes based on fluorescence-linked immunosorbent assays (tQBs-FLISA) to detect multiple inflammatory biomarkers simultaneously with tunable detection ranges. This resulted in highly sensitive detection of three inflammatory biomarkers in comparison to the single-core QD-FLISA, the sensitivities of C-reactive protein (CRP), serum amyloid A (SAA), and procalcitonin (PCT) were increased by 16-fold, 19-fold, and 5-fold, respectively, to 0.48 ng/mL, 0.42 ng/mL, and 10 pg/mL. Furthermore, the tQBs-FLISA showed good accuracy without interference from common serum factors. In this strategy, a three-color QBs suitable for multilevel sensitivity and tunable detection range was tailored using the versatile polymer and silica dual protection method, building high-performance immunosensor for in vitro diagnostics (IVD).

Bright and Stable Yellow Quantum Dot Light-Emitting Diodes Through Core-Shell Nanostructure Engineering.

Solution-processed and efficient yellow quantum dot light-emitting diodes (QLEDs) are considered key optoelectronic devices for lighting, display, and signal indication. However, limited synthesis routes for yellow quantum dots (QDs), combined with inferior stress-relaxation of the core-shell interface, pose challenges to their commercialization. Herein, a nanostructure tailoring strategy for high-quality yellow CdZnSe/ZnSe/ZnS core/shell QDs using a "stepwise high-temperature nucleation-shell growth" method is introduced. The synthesized CdZnSe-based QDs effectively smoothed the release stress of the core-shell interface and revealed a near-unit photoluminescence quantum yield, with nonblinking behavior and matched energy level, which accelerated radiative recombination and charge injection balance for device operation. Consequently, the yellow CdZnSe-based QLEDs exhibited a peak external quantum efficiency of 23.7%, a maximum luminance of 686 050 cd m-2 , and a current efficiency of 103.2 cd A-1 , along with an operating half-lifetime of 428 523 h at 100 cd m-2 . To the best of the knowledge, the luminance and operational stability of the device are found to be the highest values reported for yellow LEDs. Moreover, devices with electroluminescence (EL) peaks at 570-605 nm exhibited excellent EQEs, surpassing 20%. The work is expected to significantly push the development of RGBY-based display panels and white LEDs.

Suppressed Auger recombination and enhanced emission of InP/ZnSe/ZnS quantum dots through inner shell manipulation.

Understanding the influence of the inner shell on fluorescence blinking and exciton dynamics is essential to promote the optical performances of InP-based quantum dots (QDs). Here, the fluorescence blinking, exciton dynamics, second-order correlation function g2(τ), and ultrafast carrier dynamics of InP/ZnSe/ZnS QDs regulated by the inner ZnSe shell thickness varying from 2 to 7 monolayers (MLs) were systematically investigated. With an inner ZnSe shell thickness of 5 MLs, the photoluminescence quantum yield (PL QY) can reach 98% due to the suppressed blinking and increased probability of multiphoton emission. The exciton dynamics of InP/ZnSe/ZnS QDs with different inner shells indicates that two decay components of neural excitons and charged trions are competitive to affect the photon emission behavior. The probability density distributions of the ON and OFF state duration in the blinking traces demonstrate an effective manipulation of the inner ZnSe shell in the non-radiative processes via defect passivation. Accordingly, the radiative recombination dominates the exciton deactivation and the non-radiative Auger recombination rate is remarkably reduced, leading to a QY close to unity and a high PL stability for InP/ZnSe/ZnS QDs with 5 MLs of the ZnSe shell. These results provide insights into the photophysical mechanism of InP-based QDs and are significant for developing novel semiconductor PL core/shell QDs.

Ultrastable and High-Efficiency Deep Red QLEDs through Giant Continuously Graded Colloidal Quantum Dots with Shell Engineering.

Quantum dot (QD) based light-emitting diodes (QLEDs) hold great promise for next-generation lighting and displays. In order to reach a wide color gamut, deep red QLEDs emitting at wavelengths beyond 630 nm are highly desirable but have rarely been reported. Here, we synthesized deep red emitting ZnCdSe/ZnSeS QDs (diameter ∼16 nm) with a continuous gradient bialloyed core-shell structure. These QDs exhibit high quantum yield, excellent stability, and a reduced hole injection barrier. The QLEDs based on ZnCdSe/ZnSeS QDs have an external quantum efficiency above 20% in the luminance range of 200-90000 cd m-2 and a record T95 operation lifetime (time for the luminance to decrease to 95% of its initial value) of more than 20000 h at a luminance of 1000 cd m-2. Furthermore, the ZnCdSe/ZnSeS QLEDs have outstanding shelf stability (>100 days) and cycle stability (>10 cycles). The reported QLEDs with excellent stability and durability can accelerate the pace of QLED applications.

Minimizing heat generation in quantum dot light-emitting diodes by increasing quasi-Fermi-level splitting.

Minimizing heat accumulation is essential to prolonging the operational lifetime of quantum dot light-emitting diodes (QD-LEDs). Reducing heat generation at the source is the ideal solution, which requires high brightness and quantum efficiency at low driving voltages. Here we propose to enhance the brightness of QD-LEDs at low driving voltages by using a monolayer of large QDs to reduce the packing number in the emitting layer. This strategy allows us to achieve a higher charge population per QD for a given number of charges without charge leakage, enabling enhanced quasi-Fermi-level splitting and brightness at low driving voltage. Due to the minimized heat generation, these LEDs show a high power conversion efficiency of 23% and a T95 operation lifetime (the time for the luminance to decrease to 95% of the initial value) of more than 48,000 h at 1,000 cd m-2.

High-Radiance Shortwave Infrared Light-Emitting Diodes Based on Highly Stable PbS Colloidal Quantum Dots.

PbS quantum dot light-emitting diodes (QLEDs) emitting around 1550 nm promise important applications in optical communications. However, due to insufficient suppression of surface traps for large-size PbS quantum dots (QDs), their performance under large driving current density was not satisfactory. In this work, octanethiol surfactant was added into a PbS QD solution and adsorbed onto the dot surface. As a result, the surface traps and the continuous oxidation of the unprotected (100) facets in PbS QDs were greatly suppressed. Therefore, the PbS QDs with octanethiol doubled their photoluminescence efficiency and showed outstanding stability. The PbS-based QLEDs with benchmark device structure showed a breakthrough high radiance of 18.3 W sr-1 m-2 with >2000 mA/cm2 driving current density. The efficient passivation of surface traps with octanethiol surfactant and the suppressed coupling between excitons and surface states under large working current were the main reasons for achieving the breakthrough high radiance.

Elaborating the interplay between the detecting unit and emitting unit in infrared quantum dot up-conversion photodetectors.

The quantum dot up-conversion device combines an infrared photodetector (PD) and a visible quantum-dot light-emitting diode (QLED) to directly convert infrared targets to visible images. However, large efficiency loss is usually induced by the integration of the detecting unit and the emitting unit. One of the important reasons is the performances of the PD and QLED units restraining each other. We regulated the equilibrium between infrared absorption and visible emission by changing the thicknesses of infrared active layers in up-conversion devices. A good balance could be achieved between the absorption of 980 nm incident light and the out-coupling of the 634 nm emission when the active layer thickness is 140 nm, leading to the best performance of the up-conversion device. As more photogenerated carriers are produced with the increase of infrared illumination intensity, the external quantum efficiency (EQE) of the QLED unit in the up-conversion device remains little changed. This suggests the limited amount of photogenerated holes in the PD unit does not limit the EQE of the QLED unit. However, a PD unit with a high ratio of photogenerated holes trapped near the interconnection decreased the EQE in the QLED unit. This work provides new insights into the interplay between the PD and QLED units in up-conversion devices, which is crucial for their further improvements.

Improving the performance of quantum dot light-emitting diodes by tailoring QD emitters.

As the emitters of quantum dots (QDs) light-emitting diodes (QLEDs), QDs, which are responsible for the charge injection, charge transportation, and especially exciton recombination, play a significant role in QLEDs. With the crucial advances made in QDs, such as the advancement of synthetic methods and the understanding of luminescence mechanisms, QLEDs also demonstrate a dramatic improvement. Until now, efficiencies of 30.9%, 28.7% and 21.9% have been achieved in red, green and blue devices, respectively. However, in QLEDs, some issues are still to be solved, such as the imbalance of charge injection and exciton quenching processes (defect-assisted recombination, Auger recombination, energy transfer and exciton dissociation under a high electric field). In this review, we will provide an overview of recent advances in the study and understanding of the working mechanism of QLEDs and the exciton quenching mechanism of QDs in devices. Particular emphasis is placed on improving charge injection and suppressing exciton quenching. An in-depth understanding of this progress may help develop guidelines to direct QLED development.

Self-Assembled and Wavelength-Tunable Quantum Dot Whispering-Gallery-Mode Lasers for Backlight Displays.

Thanks to the narrow line width and high brightness, colloidal quantum dot (CQD) lasers show promising applications in next-generation displays. However, CQD laser-based displays have yet to be demonstrated because of two challenges in integrating red, green, and blue (RGB) lasers: absorption from red CQDs deteriorates the optical gain of blue and green CQDs, and imbalanced white spectra lack blue lasing due to the high lasing threshold of blue CQDs. Herein, we introduce a facile surfactant-free self-assembly method to assemble RGB CQDs into high-quality whispering-gallery-mode (WGM) RGB lasers with close lasing thresholds among them. Moreover, these RGB lasers can lase nearly independently even when they are closely integrated, and they can construct an ultrawide color space whose color gamut is 105% of that of the BT.2020 standard. These combined strategies allow us to demonstrate the first full-color liquid crystal displays using CQD lasers as the backlight source.

Establishment of a Ca(II) ion-quantum dots fluorescence signal amplification sensor for high-sensitivity biomarker detection.

Quantum dots (QDs) have been considered as the promising fluorescent labeling material, which is expected to meet the requirement of high-sensitivity detection in clinical diagnostics. Some common metal ions are known to affect the stability and fluorescence properties of QDs, but scarcely any systematic research has been done about their impacts on QD-based bio-detection. By evaluating the effect of Ca2+ metal ions on the properties of aqueous QDs, a new metal ion-QD fluorescence signal amplification sensor (i.e., Ca2+-QD-fluorescence-linked immunosorbent assay, Ca2+-QD-FLISA) has been developed for the detection of inflammatory biomarkers with high sensitivity. Compared with the common QD-FLISA, the detection sensitivity for CRP of Ca2+-QD-FLISA was improved by a 4-fold of magnitude to 0.23 ng/mL, and this assay showed good selectivity, high accuracy, and excellent repeatability. The versatility of the QD-FLISA method were also validated by using different metal ion-QD probes (Ca2+, Mg2+, Ba2+, Fe2+, and Mn2+) to detect CRP, serum amyloid A (SAA), and procalcitonin (PCT). The significant improvement in detection sensitivity was achieved due to the crosslinking of aqueous QDs by Ca2+ ions to enhance fluorescence and at the same time promote antigen-antibody binding efficiency. The present study illustrates the versatility of metal ion-QD-FLISA as a simple and effective method to detect a wide range of biomarkers with high sensitivity and accuracy.

Self-assembly strategy to reduce non-specific adsorption for the development of high sensitivity quantitative immunoassay.

The development of functionalized surfaces with low non-specific adsorption is important for their biomedical applications. To inhibit non-specific adsorption on glass substrate, we designed a novel optical biochip by modifying a layer of dense negatively charged film (SO32-) on its substrate surface via self-assembly. Compared with the untreated glass substrate, it reduced the adsorption by about 300-fold or 400-fold by poly (styrene sulfonic acid) sodium salt (PSS), or meso-tetra (4-sulfonatophenyl) porphine dihydrochloride (TSPP) on individually the modified glass substrate. Considering the effect of fluorescence resonance energy transfer (FRET) between TSPP and the QDs in solution by mixing, a strategy of 2-layer of TSPP followed by 4-layer of PSS was designed to modify the glass for preparing biochips. Under the optimized conditions, the biochip on functionalized glass substrate co-treated with TSPP and PSS realized the sensitive quantitative detection of C-reactive protein (CRP) based on a quantum dot fluorescence immunosorbent assay (QD-FLISA). The limit of detection (LOD) for CRP achieved 0.69 ng/mL with the range of 1-1,000 ng/mL using TSPP and PSS co-treated glass substrate surface, which was respectively about 1.9-fold and 7.5-fold more sensitive to the PSS-modified biochip and the TSPP-modified biochip. This work demonstrated an effective and convenient strategy to obtain biochips with low non-specific adsorption properties on functionalized surfaces, thus providing a new approach for creating ultra-high sensitivity microchannels or microarrays on glass substrates.

Green InP-based quantum dots and electroluminescent light-emitting diodes.

With the advancement toward commercialization of quantum dots (QDs) in the field of lighting and display, improving the performance of Cd-free QDs and related quantum dot light-emitting diodes (QLEDs) becomes necessary. Thus far, the performance of ZnTeSe- and InP-based blue and red QLEDs has been significantly improved by optimizing QDs emitting materials and device structure. However, as one of the three primary color sources, the performance of green InP-based QLEDs still lags behind that of blue and red Cd-free QLEDs. Herein, this review discusses the latest progress of green InP-based emitting materials and corresponding QLEDs, covering the engineering of InP core, the optimization of nanostructure and surface ligands of core/shell QDs, as well as the majorization of device architecture and carrier transport materials. Finally, some challenges and possible development directions of green InP-based QDs and related QLEDs are also identified, which may speed up the commercialization process of Cd-free QDs and corresponding QLEDs.

Size Engineering of Trap Effects in Oxidized and Hydroxylated ZnSe Quantum Dots.

Environmentally friendly blue-emitting ZnSe quantum dots (QDs) are in high demand for next-generation light-emitting devices. Yet, they suffer longstanding optical instability issues under aerobic conditions. Herein, we have demonstrated the existence of oxidization or hydroxylation on the QD surface when QDs are subjected to oxygen exposure, which potentially introduces highly localized in-gap states. Those states result in a dense number of surface-related, weak-intensity "dark" exciton states at the emission edge. Remarkably, there exists a critical diameter (Dc ≈ 8.5 nm) at which the deepest trap level reaches resonance with the highest occupied molecular orbital state. Beyond this critical diameter, the effects of those trap states are minimized, and the emission edge is dominated by high-intensity, bulk-to-bulk-like "bright" exciton states. The present work provides a novel strategy for designing highly stable QD emitters via size engineering, which are broadly applicable to other closely related QD systems.