Optimizing spectral quality with quantum dots to enhance crop yield in controlled environments

Transparent and flexible cellulose based luminescent film for multifunctional applications.

Fabricating high-performance perovskites in ambient air is desirable for low-cost and large-scale patterned manufacture of light-emitting diodes (LEDs). However, perovskites inherently exhibit high sensitivity to moisture and oxygen, which considerably hinders their fabrication under ambient air conditions. Here, we demonstrated an interface-reconstruction strategy which enabled air-processed CsPbI3 quantum dot (QD) films for high-performance LEDs. Ethyl acetate/tris(1-naphthyl)phosphine oxide (EA/TNPO) treatment was used to polish and passivate the interface between the QD film and the electron transport layer, thereby providing a protective coating against moisture and oxygen. We fabricated LEDs based on the spin-coated QD films, achieving a maximum external quantum efficiency (EQE) of 20.2% and a long half-life of over 100 days. We also fabricated LEDs based on the inkjet-printed QD films, highlighting the practical application potential for patterning techniques. Our work develops high-quality air-processed perovskite films and demonstrates their great prospect for low-cost optoelectronic devices in the future.

Improvement of color-rendering characteristics of white light emitting diodes by using red quantum dot films

The enhancement of optical properties via thermal annealing on InP/ZnS core/shell quantum dot (QD) film was investigated in this work. The increase of emission intensities of the QD films was observed after thermal annealing at 180 °C for 5 min. Through temperature dependence photoluminescence (TDPL) and power dependence photoluminescence (PL) measurement, the peak located at the low-energy shoulder was confirmed to be localized state emission and the high energy one comes from free-carrier emission. Moreover, from the TDPL spectra of the sample annealed at 180 °C for 5 min, the full width at half maximum (FWHM) of localization state emission was nearly the same before which is 250 K and then decreased with increasing temperature. However, the FWHM was decreased significantly when temperature increased in the untreated sample. We conclude that the escape of localization states with increasing temperature contributes to this anomaly phenomenon. Our studies have significance on the application of QDs in electroluminescence devices and down-conversion light-emitting devices.

Quantum dots (QDs) are investigated for photovoltaic (PV) device applications; due to safety concerns regarding the use of Cd, Pb-based materials, less-toxic copper indium sulfide (CIS) and zinc copper indium sulfide (ZCIS) QDs have gained prominence. In advanced PV structures, the formation and morphology of QD films is significant; the construction of uniform, defect-free QD films are desirable. Several strategies are employed in QD film formation, most notably spin-casting, drop-casting, and dip-coating. We studied the film deposition and self-assembly QDs in formation of device-quality films. Using dip-coating, we developed a method to deposit uniform, ordered less-toxic CIS and ZCIS QD films for large-area, scaled-up PV applications.

This Letter proposes a dual-wavelength, low-coherence digital holography system with a single light source, which utilizes a quantum dot (QD) film as a wavelength converter. By changing the size of the QDs, the proposed method easily yields higher and more uniform illumination of any target wavelength, compared with bandpass-filtered light-emitting diodes. Fabrication parameters of the QD film for better conversion efficiency are discussed. Using this light source with the dual-wavelength reconstruction method extends the efficiency and range of nanoscale three-dimensional height measurements.

Photovoltaic effects in CdSe quantum dot (QD) films have been studied using surface photovoltage spectroscopy and complementary methods. The results show that, contrary to previous studies, nonnegligible electric fields can exist in QD films. As a result, driftlike currents must be considered, in addition to the well‐known diffusionlike currents. However, it is found that the specific case of photovoltage sign reversal, observed after etching highly quantized CdSe QD films, is governed by diffusionlike transport. The latter is highly influenced by preferential trapping of one type of charge carrier. The preferential trapping is shown to be surface localized and is strongly ambient dependent. It is shown that the photovoltaic properties of these CdSe QD films are dominated by their surface state distribution.

Eliminating lattice disorder in quantum dots (QDs) is critical for achieving high‐performance quantum dot light‐emitting diodes (QLEDs), as such disorder directly disrupts the uniformity of elemental distribution and degrades their optical properties. Here, a tertiary amine‐mediated synthesis strategy is reported that utilizes nucleophilic reagents to regulate the coordination kinetics of cationic precursors during the growth of ZnCdSeS/ZnS QDs. This strategy leverages nucleophilic reagents bearing uncoordinated lone‐pair electrons to stabilize the cationic precursors and modulate the QDs surface energy of highly reactive crystal planes, thereby promoting atomic‐scale uniform growth of the QDs, minimizing lattice mismatch, preventing stacking faults, and thus enabling the synthesis of strain‐graded QDs ( sg ‐QDs). Consequently, by achieving precise control over both elemental distribution and lattice ordering in multicomponent alloy QDs, sg ‐QDs are obtained that exhibit a photoluminescence quantum yield of 98% in solution and 95% in the solid film. The sg ‐QD films further demonstrate monoexponential decay kinetics and reduced defect density, confirming effective trap‐state suppression. The resultant green QLEDs achieve a record external quantum efficiency (EQE) of 25.2%, an operational lifetime of 1 925 900 h, and sustained EQE over 20% across a luminance range of 10 2 –10 5 cd m −2 . This nucleophile‐coordination paradigm redefines the synthesis of alloy nanocrystals, providing a dual‐advantage platform for ultrastable optoelectronics and scalable QLEDs manufacturing.

Near-infrared (NIR)-emitting PbS-based quantum dots (QDs) and exfoliated layered double hydroxide (LDH) nanosheets were self-assembled to form photostable composites for NIR-emitting codes. PbS QDs prepared by a pyrolysis synthetic method or PbS–CdS (core–shell) QDs prepared by a cation exchange using PbS QDs were transferred to aqueous solutions by surface ligand exchange to dihydrolipoic acid (DHLA). The DHLA-decorated QDs and exfoliated LDH nanosheets were assembled by electrostatic interactions during vortexing. The QD–LDH composite films prepared by drop-casting exhibit more than 500% higher NIR PL intensities than the counterpart QD films (without LDH matrices) under identical conditions (i.e., QD concentration). Photostability tests under continuous light irradiation reveal that the PbS–CdS QD films are more photostable than the PbS QD films. Formation of the QD–LDH composites gives rise to further enhancement in the photostability of the film. The photostable PbS–CdS QD–LDH composite films were used to create novel NIR-emitting codes. NIR fluorescence images of a pattern of two kinds of PbS–CdS QD–LDH composite films were recorded with an InGaAs CCD camera equipped with optical filters.

Quantum dots (QDs) have received considerable attention in information displays owing to their high quantum yield, high colour purity and low-cost fabrication. However, light emission for ultra-thin QD films with low mass percentage of QDs still need to be improved because the blue light can directly transmit the films, leading to insufficient energy to excite the QDs. In this study, we report QD films based on a poly(zinc methacrylate) coating with alloyed green-emitting CdZnSeS/ZnS quantum dots (QDs@PZnMA) together with high refractive-index BaTiO3 nanoparticles to enhance the scattering coefficient of the QD films. Results demonstrate a 7.5-fold increase in the absorption coefficient, 11.3-fold increase in the scattering coefficient, 8.5-fold increase in the optical density (OD) and 8.6-fold increase in the green-light emission of QD films, compared with films that have the same mass percentage of pristine QDs. This approach provides a promising strategy for developing QD optical films with high scattering and enhanced light emission for flexible displays.

A remote-type white light-emitting diode (LED) consisting of a red quantum-dot (QD) film and a yellow phosphor plate was studied by both experiment and optical simulation. The sequence of the two color-conversion films had a substantial effect on the color-rendering properties of the vertically-stacked white LED, and the optimized configuration exhibited a high color rendering index of more than 90 thanks to the enhanced red component via the QD film. For the design of high-power white LED devices of a remote type, it was necessary to locate the color-conversion films below the diffuser plate to remove the substantial color dispersion depending on the viewing angle. The present study shows that high power and high color-rendering white LED devices can be realized in terms of two vertically-stacked color-conversion materials, which would provide long-term stability due to the remote design.

As an ideal single-photon source, quantum dots (QDs) can play a unique role in the field of quantum information. Controlling QD exciton spontaneous emission can be achieved by anti-phase coupling between QD exciton dipole field and Au dipole field after QD film has been transferred onto the Si substrate covered by Au nanoparticles. In experiment, the studied InAs/GaAs QDs are grown by molecular beam epitaxy (MBE) on a (001) semi-insulation substrate. The films containing QDs with different GaAs thickness values are separated from the GaAs substrate by etching away the AlAs sacrificial layer and transferring the QD film to the silicon wafer covered by Au nanoparticles with a diameter of 50 nm. The distance <i>D</i> (thickness of GaAs) from the surface of the Au nanoparticles to the QD layer is 10, 15, 19, 25, and 35 nm, separately. A 640-nm pulsed semiconductor laser with a 40-ps pulse length is used to excite the QD samples for measuring QD exciton photoluminescence and time-resolved photoluminescence spectra at 5 K. It is found that when the distance <i>D</i> is 15–35 nm the spontaneous emission rate of exciton is suppressed. And when <i>D</i> is close to 19 nm, the QD spontaneous emission rate decreases to <inline-formula><tex-math id="M2">\begin{document}$ ~{10}^{-3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211863_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211863_M2.png"/></alternatives></inline-formula>, which is consistent with the theoretical calculations. The physical mechanism of long-lived exciton luminescence observed in experiment lies in the fact that Au nanoparticles scatter the light field of the exciton radiation in the QD wetting layer, and the phase of the scattered field is opposite to the phase of the exciton radiation field. Therefore, the destructive interference between the exciton radiation field and scattering field of Au nanoparticles results in long-lived exciton emission observed in experiment.

Quantum dots (QDs) have extraordinary strong light absorption and size tunable bandgap. However, QD films are typically limited to ~200-300 nm due to their poor charge mobility. This severely limits the quantum efficiency of QD devices for λ <750 nm (infrared). Herein, we report a record 1 μm thick QD film using intercalated graphene layers as transparent current extractors. This overcomes QD poor mobility, ensuring both effective light absorption and charge extraction towards the near-infrared reaching quantum efficiency (EQE) of 90%. The short diffusion length (LD<200 nm) of QDs limits their useful thickness to ~200-300 nm1–4 , resulting in poor infrared light absorption. To overcome this limitation, we have built a 1 µm thick QD film with intercalated transparent graphene electrodes that keep high charge collection efficiency. As a result, the 1 µm intercalated devices show a superior EQE reaching 90% at λ ~800 nm without the drop of quantum efficiency at λ ~700 nm observed in most QD devices. The EQE of intercalated devices improves over the entire λ~ 600-1100 nm spectrum as the thickness increases from 100 nm to 1 μm, clearly breaking the restriction that the diffusion length of QDs imposes on the film thickness. This improves absorption and charge collection in the infrared.

The saturable absorption and two-photon absorption of the thin films containing the quantum dots (QD) of several metal sulfides and selenides (Ag 2 S, PbS, Ag 2 Se, and PbSe) are studied using the 1030 nm and 515 nm, 200 fs probe pulses. The prevailing role of saturable absorption and two-photon absorption in films was determined by varying the intensity of the probe pulses. The nonlinear optical parameters of the above processes are calculated. The strong two-photon absorption in PbS and PbSe QD films in the case of the 515 nm probe pulses is revealed. The growth of the nonlinear absorption in the QD films compared to the colloids containing similar species is attributed to the growing influence of the local field introduced by QDs in the films. The potential applications of the studied QD films are discussed.

Quantum dot (QD) enhanced LCDs are among today’s best‐in‐class displays exhibiting high brightness and large color gamut. In this presentation we focus on the underlying key mechanism of the embedded QD films, the down‐conversion of light, and explore the optical characteristics of a state‐of‐the‐art perovskite QD film by measuring its down‐converting properties as a function of the viewing angle. The good agreement of the experimental results with simulations of the QD film indicates that computer aided parameter optimization can be key tool for engineering QD displays.