Zhenguo Ji

Dual-Phase Glass Ceramic: Structure, Dual-Modal Luminescence, and Temperature Sensing Behaviors

Yb(3+)/Er(3+)/Cr(3+) triply doped transparent bulk glass ceramic containing orthorhombic YF3 and cubic Ga2O3 nanocrystals was fabricated by a melt-quenching route to explore its possible application in optical thermometry with high spatial and temperature resolution. It was experimentally observed that Yb(3+)/Er(3+) ions incorporated into the precipitated YF3 nanophase, while Cr(3+) ions partitioned into the crystallized Ga2O3 nanophase after glass crystallization. Importantly, such spatial isolation strategy efficiently suppressed adverse energy transfer among different active ions. As a consequence, intense green anti-Stokes luminescence originated from Er(3+): (2)H11/2,(4)S3/2 → (4)I15/2 transitions, and deep-red Stokes luminescence transitions assigned to Cr(3+): (2)E → (4)A2 radiation were simultaneously realized. Impressively, the intermediate crystal-field environment for Cr(3+) in Ga2O3 made it possible for lifetime-based temperature sensing owing to the competition of radiation transitions from the thermally coupled Cr(3+) (2)E and (4)T2 excited states. In the meantime, the low-phonon-energy environment for Er(3+) in YF3 was beneficial for upconversion fluorescence intensity ratio-based temperature sensing via thermal population between the (2)H11/2 state and (4)S3/2 state. The Boltzmann distribution theory and the two-level kinetic model were adopted to interpret these temperature-dependent luminescence of Er(3+) and Cr(3+), respectively, which gave the highest temperature sensitivities of 0.25% K(-1) at 514 K for Er(3+) and 0.59% K(-1) at 386 K for Cr(3+).

Enhanced luminescence of Mn4+:Y3Al5O12 red phosphor via impurity doping

Currently, white light-emitting-diodes converted using Ce3+:Y3Al5O12 phosphor suffer from the shortage of red component and the easy aging of the organic silicone binder. Herein, a novel and non-rare-earth doped Mn4+:Y3Al5O12 red phosphor was synthesized by a traditional solid-state reaction. This phosphor can emit red luminescence attributed to Mn4+:2E → 4A2 spin-forbidden transition in the 600–700 nm spectral region and can be efficiently excited by both the commercially available near-ultraviolet and blue chips. Impressively, Mg2+, Ca2+, and Ge4+ dopants were found to be beneficial for enhancing Mn4+ luminescence, and the related mechanisms were systematically discussed. Furthermore, Mn4+:Y3Al5O12 embedded inorganic glass ceramic was successfully fabricated to replace the phosphor in organic silicone as the color converter, and a stacking geometric configuration by sequentially coupling a Ce3+:Y3Al5O12 glass ceramic and a Mn4+:Y3Al5O12 glass ceramic with an InGaN blue chip was designed to explore its possible application in warm white light-emitting diodes.

Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage.

A strategy has been adopted for simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage. X-ray power diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectra (PL) were used to characterize the samples. The influence of molar ratio of KF to Y3+ on the crystal phase and morphology has been systematically investigated and discussed. It is found that the molar ratio of KF to Y3+ can strongly control the morphology of the as-synthesized β-NaYF4 samples because of the different capping effect of F− ions on the different crystal faces. The possible formation mechanism has been proposed on the basis of a series of time-dependent experiments. More importantly, the upconversion luminescence of β-NaYF4:Yb3+/Er3+ was greatly enhanced by increasing the molar ratio of KF to RE3+ (RE = Y, Yb, Er), which is attributed to the distortion of local crystal field symmetry around lanthanide ions through K+ ions doping. This synthetic methodology is expected to provide a new strategy for simultaneous morphology control and remarkable upconversion luminescence enhancement of yttrium fluorides, which may be applicable for other rare earth fluorides.

Garnet-based Li6CaLa2Sb2O12:Eu3+ red phosphors: a potential color-converting material for warm white light-emitting diodes

To alleviate the issues of low thermal stability and high correlated color temperature, exploring an inorganic color converter with both yellow- and red-emissions to replace the conventional resin/silicone-based phosphor converter for obtaining high-power warm white light-emitting diodes is highly desired. In this study, a series of garnet-based Li6CaLa2−2xEu2xSb2O12 (x = 0.1–1.0) red phosphors have been successfully synthesized by a conventional high-temperature solid-state method. The microstructure and luminescence properties were systematically investigated by X-ray diffraction, emission/excitation spectra, luminescence lifetimes and temperature-dependent decays. The as-synthesized phosphors exhibited a highly efficient red luminescence at 611 nm corresponding to the 5D0–7F2 electric dipole transition of Eu3+, and the luminescence monotonously enhanced as the Eu3+ content was increased to 100 mol%. The absence of concentration quenching was ascribed to the large Eu3+–Eu3+ distance (7.048–7.105 A) and subsequently the hindering of unwanted energy migration among them in the Li6CaLa2Sb2O12 crystalline lattice. Impressively, the Li6CaLaEuSb2O12 phosphor showed an excellent thermal stability with only 9.7% emission loss when the recording temperature was increased from 293 K to 553 K. To evaluate the suitability of Li6CaLa2Sb2O12:Eu3+ as a red converter, both the garnet-based Y3Al5O12:Ce3+ yellow and Li6CaLa2Sb2O12:Eu3+ red phosphor co-doped glass ceramics were successfully fabricated by a low-temperature co-sintering technique. Importantly, the adverse energy transfers between Ce3+ and Eu3+ were efficiently suppressed due to the spatial separation of Ce3+ in Y3Al5O12 and Eu3+ in Li6CaLa2Sb2O12 in the crystal lattice. As a consequence, the quantum yield of the glass ceramic reached as high as 89.3%, and the constructed white light-emitting diode exhibited an optimal luminous efficacy of 101 lm W−1, a correlated color temperature of 5449 K and a color rendering index of 73.7. It is expected that the developed Li6CaLa2−2xEu2xSb2O12 red phosphors and the related glass ceramics should have potential applications in high-power warm white light-emitting diodes.

Tuning into blue and red: europium single-doped nano-glass-ceramics for potential application in photosynthesis

A series of SiO2–Al2O3–NaF–YF3 oxyfluoride glasses and β-YF3 nanocrystals embedded glass ceramics single-doped with europium ions were prepared by high-temperature melt-quenching to explore blue/red luminescent materials for potential application in the photosynthesis of green plants. Both Eu2+ and Eu3+ activators were demonstrated to coexist in this specially designed glass fabricated under ambient atmosphere, which can be well explained based on the optical basicity model of glass and evidenced by emission, excitation and time-resolved spectra. Furthermore, the crystallization strategy has been adopted to convert the precursor glasses into nano-glass-ceramics. As a result, Eu3+ ions partitioned into the precipitated orthorhombic YF3 nanophase, while Eu2+ ions remained in the glass matrix. Such spatial isolation of the different active ions in glass ceramics can effectively suppress adverse energy transfer between Eu2+ and Eu3+, leading to both intense Eu2+ blue and Eu3+ red emissions under ultraviolet light excitation.

Achieving efficient Tb3+ dual-mode luminescence via Gd-sublattice-mediated energy migration in a NaGdF4 core–shell nanoarchitecture

A strategy to realize dual-mode luminescence from identical Tb3+ is provided via Gd-sublattice-mediated energy migration and core–shell engineering techniques. By optimizing the structure of a NaGdF4:Yb/Tm@NaGdF4:Ce/Tb nanoarchitecture, both upconversion and downshifting emissions, originating from 5D4 → 7F6,5,4,3 transitions of Tb3+, are achieved through Yb3+ → Tm3+ → [Gd3+]n → Tb3+ and Ce3+ → [Gd3+]n → Tb3+ energy transfer processes, respectively.

A dual-functional upconversion core@shell nanostructure for white-light-emission and temperature sensing

A strategy to simultaneously achieve white-light-emission and temperature sensing via a 980 nm excitable upconversion core@shell nanoarchitecture design was provided. Specifically, the prepared Yb/Ho/Ce: NaGdF4@Yb/Tm: NaYF4 active-core@active-shell nanocrystals enabled the spatially confined doping of Ho3+ in the core and Tm3+ in the shell and thus greatly reduced the adverse energy transfers between them, leading to intense upconversion emissions for both Ho3+ and Tm3+ activators. Notably, introducing Ce3+ into the core resulted in the competition of radiation transitions from the Ho3+: 5S2, 5F4 green-emitting states and Ho3+: 5F5 red-emitting one, which was beneficial to tune the red to green intensity ratio and ultimately realize white-light luminescence. Temperature-dependent upconversion emission spectra of the core@shell samples evidenced the joint contribution of Ce3+ in the core and Tm3+ in the shell to improve sensitivity for temperature detection. As a consequence, the core@shell nanostructure was demonstrated to have a high temperature sensitivity (2.4% K−1) and excellent signal discriminability (3040 cm−1), being potentially applicable as an optical thermometric material.

Highly enhanced upconversion luminescence in lanthanide-doped active-core/luminescent-shell/active-shell nanoarchitectures

We reported a new strategy to remarkably improve upconversion luminescence of Ln3+-doped nanoparticles via an active-core/luminescent-shell/active-shell engineering technique. By optimization of the structure of a NaGdF4:Yb@NaYF4:Yb/Er@NaGdF4:Yb sandwich-like nanoarchitecture, a significant enhancement of upconversion emission intensity was realized with the help of the synergistic actions of the suppression of Er-defect energy transfer and the efficient energy transfer from Yb3+ in both active-core and active-shell to Er3+ in the luminescent-shell layer.

Dual-activator luminescence of RE/TM:Y3Al5O12 (RE = Eu3+, Tb3+, Dy3+; TM = Mn4+, Cr3+) phosphors for self-referencing optical thermometry

Traditional optical temperature sensors based on the rare earth fluorescence intensity ratio of two thermally coupled energy states have intrinsic limitations such as low relative sensitivity and large detection error due to the requirement of a narrow energy gap. Herein, a strategy involving the use of rare earth and transition metal dual-emitting centers with completely different thermal-quenching behaviors has been developed to achieve high temperature sensitivity and good discrimination of signals. In particular, Eu3+/Mn4+:Y3Al5O12 with strikingly high absolute and relative sensitivities of 0.441 K−1 and 4.81% K−1 as well as a large energy gap of 2100 cm−1 was realized by taking advantage of Eu3+ luminescence as a reference signal and Mn4+ luminescence as a temperature signal. The versatility of the proposed strategy has been demonstrated by adopting other Tb3+/Mn4+, Dy3+/Mn4+, Eu3+/Cr3+ and Dy3+/Cr3+ dual-activator combinations. It is expected that this preliminary study will provide an important advance in exploring novel self-referencing optical thermometry with excellent performance.

Phase structure control and optical spectroscopy of rare-earth activated GdF3 nanocrystal embedded glass ceramics via alkaline-earth/alkali-metal doping

Hexagonal to orthorhombic phase transformation of GdF3 nanocrystals in bulk glass ceramics was achieved through alkaline-earth/alkali-metal doping and crystallization temperature controlling. Structural characterizations and spectroscopic analyses of the Eu3+ probe evidenced the incorporation of rare earth emitting-centers into the precipitated GdF3 crystals among the glass matrix. In addition, the influence of phase evolution on the upconversion luminescence of Er3+/Yb3+ co-doped glass ceramics was systematically investigated and it was evidenced that the upconversion intensity of the orthorhombic GdF3 embedded glass ceramic was two orders of magnitude higher than that of the hexagonal GdF3 containing glass ceramic. Benefiting from greatly enhanced upconversion luminescence after glass crystallization, the present glass ceramic composites were demonstrated to have promising applications in optical temperature sensors as well as tunable displays.