Sustainable next-generation color converters from P. harmala seed extracts for solid-state lighting.

Traditional solid-state lighting heavily relies on color converters, which often have a significant environmental footprint. As an alternative, natural materials such as plant extracts could be employed if their low quantum yields (QYs) in liquid and solid states were higher. With this motivation, here, we investigate the optical properties of aqueous P. harmala extract, develop efficient color-converting solids through a cost-effective and environmentally friendly method, and integrate them with light-emitting diodes (LEDs). To achieve high-efficiency solid hosts for P. harmala-based fluorophores, we optically and structurally compare two crystalline and two cellulose-based platforms. Structural analyses reveal that sucrose crystals, cellulose-based cotton, and paper platforms enable a relatively homogeneous distribution of fluorophores compared to KCl crystals. Optical characterization demonstrates that the extracted solution and the extract-embedded paper possess QYs of 75.6% and 44.7%, respectively, whereas the QYs of the cotton, sucrose, and KCl crystals remain below 10%. We demonstrated that the paper host with the highest efficiency causes a blueshift in the P. harmala fluorescence, whereas the cotton host induces a redshift. We attribute this to the passivation of nonradiative transitions related to the structure of the hosts. Subsequently, as a proof-of-concept demonstration, we integrate the as-prepared efficient solids of P. harmala for the first time with a light-emitting diode (LED) chip to produce a color-converting LED. The resulting blue-emitting LED achieves a luminous efficiency of 21.9 lm Welect -1 with CIE color coordinates of (0.139, 0.070). These findings mark a significant step toward the utilization of plant-based fluorescent biomolecules in solid-state lighting, offering promising environmentally friendly organic color conversion solutions for future lighting applications.

Tailored Synthesis of Iron Oxide Nanocrystals for Formation of Cuboid Mesocrystals.

In this work, we systematically studied the shape- and size-controlled monodisperse synthesis of iron oxide nanocrystals (IONCs) for their use as building blocks in the formation of mesocrystals. For this aim, on understanding the influence of the oleic acid concentration, iron-oleate concentration, and heating rate on the synthesis of robust and reproducible IONCs with desired sizes and shapes, we synthesized highly monodisperse ∼11 nm sized nanocubes and nanospheres. Magnetic measurements of both cubic and spherical IONCs revealed the presence of mixed paramagnetic and superparamagnetic phases in these nanocrystals. Moreover, we observed that the magnetic moments of the nanocubes are more substantial compared to their spherical counterparts. We then demonstrated a simple magnetic-field-assisted assembly of nanocubes into three-dimensional (3D) cuboid mesocrystals while nanospheres did not form any mesocrystals. These findings indicate that small cubic nanocrystals hold great promise as potential building blocks of 3D magnetic hierarchical structures with their superior magnetic properties and mesocrystal assembly capability, which may have high relevance in various fields ranging from high-density data storage to biomedical applications.

A new class of cubic SPIONs as a dual-mode T1 and T2 contrast agent for MRI.

Superparamagnetic iron oxide nanoparticles (SPIONs) are widely used as a robust negative contrast agent on conventional MRI. In this study, we (a) synthesized a new class of cubic SPIONs as a dual-mode contrast agent in MRI and (b) showed the in-vivo feasibility of these nanaoparticles as a simultaneous positive and negative contrast agent. Relaxation properties and contrast enhancement analysis of the synthesized SPIONs with two different shapes (cubic vs. spherical) and three different sizes 7nm, 11nm, and 14nm were investigated to evaluate contrast enhancement in-vitro. In-vivo MRI experiments were performed on a 3T MR scanner, where a healthy anesthetized rat was imaged before, and from 20 to 80min after intravenous injection of 1mg/kg of contrast agent. Representative transmission electron microscopy (TEM) images of the synthesized nanoparticles reveal that the particles are well dispersed in a solvent and do not aggregate. The in-vitro relaxivity and contrast enhancement analysis show that, among all six SPIONs tested, 11-nm cubic SPIONs possess optimal molar relaxivities and contrast enhancement values, which can shorten the spin-lattice and spin-spin relaxation times, simultaneously. No noticeable toxicity is observed during in-vitro cytotoxicity analysis. In-vivo T1-and T2-weighted acquisitions at 60-min post-injection of 11-nm cubic SPIONs result in 64% and 48% contrast enhancement on the T1-and T2-weighted images, respectively. By controlling the shape and size of SPIONs, we have introduced a new class of cubic SPIONs as a synergistic (dual-mode) MRI contrast agent. 11-nm cubic SPIONs with smaller size and high positive and negative contrast enhancements were selected as a promising candidate for dual-mode contrast agent. Our proof-of-concept MRI experiments on rat demonstrate the in-vivo dual-mode contrast enhancement feasibility of these nanoparticles.

Colloidal Nanocrystals Embedded in Macrocrystals: Methods and Applications.

Colloidal semiconductor nanocrystals have gained substantial interest as spectrally tunable and bright fluorophores for color conversion and enrichment solids. However, they suffer from limitations in processing their solutions as well as efficiency degradation in solid films. As a remedy, embedding them into crystalline host matrixes has stepped forward for superior photostability, thermal stability, and chemical durability while simultaneously sustaining high quantum yields. Here, we review three basic methods for loading the macrocrystals with nanocrystals, namely relatively slow direct embedding, as well as accelerated methods of vacuum-assisted and liquid-liquid diffusion-assisted crystallization. We discuss photophysical properties of the resulting composites and present their application in light-emitting diodes as well as their utilization for plasmonics and excitonics. Finally, we present a future outlook for the science and technology of these materials.

High-Stability, High-Efficiency Organic Monoliths Made of Oligomer Nanoparticles Wrapped in Organic Matrix.

Oligomer nanoparticles (OL NPs) have been considered unsuitable for solid-state lighting due to their low quantum yields and low temperature stability of their emission. Here, we address these problems by forming highly emissive and stable OL NPs solids to make them applicable in lighting. For this purpose, we incorporated OL NPs into sucrose matrix and then prepared their all-organic monoliths. We show that wrapping the OL NPs in sucrose significantly increases their quantum yield up to 44%, while the efficiency of their dispersion and direct solid-film remain only at ∼6%. We further showed ∼3-fold improved temperature stability of OL NP emission within these monoliths. Our experiments revealed that a physical passivation mechanism is responsible from these improvements. As a proof-of-concept demonstration, we successfully employed these high-stability, high-efficiency monoliths as color converters on a blue LED chip. Considering the improved optical features, low cost, and simplicity of the presented methodology, we believe that this study holds great promise for a ubiquitous use of organic OL NPs in lighting and possibly in other photonic applications.

Excitonic improvement of colloidal nanocrystals in salt powder matrix for quality lighting and color enrichment.

Here we report excitonic improvement in color-converting colloidal nanocrystal powders enabled by co-integrating nonpolar green- and red-emitting nanocrystal energy transfer pairs into a single LiCl salt matrix. This leads to nonradiative energy transfer (NRET) between the co-integrated nanocrystals in the host matrix. Here we systematically studied the resulting NRET process by varying donor and acceptor concentrations in the powders. We observed that NRET is a strong function of both of the nanocrystal concentrations and that NRET efficiency increases with increasing acceptor concentration. Nevertheless, with increasing donor concentration in the powders, NRET efficiency was found to first increase (up to a maximum level of 53.9%) but then to decrease. As a device demonstrator, we employed these NRET-improved nanocrystal powders as color-converters on blue light-emitting diodes (LEDs), with the resulting hybrid LED exhibiting a luminous efficiency >70 lm/W(elect). The proposed excitonic nanocrystal powders potentially hold great promise for quality lighting and color enrichment applications.

Implementation of High-Quality Warm-White Light-Emitting Diodes by a Model-Experimental Feedback Approach Using Quantum Dot-Salt Mixed Crystals.

In this work, a model-experimental feedback approach is developed and applied to fabricate high-quality, warm-white light-emitting diodes based on quantum dots (QDs) as color-conversion materials. Owing to their unique chemical and physical properties, QDs offer huge potential for lighting applications. Nevertheless, both emission stability and processability of the QDs are limited upon usage from solution. Incorporating them into a solid ionic matrix overcomes both of these drawbacks, while preserving the initial optical properties. Here borax (Na2B4O7·10H2O) is used as a host matrix because of its lower solubility and thereby reduced ionic strength in water in comparison with NaCl. This guarantees the stability of high-quality CdSe/ZnS QDs in the aqueous phase during crystallization and results in a 3.4 times higher loading amount of QDs within the borax crystals compared to NaCl. All steps from the synthesis via mixed crystal preparation to the warm-white LED preparation are verified by applying the model-experimental feedback, in which experimental data and numerical results provide feedback to each other recursively. These measures are taken to ensure a high luminous efficacy of optical radiation (LER) and a high color rendering index (CRI) of the final device as well as a correlated color temperature (CCT) comparable to an incandescent bulb. By doing so, a warm-white LED with a LER of 341 lm/Wopt, a CCT of 2720 K and a CRI of 91.1 is produced. Finally, we show that the emission stability of the QDs within the borax crystals on LEDs driven at high currents is significantly improved. These findings indicate that the proposed warm-white light-emitting diodes based on QDs-in-borax hold great promise for quality lighting.

Stable and efficient colour enrichment powders of nonpolar nanocrystals in LiCl.

In this work, we propose and develop the inorganic salt encapsulation of semiconductor nanocrystal (NC) dispersion in a nonpolar phase to make a highly stable and highly efficient colour converting powder for colour enrichment in light-emitting diode backlighting. Here the wrapping of the as-synthesized green-emitting CdSe/CdZnSeS/ZnS nanocrystals into a salt matrix without ligand exchange is uniquely enabled by using a LiCl ionic host dissolved in tetrahydrofuran (THF), which simultaneously disperses these nonpolar nanocrystals. We studied the emission stability of the solid films prepared using NCs with and without LiCl encapsulation on blue LEDs driven at high current levels. The encapsulated NC powder in epoxy preserved 95.5% of the initial emission intensity and stabilized at this level while the emission intensity of NCs without salt encapsulation continuously decreased to 34.7% of its initial value after 96 h of operation. In addition, we investigated the effect of ionic salt encapsulation on the quantum efficiency of nonpolar NCs and found the quantum efficiency of the NCs-in-LiCl to be 75.1% while that of the NCs in dispersion was 73.0% and that in a film without LiCl encapsulation was 67.9%. We believe that such ionic salt encapsulated powders of nonpolar NCs presented here will find ubiquitous use for colour enrichment in display backlighting.

Macrocrystals of Colloidal Quantum Dots in Anthracene: Exciton Transfer and Polarized Emission.

In this work, centimeter-scale macrocrystals of nonpolar colloidal quantum dots (QDs) incorporated into anthracene were grown for the first time. The exciton transfer from the anthracene host to acceptor QDs was systematically investigated, and anisotropic emission from the isotropic QDs in the anthracene macrocrystals was discovered. Results showed a decreasing photoluminescence lifetime of the donor anthracene, indicating a strengthening energy transfer with increasing QD concentration in the macrocrystals. With the anisotropy study, QDs inside the anthracene host acquired a polarization ratio of ~1.5 at 0° collection angle, and this increases to ~2.5 at the collection angle of 60°. A proof-of-concept application of these excitonic macrocrystals as tunable color converters on light-emitting diodes was also demonstrated.