Cd-free Quantum Dots Research Articles

Repercussions of the Inner Shell Layer on the Performance of Cd-Free Quantum Dots and Their Light-Emitting Diodes.

Indium phosphide (InP) and zinc selenium tellurium (ZnSeTe) quantum dots (QDs) as less toxic alternatives have received substantial attention. The structure of QDs generally consists of a QD core, inner shell layer, and outer shell layer. We reckon that the inner shell layer, especially its components and thickness, have a significant influence on the optical and electronic performances of QDs. In this Perspective, we compare optical properties of these QDs with different inner shells and summarize how typical inner shell components and thickness influence their optical properties. The impact of the inner shell on the performance of QD light-emitting diodes (QLEDs) has also been discussed. The appropriate components and thickness of the inner shell both contribute to alleviate valence or lattice mismatch, thereby enhancing the performance of QDs. We expect that this Perspective could heighten awareness of the significance and impact of the inner shell layer in QDs and facilitate further development of QDs and QLEDs.

Light-Emitting Diodes with Cd-Free Quantum Dots Embedded in a Carrier Transport Matrix Grown by a Wet Process

Light-Emitting Diodes with Cd-Free Quantum Dots Embedded in a Carrier Transport Matrix Grown by a Wet Process

Improvement of Cd-Free Quantum Dot Electroluminescent Characteristics by Inorganic Ion Treatment

Improvement of Cd-Free Quantum Dot Electroluminescent Characteristics by Inorganic Ion Treatment

Compact, Fast Blinking Cd-Free Quantum Dots for Super-Resolution Fluorescence Imaging.

Quantum dots (QDs) can be used as fluorescent probes in single molecule localization microscopy to achieve subdiffraction limit resolution (super-resolution fluorescence imaging). However, the toxicity of Cd in the prototypical CdSe-based QDs can limit their use in biological applications. Furthermore, commercial CdSe QDs are usually modified with relatively thick shells of both inorganic and organic materials to render them in the 10-20 nm size range, which is relatively large for biological labels. In this report, we present compact (4-6 nm) CuInS2/ZnS (CIS/ZnS) and compare them to commercially sourced CdSe/ZnS QDs for their blinking behavior, localization precision and super-resolution imaging. Although commercial CdSe/ZnS QDs are brighter than the more compact Cd-free CIS/ZnS QD, both give comparable results of 4.5-5.0-fold improvement in imaging resolution over conventional TIRF imaging of actin filaments. This likely results from the fact that CIS/ZnS QDs show very short on-times and long off times which leads to less overlap in the point spread functions of emitting CIS/ZnS QD labels on the actin filaments at the same labeling density. These results demonstrate that CIS/ZnS QDs are an excellent candidate to complement and perhaps even replace the larger and more toxic CdSe-based QDs for robust single- molecule super-resolution imaging.

Highly Bright Silica-Coated InP/ZnS Quantum Dot-Embedded Silica Nanoparticles as Biocompatible Nanoprobes.

Quantum dots (QDs) have outstanding optical properties such as strong fluorescence, excellent photostability, broad absorption spectra, and narrow emission bands, which make them useful for bioimaging. However, cadmium (Cd)-based QDs, which have been widely studied, have potential toxicity problems. Cd-free QDs have also been studied, but their weak photoluminescence (PL) intensity makes their practical use in bioimaging challenging. In this study, Cd-free QD nanoprobes for bioimaging were fabricated by densely embedding multiple indium phosphide/zinc sulfide (InP/ZnS) QDs onto silica templates and coating them with a silica shell. The fabricated silica-coated InP/ZnS QD-embedded silica nanoparticles (SiO2@InP QDs@SiO2 NPs) exhibited hydrophilic properties because of the surface silica shell. The quantum yield (QY), maximum emission peak wavelength, and full-width half-maximum (FWHM) of the final fabricated SiO2@InP QDs@SiO2 NPs were 6.61%, 527.01 nm, and 44.62 nm, respectively. Moreover, the brightness of the particles could be easily controlled by adjusting the amount of InP/ZnS QDs in the SiO2@InP QDs@SiO2 NPs. When SiO2@InP QDs@SiO2 NPs were administered to tumor syngeneic mice, the fluorescence signal was prominently detected in the tumor because of the preferential distribution of the SiO2@InP QDs@SiO2 NPs, demonstrating their applicability in bioimaging with NPs. Thus, SiO2@InP QDs@SiO2 NPs have the potential to successfully replace Cd-based QDs as highly bright and biocompatible fluorescent nanoprobes.

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.

Sensitive Immunoassay Based on Biocompatible and Robust Silica-Coated Cd-Free InP-Based Quantum Dots.

Low-toxic InP quantum dots (QDs) as an ideal candidate for Cd-based QDs have tremendous potential for next-generation commercial display and biological detection applications. However, the progress in biological detection is still far behind that of the Cd-based QDs. This is mainly because the InP-based QDs are of inferior stability and photoluminescence quantum yield (PL QY) in aqueous solution. Here, PL QY of 65% and excellent stability of InP/GaP/ZnS QD@SiO2 nanoparticles have been successfully synthesized via a silica coating method. The containing thiol-capped hydrophobic InP/GaP/ZnS QDs were pre-silanized with waterless, ammonia-free hydrolysis tetraethyl orthosilicate, and subsequently, an outer silica shell was generated in the reverse microemulsion. The corresponding QD-based fluorescence-linked immunosorbent assay exhibits a high sensitivity of 0.9 ng mL-1 for C-reactive protein and the broad detection range of 1-1000 ng mL-1, which was close to that of the state-of-the-art Cd-based QD@SiO2 nanoparticles and had the highest sensitivity of Cd-free QDs so far. This work provides a very successful silica coating method for the containing thiol-capped hydrophobic QDs and the QDs highly sensitive to water and oxygen, and the obtained InP/GaP/ZnS QD@SiO2 nanoparticles were considered as the robust, biocompatible, and promising Cd-free fluorescent labels for the further ultra-sensitive detection.

High-performance tricolored white lighting electroluminescent devices integrated with environmentally benign quantum dots.

The electroluminescent (EL) performances of quantum dot-light-emitting diodes (QLEDs) based on either high-quality CdSe- or Cd-free quantum dots (QDs) have been greatly improved during the last decade, exclusively aiming at monochromatic devices for display applications. Meanwhile, work on white lighting QLEDs integrated particularly with Cd-free QDs remains highly underdeveloped. In this work, the solution-processed fabrication of tricolored white lighting QLEDs comprising three environmentally benign primary color emitters of II-VI blue and green ZnSeTe and I-III-VI red Zn-Cu-In-S (ZCIS) QDs is explored. The emitting layer (EML) consists of two different QD layers stacked on top of the other with an ultrathin ZnMgO nanoparticle buffer layer inserted in the middle, with both blue and green QDs mixed in one layer, and red QDs placed in a separate layer. The stacking order of the bilayered EML architecture is found to control the exciton recombination zone and thus crucially determine the EL performance of the device. The optimal tricolored white device yields outstanding EL performances such as 5461 cd m-2 luminance, 5.8% external quantum efficiency, and 8.4 lm W-1 power efficiency, along with a near-ideal color rendering index of 95, corresponding to the record quantities reported among Cd-free white lighting QLEDs.

A Study on an Organic Semiconductor-Based Indirect X-ray Detector with Cd-Free QDs for Sensitivity Improvement.

In this paper, we studied the optimized conditions for adding inorganic quantum dots (QD) to the P3HT:PC70BM organic active layer to increase the sensitivity of the indirect X-ray detector. Commonly used QDs are composed of hazardous substances with environmental problems, so indium phosphide (InP) QDs were selected as the electron acceptor in this experiment. Among the three different sizes of InP QDs (4, 8, and 12 nm in diameter), the detector with 4 nm InP QDs showed the highest sensitivity, of 2.01 mA/Gy·cm2. To further improve the sensitivity, the QDs were fixed to 4 nm in diameter and then the amount of QDs added to the organic active layer was changed from 0 to 5 mg. The highest sensitivity, of 2.26 mA/Gy·cm2, was obtained from the detector with a P3HT:PC70BM:InP QDs (1 mg) active layer. In addition, the highest mobility, of 1.69 × 10−5 cm2/V·s, was obtained from the same detector. Compared to the detector with the pristine P3HT:PC70BM active layer, the detector with a P3HT:PC70BM:InP QDs (1 mg) active layer had sensitivity that was 61.87% higher. The cut-off frequency of the P3HT:PC70BM detector was 21.54 kHz, and that of the P3HT:PC70BM:InP QDs (1 mg) detector was 26.33 kHz, which was improved by 22.24%.

Research advance on cell imaging and cytotoxicity of different types of quantum Dots.

Quantum dots (QDs), as one of the emerging nanomaterials, have been widely studied by scientists due to their advantages and potential in bioimaging, especially cell imaging. Cadmium (Cd)-based QDs, which have the best photoluminescence property, have received widespread attention. However, due to the obvious toxicity problem of these QDs, their cell imaging application is hindered. Recently, the emergence of Cd-free-metal and metal-free QDs with lower toxicity makes people consider that Cd-based QDs may be substituted by these QDs. However, problems of these QDs in cytotoxicity also cannot be ignored. Some reports claimed that their prepared emerging QDs for cell imaging were low toxicity, but there still exist up-regulation of oxidative stress, inflammation, and apoptosis from other reports. And, up to now, few reports have separately summarized and discussed the issues of cell imaging and cytotoxicity of different types of QDs. Therefore, in this review, we classify QDs as following three types, Cd-based, Cd-free-metal and metal-free QDs, focus on the cell, the essential unit of life, discuss cell imaging application and cytotoxicity of QDs, and finally elucidate main mechanisms of cytotoxicity respectively. This review provides reference for the toxicity evaluation of QDs and highlights the potential cytotoxicity of Cd-free QDs.

Blue Graphene Quantum Dots with High Color Purity by Controlling Subdomain Formation for Light-Emitting Devices

As an alternative to Cd-free quantum dots (QDs), graphene quantum dots (GQDs) have potential because of their various advantages, such as color tunability, nontoxicity, cost effectiveness, and phot...

Surface Engineered Colloidal Quantum Dots for Complete Green Process

The rising demand for eradicating hazardous substances in the workplace has motivated vigorous researches on environmentally sustainable manufacturing processes of colloidal quantum dots (QDs) for their optoelectronic applications. Despite remarkable achievements witnessed in QD materials (e.g., Pb- or Cd-free QDs), the progress in the eco-friendly process is far falling behind and thus the practical use of QDs. Herein, a complete "green" process of QDs, which excludes environmentally unfriendly elements from QDs, ligands, or solvents, is presented. The implant of mono-2-(methacryloyloxy)ethyl succinate (MMES) ligands renders InP/ZnSexS1-x QDs dispersed in eco-friendly polar solvents that are widely accepted in the industry while keeping the photophysical properties of QDs unchanged. The MMES-capped QDs show exceptional colloidal stabilities in a range of green polar solvents that permit uniform inkjet printing of QD dispersion. In addition, MMES-capped QDs are also compatible with commercially available photo-patternable resins, and the cross-linkable moiety within MMES further facilitates the achievement in the formation of well-defined, micrometer-scale patterning of QD optical films. The presented materials, all composed of simple, scalable, and environmentally safe compounds, promise low environmental impact during the processing of QDs and thus will catalyze the practicable use of QDs in a variety of optoelectronic devices.

In vitro and in vivo immunotoxicity of PEGylated Cd-free CuInS2/ZnS quantum dots

The annual increase in the production and the use of engineering quantum dots (QDs) have led to concern about exposure and safety of QDs. To resolve the risk of Cd release from QDs, a series of Cd-free QDs, represented by CuInS2/ZnS QDs, has been developed in recent years. However, the toxicological profile of CuInS2/ZnS QDs has not been fully elucidated, especially, their immunotoxicity. Here, we performed a detailed in vitro cytotoxicity study on PEGylated CuInS2/ZnS QDs using the DC2.4 cell line and investigated their in vivo immunotoxicity using BALB/c mice. In vitro experiments showed that CuInS2/ZnS QDs were taken up by cells, promoted cell viability, enhanced release of tumor necrosis factor-α, and decreased the level of interleukin (IL)-6 in response to lipopolysaccharide stimulation. More than 5000 genes at the transcriptome level were observed by high-throughput RNA sequencing after CuInS2/ZnS QD exposure. In vivo study showed that CuInS2/ZnS QDs increased the levels of IL-4 on day 1 and enhanced the levels of IL-10 and IL-13 on day 28 in mice. There was no obvious difference in the number of spleen-derived lymphocytes, organic index, hematology and immune organ histology on days 1 and 28 after treatment. These findings demonstrated that PEGylated CuInS2/ZnS QDs disturbed the function of DC2.4 immune cells in vitro, but caused no obvious toxicity to immune system in vivo, suggesting that PEGylated CuInS2/ZnS QDs are biocompatible and have potential for bioapplication in the future.

Optical Properties of Cd-Free Quantum Dots-Based Fluorescent Film

CuInS2/ZnS Cd-free quantum dots (QDs) have many characteristics, such as high quantum yield (QY), wide emission peak and adjustable light color. They are suitable for optical conversion materials and applies in solid-state lighting (SSL). However, they are not easily dispersed during encapsulation, resulting in the deviation of light color out of the white light area after encapsulation. In this study, we use thermal injection synthesis method to prepare CuInS2/ZnS and CuInS2/ZnS:Al QDs. In order to improve the dispersion of QDs, solution type of QD/PS-PE-BR-PS copolymer (SEBS) mixture was prepared by mixing QD and SEBS with different ratios (10, 20, and 30 wt%) to form a fluorescent film. The experimental results show that the emission wavelengths and QY of CuInS2/ZnS and CuInS2/ZnS:Al QDs are 533 nm, 84 % and 536 nm, 97 %, respectively. The emission wavelength of CuInS2/ZnS-based fluorescent film is 574 nm, while CuInS2/ZnS:Al-based fluorescent film is 582 nm. The CRI, luminous efficacy, chromaticity coordinates and correlated color temperature of 20 wt% CuInS2/ZnS-based fluorescent film excited by blue chip is 64, 42 lm/W, (0.33, 0.32), and 5443 K, respectively. On the other hand, we find that the CRI of CuInS2/ZnS:Al-based fluorescent film can be improved from 64 to 73, and the luminous efficacy is 51 lm/W.

Critical Review—Photostable Fluorescent Cd-Free Quantum Dots Transparently Embedded in Monolithic Silica

Fluorescent Cd-free quantum dots (QDs) such as InP/ZnS and CuInS2/ZnS have attracted attention for use as color converters for blue light emitting diodes. To protect QDs from oxygen, which degrades them by photo-oxidation during excitation, we fabricated a transparent monolithic silica composite containing the QDs using an aqueous solution of tetramethylammonium silicate (TMAS) as a silica source. The TMAS solution readily dispersed the negatively charged QDs prepared by ligand exchange of 1-dodecanethiol for 3-mercatopropionic acid (MPA). A highly transparent monolithic TMAS-derived silica composite containing the MPA-capped QDs was obtained from the QD dispersion through a sol–gel process. Improved photostability of QDs was explained by the TMAS-derived silica acting as a gas barrier to protect the embedded QDs from photo-oxidation by oxygen in air.

First-principles approach to the first step of metal–phosphine bond formation to synthesize alloyed quantum dots using dissimilar metal precursors

InP-based quantum dots are promising candidates as Cd-free quantum dots; however, they require sharper fluorescence to satisfy the next-generation display standard. For this purpose, a deeper understanding of the crystal growth scheme and well-designed organometallic precursors are needed. Density functional theory calculations were used to find the activation barriers for the initial reaction of Zn, Ga, and In precursors with tris(trimethylsilyl)phosphine, and differences in reactivity between dissimilar metal precursors were revealed. The coordinations of the P precursor to Zn, In, and Ga precursors were found to differ significantly, depending on the coordination number and the ionic radius, and these results were consistent with reports that found that InZnP can be alloyed, but InGaP is not easily alloyed. The activation energies become close to each other when using thiol ligands with less steric hindrance in monodentate ligands. These results will lead to achieving precise composition control and great luminescence properties.

Improving the performance of Cd-free quantum dot light-emitting diodes by incorporating the thermally activated delayed fluorescence molecules

Cadmium-free quantum-dot light-emitting diodes (QLEDs) are the potential candidate for next generation displays due to its non-toxicity, tunable colors, saturated color emission, high luminescence efficiency, and simple fabrication process. Several methods have been proposed to improve the performance of QLEDs through synthetizing new quantum dots and charge transporting materials, optimizing the device architecture, modifying the device interfaces, and engineering the fabrication process. On the other hand, the full utilization of triplet excitons would be another important pathway to improve the efficiency of QLEDs since most triplet excitons are usually lost in fluorescence light-emitting diodes. Both the phosphorescent materials and thermally activated delayed fluorescence (TADF) materials have been proposed to realize the utilization of triplet excitons. However, in QLEDs, the use of TADF materials to achieve the use of triplet excitons to improve the luminous efficiency is still rare. Here, the TADF molecule of 2CzPN was incorporated into PVK hole-transporting layer (HTL) with the ratio of 1:5 to achieve hybrid HTL PVK: 2CzPN, through which QLEDs with the architecture ITO/PEDOT:PSS/PVK:2CzPN/InP/ZnS QDs/ZnO/Al were fabricated and measured. The results show that the incorporation of 2CzPN into PVK HTL enhances the hole transport efficiency, leading to a more balance of electrons and holes in the device. Moreover, the use of triplet excitons is achieved by the reverse intersystem crossing process in 2CzPN, and the efficiency of InP/ZnS cadmium-free QLEDs is improved by the following Forster energy transfer process between the doped HTL and quantum dots InP/ZnS. Its maximum luminous brightness is 513 cd/m2. A 26% increase was achieved compared to the maximum luminance of the undoped control device (407 cd/m2). At the same time, the maximum current efficiency is increased by 4 times compared to the undoped control device, increasing to 1.6 cd/A.

Synthesis and Degradation of Cadmium-Free InP and InPZn/ZnS Quantum Dots in Solution.

This study advances the chemical research community toward the goal of replacing toxic cadmium-containing quantum dots (QDs) with environmentally benign InP QDs. The InP QD synthesis uniquely combines the previously reported use of InP magic-sized clusters (MSCs) as a single-source precursor for indium and phosphorus to form InP QDs, with zinc incorporation and subsequent ZnS shelling, to form InPZn/ZnS QDs with luminescence properties comparable to those of commonly used cadmium-containing luminescent QDs. The resulting InPZn/ZnS QDs have an emission quantum yield of about 50% across a broad range of emission peak wavelengths and emission peaks averaging 50 nm fwhm. The emission peak wavelength can be easily tuned by varying the Zn/In ratio in the reaction mixture. The strategy of using zinc stearate to tune the emission properties is advantageous as it does not lead to a loss of emission quantum yield or emission peak broadening. Although the initial optical properties of InP and InPZn/ZnS QDs are promising, thermal stability measurements of InPZn QDs show significant degradation in the absence of a shell compared to the CdSe QDs particularly at increased temperature in the presence of oxygen, which is indicative of thermal oxidation. There is no significant difference in the degradation rate of InP QDs made from molecular precursors and from MSCs. Additionally, the emission intensity and quantum yield of InPZn/ZnS QDs when purified and diluted in organic solvents under ambient conditions decrease significantly compared to those of CdSe/ZnS QDs. This indicates instability of the ZnS shell when prepared by common literature methods. This must be improved to realize high-quality, robust Cd-free QDs with the capability of replacing CdSe QDs in QD technologies.

Tunable photoluminescence of Cd free AgInS2 quantum dots: Synthesis and application for light emitting diodes

I-III-VI semiconductor quantum dots (QDs) without Cd element have attracted considerable attention in the optoelectronics device areas. Herein, we demonstrated that AgInS2 QDs with tunable photoluminescence were used for light emitting diodes (LEDs). With the increase of reaction temperature, the photoluminescence (PL) peak was monotonically red shifted from 600 to 750 nm. The temperature-dependent lifetime of the photoluminescence emission could reach to 900 ns. The successful synthesis of the AgInS2 nanocrystals with long PL lifetime provided a potential application in LEDs. The fabricated QDs LEDs exhibited a relative low turn-on voltage of 2.9 V and a maximum luminance intensity of 122 cd/m2.

Tunable, Bright, and Narrow-Band Luminescence from Colloidal Indium Phosphide Quantum Dots

Synthesis of cadmium (Cd)-free quantum dots (QDs) with tunable emission and high color purity has been a big challenge for the academic and industrial research community. Among various Cd-free QDs, indium phosphide (InP) QDs exhibit reasonably good color purity with emission full width at half-maximum (fwhm) values between 45 and 50 nm for green and over 50 nm for red emission, which is not good enough, as values less than 35 nm are favorable in commercial display products. In this work, we present the synthesis of highly luminescent In(Zn)P/ZnSe/ZnS QDs with tunable emission from 488 to 641 nm and high color purity. We found that the addition of zinc during the conventional SILAR growth of shell (ZnSe or ZnS) deteriorated the absorption features of core InP QDs and resulted in broader emission line widths. We solved this issue by synthesizing Zn carboxylate covered In(Zn)P QDs in a single step and dramatically decreased the emission fwhm to as low as 36 nm with quantum yields (QYs) up to 67% for the gree...