Red Upconversion Emission Bands Research Articles

Growth, structure and upconversion properties of Yb3+ and Er3+ co-doped Gd3Sc2Al3O12 crystal

Upconversion luminescent materials exhibit excellent near-infrared to visible conversion properties that make them crucial for optoelectronics, visible display devices, and biological labels. Herein, we report a high-quality Yb3+ and Er3+ co-doped Gd3Sc2Al3O12(GSAG) single crystal which was grown successfully by the traditional Czochralski method with Er3+ and Yb3+ doping concentration of 1 at.% and 10 at.%, respectively. The structure parameters of the crystal were observed by the X-ray Rietveld refinement method. In addition, the transmission and upconversion emission properties of the crystal were analyzed at room temperature. The 940 nm excited upconversion energy transfer mechanism and dependence of the green-to-red upconversion intensity ratio (Ig/r) on pump power were investigated. A dominant two-photon absorption process of the green and red upconversion emission bands were revealed in Yb,Er:GSAG crystal. The results indicate the crystal is a promising upconversion single crystalline material.

Tunable green/yellow upconversion emission and enhancement of the NIR luminescence in BaLaAlO4:Er3+ phosphors by codoping with Yb3+ ions

This work reports the structural, morphological and luminescent properties of BaLaAlO4 doped with Er3+ and Yb3+ ions (BAL:Er,Yb). These phosphors were synthesized by the combustion method. The Yb concentration was varied from 0 to 12 mol% while the Er3+ concentration was fixed to 3 mol%. According to the analysis by X-ray diffraction, all the samples doped with different content of Yb presented an orthorhombic phase. The scanning electron microscopy revealed that the undoped BAL is formed by microcubes. For Yb concentrations of 0.5 and 1 mol%, the amount of cubes diminished by ≈ 70% and other morphologies such as 3D rectangles, nanorods and coalesced microparticles appeared. If the BAL:Er,Yb phosphors are doped with Yb concentrations of 3–7 mol%, only coalesced microparticles with irregular morphology are visualized. Moreover, nanorods mixed with coalesced microparticles are again observed in the sample doped with 12 mol% of Yb. On the other hand, the BAL:Er,Yb phosphors presented green and red upconversion emission bands after excitation with NIR light at 980 nm. Additionally, the BAL:Er,Yb phosphors showed an intense NIR emission band at 800 nm, which resulted from excitation of Er3+ ions. The BAL phosphors doped with 7 and 12 mol% of Yb were excited with a low power NIR LED of 70 mW and produced luminance values of 1365 and 1813 cd/m2 for the green and yellow emissions, respectively. Such high luminance and the emission of NIR light suggest that the BAL phosphors are good candidates for applications in lighting, displays and bioimaging.

Influence of transition metal ions: Broad band upconversion emission in thermally stable phosphors

Abstract Frequency upconversion (UC) study at different temperatures in Ho3+/Yb3+/Mn4+:SnO2 phosphors, prepared via solid state reaction method has been performed. The prepared phosphors have been characterized via XRD, Raman spectra, FESEM, EDAX, XPS and diffuse reflectance analysis. The decrease in bandgap has been reported due to codoping with Mn4+ ions in the developed phosphors. An enhancement of about 47 and 6 times in the green and red UC emission bands has been observed with the addition of Yb3+ ions. Moreover, incorporation of Mn4+ ions gives broad visible UC emission ranging from 400 nm to 900 nm. The mechanism responsible for the enhancement and broad band UC emission has been discussed on the basis of pump power dependence followed by the photon avalanche energy transfer process.

Upconversion luminescence quenching mechanism of single Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disk

Different amounts of Au nanoparticles decorated NaYF4: Yb3+, Er3+ hexagonal disks were synthesized by using a hydrothermal strategy. In comparsion with single NaYF4: Yb3+, Er3+ hexagonal disk, the average upconversion quenching factors for the green and red upconversion emission bands of single Au nanoparticles decorated hexagonal disk increased obviously with the increasing of the amounts of uploaded Au nanoparticles. The local field distribution accompanied with the enhancement factors were obtained by finite element method. Based on the cavity quantum optics theory, a quantitative analysis for the Au nanoparticles induced quenching was addressed. The results show that the presence of the higher mode Purcell factors induced nonradiative energy transfer from the upconverter to the Au nanoparticles, which contributed to the quenching significantly. The modest weakening of local fields only played a minor role. It could offer a novel theory guidance for the seeking of the novel plasmonic nanostructure-upconverter hybrid system.

The photo-switch effect and the energy-level population change of Li+ doping in Yb3+/Er3+ co-doped Y2O3 upconversion films

Li+–Er3+/Yb3+ co-doped Y2O3 upconversion films were prepared via a sol–gel method and spin coating. Li+ doping resulted in better crystallinity and larger crystal grains. All the films had purple, green and red upconversion emission bands. Li+ doping decreased the lattice symmetry of the matrix and enhanced energy transfer, and green and red emissions improved by more than 100 times. Meanwhile, Li+ doping also increased the lifetime of both the green and red emissions significantly via reducing the probability of non-radiation transition. Moreover, the 4S3/2 energy level tends to be populated with increasing Li+, the matrix phonon energy change and the fluorescence quenching causing it. When the mole ratio of rare earth ions to Li+ was over 1:2, the purple emission shifted from 436 to 409 nm. Here, Li+ played a switching role in the conversion of these two channels. Li+ doping up to a critical concentration value changed the matrix phonon energy. The match of the phonon energy with the energy gap between 2G7/2–2H9/2 and that of 2G7/2–4F3/2 was the reason.

Color-tunable upconversion luminescence and temperature sensing behavior of Tm3+/Yb3+ codoped Y2Ti2O7 phosphors

Tm3+/Yb3+: Y2Ti2O7 phosphors with color-tunable upconversion (UC) luminescence and optical thermometric property were synthesized and characterized. The phosphors exhibit intense blue and red UC emission bands under 980 nm excitation. The blue to red emission intensity ratio shows an enhancement with increasing excitation power. Hence, the UC color can be tuned from almost white to blue by control of the excitation power density. The temperature sensing behaviors are studied based on fluorescence intensity ratio (FIR) technique at different excitation powers. FIR-1 from the thermally coupled levels (1G4(a) and 1G4(b)) is insensitive to the excitation power, which can obtain consistent measurement results at different excitation powers. Meanwhile, the utilization of FIR-2 from non-thermally coupled levels (3F2,3 and 1G4(b)) can achieve higher sensing sensitivities with the maximal value of ∼0.81%K−1 at 398 K. Hence, Tm3+/Yb3+:Y2Ti2O7 phosphor has promising application prospects in the fields of upconverter, optical thermometer and color tunable device.

White light emission from Er2O3 nano-powder excited by infrared radiation

Phosphors of Er2O3 nano-crystalline powders were synthesized by the thermal decomposition method. The structural properties of the nano-powders were investigated with XRD and HRTEM measurements. The cubic phase with a = 10.540 Å was the only phase observed. The average crystalline sizes and the widths of the grain size distribution curves were determined to be 27.2, 18.7 and 9.7 nm, respectively. The spectroscopic properties of the Er2O3 nano-powder were studied by measuring the luminescence, decay and rise patterns under 808 and 975 nm diode laser excitations. A peculiar effect of the pressure was observed since an optically active ion (Er) is part of the complex and not a dopant. A broad band of the white light emission combined with blue, green and red up-conversion emission bands of Er3+ ions were observed at 0.03 mbar pressure under both excitation wavelengths. Only, an intense broad band white light emission was observed from these nanocrystals at atmospheric pressure. Rising patterns show that the white light intensity reaches its maximum value more rapidly under 975 nm excitation although it decays slower than that of 808 nm excitation. The color quality parameters such as the color coordinate (CRI), correlated color temperature and the color rendering index were found to vary with both the excitation wavelength and the ambient pressure indicating that these nanocrystals could be considered good white light emitting source under the infrared excitations.

Effect of the Yb3+ Concentration in Up-Conversion and Electrical Properties of Ho3+/Yb3+ Co-doped (0.94Na0.5Bi0.5TiO3-0.06BaTiO3) Ceramics

Ho3+/Yb3+ co-doped 0.94Na0.5Bi0.5TiO3–0.06BaTiO3 (NBT-BT:Ho3+/Yb3+) ceramics were synthesized by solid-state reaction and characterized by x-ray diffraction (XRD), luminescent, dielectric, ferroelectric and piezoelectric measurements. The XRD diffraction data showed that all the ceramics were single phase with a perovskite structure. Bright green up-conversion (UC) emission bands (545 nm) and weak red UC emission bands (660 nm) corresponded to the transitions from (5F4, 5S2) → 5I8 and 4I5 → 5I8, respectively. Furthermore, optimized UC emission intensity was observed in the NBT-BT:0.005Ho3+/0.03Yb3+ samples. The thermal behavior of UC emission in the ceramics was also investigated and the maximum sensitivity based on fluorescence intensity ratio (FIR) technology was approximately 0.0042 K−1 at 100 K. Moreover, relatively good dielectric properties (e = 4475) and ferroelectric properties (Pr = 32 μ/cm2 and Ec = 37 kV) were obtained in NBT-BT:0.005Ho3+/0.005Yb3+. As a multi-functional material, NBT-BT:Ho3+/Yb3+ ceramics may be useful in electro-optical devices.

White light emission and effect of annealing on the Ho3+–Yb3+ codoped BaCa2Al8O15 phosphor

The BaCa2Al8O15 (BCAO) phosphors codoped with suitable Ho3+–Yb3+ dopant concentration prepared by combustion method illuminate an intense white light upon near infrared diode laser excitation. The structural analysis of the phosphors and the detection of impurity contents have been performed by using the X-Ray Diffraction, FESEM and FTIR analysis. The purity of white light emitted from the sample has been confirmed by the CIE chromaticity diagram. Also, the white light emitted from the sample persists with the variation of pump power density. The phosphors emit upconversion (UC) emission bands in the blue, green and red region (three primary colours required for white light emission) along with one more band in the near infrared region of the electromagnetic spectrum. On annealing the white light emitting sample at higher temperatures, the sample starts to emit green colour and also the intensity of green and red UC emission bands get enhanced largely.

Rare-earth doped colour tuneable up-conversion ZBLAN phosphor for enhancing photocatalysis

Rare-earth doped ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) fluoride glasses have been successfully synthesized showing outstanding UV–VIS up-conversion luminescence of Er3+ and Tm3+, sensitized by Yb3+ ions, under near-infrared excitation at 980nm. The ratio between blue, green and red up-conversion emission bands can be adjusted by varying the pump power density of the incident infrared radiation, resulting in a controlled tuneability of the overall emitting colour from greenish to yellowish. Additionally, the observed high energy UV intense up-conversion emissions are suitable to enhance photocatalytic activity of main water-splitting semiconductor electrodes (such as TiO2) used in sustainable production of hydrogen. Photocatalysis and photolysis degradation of methylene blue in water under sun-like irradiation using benchmark photocatalyst (TiO2 Degussa P25) have been boosted by 20% and by a factor of 2.5 respectively, due to the enhancement of UV radiation that reaches the TiO2 particles by the addition of ZBLAN powder into a slurry-type photo-reactor. Hence, up-conversion ZBLAN phosphors contribute to demonstrate the possibility of transforming the incoming infrared radiation into the UV region needed to bridge the gap of photocatalytic semiconductors.