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As one of Cd-free semiconductor nanocrystals, Zn-based nanocrystals are widely studied due to their wide band gap to be a candidate for blue- and green-emitting materials. Herein, ZnInGaSe nanocrystals were synthesized with the controlled stoichiometry from the intermediate of ZnInGaSe magic size clusters. The approach via the magic size clusters greatly increases the amount of Ga in ZnInGaSe NCs. After the growth of the ZnS shell, ZnInGaSe/ZnS core/shell nanocrystals exhibited improved green emission. Furthermore, Cu as the impurity was introduced into ZnInGaSe NCs to reduce the vacancy defects to promote the photoluminescence. Finally, a photoluminescence quantum yield of 35.7% was obtained in Cu:ZnInGaSe/ZnS core/shell nanocrystals. The donor–acceptor recombination mechanism was proposed to explain the optical properties in ZnInGaSe NCs. Cu:ZnInGaSe nanocrystals were utilized in light-emitting diodes, showing the potential of ZnInGaSe nanocrystals for optoelectronic devices. [ABSTRACT FROM AUTHOR]

AgIn5S8/ZnS (AIS/ZS) core/shell quantum dots (QDs) have been synthesized using a hot injection method at relatively low temperature without any protection using inert gas. The QDs were characterized with UV visible electronic absorption spectroscopy, PL spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and temperature-dependent time-resolved PL (TRPL). The average size of the QDs was determined to be 1.73 ± 0.29 nm. The QDs exhibit a major excitonic absorption band peaked at 479–573 nm and emission bands peaked from 548 to 666 nm with excitation at 410 nm with the peak position dependent on the synthesis temperature that affects the QD size. Upon the addition of trioctylphosphine (TOP) as a ligand, the PL quantum yield (PLQY) increased to 74.1% compared to 56.7% without TOP. The QDs show high stability after hours of ultraviolet (UV) light exposure or repetitive treatment with a significant amount of polar solvents, which is attributed to surface passivation by the ligands and ZnS shell. The luminescence mechanism of the core/shell AIS/ZS QDs is ascribed to donor–acceptor pair (DAP) recombination. TRPL results suggest that the binding energy of DAP increases and the DAP recombination probability is enhanced after passivation with TOP, leading to the high PLQY. Furthermore, prototype highly pure white light-emitting diodes (LEDs) based on these AIS/ZS QDs have been successfully demonstrated, yielding ideal 1931 CIE coordinates of (0.327, 0.335). [ABSTRACT FROM AUTHOR]

Inorganic semiconductor nanocrystals (NCs) are considered potential emitters for next-generation displays due to their special quantum size confinement effect. Herein, Cd-free AgInZnS (AIZS) NCs were produced via the cation exchange reaction from AgIn5S8 NCs for light-emitting diodes as green emitters. AIZS NC-based light-emitting diodes showed a maximum current efficiency of 6.34 cd/A and an external quantum efficiency of 1.3%, which are the highest values for AIZS NC-based light-emitting diodes for green emitters so far. The composition of AIZS NCs produced with different ligands can vary the band gap structure of AIZS NCs, affecting the balance of charge injections in the device to induce different performances of AIZS NC-based light-emitting diodes. [ABSTRACT FROM AUTHOR]

Localized excitons are expected to achieve high-performance electroluminescence and have been widely investigated in GaN-based light-emitting diodes (LEDs). Although carbon nanodot (CD) based LEDs have been achieved with the radiative recombination of electrons and holes, localized excitonic electroluminescence has been not reported before. In this Letter, localized excitonic electroluminescent devices have been fabricated using fluorescent CDs as an active layer. The CDs show strong localized excitonic yellow emission with a fluorescence quantum yield of 76% and Stokes shift of 2.1 eV. The CD-based LEDs present a sub-bandgap turn-on voltage of 2.4 V and a maximum luminance of 60.2 cd m -2 , which is the lowest driving voltage among the CD-based electroluminescent devices. Localized centers trap carriers effectively, resulting in sub-bandgap light emission. The current results manifest that localized excitons may furnish a promising approach to boost the development of CD-based LEDs.

Although Mn 2+ doping in semiconductor nanocrystals (NCs) has been studied for nearly three decades, the near 100% photoluminescence (PL) quantum yield (QY) of Mn 2+ emission has never been realized so far. Herein, greatly improved PL QYs of Mn 2+ emissions are reported in Mn 2+ -doped CsPbCl 3 NCs with various Mn 2+ doping concentrations after CdCl 2 post-treatment at room temperature. Specifically, the near-unity QY and near single-exponential decay of red Mn 2+ emission peaking at 627 nm in doped CsPbCl 3 NCs are obtained for the first time. The temperature dependence of steady-state and time-resolved PL spectra reveals that the CdCl 2 post-treatment significantly reduces the nonradiative defect states and enhances the energy transfer from host to Mn 2+ ions. Moreover, the Mn 2+ :CsPbCl 3 NCs after CdCl 2 post-treatment exhibit robust stability and high PL QYs after multipurification. The results will provide an effective route to obtain doped perovskite NCs with high performance for white lighting emitting diodes.

The charge-carrier distribution has been an important parameter in determining the efficiency of quantum-dot-based light-emitting diodes (QLEDs). In this Letter, we demonstrate a new inverted device structure of ITO/ZnO/polyethylenimine/quantum dots (QDs)/1,3,5-tris( N -phenylbenzimidazole-2-yl)benzene (TPBi)/4,4'-bis(9-carbazolyl)-2,2'-biphenyl (CBP)/MoO 3 /Al for improving the efficiency of InP-QD-based QLEDs. By introducing a thin layer of electron transport materials, the hole accumulation at the hole transport layer and the QD interface is largely reduced, which suppresses the quenching effect of holes on the QD emission. Compared with the conventional device structure with the emitters at ZnO/CBP pn junction, the peak current efficiency (external quantum efficiency) increases from 3.83 (5.17 cd/A) to 6.32% (8.54 cd/A) by imbedding the QDs at the electron-dominating interface of ZnO/TPBi. The analysis reveals that an internal quantum efficiency of nearly 100% is achieved for the InP-QD-based device (with a photoluminescence quantum yield of 32%). This work provides an alternative device structure for achieving high-efficiency QLED devices.

The electroluminescence mechanisms in the Cd-free CuInS 2 /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) CuInS 2 /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/m 2 , respectively.

The effect of shell thickness on the performance of all-inorganic quantum dot light-emitting diodes (QLEDs) is explored by employing a series of green quantum dots (QDs) (Zn x Cd 1- x Se/ZnS core/shell QDs with different ZnS shell thicknesses) as the emitters. ZnO nanoparticles and sol-gel NiO are employed as the electron and hole transport materials, respectively. Time-resolved and steady-state photoluminescence results indicate that positive charging processes might occur for the QDs deposited on NiO, which results in emission quenching of QDs and poor device performance. The thick shell outside the core in QDs not only largely suppresses the QD emission quenching but also effectively preserves the excitons in QDs from dissociation of electron-hole pairs when they are subjected to an electric field. The peak efficiency of 4.2 cd/A and maximum luminance of 4205 cd/m 2 are achieved for the device based on QDs with the thickest shells (∼4.2 nm). We anticipate that these results will spur progress toward the design and realization of efficient all-inorganic QLEDs as a platform for the QD-based full-colored displays.

CsPbX 3 (X = Cl, Br, I) perovskite quantum dots (PQDs) have been intensively investigated on photoelectric devices due to their superior optical properties. To date, the stability of CsPbX 3 PQDs is still an open challenge. The previous mixed-halide CsPbX 3 PQDs were generally obtained via the anion-exchange method at 40 °C. Here, the single- and mixed-halide CsPbX 3 PQDs are synthesized at high temperature via the hot injection technique. The surface ligands could thus be strongly coordinated onto the surface of the PQDs, which dramatically improve the optical properties of the PQDs. The resulting CsPbX 3 PQDs have high quantum yield (QY, 40-95%), narrow full width at half-maximum (FWHM) (the narrowest FWHM <10 nm), tunable band gap (408-694 nm), and highly strong photostability. The variation of their emission peaks upon anion atoms is well-supported by the theoretical band gaps calculated by the density functional theory calculations with the alloy formula correction. Hence, these PQDs show great potential as good candidates for photoelectric devices.

We report carbon nanodots that can be utilized as effective color converting phosphors for the production of white light-emitting diodes (LEDs). Blue-excitable and yellow-emitting carbon nanodots, functionalized with 3-(imidazolidin-2-on-1-yl)propylmethyldimethoxysilane (IPMDS)-derived moieties (IS-CDs), are synthesized by a novel one-pot reaction in which the products from the initial reaction occurring between urea and 3-(2-aminoethylamino)propylmethyl-dimethoxysilane (AEPMDS) are further treated with citric acid. Distinctive from the majority of carbon nanodots reported previously, IS-CDs emit at 560 nm, under 460 nm excitation, with a quantum yield of 44%. Preliminary toxicity studies, assessed by the Artemia franciscana nauplii (brine shrimp larvae) bioassay, indicate that IS-CDs are largely nontoxic. Furthermore, the IS-CDs form flexible and transparent films without the need of encapsulating agents, and the solid films retain the optical properties of solvated IS-CDs. These features indicate an immense potential for the IS-CDs as an environmental-friendly, blue-excitable carbon nanodot-based phosphor in solid-state lighting devices.

A study of hybrid inverted quantum-dot (QD) light-emitting diodes constructed with and without Al 2 O 3 interlayers is presented. The Al 2 O 3 interlayers are deposited at ZnO/QDs or/and QDs/4,4'-bis(carbazol-9-yl)biphenyl interfaces, resulting in large improvement of device performance, including luminance, current efficiency, and device lifetime. Especially, the devices with QD emitters sandwiched by two Al 2 O 3 layers exhibits outstanding performance, the longest operation lifetime, and mediate efficiency. The maximum current efficiency of 15.3 cd/A is obtained, an enhancement factor of 35% in comparison to that (11.3 cd/A) of conventional device without Al 2 O 3 layer. Moreover, device lifetime is also largely enhanced, over 110 000 h for the device containing two Al 2 O 3 interlayers, nearly 40% enhancement relative to that of conventional device that shows a lifetime of only 80 000 h. On the basis of electrical property and photoluminescence spectroscopy studies, we demonstrate that the Al 2 O 3 interlayers play crucial roles in suppressing the leakage current across the device and reducing exciton quenching induced by ZnO.

Highly efficient red quantum dot light-emitting diodes (QD-LEDs) with a very high current efficiency of 16 cd/A were demonstrated by adopting stepwise hole-transport layers (HTLs) consisting of 4,4'-N,N'-dicarbazole-biphenyl (CBP) combined with N,N'-dicarbazolyl-3,5-benzene (mCP). The mCP layer plays two important roles in this kind of QD-LEDs. One is that it can block the electron to leak into the HTL due to its higher LUMO (LUMO = the lowest unoccupied molecular orbital) energy level than that of CBP; and the other is it can separate the carrier accumulation zone from the exciton formation interface, which is attributed to the stepwise hole-transport layer structure. Moreover, the lower HOMO (HOMO = the highest occupied molecular orbital) energy level of mCP decreases the hole-injection barrier from the HTL to the QD emitting layer, which improves the charge carrier balance injected into the QD layer, reducing the turn-on voltage of QD-LEDs fabricated with the stepwise HTL structure.

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB2, CB3(2), and CB4) to the lowest electronic state (CB1) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states.

The performances and spectroscopic properties of CdSe/ZnS quantum dot light-emitting diodes (QD-LEDs) with inserting a thickness-varied 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) layer between the QD emission layer and 4,4-N,N-dicarbazole-biphenyl (CBP) hole transport layer (HTL) are studied. The significant enhancement in device peak efficiency is demonstrated for the device with a 3.5 nm TPBi interlayer. The photoluminescence lifetimes of excitons formed within QDs in different devices are also measured to understand the influence of electric field on the QD emission dynamics process and device efficiency. All the excitons on QDs at different devices have nearly the same lifetime even though at different bias. The improvement of device performance is attributed to the separation of charge carrier accumulation interface from the exciton formation zone, which suppresses exciton quenching caused by accumulated carriers.

Nanotechnology, as a double-edged sword, endows gold nanoparticles (GNPs) more "power" in bioimaging and theragnostics, whereas an outstanding issue associated with the biocompatibility of GNPs should also be addressed. Especially for the silica-coated gold nanospheres (GNSs) and gold nanorods (GNRs), there is increasing attention to explore the application, because the surface silica encapsulation has been proved to be an alternative strategy for other organic surface coatings. However, among those reports there are very limited publications to focus on the toxicity of silica-coated GNSs and GNRs. Besides, the existing detoxification methods via surface chemistry on GNPs greatly improve the biocompatibility but still undergo challenges for high dose (>100 pM) demand and long-term stability. Here, we demonstrated a straightforward, low-cost, universal strategy for the surface chemistry on GNPs via silica encapsulating. Different size, shape, dose, and surface capping of GNPs for the nanotoxicity test have been carefully discussed. After silica encapsulating, the detoxification for all GNPs presents significantly from HepG2 cell proliferation results, especially for the GNRs. This new straightforward strategy will definitely rationalize the biocompatibility issue of GNPs and also provide potential for other surface chemistry methodology in biomedical fields.

Photoinduced electron transfer (ET) processes from CuInS2/CdS core/shell quantum dots (QDs) with different core sizes and shell thicknesses to TiO2 electrodes were investigated by time-resolved photoluminescence (PL) spectroscopy. The ET rates and efficiencies from CuInS2/CdS QDs to TiO2 were superior to those of CuInS2/ZnS QDs. An enhanced ET efficiency was surprisingly observed for 2.0 nm CuInS2 core QDs after growth of the CdS shell. On the basis of the experimental and theoretical analysis, the improved performances of CuInS2/CdS QDs were attributed to the passivation of nonradiative traps by overcoating shell and enhanced delocalization of electron wave function from core to CdS shell due to lower conduction band offset. These results indicated that the electron distribution regulated by the band alignment between core and shell of QDs and the passivation of surface defect states could improve ET performance between donor and acceptor.