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

One-Pot Synthesis of Color-Tunable Narrow-Bandwidth Ag-In-Ga-Zn-S Semiconductor Nanocrystals for Quantum-Dot Light-Emitting Diodes

I-III-VI type semiconductor nanocrystals (NCs) have attracted considerable attention due to their environmental friendly nature and large-scale tunable emission. Herein, we report the successful synthesis of full-spectrum (470 to 614 nm) Ag-In-Ga-Zn-S (AIGZS) NCs by precisely regulating the In/Ga ratios using a facile one-pot method. Intriguingly, the photoluminescence (PL) peak width exhibits a continuous narrowing trend with extended reaction time, ultimately reaching a full width at half-maximum (fwhm) of 34 nm for green AIGZS NCs. Furthermore, the exciton relaxation dynamics of AIGZS NCs were systematically investigated using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Remarkably, we successfully fabricated blue, green, and red quantum-dot light-emitting diodes (QLEDs), forecasting the potential of AIGZS NCs with high color purity for applications in full-spectrum QLEDs.

Building in biologically appropriate multifunctionality in aqueous copper indium selenide-based quantum dots

Advanced nanoplatforms equipped with different functional moieties for theranostics hold appealing promise for reshaping precision medicine. The reliable construction of an individual nanomaterial with intrinsic near-infrared (NIR) photofunction and magnetic domains is much desired but largely unexplored in a direct aqueous synthesis system. Herein, we develop an aqueous phase synthetic strategy for Mn2+ doping of ZnS shell grown on Zn-Cu-In-Se core quantum dots (ZCISe@ZnS:Mn QDs), providing the optimal NIR fluorescence quantum efficiency of up to 18.9% and meanwhile efficiently introducing paramagnetic domains. The relaxometric properties of the water-soluble Mn-doped QDs make them desirable for both the longitudinal and transverse (T1 and T2) magnetic resonance (MR) contrast enhancement due to the shell lattice-doped Mn2+ ions with slow tumbling rates and favoured spin-proton dipolar interactions with surrounding water molecules. Surprisingly, the incorporation of Mn2+ ions into the shell is found to significantly enhance the production of reactive oxygen species (ROS) by combining both the chemodynamic and photodynamic processes upon NIR light irradiation, showing great potential for efficient photo-assisted ablation of cancer cells. Furthermore, a broad-spectrum excitation range beneficial for bright NIR fluorescence imaging of breast cancer has been proven and offers high flexibility in the choice of incident light sources. Multiparametric MR imaging of the brain has also been successfully demonstrated in vivo.

A Review on Multiple I-III-VI Quantum Dots: Preparation and Enhanced Luminescence Properties

I-III-VI type QDs have unique optoelectronic properties such as low toxicity, tunable bandgaps, large Stokes shifts and a long photoluminescence lifetime, and their emission range can be continuously tuned in the visible to near-infrared light region by changing their chemical composition. Moreover, they can avoid the use of heavy metal elements such as Cd, Hg and Pb and highly toxic anions, i.e., Se, Te, P and As. These advantages make them promising candidates to replace traditional binary QDs in applications such as light-emitting diodes, solar cells, photodetectors, bioimaging fields, etc. Compared with binary QDs, multiple QDs contain many different types of metal ions. Therefore, the problem of different reaction rates between the metal ions arises, causing more defects inside the crystal and poor fluorescence properties of QDs, which can be effectively improved by doping metal ions (Zn²⁺, Mn²⁺ and Cu⁺) or surface coating. In this review, the luminous mechanism of I-III-VI type QDs based on their structure and composition is introduced. Meanwhile, we focus on the various synthesis methods and improvement strategies like metal ion doping and surface coating from recent years. The primary applications in the field of optoelectronics are also summarized. Finally, a perspective on the challenges and future perspectives of I-III-VI type QDs is proposed as well.

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS₂/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

Probe sonication-assisted rapid synthesis of highly fluorescent sulfur quantum dots

A new type of heavy-metal free single-element nanomaterial, called sulfur quantum dots (SQDs), has gained significant attention due to its advantages over traditional semiconductor QDs for several biomedical and optoelectronic applications. A straightforward and rapid synthesis approach for preparing highly fluorescent SQDs is needed to utilize this nanomaterial for technological applications. Until now, only a few synthesis approaches have been reported; however, these approaches are associated with long reaction times and low quantum yields (QY). Herein, we propose a novel optimized strategy to synthesize SQDs using a mix of probe sonication and heating, which reduces the reaction time usually needed from 125 h to a mere 15 min. The investigation employs cavitation and vibration effects of high energy acoustic waves to break down the bulk sulfur into nano-sized particles in the presence of highly alkaline medium and oleic acid. In contrast to previous reports, the obtained SQDs exhibited excellent aqueous solubility, desirable photostability, and a relatively high photoluminescence QY up to 10.4% without the need of any post-treatment. Additionally, the as-synthesized SQDs show excitation-dependent emission and excellent stability in different pH (2-12) and temperature (20 °C-80 °C) environments. Hence, this strategy opens a new pathway for rapid synthesis of SQDs and may facilitate the use of these materials for biomedical and optoelectronic applications.

I-III-VI Quantum Dots and Derivatives: Design, Synthesis, and Properties for Light-Emitting Diodes

Quantum dots (QDs) are important frontier luminescent materials for future technology in flexible ultrahigh-definition display, optical information internet, and bioimaging due to their outstanding luminescence efficiency and high color purity. I-III-VI QDs and derivatives demonstrate characteristics of composition-dependent band gap, full visible light coverage, high efficiency, excellent stability, and nontoxicity, and hence are expected to be ideal candidates for environmentally friendly materials replacing traditional Cd and Pb-based QDs. In particular, their compositional flexibility is highly conducive to precise control energy band structure and microstructure. Furthermore, the quantum dot light-emitting diodes (QLEDs) exhibits superior prospects in monochrome display and white illumination. This review summarizes the recent progress of I-III-VI QDs and their application in LEDs. First, the luminescence mechanism is illustrated based on their electronic-band structural characteristics. Second, focusing on the latest progress of I-III-VI QDs, the preparation mechanism, and the regulation of photophysical properties, the corresponding application progress particularly in light-emitting diodes is summarized as well. Finally, we provide perspectives on the overall current status and challenges propose performance improvement strategies in promoting the evolution of QDs and QLEDs, indicating the future directions in this field.

CuInS2 Nanocrystals Embedded PMMA Composite Films: Adjustment of Polymer Molecule Weights and Application in Remote-Type White LEDs

The commercial application of colloidal semiconductor nanocrystals has been realized owing to the development of composite film technology. Here, we demonstrated the fabrication of green and red emissive CuInS₂ nanocrystals embedded polymer composite films of equal thickness by using a precise solution casting method. The impacts of polymer molecular weight on the dispersibility of CuInS₂ nanocrystals were then systematically studied through evaluating the decrease in transmittance and red shift of emission wavelength. The composite films made from PMMA of small molecular weights exhibited higher transmittance. Applications of these green and red emissive composite films as color converters in remote-type light-emitting devices were further demonstrated.

Cation Exchange Synthesis of Aliovalent Doped InP QDs and Their ZnSexS1-x Shell Coating for Enhanced Fluorescence Properties

III-V quantum dots (QDs), in particular InP QDs, have emerged as high-performance and environmentally friendly candidates to replace cadmium based QDs. InP QDs exhibit properties of direct band gap structure, low toxicity, and high mobility, which make them suitable for high-performance optoelectronic applications. However, it is still challenging to precisely regulate the components and crystal structure of InP QDs, especially in the engineered stable aliovalent doping. In this work, we developed our original reverse cation exchange strategy to achieve Cu⁺ doped InP (InP:Cu) QDs at lower temperature. A ZnSe_xS_1-x shell was then homogeneously grown on the InP:Cu QDs as the passivation shell. The as-prepared InP:Cu@ZnSe_xS_1-x core-shell QDs exhibited better fluorescence properties with a photoluminescence quantum yield (PLQY) of 56.47%. Due to the existence of multiple luminous centers in the QDs, variable temperature-dependent fluorescence characteristics have been studied. The high photoluminescence characteristics in the near-infrared region indicate their potential applications in optoelectronic devices and biological fields.

Integration of ternary I-III-VI quantum dots in light-emitting diodes

Ternary I-III-VI quantum dots (TQDs) are semiconductor nanomaterials that have been gradually incorporated in the fabrication of light-emitting diodes (LEDs) over the last 10 years due to their physicochemical and photoluminescence properties, such as adequate quantum yield values, tunable wavelength emission, and easy synthesis strategies, but mainly because of their low toxicity that allows them to be excellent candidates to compete with conventional Cd-Pb-based QDs. This review addresses the different strategies to obtain TQDs and how synthesis conditions influence their physicochemical properties, followed by the LEDs parameters achieved using TQDs. The second part of the review summarizes how TQDs are integrated into LEDs and white light-emitting diodes (WLEDs). Furthermore, an insight into the state-of-the-art LEDs development using TQDs, including its advantages and disadvantages and the challenges to overcome, is presented at the end of the review.

Narrow-Bandwidth Blue-Emitting Ag-Ga-Zn-S Semiconductor Nanocrystals for Quantum-Dot Light-Emitting Diodes

I-III-VI type semiconductor nanocrystals (NCs) have attracted considerable attention in the display field. Herein, we realized the synthesis of narrow-bandwidth blue-emitting Ag-Ga-Zn-S (AGZS) NCs via a facile one-pot method. Intriguingly, the Ag/Zn feeding ratio and Ag/Ga feeding ratio are crucial for the realization of narrow-bandwidth AGZS NCs. By choosing a Ag/Zn feeding ratio of 4:1 and Ag/Ga feeding ratio of 1:8, AGZS NCs demonstrate a typical blue emission at 470 nm with a narrow full width at half-maximum (fwhm) of 48 nm, which is mainly generated from the band-to-hole recombination rather than the donor-acceptor pair (DAP) recombination. Furthermore, a solution-processed quantum-dot light-emitting device based on AGZS NCs exhibits a narrow electroluminescent bandwidth of 53 nm and high luminance over 123.1 cd m^-2, as well as a high external quantum efficiency (EQE) of 0.40%. Our work highlights AGZS NCs with high color purity as an important candidate for blue-light-emitting devices.

Ligand-assisted structure tailoring of highly luminescent Cu-In-Zn-S/ZnS//ZnS quantum dots for bright and stable light-emitting diodes

Harnessing environment-friendly and low-cost multinary Cu-In-Zn-S quantum dots (QDs) as emitters for light-emitting diodes (LEDs) has attracted great attention for display and lighting application. However, suboptimal QD structure is a huge obstacle, which results in serious non-radiative recombination and efficiency roll-off. Herein, we synthesized structure-tailored Cu-In-Zn-S/ZnS//ZnS QDs by improving the reactivity of shell growth by 2-ethylhexanoic acid (EHA) ligands. The EHA-assisted shell growth can boost an extended alloyed layer at the core-shell interface and a smoothed confinement barrier, which effectively passivate the interface defects and suppress Förster resonance energy transfer (FRET) process. These synthesized QDs display a bright photoluminescence emission (quantum yield of 83%) and a larger size of 8.4 nm. Moreover, the resulting LEDs based on the EHA-assisted QDs exhibit a maximum luminance of 8074 cd/m², and a current efficiency of 7.3 cd/A with a low efficiency roll-off. Our results highlight a remarkable ligand strategy to tailor the QD structure for high performance QD-based LEDs.

Sulfur-source-dependent phase-selective preparation of Cu3NiInSnS6 nanocrystals and their optical and magnetic properties

Multifunctional multinary metal chalcogenides have long been a research hotspot in the field of materials chemistry due to their rich composition, flexible structure, excellent properties and wide range of applications. However, the exploration of complex quinary chalcogenides is still challenging. In this work, for the first time, we have developed the controlled synthesis of quinary Cu₃NiInSnS₆ nanocrystals, realizing the selective preparation of hexagonal wurtzite and cubic zinc blende metastable phases by simply tuning the sulfur source. The phase structure analysis reveals that both metastable phases possess a disordered structure with a random distribution of metal atoms in the unit cells. The fabricated wurtzite and zinc blende-structure Cu₃NiInSnS₆ nanocrystals have a direct band gap of 1.82 and 1.94 eV, respectively, and both exhibit superparamagnetic behavior at low temperatures. This work is of great significance for the development of novel multifunctional materials based on metastable multinary metal chalcogenide phases.

Bright, Magnetic NIR-II Quantum Dot Probe for Sensitive Dual-Modality Imaging and Intensive Combination Therapy of Cancer

Improving the effectiveness of cancer therapy will require tools that enable more specific cancer targeting and improved tumor visualization. Theranostics have the potential for improving cancer care because of their ability to serve as both diagnostics and therapeutics; however, their diagnostic potential is often limited by tissue-associated light absorption and scattering. Herein, we develop CuInSe₂@ZnS:Mn quantum dots (QDs) with intrinsic multifunctionality that both enable the accurate localization of small metastases and act as potent tumor ablation agents. By leveraging the growth kinetics of a ZnS shell on a biocompatible CuInSe₂ core, Mn doping, and folic acid functionalization, we produce biocompatible QDs with high near-infrared (NIR)-II fluorescence efficiency up to 31.2%, high contrast on magnetic resonance imaging (MRI), and preferential distribution in 4T1 breast cancer tumors. MRI-enabled contrast of these nanoprobes is sufficient to timely identify small metastases in the lungs, which is critically important for preventing cancer spreading and recurrence. Further, exciting tumor-resident QDs with NIR light produces both fluorescence for tumor visualization through radiative recombination pathways as well as heat and radicals through nonradiative recombination pathways that kill cancer cells and initiate an anticancer immune response, which eliminates tumor and prevents tumor regrowth in 80% of mice.

Phase-Controlled Synthesis and Quasi-Static Dielectric Resonances in Silver Iron Sulfide (AgFeS2 ) Nanocrystals

Ternary metal-chalcogenide semiconductor nanocrystals are an attractive class of materials due to their tunable optoelectronic properties that result from a wide range of compositional flexibility and structural diversity. Here, the phase-controlled synthesis of colloidal silver iron sulfide (AgFeS₂ ) nanocrystals is reported and their resonant light-matter interactions are investigated. The product composition can be shifted selectively from tetragonal to orthorhombic by simply adjusting the coordinating ligand concentration, while keeping the other reaction parameters unchanged. The results show that excess ligands impact precursor reactivity, and consequently the nanocrystal growth rate, thus deterministically dictating the resulting crystal structure. Moreover, it is demonstrated that the strong ultraviolet-visible extinction peak exhibited by AgFeS₂ nanocrystals is a consequence of a quasi-static dielectric resonance (DR), analogous to the optical response observed in CuFeS₂ nanocrystals. Spectroscopic studies and computational calculations confirm that a negative permittivity at ultraviolet/visible frequencies arises due to the electronic structure of these intermediate-band (IB) semiconductor nanocrystals, resulting in a DR consisting of resonant valence-band-to-intermediate-band excitations, as opposed to the well-known localized surface plasmon resonance response typically observed in metallic nanostructures. Overall, these results expand the current library of an underexplored class of IB semiconductors with unique optical properties, and also enrich the understanding of DRs in ternary metal-iron-sulfide nanomaterials.

Wide color gamut white light-emitting diodes based on two-dimensional semiconductor nanoplatelets

II-VI colloidal semiconductor nanoplatelets (NPLs) are a kind of two-dimensional nanomaterial with uniform thickness at the atomic scale, thus leading to the characteristics of tunable emission wavelength and narrow bandwidth. Here, we report wide color gamut white light-emitting diodes (WLEDs) based on high-performance CdSe-based heterostructure NPLs. The narrow-band CdSe/CdS core/crown and CdSe/ZnCdS core/shell NPLs are chosen as green (∼521 nm) and red (∼653 nm) luminescent materials, respectively. They represent excellent PL properties, such as narrow linewidth, high quantum yields, and high photostability. Importantly, the further fabricated NPL-WLEDs exhibits an ultrawide color gamut covering up ∼141.7% of the NTSC standard in the CIE 1931 color space and excellent stability towards driving currents. These outstanding device performances indicate that the colloidal semiconductor NPLs possess huge potentiality to achieve higher color saturation and wide color gamut for applications in new-generation lightings and displays.

Tunable structural and optical properties of AgxCuyInS2 colloidal quantum dots

Herein is discussed tunable absorption and emission properties of AgxCuyInS2 quantum dots involving wurtzite to chalcopyrite phase transformation dependent on Ag+ dopant content.

Deciphering Ultrafast Carrier Dynamics of Eco-Friendly ZnSeTe-Based Quantum Dots: Toward High-Quality Blue-Green Emitters

Developing non-toxic and high-performance colloidal semiconductor quantum dots (CQDs) represents the inevitable route toward CQD-enabled technologies. Herein, the spectral and dynamic properties of heavy-metal-free ZnSeTe-based CQDs are investigated by transient absorption spectroscopy and theoretical modeling. We for the first time decode the ultrafast hot carrier trapping (<2 ps) and band-edge carrier trapping processes (∼6 ps) in the CQD system, which plagues the emission performance. The ZnSe/ZnSeS/ZnS shell engineering greatly suppresses the non-radiative trapping process and results in a high photoluminescence quantum yield of 88%. We demonstrate that the core/shell nano-heterostructure forms the quasi-type II configuration, in contrast to the presumed type I counterpart. Moreover, the Auger recombination and hot carrier cooling processes are revealed to be ∼454-405 ps and 160-370 fs, respectively, and their relationship with the composition in the spectral range of 470-525 nm is clarified. The above merits render these ZnSeTe CQDs as outstanding blue-green emitters for optoelectronic applications, exemplified by the white light-emitting diodes.

Quantum Dot Photocatalysts for Organic Transformations

Quantum dots (QDs) with tunable photo-optical properties and colloidal nature are ideal for a wide range of photocatalytic reactions. In particular, QD photocatalysts for organic transformations can provide new and effective synthetic routes to high value-added molecules under mild conditions. In this Perspective, we discuss the advances of employing QDs for visible-light-driven organic transformations categorized into net reductive reactions, net oxidative reactions, and redox neutral reactions. We then provide our outlook for potential future directions in the field: nanostructure engineering to improve charge separation efficiencies, ligand shell engineering to optimize overall catalyst performance, in situ comprehensive studies to delineate underlying reaction mechanisms, and laboratory automation with the assistance of modern computing techniques to revolutionize the reaction optimization process.

Comparative Study of Red/Green/Blue Quantum-Dot Light-Emitting Diodes by Time-Resolved Transient Electroluminescence

To understand the electronic processes in quantum-dot light-emitting diodes (QLEDs), a comparative study was performed by time-resolved transient electroluminescence (TREL). We fabricated red, green, and blue (R-, G-, and B-) QLEDs with poly(9,9-dioctylfluorene-co-N-(4-sec-butylphenyl)diphenylamine) as the hole-transporting layer with conventional structures. The external quantum efficiency (EQE) and current efficiency were 19.2% and 22.7 cd A^-1 for R-QLEDs, 21.1% and 93.3 cd A^-1 for G-QLEDs, and 10.6% and 10.4 cd A^-1 for B-QLEDs, respectively. The TREL results for B-QLEDs were remarkably different from those for R- and G-QLEDs because of the insufficient electron injection crossing the type II heterojunction between the emission layer and the electron-transporting layer. We further applied poly(N-vinylcarbazole) as the hole-transporting layer and obtained much better performance for B-QLEDs, with EQE and current efficiency of 15.9% and 15.4 cd A^-1, respectively. Concomitant with the increase in EQE are an increase in the turn-on voltage from 2.3 to 3.7 V and a transient electroluminescence spike after voltage turn-off.

The Preparation of CuInS2-ZnS-Glutathione Quantum Dots and Their Application on the Sensitive Determination of Cytochrome c and Imaging of HeLa Cells

Cytochrome c (Cyt c), one of the most significant proteins acting as an electron transporter, plays an important role during the transferring process of the energy in cells. Apoptosis, one of the major forms of cell death, has been associated with various physiological regularity and pathological mechanisms. It was found that Cyt c can be released from mitochondria to cytosol under different pathological conditions, triggering subsequent cell apoptosis. Herein, we developed a fluorescence nanoprobe based on negatively charged CuInS₂-ZnS-GSH quantum dots (QDs) for the sensitive determination of Cyt c. CuInS₂-ZnS-GSH QDs with high photochemical stability and favorable hydrophilicity were prepared by a simple hot reflux method and emit a bright orange-red light. The electron-deficient heme group in Cyt c is affiliated with the electron-rich CuInS₂-ZnS-GSH QDs through the photo-induced electron transfer process, resulting in a large decrease in fluorescence intensity of QDs. A good linearity for concentration of Cyt c in the range of 0.01-7 μmol L^-1 is obtained, and the detection limit of Cyt c is as low as 1.1 nM. The performance on the detection of Cyt c in spiked human serum and fetal bovine serum samples showed good recoveries from 85.5% to 95.0%. Furthermore, CuInS₂-ZnS-GSH QDs were applied for the intracellular imaging in HeLa cells showing an extremely lower toxicity and excellent biocompatibility.

Preparation of Graphene oxide- CuInS2/ZnS Quantum dots Nanocomposite as "Turn-On" Fluorescent Probe for the Detection of Polycyclic Aromatic Hydrocarbons in Aqueous Medium

Graphene oxide is well known for its adsorption properties with aromatic compounds. In this study, graphene oxide and eco-friendly ternary CuInS₂/ZnS QDs were used to prepare graphene oxide-qunatum dots (GO-QDs) nanocomposite via in-situ method. The composite was characterized using ultraviolet-visible (UV-Vis) spectroscopy, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectroscopy. The effect of the polycyclic aromatic hydrocarbons (PAHs) on the PL properties of the nanocomposite was investigated. The results showed that the addition of PAHs increased the PL intensity of the nanocomposite. This "turn-on" fluorescence approach can be used for the successful detection of PAHs in aqueous media.

Tapered Mach-Zehnder interferometer based on PbS quantum dots modified by polymers for copper ion sensing

An optical fiber interferometer coated with PbS quantum dots (QDs) was developed for copper ion (${{\rm{Cu}}^{2 +}}$) detection. The QDs were modified by a multifunctional copolymer that enabled QD surface ligation, dispersion, and coordination with ${{\rm{Cu}}^{2 +}}$. ${{\rm{Cu}}^{2 +}}$ coordination with the polymer induced changes in the surrounding refractive index of the interferometer. The sensor was highly selective for ${{\rm{Cu}}^{2 +}}$ and showed a linear detection range of 0-1000 µM with a limit of detection of 2.20 µM in both aqueous and biological solutions.

Dithiocarbamate Complexes as Single Source Precursors to Nanoscale Binary, Ternary and Quaternary Metal Sulfides

Nanodimensional metal sulfides are a developing class of low-cost materials with potential applications in areas as wide-ranging as energy storage, electrocatalysis, and imaging. An attractive synthetic strategy, which allows careful control over stoichiometry, is the single source precursor (SSP) approach in which well-defined molecular species containing preformed metal-sulfur bonds are heated to decomposition, either in the vapor or solution phase, resulting in facile loss of organics and formation of nanodimensional metal sulfides. By careful control of the precursor, the decomposition environment and addition of surfactants, this approach affords a range of nanocrystalline materials from a library of precursors. Dithiocarbamates (DTCs) are monoanionic chelating ligands that have been known for over a century and find applications in agriculture, medicine, and materials science. They are easily prepared from nontoxic secondary and primary amines and form stable complexes with all elements. Since pioneering work in the late 1980s, the use of DTC complexes as SSPs to a wide range of binary, ternary, and multinary sulfides has been extensively documented. This review maps these developments, from the formation of thin films, often comprised of embedded nanocrystals, to quantum dots coated with organic ligands or shelled by other metal sulfides that show high photoluminescence quantum yields, and a range of other nanomaterials in which both the phase and morphology of the nanocrystals can be engineered, allowing fine-tuning of technologically important physical properties, thus opening up a myriad of potential applications.

Near-Unity Photoluminescence Quantum Yield and Highly Suppressed Blinking in a Toxic-Metal-Free Quantum Dot

There is no literature report of simultaneously achieving near-unity PLQY (ensemble level) and highly suppressed blinking (ultrasensitive single-particle spectroscopy (SPS) level) in a toxic-metal-free QD. In this Letter we report accomplishing near-unity PLQY (96%) and highly suppressed blinking (>80% ON fraction) in a toxic-metal-free CuInS₂/ZnSeS Core/Alloy-Shell (CAS) QD. In addition, (i) gigantic enhancement of PLQY (from 15% (Core) to 96% (CAS QD)), (ii) ultrahigh stability over 1 year without significant reduction of PLQY at the ensemble level, (iii) high magnitude (nearly 3 times) of electron detrapping/trapping rate, and (iv) very long ON duration (∼2 min) without blinking at the SPS level enable this ultrasmall (∼3.3 nm) CAS QD to be quite suitable for single-particle tracking/bioimaging. A model explaining all these excellent optical properties has been provided. This ultrabright CAS QD has been successfully utilized toward fabrication of low-cost microcontroller-based stable and bright yellow and white QD-LEDs.

Controllable synthesis of non-layered two-dimensional plate-like CuGaSe2 materials for optoelectronic devices

CuGaSe2 semiconductor materials, as an important member of the I-III-VI2 family, have sparked tremendous attention due to their fascinating structure-related properties and promising applications in solar energy storage and conversion. Nevertheless, the controllable preparation of two-dimensional (2D) CuGaSe2 structures is still a daunting challenge owing to the intrinsic non-layered crystal structure and inaccessible reactivity-matching of multiple reaction precursors, which will seriously impede the much deeper research progress on their properties and applications. Herein, non-layered 2D CuGaSe2 plates possessing high crystallinity, and uniform size and morphology have been first synthesized by a feasible cation exchange strategy. Because the fabrication of 2D CuGaSe2 crystals is rarely reported, a particular highlight is laid on the compositional analysis, structural characterization, and formation mechanism. Furthermore, the optical absorption and optoelectronic measurements reveal that the as-synthesized CuGaSe2 plates exhibit high light harvesting capacity and excellent photoelectric performance. This study opens up a new avenue for the feasible fabrication of non-layered CuGaSe2 plates possessing a high-quality crystalline structure and provides a promising candidate for the development of novel solar energy conversion and storage devices.

FRET-Based Analysis of AgInS2/ZnAgInS/ZnS Quantum Dot Recombination Dynamics

Ternary quantum dots (QDs) are very promising nanomaterials with a range of potential applications in photovoltaics, light-emitting devices, and biomedicine. Despite quite intensive studies of ternary QDs over the last years, the specific relaxation channels involved in their emission mechanisms are still poorly understood, particularly in the corresponding core-shell nanostructures. In the present work, we have studied the recombination pathways of AgInS₂ QDs stabilized with the ZnAgInS alloy layer and the ZnS shell (AIS/ZAIS/ZnS QDs) using time-resolved fluorescence spectroscopy. We have also investigated FRET in complexes of AIS/ZAIS/ZnS QDs and cyanine dyes with the absorption bands overlapping in the different regions of the QD emission spectrum, which allowed us to selectively quench the radiative transitions of the QDs. Our studies have demonstrated that FRET from QDs to dyes results in decreasing of all QD PL decay components with the shortest lifetime decreasing the most and the longest one decreasing the least. This research presents important approaches for the investigation of ternary QD luminescence mechanisms by the selective quenching of recombination pathways. These studies are also essential for potential applications of ternary QDs in photodynamic therapy, multiplex analysis, and time-resolved FRET sensing.

Improving power conversion efficiency in luminescent solar concentrators using nanoparticle fluorescence and scattering

Large-size luminescent solar concentrators (LSCs), which act as a complement to silicon-based photovoltaic (Si-PV) systems, still suffer from low power conversion efficiency (PCE). How to improve the performance of LSCs, especially large ones, is currently a hot research topic. Traditional LSCs have only a single transmission mode of fluorescence from the luminescent materials to the Si-PV, but here we introduce a new idea to improve the absorption of Si-PV by employing dual transmission modes of both fluorescence and scattering light. To prepare LSCs with dual mode transmission, Si-PV systems are coupled around the edges of a light-harvesting slice, which is prepared by ultraviolet light-induced polymerization of methyl methacrylate (MMA) solution containing both luminescent CsPbBr₃ and TiO₂ nanocrystals (NCs). When the sun light or incident light is coupled into the light-harvesting slice, CsPbBr₃ NCs can convert the incident light into fluorescence, and then partly transmit to Si-PV at the edges, where the light is finally converted into electrical energy. Besides the traditional fluorescence transmission mode, the addition of TiO₂ brings another transmission mode, namely the scattering of incident light to Si-PV, leading to an increase in PCE. In comparison to that of pure CsPbBr₃-based LSCs without the addition of TiO₂ (0.97%), the PCE of TiO₂-doped LSCs with a large size of 20 cm × 20 cm is improved to 1.82%. The maximal PCE appears for LSCs with a size of 5 cm × 5 cm, reaching 2.62%. The reported method of dual transmission modes is a new alternative way to improve the performance of LSC devices, which does not need to change the optical properties of luminescent materials. Moreover, the production process is simple, low-cost and suitable for preparing large area LSCs, further promoting the application of LSCs.

Lock-in Ultrafast Electron Microscopy Simultaneously Visualizes Carrier Recombination and Interface-Mediated Trapping

Visualizing charge carrier flow over interfaces or near surfaces meets great challenges concerning resolution and vastly different time scales of bulk and surface dynamics. Ultrafast or four-dimensional scanning electron microscopy (USEM) using a laser pump electron probe scheme circumvents the optical diffraction limit, but disentangling surface-mediated trapping and ultrafast carrier dynamics in a single measurement scheme has not yet been demonstrated. Here, we present lock-in USEM, which simultaneously visualizes fast bulk recombination and slow trapping. As a proof of concept, we show that the surface termination on GaAs, i.e., Ga or As, profoundly influences ultrafast movies. We demonstrate the differences can be attributed to trapping-induced surface voltages of approximately 100-200 mV, which is further supported by secondary electron particle tracing calculations. The simultaneous visualization of both competing processes opens new perspectives for studying carrier transport in layered, nanostructured, and two-dimensional semiconductors, where carrier trapping constitutes a major bottleneck for device efficiency.

Simple Synthesis of CuInS2/ZnS Core/Shell Quantum Dots for White Light-Emitting Diodes

In this study, the CuInS2/ZnS core/shell quantum dots (QDs) were prepared via simple and environmentally friendly solvothermal synthesis and were used as phosphors for white light-emitting diodes (WLEDs). The surface defect of the CuInS2 core QDs were passivated by the ZnS shell by forming CuInS2/ZnS core/shell QDs. By adjusting the Cu/In ratio and the nucleation temperature, the photoluminescence (PL) peak of the CuInS2 QDs was tunable in a range of 651-775 nm. After coating the ZnS layer and modifying oleic acid ligands, the PL quantum yield increased to 85.06%. The CuInS2/ZnS QD powder thermal stability results showed that the PL intensity of the QDs remained 91% at 100°C for 10 min. High color rendering index values (CRI, 90) and correlated color temperature of 4360 K for the efficient WLEDs were fabricated using CuInS2/ZnS QDs and (Ba,Sr)2SiO4:Eu2+ as color converters in combination with a blue GaN light-emitting diode chip.

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.

Flexible translucent chitosan-glycerin/QD nanocomposite glue for anti-counterfeiting films with strong adhesion and stability

With the rapid development of commodity circulation, more attention has been paid to the anticounterfeiting technology of commodities, including stability, universality and ease of distinguishing. The authors report the use of gelatin-chitosan-glycerin/QD nanocomposite-functionalized glue for luminescent anti-counterfeiting labels. As the blend and plasticizer, the addition of chitosan and glycerin effectively improved the flexibility and formability of the gelatin-chitosan-glycerin/QD composite films, which show excellent mechanical properties, including high transparency, luminescence and flexibility, and they are easy to prepare on a large scale, providing certain reference values for new anticounterfeiting technology applying a variety of morphologies.

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.

High-Brightness Blue InP Quantum Dot-Based Electroluminescent Devices: The Role of Shell Thickness

InP quantum dots (QDs) are considered as one of the most promising candidates of Cd- or Pb-based QDs in the applications of display and lighting. However, the performances of blue InP QDs and the corresponding light emitting devices (LEDs) are far inferior to those of their red and green counterparts, which strongly limits the development of InP QD based LEDs (QLEDs) technology. Here, high quantum yield (∼81%) and large size (∼7.0 ± 0.9 nm) InP/GaP/ZnS//ZnS QDs with a thick shell have been successfully synthesized by a shell engineering approach, and the corresponding QLEDs exhibit a record brightness and external quantum efficiency of 3120 cd·m^-2 and 1.01%, respectively. Large-scale density functional theory calculations on thousands-of-atoms QDs indicate that thicker-shell ones favor a more balanced carrier injection in the QD film and simultaneously suppress the FRET between closely packed QDs, which collectively contribute to the improved blue device performances.

A fluorometric optical fiber nanoprobe for copper(II) by using AgInZnS quantum dots

An optical fiber nanoprobe is presented for fluorometric determination of copper(II). The method based on the use of water-dispersible AgInZnS quantum dots (QDs) deposited at the end of an optical fiber in a poly(vinyl alcohol) matrix. The fluorescnece of the QDs, best measured at excitation/emisssion wavelengths of 365/570 nm, is quenched by Cu(II) due to both static and electron transfer from the QDs to Cu(II). This is experimentally confirmed by photoluminescence and UV-vis absorption spectra, and measurement of luminescence lifetimes. The probe is highly selective and possesses a linear detection range that extends from 2.5 to 800 nM. Graphical abstractSchematic representation of an optical fiber nanoprobe based on hydrophilic AgInZnS quantum dots for fluorometric determination of copper(II). The fluorescence is quenched by Cu(II) due to static quenching and dynamic quenching. It has a detection range of 2.5-800 nM.

Al/Zn co-incorporated Cu–In–Se quantum dots for high efficiency quantum dot sensitized solar cells

A facile method for synthesizing high-quality Cu–In–Se quantum dots (QDs) was developed by Al/Zn co-incorporation. Benefiting from the reduction of trap-state defects in QDs, the efficiency of solar cells basing prepared QDs is obviously improved.

Unique Luminescence of Hexagonal Dominant Colloidal Copper Indium Sulphide Quantum Dots in Dispersed Solutions

Luminescent hexagonal dominant copper indium sulphide (h-dominant CIS) quantum dots (QDs) by precursor-injection of mixed metal-dialkyldithiocarbamate precursors. Owing to the different reactivity of the precursors, this method allowed the CIS QDs to grow while retaining the crystallinity of the hexagonal nucleus. The photoluminescence (PL) spectra exhibited dual emission (600–700 nm red emission and 700–800 nm NIR emission) resulting from the combined contributions of the hexagonal (wurtzite) h-CIS and tetragonal (chalcopyrite) t-CIS QDs, i.e. the NIR and red emissions were due to the h-CIS QDs and coexisting t-CIS QDs (weight ratio of h-CIS/t-CIS ~ 10), respectively. The PL intensities of the h-CIS as well as t-CIS QDs were enhanced by post-synthetic heat treatment; the t-CIS QDs were particularly sensitive to the heat treatment. By separating h-CIS and t-CIS successfully, it was demonstrated that this phenomenon was not affected by size and composition but by the donor-acceptor pair states and defect concentration originating from their crystal structure. The h-dominant CIS QDs in this work provide a new technique to control the optical property of Cu-In-S ternary NCs.

Synthesis of Alloyed ZnSeTe Quantum Dots as Bright, Color-Pure Blue Emitters

Considering a strict global environmental regulation, fluorescent quantum dots (QDs) as key visible emitters in the next-generation display field should be compositionally non-Cd. When compared to green and red emitters obtainable from size-controlled InP QDs, development of non-Cd blue QDs remains stagnant. Herein, we explore the synthesis of non-Cd, ZnSe-based QDs with binary and ternary compositions toward blue photoluminescence (PL). First, the size increment of binary ZnSe QDs is attempted by a multiply repeated growth until blue PL is attained. Although this approach offers a relevant blue color, excessively large-sized ZnSe QDs inevitably entail a low PL quantum yield. As an alternative strategy to the above size enlargement, the alloying of high-band gap ZnSe with lower-band gap ZnTe in QD synthesis is carried out. These alloyed ternary ZnSeTe QDs after ZnS shelling exhibit a systematically tunable PL of 422-500 nm as a function of Te/Se ratio. Analogous to the state-of-the-art heterostructure of InP QDs with a double-shelling scheme, an inner shell of ZnSe is newly inserted with different thicknesses prior to an outer shell of ZnS, where the effects of the thickness of ZnSe inner shell on PL properties are examined. Double-shelled ZnSeTe/ZnSe/ZnS QDs with an optimal thickness of the ZnSe inner shell are then employed for all-solution-processed fabrication of a blue QD light-emitting diode (QLED). The present blue QLED as the first ZnSeTe QD-based device yields a peak luminance of 1195 cd/m², a current efficiency of 2.4 cd/A, and an external quantum efficiency of 4.2%, corresponding to the record values reported from non-Cd blue devices.

Ternary Metal Chalcogenides: Into the Exciton and Biexciton Dynamics

Intra-band-gap state-induced low-toxicity colloidal I-III-VI ternary metal chalcogenide nanocrystals (NCs) have emerged as promising alternatives to the toxic Cd- and Pb-chalcogenides for different optoelectronic and bioimaging applications. In this Perspective, we provide the primary understanding of the intra-band-gap state-induced photoluminescence (PL) of I-III-VI NCs, specifically CuInS₂ and AgInS₂, as a function of particle size and composition and correlated with time-resolved PL measurements. The intra-band-gap state-induced ultrafast exciton and biexciton dynamics are discussed in detail to unravel the subpicosecond carrier relaxation dynamics through transient absorption measurement. Furthermore, ultrafast dissociation of the biexciton on Au@CuInS₂ hybrid NCs has been revealed to be due to the presence of Au, which has direct relevance to the improvement of the solar cell efficiency. The proper fundamental insight of the ultrafast exciton and biexciton dynamics of these materials will enable utilization of ternary metal chalcogenides in photovoltaic as well as light-emitting devices.

Direct Observation of Surface Polarons in Capped CuInS2 Quantum Dots by Ultrafast Pump-Probe Spectroscopies

Semiconductor nanocrystals are mostly prepared by colloid chemistry with organic surfactant molecules, and their surface polarization effect on the carrier relaxations are critical to their optoelectronic applications. Until now, the surface polarization effect and detailed photophysical processes of these capped quantum dots (QDs) are still unclear. Here, we studied the dynamics of the photoinduced carriers and capping molecule  vibrations of capped CuInS₂ quantum dots by using the femtosecond pump-probe system in both visible and IR zones. It is identified that the capping  molecular vibrations exhibit significant Fermion bleaching nature, whose relaxation profile is in good agreement with the radiative recombination dynamics of QDs in the visible region. These results demonstrate that the extrinsic surface polarons form by the coupling of photoinduced carriers and surface ligand vibrations, and take part in the photophysical processes of these QDs. This finding is helpful for the QD design and applications in photoelectronic devices.

Emission Enhancement of Cu-Doped InP Quantum Dots through Double Shelling Scheme

The doping of transition metal ions, such as Cu⁺ and Mn²⁺ into a quantum dot (QD) host is one of the useful strategies in tuning its photoluminescence (PL). This study reports on a two-step synthesis of Cu-doped InP QDs double-shelled with ZnSe inner shell/ZnS outer shell. As a consequence of the double shelling-associated effective surface passivation along with optimal doping concentrations, Cu-doped InP/ZnSe/ZnS (InP:Cu/ZnSe/ZnS) QDs yield single Cu dopant-related emissions with high PL quantum yields of 57–58%. This study further attempted to tune PL of Cu-doped QDs through the variation of InP core size, which was implemented by adopting different types of Zn halide used in core synthesis. As the first application of doped InP QDs as electroluminescent (EL) emitters, two representative InP:Cu/ZnSe/ZnS QDs with different Cu concentrations were then employed as active emitting layers of all-solution-processed, multilayered QD-light-emitting diodes (QLEDs) with the state-of-the-art hybrid combination of organic hole transport layer plus inorganic electron transport layers. The EL performances, such as luminance and efficiencies of the resulting QLEDs with different Cu doping concentrations, were compared and discussed.

Quantum-confined stark effect in the ensemble of phase-pure CdSe/CdS quantum dots

Colloidal semiconductor quantum dots (QDs) have recently attracted great attention in electric field sensing via the quantum-confined Stark effect (QCSE), but they suffer from the random local electric field around the charged QDs through the Auger process or defect traps. Here, QCSE in the ensemble of phase-pure wurtzite CdSe/CdS QDs was studied by applying a uniform external electric field. We observed clear field-dependent photoluminescence (PL) and absorption characteristics in thick-shell CdSe/CdS QDs with 11 CdS monolayers (11 MLs) including a pronounced spectral redshift in PL of ∼2.3 nm and absorption of ∼2.1 nm. The time-dependent PL intensity traces implied that the thick-shell QDs were conducive to realize the Stark shift in QD ensembles due to the effective suppression of the main sources of the local field. These findings were in stark contrast to those of moderate-shell (5 MLs) and ultrathick-shell (15 MLs) CdSe/CdS QDs. The measurement value of exciton polarizability was smaller than the theoretical value, which may be influenced by very few exciton traps. Moreover, the amplified stimulated emission also exhibited obvious optical modulations under the electric field with decreased emission intensity and an increased ultrafast lifetime. Finally, the temporal evolution of the multiexciton process in thick-shell CdSe/CdS QDs indicated that the multiexciton state induced a higher energy state near the band edge, which may weaken the QCSE of a single exciton. Therefore, it was demonstrated that efficient field control over the optical properties of these nanomaterials is feasible and this can open up potential applications in field-controlled electro-optic modulators.

Bright Quantum Dot Light-Emitting Diodes Enabled by Imprinted Speckle Image Holography Nanostructures

Super-bright all-solution-processed quantum dot light-emitting diodes (QLEDs) with an inverted structure are achieved by imprinting speckle image holography (SIH) structures inside the devices. QLEDs with imprinted random grating structures can reach a luminance of up to 146 000 Cd/m² at driving voltage of 8 V, which is 1.76 times higher than the value of control devices with planar architecture, setting a new brightness record for all-solution-processed inverted red QLEDs. The luminous power efficiency and external quantum efficiency of the QLEDs with imprinted structures are 1.8 and 1.65 times higher to those of the control devices, respectively. Further optical simulation results reveal that not only can the structure help extract the trapped internal photon energy but also the mechanical pressure during the imprinting process plays a crucial role in improving the device performance.

Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins

Colloidal nanocrystals of ternary I-III-VI₂ semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential has been hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, we provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I-III-VI₂ nanocrystals. From our analysis, we conclude that the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, we highlight the remaining open questions and propose experiments to address them.