Probing Eu²⁺ Luminescence from Different Crystallographic Sites in Ca₁₀M(PO₄)₇:Eu²⁺ (M = Li, Na, and K) with β‑Ca₃(PO₄)₂‑Type Structure

Eu²⁺ local environments in various crystallographic sites enable the different distributions of the emission and excitation energies and then realize the photoluminescence tuning of the Eu²⁺ doped solid state phosphors. Herein we report the Eu²⁺-doped Ca₁₀M­(PO₄)₇ (M = Li, Na, and K) phosphors with β-Ca₃(PO₄)₂-type structure, in which there are five cation crystallographic sites, and the phosphors show a color tuning from bluish-violet to blue and yellow with the variation of M ions. The difference in decay rate monitored at selected wavelengths is related to multiple luminescent centers in Ca₁₀M­(PO₄)₇:Eu²⁺, and the occupied rates of Eu²⁺ in Ca(1), Ca(2), Ca(3), Na(4), and Ca(5) sites from Rietveld refinements using synchrotron power diffraction data confirm that Eu²⁺ enters into four cation sites except for Ca(5). Since the average bond lengths <i>d</i>(Ca–O) remain invariable in the Ca₁₀M­(PO₄)₇:Eu²⁺, the drastic changes of bond lengths <i>d</i>(M–O) and Eu²⁺ emission depending on the variation from Li to Na and K can provide insight into the distribution of Eu²⁺ ions. It is found that the emission band at 410 nm is ascribed to the occupation of Eu²⁺ in the Ca(1), Ca(2), and Ca(3) sites with similar local environments, while the long-wavelength band (466 or 511 nm) is attributed to Eu²⁺ at the M(4) site (M = Na and K). We show that the crystal-site engineering approach discussed herein can be applied to probe the luminescence of the dopants and provide a new method for photoluminescence tuning

Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K₂GeF₆:Mn⁴⁺

This study explored optimal preparation conditions for K₂GeF₆:Mn⁴⁺ red phosphors by using chemical coprecipitation method. The prepared hexagonal <i>P</i>3̅m1 K₂GeF₆:Mn⁴⁺ exhibited efficient red emission, high color purity, good Mn⁴⁺ concentration stability, and low thermal quenching. Structural evolution from hexagonal <i>P</i>3̅<i>m</i>1 to <i>P</i>6₃mc and then <i>P</i>6₃<i>mc</i> to cubic <i>Fm</i>3<i>m</i> occurred after thermal treatment at approximately 400 and 500 °C, respectively. Hexagonal <i>P</i>6₃mc phase showed an obvious zero phonon line peak at 621 nm, whereas cubic <i>Fm</i>3<i>m</i> phase showed no red emission. Yellowish K₂GeF₆:Mn⁴⁺ with both hexagonal <i>P</i>3̅<i>m</i>1 and <i>P</i>6₃<i>mc</i> symmetries are promising commercial red phosphors for white light-emitting diodes

All-In-One Light-Tunable Borated Phosphors with Chemical and Luminescence Dynamical Control Resolution

Single-composition white-emitting phosphors with superior intrinsic properties upon excitation by ultraviolet light-emitting diodes are important constituents of next-generation light sources. Borate-based phosphors, such as NaSrBO₃:Ce³⁺ and NaCaBO₃:Ce³⁺, have stronger absorptions in the near-ultraviolet region as well as better chemical/physical stability than oxides. Energy transfer effects from sensitizer to activator caused by rare-earth ions are mainly found in the obtained photoluminescence spectra and lifetime. The interactive mechanisms of multiple dopants are ambiguous in most cases. We adjust the doping concentration in NaSrBO₃:RE (RE = Ce³⁺, Tb³⁺, Mn²⁺) to study the energy transfer effects of Ce³⁺ to Tb³⁺ and Mn²⁺ by comparing the experimental data and theoretical calculation. The vacuum-ultraviolet experimental determination of the electronic energy levels for Ce³⁺ and Tb³⁺ in the borate host regarding the 4f–5d and 4f–4f configurations are described. Evaluation of the Ce³⁺/Mn²⁺ intensity ratios as a function of Mn²⁺ concentration is based on the analysis of the luminescence dynamical process and fluorescence lifetime measurements. The results closely agree with those directly obtained from the emission spectra. Density functional calculations are performed using the generalized gradient approximation plus an on-site Coulombic interaction correction scheme to investigate the forbidden mechanism of interatomic energy transfer between the NaSrBO₃:Ce³⁺ and NaSrBO₃:Eu²⁺ systems. Results indicate that the NaSrBO₃:Ce³⁺, Tb³⁺, and Mn²⁺ phosphors can be used as a novel white-emitting component of UV radiation-excited devices

Water-Resistant Efficient Stretchable Perovskite-Embedded Fiber Membranes for Light-Emitting Diodes

Cesium lead halide perovskite nanocrystals (NCs) with excellent intrinsic properties have been employed universally in optoelectronic applications but undergo hydrolysis even when exposed to atmospheric moisture. In the present study, composite CsPbX₃ (X = Cl, Br, and I) perovskite NCs were encapsulated with stretchable (poly­(styrene-butadiene-styrene); SBS) fibers by electrospinning to prepare water-resistant hybrid membranes as multicolor optical active layers. Brightly luminescent and color-tunable hydrophobic fiber membranes (FMs) with perovskite NCs were maintained for longer than 1 h in water. A unique remote FMs packaging approach was used in high-brightness perovskite light-emitting diodes (PeLEDs) for the first time

Evaluations of the Chemical Stability and Cytotoxicity of CuInS₂ and CuInS₂/ZnS Core/Shell Quantum Dots

Recently, CuInS₂ quantum dots (CIS QDs) are extensively applied in biological applications because of their distinctive optical property. These novel ternary semiconductor CIS QDs can be developed into good biomarkers or trackers because they do not contain cadmium, unlike CdTe and CdSe QDs with high risk for cytotoxicity. However, reports on toxicity and effective factors affecting CIS QDs are seldom developed, and in vivo chemical stability has not been clearly investigated. In this study, we focused on the fate, degradation, and exposure time of CIS QDs in Caenorhabditis elegans (C. elegans), which is used as a model organism in biology. Moreover, X-ray absorption near-edge structure (XANES) is used to identify the oxidation state of CIS and CIS/ZnS QDs in various exposure times. The purpose was to use different oxidation states of copper and zinc ions of QDs to achieve chemical stability in C. elegans. CIS and CIS/ZnS QDs were synthesized by hydrothermal method, and QDs were transferred to aqueous solution by coating with <i>O</i>-carboxymethylchitosan (OCMCS). Moreover, intracellular uptake and cell viability tests were estimated as preliminary experiments for in vitro cytotoxicity testing. Our results showed that the supported QD materials can be applied in biological systems. Consequently, we further considered the function of QD materials in C. elegans. The QD materials of coating OCMCS could be successfully delivered to the interior of the C. elegans through the alimentary system in a manner dependent on the exposure time. Most importantly, XANES results revealed that the oxidative state of CIS QDs did not change without an outer layer after treatment for 96 h in C. elegans. Therefore, the extreme chemical stability of CIS QDs may explain the low cytotoxicity in the organism and thus has potential biomedical applications

Defect Engineering Strategy for Superior Integration of Metal–Organic Framework and Halide Perovskite as a Fluorescence Sensing Material

Combining halide perovskite quantum dots (QDs) and metal–organic frameworks (MOFs) material is challenging when the QDs’ size is larger than the MOFs’ nanopores. Here, we adopted a simple defect engineering approach to increase the size of zeolitic imidazolate framework 90 (ZIF-90)’s pores size to better load CH3NH3PbBr3 perovskite QDs. This defect structure effect can be easily achieved by adjusting the metal-to-ligand ratio throughout the ZIF-90 synthesis process. The QDs are then grown in the defective structure, resulting in a hybrid ZIF-90-perovskite (ZP) composite. The QDs in ZP composites occupied the gap of 10–18 nm defective ZIF-90 crystal and interestingly isolated the QDs with high stability in aqueous solution. We also investigated the relationship between defect engineering and fluorescence sensing, finding that the aqueous Cu2+ ion concentration was directly correlated to defective ZIF-90 and ZP composites. We also found that the role of the O–Cu coordination bonds and CH3NHCu+ species formation in the materials when they reacted with Cu2+ was responsible for this relationship. Finally, this strategy was successful in developing Cu2+ ion fluorescence sensing in water with better selectivity and sensitivity

Highly Efficient Blue Emission and Superior Thermal Stability of BaAl₁₂O₁₉:Eu²⁺ Phosphors Based on Highly Symmetric Crystal Structure

Highly efficient phosphor materials with superior thermal stability are indispensable for phosphor-converted white light-emitting diodes (pc-WLEDs) solid state lighting. In order to obtain a high quality warm white light, near-ultraviolet (n-UV) chips combined with trichromatic phosphors have be extensively studied. Among them, the development of efficient blue phosphor remains a challenging task. In view of the close correlation between 5d–4f transitions of rare earth ions and the coordination environment of host lattice, many studies have been dedicated to improving the photoluminescence performances by modifying the lattice coordination environment including the lattice rigidity and symmetry. In this work, we reported highly efficient blue-emitting Eu²⁺-doped BaAl₁₂O₁₉ (BAO) phosphors with excellent thermal stability, which were prepared via the traditional high-temperature solid state reaction routes. According to the X-ray powder diffraction (XRD) Rietveld refinement analysis, BAO owned a highly symmetric layer structure with two Ba polyhedrons, marked as Ba(1)­O₉ and Ba(2)­O₁₀, respectively. The diffuse reflectance spectra revealed the optical band gap to be 4.07 eV. Due to the suitable optical bandgap, the Eu²⁺ ions could realize a highly efficient doping in the BAO matrix. The photoluminescence excitation (PLE) spectra for as-prepared BAO:Eu²⁺ phosphors exhibited a broad absorption band in the region from 250 to 430 nm, matching well with the n-UV LED chip. Under the UV radiation, it is highly luminous (internal quantum yields (IQYs) = 90%) with the peak around 443 nm. Furthermore, the color purity of BAO:Eu²⁺ phosphors could achieve 92%, ascribing to the narrow full width at half-maximum (fwhm = 52 nm), which was even much better than that of commercially available BAM:Eu²⁺ phosphor (color purity = 91.34%, fwhm = 51.7 nm). More importantly, the as-prepared BAO:Eu²⁺ phosphor showed extra high thermal stability when working in the region of 298–550 K, which was a bit better than that of commercial BAM:Eu²⁺ phosphors. According to the distortion calculation of Ba crystallographic occupation, the superior thermal stability could be attributed to the highly symmetric crystal structure of BAO host. In view of the excellent luminescence performances of BAO:Eu²⁺, it is a promising blue-emitting phosphor for n-UV WLED

Multi-Bandgap-Sensitized ZnO Nanorod Photoelectrode Arrays for Water Splitting: An X-ray Absorption Spectroscopy Approach for the Electronic Evolution under Solar Illumination

This investigation demonstrates an environmentally friendly inorganic light-harvesting nanostructure. This system provides a stable photoelectrochemical platform for the photolysis of water. The device is constructed by first building up an array of ZnO nanowires and then incorporating indium phosphide (InP) nanocrystals into them. A different-sized quantum dots (QDs) sensitization of the ZnO nanowire array for splitting water with a substantially enhanced photocurrent was demonstrated. InP QDs of various sizes are utilized as simultaneous sensitizers of the array of ZnO nanowires, and this multi-bandgap sensitization layer of InP QDs can harvest complementary solar light in the visible region while the ZnO nanostructures absorb the UV part of solar light. A photocurrent of 1.2 mA/cm² at +1.0 V was observed; it was more than 108% greater than the photocurrent achieved by bare ZnO nanowires. Solar illumination measurements investigated the contribution from photoelectrochemical response and effect in unoccupied states of conduction band. ZnO decorated with single/three-sized InP QDs had a significant increase in photogenerating electrons in 4p orbital, which indicated this increase of photogenerating electrons could be attributable to the absorption of InP QDs in visible region and the photogenerating electrons transfer from conduction band of InP to that of ZnO. The photogenerating electron in conduction band can significantly response to the photoactivity collected in photoelectrochemical measurement, and the contribution of photoresponse from ZnO nanowire or InP quantum dots can be distinguished by comparing the spectra collected under dark/illumination condition

Photoluminescence Tuning via Cation Substitution in Oxonitridosilicate Phosphors: DFT Calculations, Different Site Occupations, and Luminescence Mechanisms

Tuning and optimizing luminescent properties of oxonitridosilicates phosphors are important for white light-emitting diode (WLED) applications. To improve the color rendering index, correlated color temperature and thermal stability of layer-structured <i>M</i>Si₂O₂N₂:Eu (M = Sr, Ba) phosphors, cation substitutions have been used to adjust their luminescent properties. However, the underlying mechanisms are still unclear. In this research, a series of (Sr_1–<i>x</i>Ba_<i>x</i>)­Si₂O₂N₂:Eu (0 ≤ <i>x</i> ≤ 1) compounds were prepared by solid-state reaction, after which systematic emission variations were investigated. The crystal structures of (Sr_1–<i>x</i>Ba_<i>x</i>)­Si₂O₂N₂:Eu (0 ≤ <i>x</i> ≤ 1) are nominally divided into three sections, namely, Phase 1 (0 ≤ <i>x</i> ≤ 0.65), Phase 2 (0.65 < <i>x</i> < 0.80), and Phase 3 (0.80 ≤ <i>x</i> ≤ 1) based on the X-ray diffraction measurements. These experimental results are further confirmed by optimizing the crystal structure data with first-principle calculations. Continuous luminescence adjustments from green to yellow are observed in Phase 1 with gradual replacement of Sr²⁺ with Ba²⁺, and the abnormal redshift is clarified through extended X-ray absorption fine structure analysis. Sr­(Eu)–O/N bond length shrinkage in local structure causes the redshift emission, and the corresponding luminescence mechanism is proposed. Controllable luminescence in Phase 2 (from blue to white) and Phase 3 (from cyan to yellowish green) are observed. Based on the high-resolution transmission electron microscopy and selected area electron diffraction analysis, the two kinds of luminescence tuning are attributed to phase segregation. This study serves as a guide in developing oxonitride luminescent materials with controllable optical properties based on variations in local coordination environments through cation substitutions