Preparation, Growth Mechanism, Upconversion, and Near-Infrared Photoluminescence Properties of Convex-Lens-like NaYF₄ Microcrystals Doped with Various Rare Earth Ions Excited at 808 nm

Preparation of rare earth ions doped photoluminescence materials with controlled morphology was desired to fulfill the requirement of different applications. In the work, convex-lens-like NaYF₄ microcrystals doped with various rare earth ions were prepared by adjusting preparation parameters including the reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene, and types of doping ions. A possible growth mechanism of convex-lens-like NaYF₄ microcrystals is proposed based on reaction time and temperature-dependent morphology evolution. The formation of micro-convex-lens includes the three processes of NaYF₄ nanoparticles self-assemble, dissolution–nucleation, and regrowth. Doping ions dependent near-infrared and upconversion luminescence properties of convex-lens-like NaYF₄ microcrystals were investigated excited at 808 nm. The visible upconversion luminescence was observed in the Er³⁺, Yb³⁺/Er³⁺, Nd³⁺/Er³⁺, Nd³⁺/Yb³⁺/Er³⁺ doped convex-lens-like NaYF₄ microcrystals, and near-infrared luminescence was obtained in the Nd³⁺, Nd³⁺/Er³⁺, Yb³⁺/Er³⁺, Nd³⁺/Yb³⁺, Nd³⁺/Yb³⁺/Er³⁺ doped NaYF₄ convex-lens-like NaYF₄ microcrystals. The Er³⁺, Yb³⁺/Er³⁺, Nd³⁺/Er³⁺, Nd³⁺/Yb³⁺/Er³⁺ doped convex-lens-like NaYF₄ microcrystals exhibit various upconversion luminescence mechanisms. The energy transfer of the Er³⁺ → Yb³⁺ and the Nd³⁺ → Er³⁺ was observed in the Yb³⁺/Er³⁺ and Nd³⁺/Er³⁺ doped convex-lens-like NaYF₄ microcrystals, respectively. The upconversion emission of Nd³⁺/Yb³⁺/Er³⁺ doped convex-lens-like NaYF₄ microcrystals is from the energy transfer mechanisms of Nd³⁺ → Yb³⁺ → Er³⁺

Effect of Defect Distribution on the Optical Storage Properties of Strontium Gallates with a Low-Dimensional Chain Structure

The low-dimensional structure of the SrGa₂O₄ host exhibits a self-activated long persistent luminescence related to the creation of the oxygen vacancies. Because of the unique structure of the SrGa₂O₄ with a chain of cations along the <i>a</i> crystal direction, the emission and trapping centers could be introduced easily when the metal ions of Bi³⁺ are doped. Both the photoluminescence and long persistent luminescence are related to two efficient emission centers of Bi³⁺ in the two different crystallographic Sr sites, while the photostimulated luminescence spectra exhibit only one emission center of Bi₁ ions under excitation at 980 or 808 nm. The results indicate that the distribution of defects in the low-chain structure of the SrGa₂O₄ host plays a vital role in the capture and transfer processes of carriers, which has a profound influence on the luminescence performance of SrGa₂O₄:Bi³⁺ as one of the electron-trapping materials

Tunable LLP via Energy Transfer between Na_2–<i>y</i>(Zn_1–<i>x</i>Ga_<i>x</i>)GeO₄ Sosoloid Host and Emission Centers with the Assistance of Zn Vacancies

A series of sodium zinc gallogermanate sosoloid Na_2–<i>y</i>(Zn_1–<i>x</i>Ga_<i>x</i>)­GeO₄: <i>y</i>Tb³⁺ (<i>x</i> = 0, 0.1, 0.12, 0.15, 0.18, and 0.20; <i>y</i> = 0, 0.01, 0.02, 0.03, and 0.04) samples were successfully synthesized and their luminescence properties were investigated. It was found that Zn vacancies could be the predominant contribution to the enhancement of PL and LLP intensities when Ga³⁺ ions were introduced as solute atoms substituted the crystal sites of Zn²⁺ in Na₂ZnGeO₄ host. Zn vacancies act not only as an exciton energy-level participated in the PL process but also as trapping centers contributed to the LLP process. Furthermore, the energy transfer processes from Na₂(Zn_0.8Ga_0.2)­GeO₄ host to Tb³⁺ ions in PL and LLP were identified. Accordingly, the LLP colors changed from blue to green with increasing concentration of Tb³⁺ and the mechanism of which was proposed. Besides, after irradiation with 800 nm femtosecond pulsed laser, a blue upconversion LLP phenomenon was clearly observed in Na₂(Zn_0.8Ga_0.2)­GeO₄ for the first time. Analysis suggested that multiphoton absorption process was dominant in this upconversion luminescence, and subsequently the carriers excited into conduction band were captured by the trapping centers. Then the realization of LLP was due to the thermal stimulated recombination of holes and electrons at traps. Our results indicated that the blue-green emitting phosphor of Na₂(Zn_0.8Ga_0.2)­GeO₄: Tb³⁺ could be a new member in the family of LLP materials, which could also provide potential applications in the fabrication of optical memory

White-Light Whispering-Gallery-Mode Lasing from Lanthanide-Doped Upconversion NaYF₄ Hexagonal Microrods

We demonstrate simultaneous red, green, and blue emission from a Yb³⁺–Er³⁺–Tm³⁺ tridoped hexagonal β-NaYF₄ microrod, which supports whispering-gallery-mode (WGM) resonance, to realize white-light lasing under near-infrared excitation at room temperature. This can be done by optimizing the upconversion efficiency and emission intensity balance of the blue, green, and red peaks through the proper tuning of sensitizer (Yb³⁺) and activator (Er³⁺, Tm³⁺) concentration in the host matrix. In addition, we minimize the difference of lasing threshold and maintain stable single-mode operation to achieve simultaneous red, green, and blue lasing by optimizing the radius of the hexagonal microrods

Coupling of Ag Nanoparticle with Inverse Opal Photonic Crystals as a Novel Strategy for Upconversion Emission Enhancement of NaYF₄: Yb³⁺, Er³⁺ Nanoparticles

Rare-earth-ion-doped upconversion (UC) nanoparticles have generated considerable interest because of their potential application in solar cells, biological labeling, therapeutics, and imaging. However, the applications of UC nanoparticles were still limited because of their low emission efficiency. Photonic crystals and noble metal nanoparticles are applied extensively to enhance the UC emission of rare earth ions. In the present work, a novel substrate consisting of inverse opal photonic crystals and Ag nanoparticles was prepared by the template-assisted method, which was used to enhance the UC emission of NaYF₄: Yb³⁺, Er³⁺ nanoparticles. The red or green UC emissions of NaYF₄: Yb³⁺, Er³⁺ nanoparticles were selectively enhanced on the inverse opal substrates because of the Bragg reflection of the photonic band gap. Additionally, the UC emission enhancement of NaYF₄: Yb³⁺, Er³⁺ nanoparticles induced by the coupling of metal nanoparticle plasmons and photonic crystal effects was realized on the Ag nanoparticles included in the inverse opal substrate. The present results demonstrated that coupling of Ag nanoparticle with inverse opal photonic crystals provides a useful strategy to enhance UC emission of rare-earth-ion-doped nanoparticles

X‑ray-Irradiation-Induced Discoloration and Persistent Radioluminescence for Reversible Dual-Mode Imaging and Detection Applications

The combination of X-ray-irradiation-induced photochromism and persistent radioluminescence in a single material presents an exciting avenue for multi-functional applications such as optical memory, anti-counterfeiting, and X-ray detection and imaging. However, developing such a material remains a significant challenge. Here, a white Ba3MgSi2O8:Mn2+ photochromic phosphor was prepared, exhibiting a white-to-orange color change (>20 h for bright field) and good persistent radioluminescence emission (>90 min for dark field) in response to X-ray radiation. The photochromic phosphor also demonstrated accelerated bleaching and recovery after 14 min of 254 nm UV light stimulation. This Ba3MgSi2O8:Mn2+-based flexible film displayed simultaneous reversible photochromism and recoverable persistent luminescence, providing dual-mode X-ray imaging and detection capabilities, as well as good reproducibility and read/write erasability. This study suggests that combining X-ray-induced photochromism and persistent radioluminescence in a single material is a promising approach to design advanced photonic materials for information security, cryptography, and smart anti-counterfeiting applications

Upconversion Emission Enhancement of NaYF₄:Yb,Er Nanoparticles by Coupling Silver Nanoparticle Plasmons and Photonic Crystal Effects

Metal nanoparticle plasmons or the photonic crystal effect are being widely used to modify luminescence properties of materials. However, coupling of surface plasmons with photonic crystals are seldom reported for enhancing luminescence of materials. In this paper, a new method for upconversion emission enhancement of rare-earth doped nanoparticles is reported, attributed to the coupling of surface plasmons with photonic band gap effects. Opal/Ag hybrid substrates were prepared by depositing Ag nanoparticles on the top layer of opals by magnetron sputtering. The selective enhancement of red or green upconversion emission of NaYF₄:Yb³⁺,Er³⁺ nanoparticles on the opal/Ag hybrid substrates is attributed to the coupling effect of surface plasmons and Bragg reflection of the photonic band gap. In addition, the upconversion emission enhancement of NaYF₄:Yb³⁺,Er³⁺ nanoparticles on the opal/Ag hybrid substrate is attributed to the excitation enhancement was obtained when the excitation light wavelengths overlap with the photonic band gaps of opal/Ag hybrid substrates. We believe that these enhancement effects based on the coupling of metal nanoparticles with the photonic band gap could be extended to other light-emitting materials, which may result in a new generation of lighting devices

Enhanced Storage Capacity via Anion Substitution for Advanced Delayed X‑ray Detection

X-ray radiation information storage, characterized by its ability to detect radiation with delayed readings, shows great promise in enabling reliable and readily accessible X-ray imaging and dosimetry in situations where conventional detectors may not be feasible. However, the lack of specific strategies to enhance the memory capability dramatically hampers its further development. Here, we present an effective anion substitution strategy to enhance the storage capability of NaLuF4:Tb3+ nanocrystals attributed to the increased concentration of trapping centers under X-ray irradiation. The stored radiation information can be read out as optical brightness via thermal, 980 nm laser, or mechanical stimulation, avoiding real-time measurement under ionizing radiation. Moreover, the radiation information can be maintained for more than 13 days, and the imaging resolution reaches 14.3 lp mm–1. These results demonstrate that anion substitution methods can effectively achieve high storage capability and broaden the application scope of X-ray information storage