Color-Tunable Highly Bright Photoluminescence of Cadmium-Free Cu-Doped Zn–In–S Nanocrystals and Electroluminescence

A series of Cu doped Zn–In–S quantum dots (Cu:Zn–In–S d-dots) were synthesized via a one-pot noninjection synthetic approach by heating up a mixture of corresponding metal acetate salts and sulfur powder together with dodecanethiol in oleylamine media. After overcoating the ZnS shell around the Cu:Zn–In–S d-dot cores directly in the crude reaction solution, the resulting Cu:Zn–In–S/ZnS d-dots show composition-tunable photoluminescence (PL) emission over the entire visible spectral window and extending to the near-infrared spectral window (from 450 to 810 nm), with the highest PL quantum yield (QY) up to 85%. Importantly, the initial high PL QY of the obtained Cu:Zn–In–S/ZnS d-dots in organic media can be preserved when transferred into aqueous media via ligand exchange. Furthermore, electroluminescent devices with good performance (with a maximum luminance of 220 cd m^–2, low turn-on voltages of 3.6 V) have been fabricated with the use of these Cd-free low toxicity yellow-emission Cu:Zn–In–S/ZnS d-dots as an active layer in these QD-based light-emitting diodes

Efficient and Stable Red Emissive Carbon Nanoparticles with a Hollow Sphere Structure for White Light-Emitting Diodes

Red-emissive solid-state carbon nanoparticles (CNPs) with a hollow sphere structure for white light-emitting diodes (WLEDs) were designed and synthesized by molecular self-assembly and microwave pyrolysis. Highly ordered graphite-like structures for CNPs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible (UV–vis) spectroscopy. The emission mechanism of the red-emissive solid-state CNPs was investigated in detail by steady-state and time-resolved photoluminescence (PL) spectroscopy. The as-prepared CNPs showed a red emission band centered at 620 nm with excitation wavelength independence, indicating uniform size of sp² carbon domains in the CNPs. The CNPs also had a PL quantum yield (QY) of 17% under 380 nm excitation. Significantly, the PL QY of the organosilane-functionalized CNPs was 47%, which is the highest value recorded for red-emissive solid-state carbon-based materials under UV-light excitation. More importantly, the red-emissive CNPs exhibited a PL QY of 25% after storage in air for 12 months, indicating their excellent stability. The red-emissive CNP powders were used as environmentally friendly and low-cost phosphors on a commercial 460 nm blue GaN-based chip, and a pure white light with CIE coordinates of (0.35, 0.36) was achieved. The experimental results indicated that the red-emissive CNP phosphors have potential applications in WLEDs

Exploring the Effect of Band Alignment and Surface States on Photoinduced Electron Transfer from CuInS₂/CdS Core/Shell Quantum Dots to TiO₂ Electrodes

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

Highly Efficient and Low Turn-On Voltage Quantum Dot Light-Emitting Diodes by Using a Stepwise Hole-Transport Layer

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′-<i>N</i>,<i>N</i>′-dicarbazole-biphenyl (CBP) combined with <i>N</i>,<i>N</i>′-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

Photoinduced Charge Separation and Recombination Processes in CdSe Quantum Dot and Graphene Oxide Composites with Methylene Blue as Linker

The charge separation and recombination processes between CdSe quantum dot (QD) and graphene oxide (GO) composites with linking molecule methylene blue (MB⁺) were studied by femtosecond transient absorption spectroscopy. Anchoring MB⁺ molecules on GO results in significant changes in steady-state and transient absorption spectra, where the exciton dissociation time in the CdSe QD-MB⁺-GO composite was determined to be 1.8 ps. Surprisingly, the ground state bleaching signal increased for MB⁺-GO complex was found to be 5.2 ps, in relation with electron transfer from QD to GO. On the other hand, the strong electronic coupling between MB<b>^•</b>-GO radical and GO prolonged charge recombination process (≥5 ns) in QD-MB⁺-GO composites. Charge separation and recombination processes at the interface between semiconductor QDs and graphene can thus be modulated by the functionalized dye molecules

Photoinduced Charge Separation and Recombination Processes in CdSe Quantum Dot and Graphene Oxide Composites with Methylene Blue as Linker

The charge separation and recombination processes between CdSe quantum dot (QD) and graphene oxide (GO) composites with linking molecule methylene blue (MB⁺) were studied by femtosecond transient absorption spectroscopy. Anchoring MB⁺ molecules on GO results in significant changes in steady-state and transient absorption spectra, where the exciton dissociation time in the CdSe QD-MB⁺-GO composite was determined to be 1.8 ps. Surprisingly, the ground state bleaching signal increased for MB⁺-GO complex was found to be 5.2 ps, in relation with electron transfer from QD to GO. On the other hand, the strong electronic coupling between MB<b>^•</b>-GO radical and GO prolonged charge recombination process (≥5 ns) in QD-MB⁺-GO composites. Charge separation and recombination processes at the interface between semiconductor QDs and graphene can thus be modulated by the functionalized dye molecules

Photoluminescence Temperature Dependence, Dynamics, and Quantum Efficiencies in Mn²⁺-Doped CsPbCl₃ Perovskite Nanocrystals with Varied Dopant Concentration

A series of Mn²⁺-doped CsPbCl₃ nanocrystals (NCs) was synthesized using reaction temperature and precursor concentration to tune Mn²⁺ concentrations up to 14%, and then studied using variable-temperature photoluminescence (PL) spectroscopy. All doped NCs show Mn²⁺ ⁴T_1g → ⁶A_1g d–d luminescence within the optical gap coexisting with excitonic luminescence at the NC absorption edge. Room-temperature Mn²⁺ PL quantum yields increase with increased doping, reaching ∼60% at ∼3 ± 1% Mn²⁺ before decreasing at higher concentrations. The low-doping regime is characterized by single-exponential PL decay with a concentration-independent lifetime of 1.8 ms, reflecting efficient luminescence of isolated Mn²⁺. At elevated doping, the decay is shorter, multiexponential, and concentration-dependent, reflecting the introduction of Mn²⁺–Mn²⁺ dimers and energy migration to traps. A large, anomalous decrease in Mn²⁺ PL intensity is observed with decreasing temperature, stemming from the strongly temperature-dependent exciton lifetime and slow exciton-to-Mn²⁺ energy transfer, which combine to give a strongly temperature-dependent branching ratio for Mn²⁺ sensitization

Size- and Composition-Dependent Energy Transfer from Charge Transporting Materials to ZnCuInS Quantum Dots

We studied the energy transfer processes from organic charge transporting materials (CTMs) to ZnCuInS (ZCIS) quantum dots (QDs) with different emission wavelength by steady-state and time-resolved photoluminescence (PL) spectroscopy. The change in the PL excitation intensity of the ZCIS QDs and the PL decay time of the CTMs clearly demonstrated an efficient energy transfer process in the ZCIS/CTM blend films. It was found that the efficiency of Förster resonance energy transfer significantly increases with increasing the particle size and decreasing the Zn content in the QDs, which is well consistent with the estimated Förster radii (<i>R</i>₀) varying from 3 to 5 nm. In addition, the PL quenching of the QDs related to the charge separation process was also observed in some of the samples. The energy transfer and charge separation processes in the films were well explained based on the band alignment between the ZCIS QDs and CTMs

Broadband MoS₂ Square Nanotube-Based Photodetectors

Although the research on layered MoS2 photodetectors has made great progress, their poor light absorption ability and complex preparation process hinder their further commercial application. In the present work, we report the growth of MoS2 square nanotubes with high purity via a facile hydrothermal method for the first time. Microstructure characterization demonstrates that the cavity structure of the nanotubes can bring about a light trapping effect, thus obtaining a strong photoelectric performance. The as-constructed MoS2 square nanotube photodetector with a paper substrate displays a broadband response with a detection range of 375 to 915 nm. It exhibits excellent performance with a high responsivity of 2.33 mA/W under 915 nm light irradiation, which is comparable to the best ones ever reported for polycrystalline MoS2 photodetectors

Dual Emissive Manganese and Copper Co-Doped Zn–In–S Quantum Dots as a Single Color-Converter for High Color Rendering White-Light-Emitting Diodes

Novel white light emitting diodes (LEDs) with environmentally friendly dual emissive quantum dots (QDs) as single color-converters are one of the most promising high-quality solid-state lighting sources for meeting the growing global demand for resource sustainability. A facile method was developed for the synthesis of the bright green–red-emitting Mn and Cu codoped Zn–In–S QDs with an absorption bangdgap of 2.56 eV (485 nm), a large Stokes shift of 150 nm, and high emission quantum yield up to 75%, which were suitable for warm white LEDs based on blue GaN chips. The wide photoluminescence (PL) spectra composed of Cu-related green and Mn-related red emissions in the codoped QDs could be controlled by varying the doping concentrations of Mn and Cu ions. The energy transfer processes in Mn and Cu codoped QDs were proposed on the basis of the changes in PL intensity and lifetime measured by means of steady-state and time-resolved PL spectra. By integrating these bicolor QDs with commercial GaN-based blue LEDs, the as-fabricated tricolor white LEDs showed bright natural white light with a color rendering index of 95, luminous efficacy of 73.2 lm/W, and color temperature of 5092 K. These results indicated that (Mn,Cu):Zn–In–S/ZnS QDs could be used as a single color-converting material for the next generation of solid-state lighting