Simultaneous Tailoring of Dual-Phase Fluoride Precipitation and Dopant Distribution in Glass to Control Upconverting Luminescence

Low-Temperature Fluoro-Borosilicate Glass for Controllable Nano-Crystallization in Glass Ceramic Fibers

A fluorosilicate (FS) nano-crystallized glass ceramic (NGC) is one of the most commonly used gain materials for applications in optical devices due to its excellent thermal stability as well as high-efficiency luminescence. However, FS glass can hardly be used to prepare NGC fibers due to its high preparation temperature. Here, a series of low-temperature fluoro-borosilicate (FBS) glasses were designed for the fabrication of active NGC fibers. By modulating B₂O₃, the preparation temperature of FBS glass was reduced to 1050 °C, and the crystallization in FBS NGCs was more controllable than in FS NGC. The crystallization of the impure phase was inhibited, and single-phase rare earth (RE)-fluoride nanocrystals were controllably precipitated in the FBS NGCs. The 40Si-20B FBS NGC not only exhibited a higher optical transmittance, but the luminescence efficiency was also much higher than traditional FS NGCs. More importantly, NGC fibers were successfully fabricated by using the designed FBS glass as core glass. Nanocrystals were controllably precipitated and greatly enhanced, and upconversion luminescence was observed in NGC fibers. The designed FBS NGCs provided high-quality optical gain materials and offered opportunities for fabricating a wide range of NGC fibers for multiple future applications, including fiber lasers and sensors.

Polymorphs of Rb3ScF6: X-ray and Neutron Diffraction, Solid-State NMR, and Density Functional Theory Calculations Study

The crystal structures of three polymorphs of Rb₃ScF₆ have been determined through a combination of synchrotron, laboratory X-ray, and neutron powder diffraction, electron diffraction, and multinuclear high-field solid-state NMR studies. The room temperature (RT; α) and medium-temperature (β) structures are tetragonal, with space groups I4₁/a (Z = 80) and I4/m (Z = 10) and lattice parameters a = 20.2561(4) Å, c = 36.5160(0) Å and a = 14.4093(2) Å, c = 9.2015(1) Å at RT and 187 °C, respectively. The high-temperature (γ) structure is cubic space group Fm3̅m (Z = 4) with a = 9.1944(1) Å at 250 °C. The temperatures of the phase transitions were measured at 141 and 201 °C. The three α, β, and γ Rb₃ScF₆ phases are isostructural with the α, β, and δ forms of the potassium cryolite. Detailed structural characterizations were performed by density functional theory as well as NMR. In the case of the β polymorph, the dynamic rotations of the ScF₆ octahedra of both Sc crystallographic sites have been detailed.

Nano-Crystallization of Ln-Fluoride Crystals in Glass-Ceramics via Inducing of Yb3+ for Efficient Near-Infrared Upconversion Luminescence of Tm3

Transparent glass-ceramic composites embedded with Ln-fluoride nanocrystals are prepared in this work to enhance the upconversion luminescence of Tm³⁺. The crystalline phases, microstructures, and photoluminescence properties of samples are carefully investigated. KYb₃F₁₀ nanocrystals are proved to controllably precipitate in the glass-ceramics via the inducing of Yb³⁺ when the doping concentration varies from 0.5 to 1.5 mol%. Pure near-infrared upconversion emissions are observed and the emission intensities are enhanced in the glass-ceramics as compared to in the precursor glass due to the incorporation of Tm³⁺ into the KYb₃F₁₀ crystal structures via substitutions for Yb³⁺. Furthermore, KYb₂F₇ crystals are also nano-crystallized in the glass-ceramics when the Yb³⁺ concentration exceeds 2.0 mol%. The upconversion emission intensity of Tm³⁺ is further enhanced by seven times as Tm³⁺ enters the lattice sites of pure KYb₂F₇ nanocrystals. The designed glass ceramics provide efficient gain materials for optical applications in the biological transmission window. Moreover, the controllable nano-crystallization strategy induced by Yb³⁺ opens a new way for engineering a wide range of functional nanomaterials with effective incorporation of Ln³⁺ ions into fluoride crystal structures.

Glass crystallization making red phosphor for high-power warm white lighting

Rapid development of solid-state lighting technology requires new materials with highly efficient and stable luminescence, and especially relies on blue light pumped red phosphors for improved light quality. Herein, we discovered an unprecedented red-emitting Mg₂Al₄Si₅O₁₈:Eu²⁺ composite phosphor (λ_ex = 450 nm, λ_em = 620 nm) via the crystallization of MgO-Al₂O₃-SiO₂ aluminosilicate glass. Combined experimental measurement and first-principles calculations verify that Eu²⁺ dopants insert at the vacant channel of Mg₂Al₄Si₅O₁₈ crystal with six-fold coordination responsible for the peculiar red emission. Importantly, the resulting phosphor exhibits high internal/external quantum efficiency of 94.5/70.6%, and stable emission against thermal quenching, which reaches industry production. The maximum luminous flux and luminous efficiency of the constructed laser driven red emitting device reaches as high as 274 lm and 54 lm W^-1, respectively. The combinations of extraordinary optical properties coupled with economically favorable and innovative preparation method indicate, that the Mg₂Al₄Si₅O₁₈:Eu²⁺ composite phosphor will provide a significant step towards the development of high-power solid-state lighting.

LiYF4-nanocrystal-embedded glass ceramics for upconversion: glass crystallization, optical thermometry and spectral conversion

Glass ceramics (GCs) can perfectly integrate nanocrystals (NCs) into bulk materials. Herein, GCs containing LiYF4 NCs were fabricated via a traditional melt-quenching method and subsequent glass crystallization. Structural characterization was carried out via X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) analysis, suggesting the precipitation of LiYF4 NCs from a glass matrix. Taking Eu3+ as a structural probe, the spectrographic features provide compelling evidence for the partition of dopants. In particular, intense upconversion (UC) emission was achieved when co-doped with Yb3+ and Er3+. Temperature-dependent UC emission behaviour was also established based on the fluorescence intensity ratio (FIR) of Er3+, to study its properties for optical thermometry. Furthermore, spectral conversion was attained through cross relaxation (CR) between Ce3+ and Ho3+, tuning from green to red with various Ce3+ doping concentrations. There is evidence that LiYF4 NC-embedded GCs were favorable for UC, which may be extremely promising for optical thermometry and spectral conversion applications. This work may open up new avenues for the exploration of GC materials for expansive applications.

Enhanced IR-driven photoelectrochemical responses of CdSe/ZnO heterostructures by up-conversion UV/visible light irradiation

We, for the first time, report the development of infrared (IR)-driven photoelectrochemical (PEC) cells using up-conversion glass-ceramics as substrates, which is different from the previous strategies of decorating photocatalysts with up-conversion (UC) rare earth-doped fluoride nanoparticles to utilize IR light. Our approach is more efficient since the use of UC glass-ceramics as substrates of photocatalysts could overcome the chemical instability of fluoride nanoparticles, the blockage of incident light, and the limited exposure of photocatalysts to liquid electrolytes. Oxyfluoride glass-ceramics bearing (Yb,Er)-doped YF₃ and (Yb,Tm)-doped YF₃ nanocrystals turned out to generate UC green and ultraviolet/blue emissions, respectively, under 980 nm illumination. High-density ZnO nanorods were grown on the up-conversion glass-ceramic substrates by the hydrothermal method and they were subsequently overcoated with CdSe nanocrystals to obtain CdSe/ZnO heterostructures by the chemical bath deposition method. CdSe nanoparticles were excited by both the UC UV emission from Tm and the visible emission from Er and Tm, while ZnO nanorods were excited mostly by the UC UV emission from Tm. Because of the difference in the UC emissions from Er and Tm, two distinct carrier transportations, sensitization and type-II cascade, occurred in the identical CdSe/ZnO heterostructures. Eventually, CdSe/ZnO fabricated on the glass-ceramics bearing (Yb,Tm)-doped YF₃ showed increased photocurrent density compared to that fabricated on the glass-ceramics bearing (Yb,Er)-doped YF₃ due to the charge separation activated by the type-II cascade structure.

Morphology control, spectrum modification and extended optical applications of rare earth ion doped phosphors

This review summarizes the morphology control strategy, phase transfer theory, spectrum modulation, and extended optical applications of RE³⁺-doped phosphors.

CsRe2F7@glass nanocomposites with efficient up-/down-conversion luminescence: from in situ nanocrystallization synthesis to multi-functional applications

Recently, lanthanide-doped luminescent materials have been widely studied and most investigations have been limited to rare-earth-containing fluorides formed with lighter alkali metals (Li, Na and K). Hence, it is important to understand the luminescence properties of cesium rare-earth fluorides. Herein, a novel type of multi-functional luminescent material, hexagonal β-CsRe2F7 (Re = La-Lu, Y, Sc) nanocrystals, is successfully prepared via in situ crystallization inside glass. Specifically, Yb/Er:β-CsLu2F7@glass exhibits a much higher upconversion quantum yield than Yb/Er:β-NaYF4@glass (about 6 times), which is believed to be one of the most efficient upconversion materials so far. Impressively, Er:CsYb2F7@glass shows a significant photothermal effect, which can produce variable upconversion emission colors induced by an incident 980 nm laser diode, enabling it to find practical application in novel/high-precision anti-counterfeiting. In addition, Ce:CsLu2F7@glass with a maximal photoluminescence quantum yield reaching 67% can yield intense X-ray excitable radioluminescence, which is even higher than that of a commercial Bi4Ge3O12 scintillator. Benefitting from the effective protection of robust oxide glass, lanthanide-doped CsRe2F7 nanocrystals show long-term stability in harsh environments, retaining near 100% luminescence after directly immersing them in water/oil for 30 days. It is expected that the present nanocomposites have potential applications in the fields of high-end upconversion anti-counterfeiting and high-energy radiation detection.