Design of Core-Shell Quantum Dots-3D WS2 Nanowall Hybrid Nanostructures with High-Performance Bifunctional Sensing Applications.

Transition metal dichalcogenides (TMDCs) have recently attracted a tremendous amount of attention owing to their superior optical and electrical properties as well as the interesting and various nanostructures that are created by different synthesis processes. However, the atomic thickness of TMDCs limits the light absorption and results in the weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate the approach to increase the surface area of TMDCs by a one-step synthesis process of TMDC nanowalls from WOx into three-dimensional (3D) WS2 nanowalls. By utilizing a rapid heating and rapid cooling process, the formation of 3D nanowalls with a height of approximately 150 nm standing perpendicularly on top of the substrate can be achieved. The combination of core-shell colloidal quantum dots (QDs) with three different emission wavelengths and 3D WS2 nanowalls further improves the performance of WS2-based photodetector devices, including a photocurrent enhancement of 320-470% and shorter response time. The significant results of the core-shell QD-WS2 hybrid devices can be contributed by the high nonradiative energy transfer efficiency between core-shell QDs and the nanostructured material, which is caused by the spectral overlap between the emission of core-shell QDs and the absorption of WS2. Besides, outstanding NO2 gas-sensing performance of core-shell QDs/WS2 devices can be achieved with an extremely low detection limit of 50 ppb and a fast response time of 26.8 s because of local p-n junctions generated by p-type 3D WS2 nanowalls and n-type core-shell CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in core-shell QD-WS2 hybrid devices and shows great potential in commercial multifunctional sensing applications.

Electrically Pumped White-Light-Emitting Diodes Based on Histidine-Doped MoS2 Quantum Dots.

MoS2 quantum dots (QDs)-based white-light-emitting diodes (QD-WLEDs) are designed, fabricated, and demonstrated. The highly luminescent, histidine-doped MoS2 QDs synthesized by microwave induced fragmentation of 2D MoS2 nanoflakes possess a wide distribution of available electronic states as inferred from the pronounced excitation-wavelength-dependent emission properties. Notably, the histidine-doped MoS2 QDs show a very strong emission intensity, which exceeds seven times of magnitude larger than that of pristine MoS2 QDs. The strongly enhanced emission is mainly attributed to nitrogen acceptor bound excitons and passivation of defects by histidine-doping, which can enhance the radiative recombination drastically. The enabled electroluminescence (EL) spectra of the QD-WLEDs with the main peak around 500 nm are found to be consistent with the photoluminescence spectra of the histidine-doped MoS2 QDs. The enhanced intensity of EL spectra with the current increase shows the stability of histidine-doped MoS2 based QD-WLEDs. The typical EL spectrum of the novel QD-WLEDs has a Commission Internationale de l'Eclairage chromaticity coordinate of (0.30, 0.36) exhibiting an intrinsic broadband white-light emission. The unprecedented and low-toxicity QD-WLEDs based on a single light-emitting material can serve as an excellent alternative for using transition metal dichalcogenides QDs as next generation optoelectronic devices.

Greener Luminescent Solar Concentrators with High Loading Contents Based on in Situ Cross-Linked Carbon Nanodots for Enhancing Solar Energy Harvesting and Resisting Concentration-Induced Quenching.

A luminescent solar concentrator (LSC) is composed of loaded luminophores and a waveguide that can be employed to harvest and concentrate both direct and diffused sunlight for promising applications in solar windows. Thus far, most of efficient LSCs still relied on the heavy-metal-containing colloidal quantum dots (CQDs) dispersed into a polymer matrix with a very low loading (typically <1 wt %). Such low-loading constraint is required to mitigate the concentration-induced quenching (CIQ) and maintain high optical quality and film uniformity, but this would strongly reduce the light-absorbing efficiency. To address all issues, greener LSCs with high loading concentration were prepared by in situ cross-linking organosilane-functionalized carbon nanodots (Si-CNDs), and their photophysical properties relevant to LSC operation were studied. The PL emission is stable and does not suffer from the severe CIQ effect for cross-linked Si-CNDs even with 25 wt % loadings, thus exhibiting high solid-state quantum yields (QYs) up to 45 ± 5% after the calibration of the reabsorption losses. Furthermore, such LSCs can still hold high optical quality and film uniformity, leading to low scattering losses and high internal quantum efficiency of ∼22%. However, the reabsorption losses need to be further addressed to realize large-area LSCs based on earth-abundant, cost-effective CNDs.

Tunnel injection from WS2 quantum dots to InGaN/GaN quantum wells.

We propose a tunnel-injection structure, in which WS2 quantum dots (QDs) act as the injector and InGaN/GaN quantum wells (QWs) act as the light emitters. Such a structure with different barrier thicknesses has been characterized using steady-state and time-resolved photoluminescence (PL). A simultaneous enhancement of the PL intensity and PL decay time for the InGaN QW were observed after transfer of charge carriers from the WS2-QD injector to the InGaN-QW emitter. The tunneling time has been extracted from the time-resolved PL, which increases as the barrier thickness is increased. The dependence of the tunneling time on the barrier thickness is in good agreement with the prediction of the semiclassical Wentzel-Kramers-Brillouin model, confirming the mechanism of the tunnel injection between WS2 QDs and InGaN QWs.

Correction: Tunnel injection from WS2 quantum dots to InGaN/GaN quantum wells.

[This corrects the article DOI: 10.1039/C7RA13108A.].

Effect of nitrogen doping on the photoluminescence intensity of graphene quantum dots.

We have developed a facile, fast, and one-step synthetic method to prepare graphene quantum dots (GQDs) simultaneously with nitrogen (N) doping via pulsed laser ablation. The N-doped GQDs (N-GQDs) with an average size around 3 nm and an N/C atomic ratio of 33% have been obtained. The N-GQDs emit blue photoluminescence (PL), where the PL intensity enhances as the N doping increases. The PL enhancement for the N-GQDs with a factor as high as 25 has been achieved as compared to GQDs. The origin of the PL enhancement in GQDs after N doping is attributed to the increased densities of pyridinic and graphitic N.

Enhanced Performance of GaN-based Ultraviolet Light Emitting Diodes by Photon Recycling Using Graphene Quantum Dots.

Graphene quantum dots (GQDs) with an average diameter of 3.5 nm were prepared via pulsed laser ablation. The synthesized GQDs can improve the optical and electrical properties of InGaN/InAlGaN UV light emitting diodes (LEDs) remarkably. An enhancement of electroluminescence and a decrease of series resistance of LEDs were observed after incorporation of GQDs on the LED surface. As the GQD concentration is increased, the emitted light (series resistance) in the LED increases (decreases) accordingly. The light output power achieved a maximum increase as high as 71% after introducing GQDs with the concentration of 0.9 mg/ml. The improved performance of LEDs after the introduction of GQDs is explained by the photon recycling through the light extraction from the waveguide mode and the carrier transfer from GQDs to the active layer.

Nano-sized graphene flakes: insights from experimental synthesis and first principles calculations.

In this study, we proposed a cost-effective method for preparing graphene nano-flakes (GNFs) derived from carbon nanotubes (CNTs) via three steps (pressing, homogenization and sonication exfoliation processes). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), laser scattering, as well as ultraviolet-visible and photoluminescence (PL) measurements were carried out. The results indicated that the size of as-synthesized GNFs was approximately 40-50 nm. Furthermore, we also used first principles calculations to understand the transformation from CNTs to GNFs from the viewpoints of the edge formation energies of GNFs in different shapes and sizes. The corresponding photoluminescence measurements of GNFs were carried out in this work.

Carbon Nanodots with Sub-Nanosecond Spontaneous Emission Lifetime.

Compared with most mature cadmium-containing quantum dots (QDs), carbon nanodots (CNDs) are a new class of colloidal nanomaterials that exhibit unique photoluminescence (PL) properties while being nontoxic and easily manufactured using low-cost precursor materials. However, solid-state CNDs exhibit poor PL quantum yields (PL-QYs) and inefficient radiative transition, which significantly hinders their practical use in optoelectronic devices. To address this issue, plasmonic nanoantennas consisting of Au nanorods (Au-NRs) deposited on a flat Au film with inserted dielectric layers were used to enhance the spontaneous emission of solid-state CNDs with broad spectral linewidth. Using steady-state, time-resolved, and spatial-resolved PL measurements, we found that after coupling to plasmonic nanogaps (PNGs), the PL emission was significantly enhanced, accompanied by a PL lifetime shortening to the sub-nanosecond range (≈140 ps). According to the experimental data, the radiative transition is strongly accelerated and can thus overcome the metal loss, leading to a large PL enhancement. Our demonstration can pave the way to the design of eco-friendly nanoemitters with sub-nanosecond PL lifetime for promising applications in light-emitting devices.

Enhanced Conversion Efficiency of III-V Triple-junction Solar Cells with Graphene Quantum Dots.

Graphene has been used to synthesize graphene quantum dots (GQDs) via pulsed laser ablation. By depositing the synthesized GQDs on the surface of InGaP/InGaAs/Ge triple-junction solar cells, the short-circuit current, fill factor, and conversion efficiency were enhanced remarkably. As the GQD concentration is increased, the conversion efficiency in the solar cell increases accordingly. A conversion efficiency of 33.2% for InGaP/InGaAs/Ge triple-junction solar cells has been achieved at the GQD concentration of 1.2 mg/ml, corresponding to a 35% enhancement compared to the cell without GQDs. On the basis of time-resolved photoluminescence, external quantum efficiency, and work-function measurements, we suggest that the efficiency enhancement in the InGaP/InGaAs/Ge triple-junction solar cells is primarily caused by the carrier injection from GQDs to the InGaP top subcell.

Enhanced external quantum efficiency in GaN-based vertical-type light-emitting diodes by localized surface plasmons.

Enhancement of the external quantum efficiency of a GaN-based vertical-type light emitting diode (VLED) through the coupling of localized surface plasmon (LSP) resonance with the wave-guided mode light is studied. To achieve this experimentally, Ag nanoparticles (NPs), as the LSP resonant source, are drop-casted on the most top layer of waveguide channel, which is composed of hydrothermally synthesized ZnO nanorods capped on the top of GaN-based VLED. Enhanced light-output power and external quantum efficiency are observed, and the amount of enhancement remains steady with the increase of the injected currents. To understand the observations theoretically, the absorption spectra and the electric field distributions of the VLED with and without Ag NPs decorated on ZnO NRs are determined using the finite-difference time-domain (FDTD) method. The results prove that the observation of enhancement of the external quantum efficiency can be attributed to the creation of an extra escape channel for trapped light due to the coupling of the LSP with wave-guided mode light, by which the energy of wave-guided mode light can be transferred to the efficient light scattering center of the LSP.