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Colloidal quantum dots are promising emitters for quantum-dot-based light-emitting-diodes. Though quantum dots have been synthesized with efficient, stable, and high colour-purity photoluminescence, inheriting their superior luminescent properties in light-emitting-diodes remains challenging. This is commonly attributed to unbalanced charge injection and/or interfacial exciton quenching in the devices. Here, a general but previously overlooked degradation channel in light-emitting-diodes, i.e., operando electrochemical reactions of surface ligands with injected charge carriers, is identified. We develop a strategy of applying electrochemically-inert ligands to quantum dots with excellent luminescent properties to bridge their photoluminescence-electroluminescence gap. This material-design principle is general for boosting electroluminescence efficiency and lifetime of the light-emitting-diodes, resulting in record-long operational lifetimes for both red-emitting light-emitting-diodes (T 95  > 3800 h at 1000 cd m −2 ) and blue-emitting light-emitting-diodes (T 50  > 10,000 h at 100 cd m −2 ). Our study provides a critical guideline for the quantum dots to be used in optoelectronic and electronic devices. New design principles for bridging the photoluminescence and electroluminescence of colloidal quantum dots are needed. In this work, the authors report electrochemically-inert ligands as a general material-design strategy for realizing high-performance LEDs based on quantum dots.

Photonic quantum information requires high-purity, easily accessible, and scalable single-photon sources. Here, we report an electrically driven single-photon source based on colloidal quantum dots. Our solution-processed devices consist of isolated CdSe/CdS core/shell quantum dots sparsely buried in an insulating layer that is sandwiched between electron-transport and hole-transport layers. The devices generate single photons with near-optimal antibunching at room temperature, i.e., with a second-order temporal correlation function at zero delay ( g (2) (0)) being <0.05 for the best devices without any spectral filtering or background correction. The optimal g (2) (0) from single-dot electroluminescence breaks the lower g (2) (0) limit of the corresponding single-dot photoluminescence. Such highly suppressed multi-photon-emission probability is attributed to both novel device design and carrier injection/recombination dynamics. The device structure prevents background electroluminescence while offering efficient single-dot electroluminescence. A quantitative model is developed to illustrate the carrier injection/recombination dynamics of single-dot electroluminescence. Single-photon sources are one of the most basic devices for quantum optical experiments and applications. Here, Lin et al. present an electrically driven single-photon source based on solution-processed colloidal quantum dots with near-optimal antibunching at room temperature.

This work studies extinction properties of ZnSe quantum dots terminated with either Se-surface or Zn-surface (Se-ZnSe or Zn-ZnSe QDs). In addition to commonly observed photoluminescence quenching by anionic surface sites, Se-ZnSe QDs are found to show drastic signatures of Se-surface states in their UV-visible (Vis) absorption spectra. Similar to most QDs reported in literature, monodisperse Zn-ZnSe QDs show sharp absorption features and blue-shifted yet steep absorption edge respect to the bulk bandgap. However, for monodisperse Se-ZnSe QDs, all absorption features are smeared and a low-energy tail is identified to extend to an energy window below the bulk ZnSe bandgap. Along increasing their size, a cyclic growth of ZnSe QDs switches their surface from Zn-terminated to Se-terminated ones, which confirms that the specific absorption signatures are reproducibly repeated between those of two types of the QDs. Though the extinction coefficients per unit of Se-ZnSe QDs are always larger than those of Zn-ZnSe QDs with the same size, both of them approach the same bulk limit. In addition to contribution of the lattice, extinction coefficients per nanocrystal of Zn-ZnSe QDs show an exponential term against their sizes, which is expected for quantum-confinement enhancement of electron-hole wavefunction overlapping. For Se-ZnSe QDs, there is the third term identified for their extinction coefficients per nanocrystal, which is proportional to the square of size of the QDs and consistent with surface contribution.

Against general wisdom in crystallization, the nucleation of InP and III-V quantum dots （QDs） often dominates their growth. Systematic studies on InP QDs identified the key reason for this： the dense and tight alkanoate-ligand shell around each nanocrystal. Different strategies were explored to enable necessary ligand dynamics--i.e., ligands rapidly switching between being bonded to and detached from a nanocrystal upon thermal agitation--on nanocrystals to simultaneously retain colloidal stability and allow appreciable growth. Among all the surface-activation reagents tested, 2,4-diketones （such as acetylacetone） allowed the full growth of InP QDs with indium alkanoates and trimethylsilylphosphine as precursors. While small fatty acids （such as acetic acid） were partially active, common neutral ligands （such as fatty amines, organophosphines, and phosphine oxides） showed limited activation effects. The existing amine-based synthesis of InP QDs was activated by acetic acid formed in situ. Surface activation with common precursors enabled the growth of InP QDs with a distinguishable absorption peak between ~450 and 650 nm at mild temperatures （140-180 ~C）. Furthermore, surface activation was generally applicable for InAs and III-V based core/shell QDs.

Alkanoate-coated CdSe/CdS core/shell quantum dots (QDs) with near-unity photoluminescence (PL) quantum yield and mono-exponential PL decay dynamics are applied for studying quasi-stationary charge transfer from photo-excited QDs to quinone derivatives physically-adsorbed within the ligand monolayer of a QD. Though PL quenching efficiency due to electron transfer can be up to > 80%, transient PL and transient absorption spectra reveal that the charge transfer rate ranges from single-digit nanoseconds to sub-nanoseconds, which is ∼ 3 orders of magnitude slower than that of static charge transfer and ∼ 2 orders of magnitude faster than that of collisional charge transfer. The physically-adsorbed acceptors can slowly (500–1,000 min dependent on the size of the quinone derivatives) desorb from the ligand monolayer after removal of the free acceptors. Contrary to collisional charge transfer, the efficiency of quasi-stationary charge transfer increases as the ligand length increases by providing additional adsorption compartments in the elongated hydrocarbon chain region. Because ligand monolayer commonly exists for a typical colloidal nanocrystal, the quasi-stationary charge transfer uncovered here would likely play an important role when colloidal nanocrystals are involved in photocatalysis, photovoltaic devices, and other applications related to photo-excitation.

A one-pot/three-step synthetic scheme was developed for phase-pure epitaxy of CdS shells on zinc-blende CdSe nanocrystals to yield shells with up to sixteen monolayers. The key parameters for the epitaxy were identified, including the core nanocrystal concentration, solvent type/composition, quality of the core nanocrystals, epitaxial growth temperature, type/concentration of ligands, and composition of the precursors. Most of these key parameters were not influential when the synthetic goal was thin-shell CdSe/CdS core/shell nanocrystals. The finalized synthetic scheme was reproducible at an almost quantitative level in terms of the crystal structure, shell thickness, and optical properties.

The diffusion-controlled growth mode is widely used to narrow the size distribution of colloidal quantum dots. However, this growth mode always suffers from size broadening at the later growth stage. By monitoring the growth process of CdS colloidal quantum dots, we show the size broadening is a result of different growth rates of CdS colloidal quantum dots (CQDs) with different morphologies. Monomer concentration-dependent growth experiments demonstrate the different growth rates are caused by the different ligand permeabilities of CdS CQDs. The cubic ones have lower ligand permeability but higher saturated surface reaction rate than the noncubic ones, leading to unexpected narrower size distribution under higher monomer concentration. More efficient narrowing can be obtained by the addition of chloride ions, which can increase the ligand permeability of all CdS CQDs, as well as the opposite discrepancies in ligand permeability and surface reaction between cubic and noncubic CdS CQDs. The photoluminescence (PL) full width at half maximum (FWHM) of CdS CQDs can be narrowed down to below 80 meV for PL peaks from 430 to 500 nm. Given the inevitable usage of the ligands in the solution synthesis of colloidal nanocrystals, the influence of morphology difference on growth rate should be common. Our results can provide an alternative solution to realize size focusing for the synthesis of colloidal nanocrystals.

The impact of alkali metal carboxylates on the synthesis of colloidal quantum dots (CQDs) was investigated. Through a ligand removal experiment, we demonstrated that due to its high hydrophilic nature, sodium oleate dispersed in n-octadecene (ODE) with the formation of micelles with the help of other polar molecules, which resulted in reduced concentration of oleic acid and cadmium oleate both in the solution and on the surface of CQDs. These effects allow for control the size of CdSe CQDs in a wide range when synthesizing them by solely changing the amount of sodium oleate, under either cation-rich or anion-rich conditions. Additionally, enhanced ligand dynamics promote morphology transformation and suppress size deviation caused by different morphologies’ existence in CQDs synthesis. Alkali metal oleate not only stabilized anion-rich CdSe CQDs but also results in highly crystallized wurtzite structure of CdSe CQDs when synthesizing them with excess anions. Furthermore, under anion-rich synthetic condition, anisotropic growth can be realized, leading to nanorods and nanoplatelets based on the alkali metal ions used. Given their outstanding effects and widely applicable synthetic conditions, alkali metal carboxylates offer new possibilities for designing efficient methods for synthesizing CQDs.

A suspension of fine selenium powder (100 or 200 mesh) in octadecene (Se-SUS) has proven to be a high-performance, versatile, convenient, reproducible, yet green selenium precursor. The advantages of Se-SUS arise from its highly reactive chemical nature and flexibility. These two features made it possible to carry out the synthesis of high quality metal selenide nanocrystals with diverse compositions and structures, including binary, core/shell, transition metal doped, and complex composition nanocrystals. These successes further demonstrated that Se-SUS is a powerful Se precursor for solving a few long-standing challenges in the synthesis of high quality selenide nanocrystals. For instance, Se-SUS was successfully employed as a Se precursor for shell growth in high quality core/shell nanocrystals to replace expensive and highly toxic precursors, such as Se-phosphine and bis-trimethylsilyl selenide, with greatly lowered epitaxial temperatures (as low as 150 °C) to avoid alloying. As another example, Se-SUS enabled “co-nucleation doping” as a means of preparing high quality Mn doped ZnSe nanocrystals with pure, stable, and highly efficient dopant fluorescence.

Water pollution has always been a serious problem across the world; therefore, facile pollutant degradation via light irradiation has been an attractive issue in the field of environmental protection. In this study, a type of Zn-based metal–organic framework (ZIF−8)-wrapped BiVO4 nanorod (BiVO4@ZIF−8) with high efficiency for photocatalytic wastewater treatment was synthesized through a two-step hydrothermal method. The heterojunction structure of BiVO4@ZIF−8 was confirmed by morphology characterization. Due to the introduction of mesoporous ZIF−8, the specific surface area reached up to 304.5 m2/g, which was hundreds of times larger than that of pure BiVO4 nanorods. Furthermore, the band gap of BiVO4@ZIF−8 was narrowed down to 2.35 eV, which enabled its more efficient utilization of visible light. After irradiation under visible light for about 40 min, about 80% of rhodamine B (RhB) was degraded, which was much faster than using pure BiVO4 or other BiVO4-based photocatalysts. The synergistic photocatalysis mechanism of BiVO4@ZIF−8 is also discussed. This study might offer new pathways for effective degradation of wastewater through facile design of novel photocatalysts.