Synthesis and characterization of green Zn-Ag-In-S and red Zn-Cu-In-S quantum dots for ultrahigh color quality of down-converted white LEDs

Enhancement mechanism of quantum yield in core/shell/shell quantum dots of ZnS-AgIn5S8/ZnIn2S4/ZnS

To achieve a high quantum yield (QY) of nanomaterials suitable for optical applications, we improved the optical properties of AgIn5S8 (AIS) quantum dots (QDs) by employing an alloyed-core/inner-shell/outer-shell (ZAIS/ZIS/ZnS) structure. We also investigated the mechanism of optical transitions to clarify the improvement of QYs. In AIS, the low-energy absorption near the band edge region is attributed to the weakly allowed band gap transition, which gains oscillator strength through state intermixing and electron-phonon coupling. The main photoluminescence is also ascribed to the weakly allowed band gap transition with characteristics of self-trapped excitonic emission. With alloying/shelling processes, the weakly allowed transition is enhanced by the evolution of the electronic structures in the alloyed core, which improves the band gap emission. In shelled structures, the nonradiative process is reduced by the reconstructed lattice and passivated surface, ultimately leading to a high QY of 85% in ZAIS/ZIS/ZnS. These findings provide new insights into the optical transitions of AIS because they challenge previous conclusions. In addition, our work elucidates the mechanism behind the enhancement of QY accomplished through alloying/shelling processes, providing strategies to optimize nontoxic QDs for various applications using a green chemistry approach.

Multiple underlying images tuned by Mn-doped Zn-Cu-In-S quantum dots

In this study, ZnS capped Cu-In-S (ZCIS) quantum dots doped with Mn ions are synthesized by a thermal injection method, with luminescence covering almost the entire visible area. The large Stokes shift effectively inhibits the self-absorption effect under luminescence, and the quantum yield of ZCIS quantum dots increased from 38% to 50% after ZnS capping and further to 69% after doping with Mn. First, red-, yellow-, and blue-emitting quantum dots were synthesized and then, polychromatic ensembles were obtained by mixing the trichromatic quantum dots in a different ratio. Using the home-built inkjet printer, multilayered and multicolor mixed patterns were obtained for information pattern storage and multilayer pattern recognition and reading.

Selected I-III-VI2 Semiconductors: Synthesis, Properties and Applications in Photovoltaic Cells

I-III-VI2 group quantum dots (QDs) have attracted high attention in photoelectronic conversion applications, especially for QD-sensitized solar cells (QDSSCs). This group of QDs has become the mainstream light-harvesting material in QDSSCs due to the ability to tune their electronic properties through size, shape, and composition and the ability to assemble the nanocrystals on the surface of TiO2. Moreover, these nanocrystals can be produced relatively easily via cost-effective solution-based synthetic methods and are composed of low-toxicity elements, which favors their integration into the market. This review describes the methods developed to prepare I-III-VI2 QDs (AgInS2 and CuInS2 were excluded) and control their optoelectronic properties to favor their integration into QDSSCs. Strategies developed to broaden the optoelectronic response and decrease the surface-defect states of QDs in order to promote the fast electron injection from QDs into TiO2 and achieve highly efficient QDSSCs will be described. Results show that heterostructures obtained after the sensitization of TiO2 with I-III-VI2 QDs could outperform those of other QDSSCs. The highest power-conversion efficiency (15.2%) was obtained for quinary Cu-In-Zn-Se-S QDs, along with a short-circuit density (JSC) of 26.30 mA·cm-2, an open-circuit voltage (VOC) of 802 mV and a fill factor (FF) of 71%.

Amplified Spontaneous Emission and Lasing from Zn-Processed AgIn5S8 Core/Shell Quantum Dots

I-III-VI ternary quantum dots (QDs) have emerged as favorable alternatives to the toxic II-VI QDs for optoelectronic and biological applications. However, their use as optical gain media for microlasers is still limited by a low fluorescence efficiency. Here, we demonstrate amplified spontaneous emission (ASE) and lasing from colloidal QDs of Zn-processed AgIn₅S₈ (AIS) for the first time. The passivation treatment on the AIS QDs yields a 3.4-fold enhancement of fluorescence quantum efficiency and a 30% increase in the two-photon absorption cross section. ASE is achieved from the AIS/ZnS core/shell QD films under both one- and two-photon pumping with a threshold fluence of ∼84.5 μJ/cm² and 3.1 mJ/cm², respectively. These thresholds are comparable to the best optical gain performance of Cd based-QDs reported in the literature. Moreover, we demonstrate a facile whispering-gallery-mode microlaser of the core/shell QDs with a lasing threshold of ∼233 μJ/cm². The passivated AIS QDs can be promising optical gain media for photonic applications.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

Passivation and Interlayer Effect of Zr(i-PrO)4 on Green CuGaS2/ZnS/Zr(i-PrO)4@Al2O3 and Red CuInS2/ZnS/Zr(i-PrO)4@Al2O3 QD Hybrid Powders

Broadband emissive I-III-VI quantum dots (QDs) are synthesized as efficient and stable I-III-VI QDs to be used as eco-friendly luminescent materials in various applications. Here, we introduce the additional passivation of zirconium isopropoxide (Zr(i-PrO)₄) to improve the optical properties and environmental stability of green-emitting CuGaS₂/ZnS (G-CGS/ZnS) and red-emitting CuInS₂/ZnS (R-CIS/ZnS) QDs. The photoluminescence quantum yield (PLQY) of both resultant Zr(i-PrO)₄-coated G-CGS/ZnS and R-CIS/ZnS QDs reaches similar values of ~ 95%. In addition, the photostability and thermal-stability of G-CGS/ZnS/Zr(i-PrO)₄ and R-CIS/ZnS/Zr(i-PrO)₄ QDs are improved by reducing the ligand loss via encapsulation of the ligand-coated QD surface with Zr(i-PrO)₄. It is also proved that the Zr(i-PrO)₄-passivated interlayer mitigates the further degradation of I-III-V QDs from ligand loss even under harsh conditions during additional hydrolysis reaction of aluminum tri-sec-butoxide (Al(sec-BuO)₃), forming easy-to-handle G-CGS/ZnS and R-CIS/ZnS QD-embedded Al₂O₃ powders. Therefore, the introduction of a Zr(i-PrO)₄ complex layer potentially provides a strong interlayer to mitigate degradation of I-III-VI QD-embedded Al₂O₃ hybrid powders as well as passivation layer for protecting I-III-VI QD.

Surface ligand chemistry on quaternary Ag(In x Ga1-x )S2 semiconductor quantum dots for improving photoluminescence properties

Ternary and quaternary semiconductor quantum dots (QDs) are candidates for cadmium-free alternatives. Among these, semiconductors containing elements from groups 11, 13, and 16 (i.e., I-III-VI₂) are attracting increasing attention since they are direct semiconductors whose bandgap energies in the bulk state are tunable between visible and near infrared. The quaternary system of alloys consisting of silver indium sulfide (AgInS₂; bandgap energy: E _g = 1.8 eV) and silver gallium sulfide (AgGaS₂; E _g = 2.4 eV) (i.e., Ag[In _x Ga_1-x ]S₂ (AIGS)) enables bandgap tuning over a wide range of visible light. However, the photoluminescence (PL) quantum yield (10-20%) of AIGS QDs is significantly lower than that of AgInS₂ (60-70%). The present study investigates how to improve the PL quantum yield of AIGS QDs via surface ligand engineering. Firstly, the use of a mixture of oleic acid and oleylamine, instead of only oleylamine, as the solvent for the QD synthesis was attempted, and a threefold improvement of the PL quantum yield was achieved. Subsequently, a post-synthetic ligand exchange was performed. Although the addition of alkylphosphine, which is known as an L-type ligand, improved the PL efficiency only by 20%, the use of metal halides, which are categorized as Z-type ligands, demonstrated a twofold to threefold improvement of the PL quantum yield, with the highest value reaching 73.4%. The same procedure was applied to the band-edge emitting core/shell-like QDs that were synthesized in one batch based on our previous findings. While the as-prepared core/shell-like QDs exhibited a PL quantum yield of only 9%, the PL quantum yield increased to 49.5% after treatment with metal halides.

Luminescent Quaternary Ag(InxGa1-x)S2/GaSy Core/Shell Quantum Dots Prepared Using Dithiocarbamate Compounds and Photoluminescence Recovery via Post Treatment

Cadmium-free quantum dots (QDs) consisting of silver-indium-gallium-sulfide (AIGS) quaternary semiconductors were successfully synthesized using a metal-dithiocarbamate complex with sufficiently high reactivity to produce metal sulfides. The introduction of a gallium diethyldithiocarbamate precursor decreased the reaction temperature to produce active intermediates, which were subsequently converted into AIGS QDs at 150 °C with silver and indium acetates. Because of the low reaction temperature, AIGS QDs with a tetragonal crystal phase were produced selectively, which favorably generated band-edge emission whose full width at half-maximum is smaller than 40 nm after they were coated with gallium sulfide (GaS_y) shells. The compositional indium/gallium ratio was varied by changing the mixing ratio of the precursors used for the synthesis of the AIGS core, and the band-edge photoluminescence (PL) generated from the AIGS/GaS_y core/shell QDs was blue-shifted with an increase in the gallium content in the core. Consequently, a pure green emission centered at 518 nm was obtained with a PL quantum yield as high as 68%.

Enhancing the Water Resistance and Stability of CsPbBr3 Perovskite Quantum Dots for Light-Emitting-Diode Applications through Encapsulation in Waterproof Polymethylsilsesquioxane Aerogels

Highly stable all-inorganic perovskite quantum dot/polymethylsilsesquioxane aerogel (CsPbBr₃/PMSQ AG) composites were first produced using two-step hot-injection and encapsulation processes by embedding green-emitting CsPbBr₃ PQDs into modified hydrophobic mesoporous silica AGs. The unique structure of the composites not only considerably enhances the chemical stability of CsPbBr₃ PQDs against moisture, humidity, and blue-light irradiation in air but also prevents anion exchange reactions during light-emitting diode (LED) manufacturing processes. In addition, the composition-optimized CsPbBr₃/PMSQ AG exhibited excellent stability when soaked in water for more than 14 days and retained half of its initial intensity. Finally, white LED devices were fabricated by combining a blue-emitting GaN-based chip, green-emitting CsPbBr₃/PMSQ AG, and red-emitting K₂SiF₆:Mn⁴⁺ phosphors.

Multinary copper-based chalcogenide nanocrystal systems from the perspective of device applications

Multinary chalcogenide semiconductor nanocrystals are a unique class of materials as they offer flexibility in composition, structure, and morphology for controlled band gap and optical properties. They offer a vast selection of materials for energy conversion, storage, and harvesting applications. Among the multinary chalcogenides, Cu-based compounds are the most attractive in terms of sustainability as many of them consist of earth-abundant elements. There has been immense progress in the field of Cu-based chalcogenides for device applications in the recent years. This paper reviews the state of the art synthetic strategies and application of multinary Cu-chalcogenide nanocrystals in photovoltaics, photocatalysis, light emitting diodes, supercapacitors, and luminescent solar concentrators. This includes the synthesis of ternary, quaternary, and quinary Cu-chalcogenide nanocrystals. The review also highlights some emerging experimental and computational characterization approaches for multinary Cu-chalcogenide semiconductor nanocrystals. It discusses the use of different multinary Cu-chalcogenide compounds, achievements in device performance, and the recent progress made with multinary Cu-chalcogenide nanocrystals in various energy conversion and energy storage devices. The review concludes with an outlook on some emerging and future device applications for multinary Cu-chalcogenides, such as scalable luminescent solar concentrators and wearable biomedical electronics.

Strongly emitting and long-lived silver indium sulfide quantum dots for bioimaging: Insight into co-ligand effect on enhanced photoluminescence

Nanoscale ternary chalcogenides have attracted increasing research interest due to their merits of tunable properties and diverse applications in energy and biomedical fields. In this article, silver indium sulfide quantum dots supported by glutathione and polyethyleneimine as dual-ligands have been synthesized through an environmentally friendly and reproducible aqueous method. An emission quantum yield up to 37.2% has been achieved by glutathione as co-ligand bearing electron-rich groups, much higher than that of polyethyleneimine coated quantum dots (4.97%). Both spectroscopic and structural characterizations demonstrate that the photoluminescence enhancement is attributed to change of surface properties by glutathione as co-ligand. Dynamic light scattering (DLS) results and thermogravimetric analysis (TGA) reveal that glutathione covers the QDs with a higher density on the nanocrystal surface than other co-ligands. Therefore, it can effectively passivate the surface trap centers, thus decreasing the non-radiative emission. Moreover, the resultant silver indium sulfide quantum dots present surprisingly long lifetime of 3.69 μs, excellent fluorescent stability and low cytotoxicity, which enables them to be ideal candidate for real-time bioimaging.

Influence of halide ions on the structure and properties of copper indium sulphide quantum dots

In the synthesis of CuInS2 quantum dots (QDs), the halide ions present in the copper salts influence the QD growth and optical properties. X-ray absorption spectroscopy allowed rationalizing the halide incorporation in the lattice and the dependence of electronic properties of the material on the ion's polarizability and interaction with hydrophobic moieties.

Cu–Cd–Zn–S/ZnS core/shell quantum dot/polyvinyl alcohol flexible films for white light-emitting diodes

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation. The PL spectra of Cu–Cd–Zn–S/ZnS core/shell quantum dots can cover the whole visible light region in the case of only two ratios of Cu/Cd/Zn. The emission wavelength of Cu–Cd–Zn–S/ZnS QDs can be conveniently tuned from 474 to 515 and 548 to 629 nm by adjusting the pH value when the ratios of Cu/Cd/Zn are fixed at 1 : 5 : 80 and 1 : 5 : 10, respectively. It is worth noting that under the condition of a constant Cu/Cd/Zn ratio, the UV-vis absorption spectra do not change with the fluorescence spectra, indicating that the band gap of QDs remains unchanged during the change of pH value. The photoluminescence (PL) quantum yield of the as-prepared QDs with yellow emission is up to 76%. The QDs also show excellent chemical stability after the deposition of the ZnS shell. Luminescent and flexible films are fabricated by combining Cu–Cd–Zn–S QDs with polyvinyl alcohol (PVA). The QD/PVA flexible hybrid films are successfully applied on top of a conventional blue InGaN chip for remote-type warm-white LEDs. As-fabricated warm-white LEDs exhibit a higher color rendering index (CRI) of about 89.2 and a correlated color temperature (CCT) of 4308 K.

We present a facile route for the synthesis of water-soluble Cu–Cd–Zn–S/ZnS core/shell quantum dots (QDs) by simple pH regulation.

Exploring Electronic and Excitonic Processes toward Efficient Deep-Red CuInS2/ZnS Quantum-Dot Light-Emitting Diodes

The electroluminescence mechanisms in the Cd-free CuInS₂/ZnS quantum dot-based light-emitting diodes (QLEDs) are systematically investigated through transient electroluminescence measurements. The results demonstrate that the characteristics of hole transporting layers (HTLs) determine the QLEDs to be activated by the direct charge injection or the energy transfer. Moreover, both the energy level alignment between the HTL and quantum dot and the carrier mobility properties of the HTLs are critical factors to affect the device performance. By choosing the suitable HTL, such as 4,4'-bis(9-carbazolyl)-2,2'-biphenyl, highly efficient deep-red (emission peak at ∼650 nm) CuInS₂/ZnS QLEDs based on the single HTL can be obtained with a peak current efficiency and luminance of ∼2.0 cd/A and nearby 3000 cd/m², respectively.

Quaternary quantum dots with gradient valence band for all-inorganic perovskite solar cells

The severe interface charge recombination caused by the large energy difference between perovskite material and carbon electrode significantly limits the further performance improvement of the all-inorganic perovskite solar cells (PSCs). We apply innovatively multilayer of quaternary Ag-In-Ga-S (AIGS) quantum dots (QDs) with cascade-like valence bands as hole-transport materials to assemble all-inorganic PSCs, and the resultant all-inorganic PSCs exhibit a power conversion efficiency (PCE) of 8.46%, which is enhanced by 20.9% in comparison with 7% for the pristine device. The high performance of the PSCs indicates that sequential layers of AIGS QDs with cascade-like energy levels can facilitate the charge separation, reduce the barrier the holes crossing and suppress the charge recombination. Stack of QDs with cascade-like energy levels provide solution-processed PSCs with a new method to enhance device performance.

Semiconductor nanocrystal-polymer hybrid nanomaterials and their application in molecular imprinting

Quantum dots (QDs) are attractive semiconductor fluorescent nanomaterials with remarkable optical and electrical properties. The broad absorption spectra and high stability of QD transducers are advantageous for sensing and bioimaging. Molecular imprinting is a technique for manufacturing synthetic polymeric materials with a high recognition ability towards a target analyte. The high selectivity of the molecularly imprinted polymers (MIPs) is a result of the fabrication process based on the template-tailored polymerization of functional monomers. The three-dimensional cavities formed in the polymer network can serve as the recognition elements of sensors because of their specificity and stability. Appending specific molecularly imprinted layers to QDs is a promising strategy to enhance the stability, sensitivity, and selective fluorescence response of the resulting sensors. By merging the benefits of MIPs and QDs, inventive optical sensors are constructed. In this review, the recent synthetic strategies used for the fabrication of QD nanocrystals emphasizing various approaches to effective functionalization in aqueous environments are discussed followed by a detailed presentation of current advances in QD conjugated MIPs (MIP-QDs). Frontiers in manufacturing of specific imprinted layers of these nanomaterials are presented and factors affecting the specific behaviour of an MIP shell are identified. Finally, current limitations of MIP-QDs are defined and prospects are outlined to amplify the capability of MIP-QDs in future sensing.

The Composition-Dependent Photoluminescence Properties of Non-Stoichiometric ZnxAgyInS1.5+x+0.5y Nanocrystals

A facile hot injection approach to synthesize high-quality non-stoichiometric Zn_xAg_yInS_1.5+x+0.5y nanocrystals (NCs) in the size range of 2.8–3.1 nm was presented. The fluorescence spectra had single band gap features, and indicated the formation of alloy states rather than simple composite structures. The chemical compositions, photoluminescence (PL) emission wavelengths, and quantum yields of Zn_xAg_yInS_1.5+x+0.5y nanocrystals were significantly influenced by the concentration of an organic capping agent. The appropriate proportion of 1-dodecanthiol in the precursor prevented the precipitation, increased the fluorescence quantum yield, and improved their optical properties. The proper ratio of capping agent allowed Zn, Ag, and In to form a better crystallinity and compositional homogeneity of Zn_xAg_yInS_1.5+x+0.5y nanocrystals. The photoluminescence was tunable from blue to red in the range of 450–700 nm as the Ag content changed independently. The PL and absorption spectra of Zn_xAg_yInS_1.5+x+0.5y nanocrystals showed a significant blue shift with the decrease of Ag content in the precursor. As there were no obvious differences on the average particle sizes of Zn_xAg_yInS_1.5+x+0.5y samples, these results fully revealed the composition-dependent photoluminescence properties of Zn_xAg_yInS_1.5+x+0.5y nanocrystals. The relative quantum yield reached 35%. The fluorescence lifetimes (τ₁=115–148 ns and τ₂=455–483 ns) were analogous to those of AgInS₂ and (AgIn)_xZn_2(1−x)S₂.

Polyelectrolyte-Mediated Nontoxic AgGa xIn1- xS2 QDs/Low-Density Lipoprotein Nanoprobe for Selective 3D Fluorescence Imaging of Cancer Stem Cells

Cancer stem cells, which are a population of cancer cells sharing common properties with normal stem cells, have strong self-renewal ability and multi-lineage differentiation potential to trigger tumor proliferation, metastases, and recurrence. From this, targeted therapy for cancer stem cells may be one of the most promising strategies for comprehensive treatment of tumors in the future. We design a facile approach to establish the colon cancer stem cells-selective fluorescent probe based on the low-density lipoprotein (LDL) and the novel AgGa _xIn_{(1- x)}S₂ quantum dots (AGIS QDs). The AGIS QDs with a high crystallinity are obtained for the first time via cation-exchange protocol of Ga³⁺ to In³⁺ starting from parent AgInS₂ QDs. Photoluminescence peak of AGIS QDs can be turned from 502 to 719 nm by regulating the reaction conditions, with the highest quantum yield up to 37%. Subsequently, AGIS QDs-conjugated LDL nanocomposites (NCs) are fabricated, in which a cationic polyelectrolyte was used as a coupling reagent to guarantee the electrostatic self-assembly. The structural integrity and physicochemical properties of the LDL-QDs NCs are found to be maintained in vitro, and the NCs exhibit remarkable biocompatibility. The LDL-QDs can be selectively delivered into cancer stem cells that overexpress LDL receptor, and three-dimensional imaging of cancer stem cells is realized. The results of this study not only demonstrate the versatility of nature-derived lipoprotein nanoparticles, but also confirm the feasibility of electrostatic conjugation using cationic polyelectrolyte, allowing reseachers to design nanoarchitectures for targeted diagnosis and treatment of cancer.

Tunable Emission of Bluish Zn-Cu-Ga-S Quantum Dots by Mn Doping and Their Electroluminescence

On the basis of bluish-emitting double-shelled quantum dots (QDs) of Zn-Cu-Ga-S (ZCGS)/ZnS/ZnS, Mn doping into ZCGS host with different Mn/Cu concentrations is implemented via surface adsorption and lattice diffusion. The resulting double-shelled Mn-doped ZCGS (ZCGS/Mn) QDs exhibit a distinct Mn^2+ 4T₁-⁶A₁ emission as a consequence of effective lattice incorporation simultaneously with host intragap states-involving emissions of free-to-bound and donor-acceptor pair recombinations. The relative contribution of Mn emission to the overall photoluminescence (PL) is consistently proportional to its concentration, resulting in tunable PL from bluish, white, to reddish white. Regardless of Mn doping and its concentration, all QDs possess high PL quantum yield levels of 74-79%. Those undoped and doped QDs are then employed as an emitting layer (EML) of all-solution-processed QD-light-emitting diodes (QLEDs) with hybrid charge transport layers and their electroluminescence (EL) is compared. Compared to undoped QDs, doped analogues give rise to a huge spectral disparity of EL versus PL, specifically showing a near-complete quenching of Mn²⁺ EL. This unexpected observation is rationalized primarily by considering unbalanced carrier injection to QD EML on the basis of energetic alignment of the present QLED and rapid trapping of holes injected at intragap states of QDs.

Colloidal Phase-Change Materials: Synthesis of Monodisperse GeTe Nanoparticles and Quantification of Their Size-Dependent Crystallization

Phase-change memory materials refer to a class of materials that can exist in amorphous and crystalline phases with distinctly different electrical or optical properties, as well as exhibit outstanding crystallization kinetics and optimal phase transition temperatures. This paper focuses on the potential of colloids as phase-change memory materials. We report a novel synthesis for amorphous GeTe nanoparticles based on an amide-promoted approach that enables accurate size control of GeTe nanoparticles between 4 and 9 nm, narrow size distributions down to 9-10%, and synthesis upscaling to reach multigram chemical yields per batch. We then quantify the crystallization phase transition for GeTe nanoparticles, employing high-temperature X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. We show that GeTe nanoparticles crystallize at higher temperatures than the bulk GeTe material and that crystallization temperature increases with decreasing size. We can explain this size-dependence using the entropy of crystallization model and classical nucleation theory. The size-dependences quantified here highlight possible benefits of nanoparticles for phase-change memory applications.

A photoelectrochemical immunosensor based on gold nanoparticles/ZnAgInS quaternary quantum dots for the high-performance determination of hepatitis B virus surface antigen

ZnAgInS quaternary quantum dots were prepared using glutathione as the capped reagent. Gold nanoparticles (GNPs) were integrated with ZnAgInS QDs to provide a GNPs/ZnAgInS QDs nanocomposite. The morphological image, component and crystal structure of GNPs/ZnAgInS QDs were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A glassy carbon electrode surface was coated with GNPs/ZnAgInS QDs nanocomposites to construct an interface for immobilizing the antibody of hepatitis B virus surface antigen (anti-HBsAg). By employing GNPs/ZnAgInS QDs as a photoactive element, a photoelectrochemical immunosensor for hepatitis B virus surface antigen (HBsAg) was developed. The results indicate that gold nanoparticles can dramatically enhance the photocurrent response of ZnAgInS QDs and thus improving the sensing performances of the immunosensor. The experimental conditions including incubation time, incubation temperature, and ascorbic acid concentration were optimized. The relative photocurrent decline [R_i = ΔI/I₀= (I₀ - I)/I₀] shows a linear relationship to the logarithm of HBsAg concentration [lg(c, ng mL^-1)] in the range from 0.005 to 30 ng mL^-1. A detection limit of 0.5 pg mL^-1 was obtained. The immunosensor shows excellent sensitivity, selectivity, stability and reproducibility. The HBsAg concentrations in clinical serum samples were also accurately determined with this new photoelectrochemical immunosensor.

From Large-Scale Synthesis to Lighting Device Applications of Ternary I-III-VI Semiconductor Nanocrystals: Inspiring Greener Material Emitters

Quantum dots with fabulous size-dependent and color-tunable emissions remained as one of the most exciting inventories in nanomaterials for the last 3 decades. Even though a large number of such dot nanocrystals were developed, CdSe still remained as unbeatable and highly trusted lighting nanocrystals. Beyond these, the ternary I-III-VI family of nanocrystals emerged as the most widely accepted greener materials with efficient emissions tunable in visible as well as NIR spectral windows. These bring the high possibility of their implementation as lighting materials acceptable to the community and also to the environment. Keeping these in mind, in this Perspective, the latest developments of ternary I-III-VI nanocrystals from their large-scale synthesis to device applications are presented. Incorporating ZnS, tuning the composition, mixing with other nanocrystals, and doping with Mn ions, light-emitting devices of single color as well as for generating white light emissions are also discussed. In addition, the future prospects of these materials in lighting applications are also proposed.

One-Pot Aqueous Synthesis of Fluorescent Ag-In-Zn-S Quantum Dot/Polymer Bioconjugates for Multiplex Optical Bioimaging of Glioblastoma Cells

Cancer research has experienced astonishing advances recently, but cancer remains a major threat because it is one of the leading causes of death worldwide. Glioblastoma (GBM) is the most malignant brain tumor, where the early diagnosis is vital for longer survival. Thus, this study reports the synthesis of novel water-dispersible ternary AgInS2 (AIS) and quaternary AgInS2-ZnS (ZAIS) fluorescent quantum dots using carboxymethylcellulose (CMC) as ligand for multiplexed bioimaging of malignant glioma cells (U-87 MG). Firstly, AgInS2 core was prepared using a one-pot aqueous synthesis stabilized by CMC at room temperature and physiological pH. Then, an outer layer of ZnS was grown and thermally annealed to improve their optical properties and split the emission range, leading to core-shell alloyed nanostructures. Their physicochemical and optical properties were characterized, demonstrating that luminescent monodispersed AIS and ZAIS QDs were produced with average sizes of 2.2 nm and 4.3 nm, respectively. Moreover, the results evidenced that they were cytocompatible using in vitro cell viability assays towards human embryonic kidney cell line (HEK 293T) and U-87 MG cells. These AIS and ZAIS successfully behaved as fluorescent nanoprobes (red and green, resp.) allowing multiplexed bioimaging and biolabeling of costained glioma cells using confocal microscopy.

Cd-free Cu-Zn-In-S/ZnS quantum dots@SiO2 multiple cores nanostructure: preparation and application for white LEDs

The work reports the fabrication of Cu doped Zn-In-S (CZIS) alloy quantum dots (QDs) using dodecanethiol and oleic acid as stabilizing ligands. With the increase of doped Cu element, the photoluminescence (PL) peak is monotonically red shifted. After coating ZnS shell, the PL quantum yield of CZIS QDs can reach 78%. Using reverse micelle microemulsion method, CZIS/ZnS QDs@SiO₂ multi-core nanospheres were synthesized to improve the colloidal stability and avoid the aggregation of QDs. The obtained multi-core nanospheres were dispersed in curing adhesive, and applied as a color conversion layer in down converted light-emitting diodes. After encapsulation in curing adhesive, the newly designed LEDs show artifically regulated color coordinates with varying the weight ratio of green QDs and red QDs, and the concentrations of these two types of QDs. Moreover, natural white and warm white LEDs with correlated color temperature of 5287, 6732, 2731, and 3309 K can be achieved, which indicates that CZIS/ZnS QDs@SiO₂ nanostructures are promising color conversion layer material for solid-state lighting application.

AgInS2-ZnS Quantum Dots: Excited State Interactions with TiO2 and Photovoltaic Performance

Multinary quantum dots such as AgInS₂ and alloyed AgInS₂-ZnS are an emerging class of semiconductor materials for applications in photovoltaic and display devices. The nanocrystals of (AgInS₂)_x-(ZnS)_1-x (for x = 0.67) exhibit a broad emission with a maximum at 623 nm and interact strongly with TiO₂ nanostructures by injecting electrons from the excited state. The electron transfer rate constant as determined from transient absorption spectroscopy was 1.8 × 10¹⁰ s^-1. The photovoltaic performance was evaluated over a period of a few weeks to demonstrate the stability of AgInS₂-ZnS when utilized as sensitizers in solar cells. We report a power conversion efficiency of 2.25% of our champion cell 1 month after its fabrication. The limitations of AgInS₂-ZnS nanocrystals in achieving greater solar cell efficiency are discussed.

Origin of highly efficient photoluminescence in AgIn5S8 nanoparticles

The photoluminescence of AgIn₅S₈ nanoparticles was examined to clarify the emissive relaxation processes of defect states and to explain the highly efficient photoluminescence of defect states. The large Stokes shift of the defect emission was explained by strong electron-phonon coupling in the nanoparticles. Steady-state and time-resolved photoluminescence spectroscopy indicated two emissive defect states with characteristic emission energies and lifetimes. Change of the surface-to-volume ratio in the nanoparticles affected the relative contribution of the two states, implying that defect emission in higher energy was attributable to surface-related defects. The defect emission in lower energy was attributable to intrinsic defects, which were also present in bulk. The quantum yield of the surface defects was larger than that of the intrinsic defects, which accounted for the unusually high quantum yield of AgIn₅S₈ nanoparticles, although the origin of emission was the defect states, not the exciton recombination found in typical semiconductor nanoparticles.

Circadian-tunable Perovskite Quantum Dot-based Down-Converted Multi-Package White LED with a Color Fidelity Index over 90

New metrics of the color and circadian performances of down-converted white light-emitting diodes (DC-WLEDs) are rapidly becoming popular in smart lighting systems. This is due to the increased desire for accurate analytical methods to measure the effects of newly developed quantum dot (QD)-based lighting on the vision, color, and circadian sensors of retina cells in the human eye. In this regard, a two-measure system known as technical memorandum TM-30-2015 (Illuminating Engineering Society of North America), encompassing the color fidelity index (CFI, R_f) and the color gamut index (CGI, R_g), has been developed as a new metrics of color to replace the currently utilized color rendering index (CRI, R_a). In addition, the tunability of the circadian efficacy of radiation (CER) is now more important due to its effect on the control of melatonin suppression/secretion, resetting of the central/local clocks of individuals given their daily cycles, and benefits to human health. In this paper, we developed and analyzed six-colored perovskite (Pe; cyan, green, yellowish green, amber, orange, and red colors) QDs-based multi-package WLED, and optimized the SPDs of tunable PeQD-based multi-package WLEDs in terms of promising human-centric lighting device, given its optimized visual energy, color qualities and health-promoting effects.

Gram-Scale Synthesis of Hydrophilic PEI-Coated AgInS2 Quantum Dots and Its Application in Hydrogen Peroxide/Glucose Detection and Cell Imaging

Assisted with polyethylenimine, 4.0 L of water-soluble AgInS₂ quantum dots (AIS QDs) were successfully synthesized in an electric pressure cooker. As-prepared QDs exhibit yellow emission with a photoluminescence (PL) quantum yield up to 32%. The QDs also show excellent water/buffer stability. The highly luminescent AIS QDs are used to explore their dual-functional behavior: detection of hydrogen peroxide (H₂O₂)/glucose and cell imaging. The amino-functionalized AIS QDs show high sensitivity and specificity for H₂O₂ and glucose with detection limits of 0.42 and 0.90 μM, respectively. A linear correlation was established between PL intensity and concentration of H₂O₂ in the ranges of 0.5-10 μM and 10-300 μM, while the linear ranges were 1-10 μM and 10-1000 μM for detection of glucose. The AIS QDs reveal negligible cytotoxicity on HeLa cells. Furthermore, the luminescence of AIS QDs gives the function of optical imaging.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Engineering Quantum Dots with Different Emission Wavelengths and Specific Fluorescence Lifetimes for Spectrally and Temporally Multiplexed Imaging of Cells

In this work, a proof-of-concept study was performed to examine the potential of spectrally and temporally multiplexed imaging of cells by using quantum dots (QDs). The CdSe and ZAIS QDs with different emission wavelengths and well-separated fluorescence lifetimes were prepared to provide 2-dimensional information. After incubation with cells, the same type of QDs with different emission wavelengths were distinguishable in spectral imaging while different types of QDs with similar emission wavelengths but well-separated fluorescence lifetimes were resolvable in fluorescence lifetime imaging. For cells co-stained with dye and different types of QDs, the fluorescence lifetime imaging microscopy (FLIM) images showed spatially separated patterns that can be split into channel images by using the software-based time gates. Overall, the results demonstrate the feasibility of combining the 2-dimensional encoded QDs for spectrally and temporally multiplexed imaging. This method can be extended to other QDs and organic dyes to maximize the number of measurable species in multiplexed imaging and sensing applications.

Solid Solution Quantum Dots with Tunable Dual or Ultrabroadband Emission for LEDs

Quantum dots that efficiently emit white light directly or feature a "candle-like" orange photoluminescence with a high Stokes shift are presented. The key to obtaining these unique emission properties is through controlled annealing of the core Cu-In-Ga-S quantum dots in the presence of zinc ions, thus forming Zn-Cu-In-Ga-S solid solutions with different distributions of the substitution and dopant elements. The as-obtained nanocrystals feature excellent quantum yields of up to 82% with limited or even eliminated reabsorption and a color rendering index of bare particles of up to 88, enabling the production of high-quality white LEDs using a single color converter layer. Furthermore, the color properties can be tuned by changing the experimental conditions as well as by varying the excitation wavelength. The multicomponent luminescence mechanism is discussed in detail based on similar literature reports. White LEDs with unparalleled color quality and competitive luminous efficacies are presented herein.

Synthetic strategies and biomedical applications of I–III–VI ternary quantum dots

In this review, we discuss recent advances of I–III–VI QDs with a major focus on synthesis and biomedical applications; advantages include low toxicity and fluorescent tuning in the biological window.

Luminophores of tunable colors from ternary Ag–In–S and quaternary Ag–In–Zn–S nanocrystals covering the visible to near-infrared spectral range

We report an efficient synthesis of Ag–In–S and Ag–In–Zn–S nanocrystals with strong photoluminescence (QY = 59%) in the visible to near-infrared range.

Partial Ion Exchange Derived 2D Cu-Zn-In-S Nanosheets as Sensitizers of 1D TiO2 Nanorods for Boosting Solar Water Splitting

A facile route for the fabrication of a novel ZnS shell/Cu-Zn-In-S nanosheets/TiO₂ nanorods heterojunction was reported in this work. Especially, the quaternary Cu-Zn-In-S nanosheets were synthesized creatively from the ternary ZnIn₂S₄ nanosheets by partial exchange reaction, leading to substantial enhancement on the light absorbance. Such heterojunction could increase the surface area and accelerate the charge transfer resulting from its hierarchical 2D/1D structure and favorable energy bands. Moreover, the ZnS coating acted as a passivation layer as well as a potential barrier, significantly suppressing the interface recombination. The above synergistic effects resulted in the largely increased photocurrent density from 0.34 mA cm^-2 for the pristine TiO₂ to 0.81 mA cm^-2 for the heterojunction at 0.8 V vs RHE.

Radiative and Nonradiative Recombination in CuInS2 Nanocrystals and CuInS2-Based Core/Shell Nanocrystals

Luminescent copper indium sulfide (CIS) nanocrystals are a potential solution to the toxicity issues associated with Cd- and Pb-based nanocrystals. However, the development of high-quality CIS nanocrystals has been complicated by insufficient knowledge of the electronic structure and of the factors that lead to luminescence quenching. Here we investigate the exciton decay pathways in CIS nanocrystals using time-resolved photoluminescence and transient absorption spectroscopy. Core-only CIS nanocrystals with low quantum yield are compared to core/shell nanocrystals (CIS/ZnS and CIS/CdS) with higher quantum yield. Our measurements support the model of photoluminescence by radiative recombination of a conduction band electron with a localized hole. Moreover, we find that photoluminescence quenching in low-quantum-yield nanocrystals involves initially uncoupled decay pathways for the electron and hole. The electron decay pathway determines whether the exciton recombines radiatively or nonradiatively. The development of high-quality CIS nanocrystals should therefore focus on the elimination of electron traps.

Highly pure green light emission of perovskite CsPbBr3 quantum dots and their application for green light-emitting diodes

All-inorganic perovskite CsPbBr₃ quantum dots (QDs) with an emission peak of around 520 nm were synthesized by a hot-injection method, and were systematically studied as green phosphor for light-emitting diodes (LEDs). Highly pure green light with an emission peak of 534 nm and a full-width at half-maximum (FWHM) about 20 nm was achieved using CsPbBr₃ QDs and GaN LEDs. Commission Internationale Ed I'Eclairage coordinate of the fabricated green LEDs was (0.203, 0.757). Compared to GaN LEDs, the current-voltage characteristic of the green LED did not show any degradation. Moreover, the green LEDs displayed a luminous efficiency of 31.92 lm/W under an injection current of 10 mA.

Colloidal nanocrystals for quality lighting and displays: milestones and recent developments

Recent advances in colloidal synthesis of nanocrystals have enabled high-quality high-efficiency light-emitting diodes, displays with significantly broader color gamut, and optically-pumped lasers spanning the whole visible regime. Here we review these colloidal platforms covering the milestone studies together with recent developments. In the review, we focus on the devices made of colloidal quantum dots (nanocrystals), colloidal quantum rods (nanorods), and colloidal quantum wells (nanoplatelets) as well as those of solution processed perovskites and phosphor nanocrystals. The review starts with an introduction to colloidal nanocrystal photonics emphasizing the importance of colloidal materials for light-emitting devices. Subsequently,we continue with the summary of important reports on light-emitting diodes, in which colloids are used as the color converters and then as the emissive layers in electroluminescent devices. Also,we review the developments in color enrichment and electroluminescent displays. Next, we present a summary of important reports on the lasing of colloidal semiconductors. Finally, we summarize and conclude the review presenting a future outlook.

Synthesis of Cu-Deficient and Zn-Graded Cu-In-Zn-S Quantum Dots and Hybrid Inorganic-Organic Nanophosphor Composite for White Light Emission

Cu-deficient graded-zinc Cu-In-Zn-S (CIZS) quantum dots (QDs) were synthesized by a two-step solvothermal method. These CIZS QDs exhibited size and composition tunable photoluminescence characteristics with emission color tunable from greenish-yellow to orange to red with a relatively high quantum yield between 45 and 60%. Novel white-light-emitting (WLE) hybrid composite is fabricated by integrating the blue-emissive 1,4-bis-2-(5-phenyl oxazolyl)-benzene (POPOP) organic fluorophore and quaternary CIZS inorganic QDs. Integrating CIZS QDs with POPOP fluorophore resulted in series of tunable emission colors with CIE coordinates lying in a straight line between the coordinates of the end member. WLE was shown for hybrid mixture comprising 0.5 nM of POPOP and 3 mg/mL of CIZS QDs with color coordinates (0.3312, 0.3324). Thin films of this hybrid mixture in PMMA matrix coated on UV-LED or on glass substrates with UV backlit light also showed broadband WLE with ideal CIE color coordinates of (0.34, 0.33), high color-rendering index value of 92, and correlated color temperature value of 5143 K. The hybrid composite exhibit Forster resonance energy transfer cascading from POPOP to CIZS which results in emission covering the entire visible spectral range. POPOP and CIZS QDs hybrid composite is a versatile material for WLED applications.

Facile Synthesis of Cadmium-Free Zn-In-S:Ag/ZnS Nanocrystals for Bio-Imaging

High quality cadmium-free Zn-In-S:Ag doped-nanocrystals (d-NCs) were synthesized via a simple one-step noninjection route using silver nitrate, indium acetate, zinc acetate, oleylamine, S powder and 1-dodecanethiol as starting materials in an organic phase. The size and optical properties can be effectively tailored by controlling the reaction time, reaction temperature, Ag(+) dopant concentration, and the molar ratio of In to Zn. The photoluminescence wavelength of as-prepared Zn-In-S:Ag NCs covered a broad visible range from 458 nm to 603 nm. After being passivated by protective ZnS shell, the photoluminescence quantum yield (PLQY) of Zn-In-S:Ag(+) /ZnS was greatly improved to 43.5%. More importantly, the initial high PLQY of the obtained core/shell d-NCs in organic media can be preserved when being transferred into the aqueous media via ligand exchange. Finally, high quality Zn-In-S:Ag(+) /ZnS d-NCs in aqueous phase were applied as bio-imaging agents for identifying living KB cells.

White Light-Emitting Diodes Based on AgInS₂/ZnS Quantum Dots with Improved Bandwidth in Visible Light Communication

Quantum dot white light-emitting diodes (QD-WLEDs) were fabricated from green- and red-emitting AgInS₂/ZnS core/shell QDs coated on GaN LEDs. Their electroluminescence (EL) spectra were measured at different currents, ranging from 50 mA to 400 mA, and showed good color stability. The modulation bandwidth of previously prepared QD-WLEDs was confirmed to be much wider than that of YAG:Ce phosphor-based WLEDs. These results indicate that the AgInS₂/ZnS core/shell QDs are good color-converting materials for WLEDs and they are capable in visible light communication (VLC).