Red Upconversion Emission Research Articles

Benefit of Pr3+ ions to the spectral properties of Er3+/Pr3+:SrLaAlO4 crystal for use in 2.7 μm mid-infrared laser

High optical quality Er3+ single doped and Er3+/Pr3+ co-doped SrLaAlO4 crystals were grown by the Czochralski method. Detailed spectroscopic analyses of Er3+:SrLaAlO4 and Er3+/Pr3+:SrLaAlO4 crystals were carried out. With the presence of Pr3+, the intensities of the red up-conversion and near-infrared emissions reduce to 1/4 and 1/10, while the intensity of mid-infrared emission increases to 1.6 times of those for Er3+:SrLaAlO4. Furthermore, the lifetimes of 4I11/2 upper and 4I13/2 lower laser levels for 2.7 μm emission fall from 1.06 ms to 4.42 ms–0.73 ms and 0.93 ms. It means that the self-termination bottleneck in Er3+ 2.7 μm laser was inhibited in a great degree. Deactivation effect of Pr3+ ions as well as the energy transfer mechanism in Er3+/Pr3+:SrLaAlO4 crystal are discussed in detail. The results indicate that the introduce of deactivation ion Pr3+ can be beneficial for achieving an enhanced 2.7 μm emission in Er3+/Pr3+:SrLaAlO4.

Enhanced upconversion luminescence and single-band red emission in β-Ba2ScAlO5: Yb3+/Er3+ phosphor through doping Cu2+

This study reports on a single-band red upconversion emission of Er3+ in a pure-phase β-Ba2ScAlO5: Yb3+/Er3+ fluorescent powder by doping Cu2+. The optimum concentration of Cu2+ ions in β-Ba2ScAlO5: Yb3+/Er3+ powder was 0.20. The red and green emission intensity of β-Ba2ScAlO5: Yb3+/Er3+ sample with optimum concentration of Cu2+ ions increased by 19.1 and 3.6 times, respectively, relative to the β-Ba2ScAlO5:Yb3+/Er3+ material lacking Cu2+ ions. The red-green integral intensity ratio of the β-Ba2ScAlO5:Yb3+/Er3+/Cu2+ phosphor reached 141.9. The potential upconversion luminescence (UCL) mechanism was explored by analyzing the down-shift luminescence (DS) spectra, upconversion luminescence spectra, and upconversion fluorescence lifetime decay curves of Yb3+/Er3+/Cu2+ triple-doped β-Ba2ScAlO5. Enhanced red emission and green upconversion emission are achieved by energy transfer between defect band by the oxygen vacancy and Er3+ energy levels. This study's findings establish the foundation for developing novel phosphors with single UC red light emission, which has potential applications in illumination displays, optical thermometry, and biomedicine.

Activating ultralow upconversion nanothermometry in neodymium sublattice for heart tissue imaging rapid-responsive

The fields of biosensitivity and biological imaging have received a lot of attention from rare earth-doped upconversion nanoparticles (UCNPs). However, owing to the relatively large energy difference of rare earth ions, biological sensitivity based on UCNPs is restricted to detect at low temperature. Here, we design core-shell-shell NaErF4:Yb@Nd2O3@SiO2 UCNPs as a dual-mode bioprobe that produces blue, green, and red multi-color upconversion emissions at extremely low temperatures between 100 K and 280 K. Based on the thermally coupled energy levels (TCELs) of Er3+ (2H11/2 and 4S3/2) and Nd3+ (4F5/2 and 4F3/2) at 100 K, the greatest relative sensitivity (SR) approaches 12.7% K−1. NaErF4:Yb@Nd2O3@SiO2 injection is used to achieve blue upconversion emission imaging of frozen heart tissue, showing that this UCNP can serve as a low-temperature sensitive biological fluorescence.

Near-infrared laser-irradiated upconversion nanoparticles with dexamethasone precise released for alleviating lung ischemia-reperfusion injury.

Introduction: Dexamethasone (DEX), as an important enduring-effect glucocorticoid (GC), holds great promise in the field of lung ischemia-reperfusion injury (LIRI) comprehensive therapy owing to its immunomodulatory properties, such as inducing apoptosis and cell cycle distribution. However, its potent anti-inflammatory application is still restricted because of multiple internal physiologic barriers. Methods: Herein, we developed upconversion nanoparticles (UCNPs) coated with photosensitizer/capping agent/fluorescent probe-modified mesoporous silica (UCNPs@mSiO2[DEX]-Py/β-CD/FITC, USDPFs) for precise DEX release synergistic LIRI comprehensive therapy. The UCNPs were designed by covering an inert YOF:Yb shell on the YOF:Yb, Tm core to achieve high-intensity blue and red upconversion emission upon Near-Infrared (NIR) laser irradiation. Results: Under suitable compatibility conditions, the molecular structure of photosensitizer can be damaged along with capping agent shedding, which endowed USDPFs with an outstanding capability to carry out DEX release controlling and fluorescent indicator targeting. Furthermore, the hybrid encapsulating of DEX significantly increased utilization of nano-drugs, improving the water solubility and bioavailability, which was conducive to developing the anti-inflammatory performance of USDPFs in the complex clinical environment. Discussion: The response-controlled release of DEX in the intrapulmonary microenvironment can reduce normal cell damage, which can effectively avoid the side effects of nano-drugs in anti-inflammatory application. Meanwhile, the multi-wavelength of UCNPs endowed nano-drugs with the fluorescence emission imaging capacity in an intrapulmonary microenvironment, providing precise guidance for LIRI.

Polymorphism in LiDy(WO4)2: Phase tunable synthesis, neutron diffraction, and symmetry-driven upconversion red emission

Tunable polymorphic structures to achieve novel properties are of great concern for energy-related applications. Herein, we demonstrate temperature-driven irreversible structural phase transition in LiDy(WO4)2 (β-LiDyW and α-LiDyW). A facile sol-gel method has been employed to achieve phase pure crystalline polymorphs at relatively lower temperatures and time. LiDy(WO4)2 crystallizes in a monoclinic wolframite-type structure (space group, P21/n, No = 14) at ambient temperature and a Scheelite-like tetragonal structure (space group, I41/a, No = 88) upon heating at high temperature. Crystal structure analysis shows that Li+ and Dy3+ occupy the distinct site, and W forms a distorted WO6 octahedron in the β-LiDyW phase. In contrast, Li+ and Dy3+ are statistically distributed on a dodecahedral S4 site sharing a similar crystallographic lattice site, and W forms a WO4 tetrahedron in the α-LiDyW phase. The Wolframite to Scheelite (β → α) transformation is due to the increase in the crystal symmetry on the heating function where distorted WO6 transformed to free WO4 going from β → α phase and coordination of W+6 is lowered. Density functional theory calculations at 0 K revealed that the β-LiDyW is energetically more favorable than α-LiDyW by 337.3 meV per formula unit. The co-substitution of the Yb3+−Er3+ pair in LiDy(WO4)2 lattice displays a concentration-dependent upconversion red emission when excited with 980 nm. The monoclinic phase of LiDy0.7Yb0.2Er0.1(WO4)2 shows an intense red emission at 656 nm due to inherent lower crystal symmetry. Distorted WO6 and chemically induced lattice distortions in β-LiDy0.7Yb0.2Er0.1(WO4)2 would have strongly influenced the coordination environment of the Er3+ to achieve red emission. This study presents the structural relationship among the tunable crystallographic phases of LiDy(WO4)2 with the observed red emission induced by Yb3+−Er3 upon 980 nm irradiation.

Control of red upconversion emission in Er3+–Yb3+– Fe3+ tri–doped biphasic calcium phosphate

This paper reports a novel red upconversion (UC) emission in Er/Yb/Fe tri-doped biphasic calcium phosphate (BCP) synthesized using a solid-state reaction method. The crystal structure of BCP was analyzed using Rietveld refinement, which confirmed the hexagonal structure of hydroxyapatite and the rhombohedral structure of beta-tricalcium phosphate with good crystallinity. The results of Rietveld refinement, Fourier-transform infrared spectroscopy, and diffuse reflectance spectroscopy revealed that the presence of Fe3+ in the host lattices strongly influenced the crystal structure of the phosphors. The phosphors showed intense red UC emission with a maximum at 660 nm upon the near-infrared excitation of a diode laser at 975 nm. Significantly, the red emission intensities of the Fe3+-doped sample were 60-times greater than those of the undoped sample. The preferential enhancement of red UC emission was due to the energy transfer from the high-energy excited state |2F7/2, 4T1g > of the Yb3+-Fe3+ dimer to the 4F9/2 state of the Er3+ ions. The UC emission mechanism was primarily estimated by the dependence of the UC spectral intensities on the laser 975 nm excitation power. The BCP: Er/Yb/Fe phosphor generating strong red UC emission could be used to develop a highly effective bioimaging tool in biomedicine.

Optimized core-shell lanthanide nanoparticles with ultrabright Ce3+-modulated second near-infrared emission for "lighting" plants.

Second near-infrared (NIR-II, 1000-1700 nm) photon-mediated fluorescence imaging has attracted extensive interest in the field of bioimaging. However, NIR-II fluorescent nanoprobes competent for plant imaging have been rarely developed. Herein, lanthanide-doped nanoparticle (LDNP) optimal core-shell structure and ultrabright NIR-II emission were developed for "lighting" plants. The Ce3+-doped active shell coated on the NaErF4:Tm core enables dual-mode red upconversion (UC) and NIR-II downconversion (DC) emission of LDNPs upon 980 nm laser excitation. Under the optimized doping content, the intensities of red UC and NIR-II DC emission were respectively boosted by 5- and 19-fold those of the core nanoparticles, which endowed LDNPs with ideal NIR-II emissive capabilities for optical imaging of plants. Significantly, the NIR-II fluorescent signal affords much higher signal-to-noise rate than the red UC. LDNPs were modified with polyethyleneimine to enable outstanding hydrophilicty and facilitate their uptake by plants. Arabidopsis thaliana and Nicotiana benthamiana were chosen as plant models for NIR-II imaging studies. The toxic effect of LDNPs after being transported into Brassica rapa chinensis was systematically studied on mice. The NIR-II imaging strategy offers a promising method for studying the uptake and transport of nanoparticles in plants.

Red upconversion emission of Er<sup>3+</sup> enhanced by building NaErF<sub>4</sub>@ NaYbF<sub>4</sub>:2%Er<sup>3+</sup> core-shell structure

Building core-shell structures are widely used to enhance and regulate the luminescence properties of rare-earth-doped micro/nano materials. In this work, a variety of different NaErF<sub>4</sub> core-shell and core-shell-shell nanocrystals are successfully constructed based on high temperature co-precipitation method by epitaxial growth technology. The upconversion red emission intensities of Er<sup>3+</sup> ions in different core-shell structures are effectively enhanced by regulating their structures and doping ions. The experimental structures show that the constructed core-shell nanocrystals each have a hexagonal phase structure, and core-shell structure of about 40 nm. In the near infrared 980 nm laser excitation, the NaErF<sub>4</sub> core-shell nanocrystal shows a strong single-band red emission. And the single-band red emission intensity of Er<sup>3+</sup> ions is enhanced through constructing the NaErF<sub>4</sub>@NaYbF<sub>4</sub>:2%Er<sup>3+</sup> core-shell structure. The experimental results show that red emission intensity of Er<sup>3+</sup> ions is about 1.4 times higher than that of the NaErF<sub>4</sub>@NaYbF<sub>4</sub> core-shell structure by constructing the NaErF<sub>4</sub>@NaYbF<sub>4</sub>:2%Er<sup>3+</sup> core-shell structures under 980 nm excitation, and its red/green emission intensity ratio increases from 5.4 to 6.5. Meanwhile, when NaErF<sub>4</sub>@NaYbF<sub>4</sub>:2%Er<sup>3+</sup> core-shell structure recoats the NaYF<sub>4</sub> inert shell and is added with a small quantity of Tm<sup>3+</sup> ions, their red emission intensities of Er<sup>3+</sup> ions are 23.2 times and 40.3 times that of NaErF<sub>4</sub>@NaYbF<sub>4</sub> core-shell structures, and their red/green emission intensity ratios reach 7.5 and 10.2, respectively. The red emission enhancement of Er<sup>3+</sup> ions is mainly caused by bidirectional energy transfer process of high excitation energy of Yb<sup>3+</sup> ions and energy trapping center of Tm<sup>3+</sup> ions which effectively change the density of population of luminescent energy levels of Er<sup>3+</sup> ions. Furthermore, the coated NaYF<sub>4</sub> inert shell also effectively weakens the surface quenching effect of nanocrystals. The mechanisms of red enhancement in different core-shell structures are discussed based on the spectral properties, the process of interion energy transfer, and luminescence kinetics. The constructed NaErF<sub>4</sub>@NaYbF<sub>4</sub>:2%Er<sup>3+</sup>@NaYF<sub>4</sub> core-shell structures with high-efficiency red emission in this work have great potential applications in the fields of colorful anti-counterfeiting, display and biological imaging.

Enhancing red upconversion emission in NaErF4@NaYF4 core-shell nanoparticles by introducing Yb3+ ions as energy trapping centers

Rare-earth-doped red upconversion nanocrystals (UCNPs) have shown excellent prospects for application in biological imaging, labeling and other fields. In this work, NaErF4@NaYF4 core-shell (CS) UCNPs were successfully constructed to enhance red emission by introducing Yb3+ ions as energy trapping centers under different near-infrared excitation wavelengths. The experimental results showed that the red upconversion emission intensity of NaErF4@ NaYF4 CS UCNPs doped with 15%Yb3+ ions in the core is nearly 202 times higher than that of NaErF4 UCNPs under 980 nm excitation wavelength, with a red/green emission intensity ratio of 18.7. When Yb3+ ions were introduced into the core and shell of NaErF4@NaYF4 CS UCNPs, NaErF4:15%Yb3+@NaYF4:10%Yb3+ CS UCNPs showed the best red emission due to the energy network characteristics of the Yb3+ ions. Furthermore, the enhancement mechanism of red emission in NaErF4@NaYF4 CS UCNPs with different Yb3+ ion doping concentrations were discussed, based on the spectral characteristics and lifetime. It was found that doping of Yb3+ ions in NaErF4@ NaYF4 CS UCNPs can significantly improve the red emission intensity of Er3+ ions through controlling energy migration (EM) and energy back transfer (EBT) processes, which has considerable potential for applications in bioimaging and anti-counterfeiting.

Phase evolution in Bi2O3:Yb3+/Er3+ nanoparticles with nearly single-band red upconversion luminescence

Bi2O3:Yb3+/Er3+ nanoparticles with flower-like morphology were easily synthesized by urea-assisted coprecipitation reactions. The influences of calcination temperature and doping concentration on the crystal phase structures of Bi2O3 and Bi2O3:x%Yb3+/2%Er3+ (x = 0–30) were systematically investigated by XRD analysis. The experimental results revealed that lanthanide doping could effectively improve the thermal stability of Bi2O2CO3 samples, and the monoclinic-to-tetragonal-to-cubic phase transitions of Bi2O3 were implemented by controlling calcination temperatures and introducing smaller lanthanide ions (Ln3+) into the host lattices. Based on the analysis of TG and DSC curves, we found that the fundamental reason for this phase transition was the different stabilities of each crystalline phase under different doping conditions. Upon 980 nm laser excitation, Bi2O3:x%Yb3+/2%Er3+ samples presented near single-band red upconversion emission owing to the efficient energy reabsorption of the Bi2O3 host to Er3+ emission.

Ultraviolet and visible upconversion in Yb/Er-CaSiO3 β-wollastonite phosphors

Wollastonite (CaSiO3) is a well-known rock-forming mineral and an important constituent in ceramics and cement industries due to its outstanding mechanical, chemical, and thermal stabilities. Despite technological importance, functional properties such as photon upconversion in CaSiO3 wollastonite ceramics have not been studied. In this contribution, Yb- and Er-doped CaSiO3 (Yb/Er–CaSiO3) wollastonite ceramics were synthesized via microwave hydrothermal technique followed up by heat-treatment in an air environment. X-ray diffraction and transmission electron microscopy studies confirmed the β-wollastonite (2M) phase in the synthesized samples heat-treated at 1050 °C. X-ray photoelectron spectroscopy analysis has shown that the binding energy of Ca 2p orbitals decreases after doping, indicating a change in the crystal environment of Ca in the CaSiO3 and hence a successful incorporation of Yb3+ and Er3+ ions in the lattice. The 980 nm excitation resulted in ultraviolet, violet and strong green and red upconversion emissions as well as downshifting infrared emissions due to the energy transfer between Yb3+ and Er3+ ions. An absolute upconversion quantum yield in the 400–800 nm range is 0.04%. The most intense phonon band was observed at 969 cm−1 in the Yb/Er–CaSiO3 system. This study demonstrates that the β-wollastonite can be developed as a new kind of efficient upconversion phosphor material.

Fe/Mn Bimetal-Doped ZIF-8-Coated Luminescent Nanoparticles with Up/Downconversion Dual-Mode Emission for Tumor Self-Enhanced NIR-II Imaging and Catalytic Therapy.

ZIF-8, as an important photoresponsive metal-organic framework (MOF), holds great promise in the field of cancer theranostics owing to its versatile physiochemical properties. However, its photocatalytic anticancer application is still restricted because of the wide bandgap and specific response to ultraviolet light. Herein, we developed lanthanide-doped nanoparticles (LDNPs) coated with Fe/Mn bimetal-doped ZIF-8 (LDNPs@Fe/Mn-ZIF-8) for second near-infrared (NIR-II) imaging-guided synergistic photodynamic/chemodynamic therapy (PDT/CDT). The LDNPs were synthesized by encapsulating an optimal Yb3+/Ce3+-doped active shell on the NaErF4:Tm core to achieve dual-mode red upconversion (UC) and NIR-II downconversion (DC) emission upon NIR laser irradiation. At the optimal doping concentration, the UC and DC NIR-II emission intensities of LDNPs were increased 30.2- and 13.2-fold above those of core nanoparticles, which endowed LDNPs@Fe/Mn-ZIF-8 with an outstanding capability to carry out UC-mediated PDT and NIR-II optical imaging. In addition, the dual doping of Fe2+/Mn2+ markedly decreased the bandgap of the ZIF-8 photosensitizer from 5.1 to 1.7 eV, expanding the excitation threshold of ZIF-8 to the visible light region (∼650 nm), which enabled Fe/Mn-ZIF-8 to be efficiently excited by UC photons to achieve photocatalytic-driven PDT. Furthermore, Fe2+/Mn2+ ions could be responsively released in the tumor microenvironment through degradation of Fe/Mn-ZIF-8, thereby producing hydroxyl radicals (·OH) by Fenton/Fenton-like reactions to realize CDT. Meanwhile, the degradation of Fe/Mn-ZIF-8 endowed the nanosystems with tumor self-enhanced NIR-II imaging function, providing precise guidance for CDT/PDT.

One-step hydrothermal synthesis of Cit-NaYbF4:Er3+ nanocrystals with enhanced red upconversion emission for in vivo fluorescence molecular tomography

Fluorescence imaging techniques represent essential tools in in vitro, preclinical, and clinical studies. In this study, an improved one-step hydrothermal method to synthesize citric acid (CA) modified α-NaYbF4:2%Er3+ nanocrystals was proposed. The introduction of various doping ions into NaYbF4:2%Er3+ and the different valence states of the same ions affect both the crystal size and upconversion luminescence. Therefore, we investigated the upconversion luminescence enhancement of NaYbF4:2%Er3+ by ion doping and find that the upconversion luminescence intensity of the upconversion nanoparticles (UCNPs) co-doped with 5 mol% Fe2+ ions shows the greatest enhancement, especially for red emission at 654 nm. Furthermore, HeLa cells incubated with UCNPs allow for imaging with strong red upconversion emission detection. Confocal laser scanning microscope (CLSM) fluorescent images of HeLa cells indicate that NaYbF4:2%Er/5%Fe2+ leads to a clear outline and improves visualization of the cell morphology. In addition, the CA coated NaYbF4:2%Er3+/5%Fe2+ nanoparticles and NaYbF4:2%Er3+/5%Fe2+ show low cytotoxicity in HeLa cells. Organ imaging reveals the efficiency of these UCNPs to analyze the lungs, liver, and spleen. Together, these results indicate that the Cit-NaYbF4:2%Er3+/5%Fe2+ UCNPs are efficient nanoprobes for fluorescence molecular tomography.

Anomalous upconversion behavior and high-temperature spectral properties of Yb/Ho-SiAlON ceramics

Traditionally, NaYF4 and glass-based materials are considered the most efficient upconversion materials. However, those materials may show signs of thermal warping or risks of fracture during long-term service life in extreme environments. In contrast, SiAlON ceramics preserve their spectral and physical integrity during continuous use due to their ultra-low thermal expansion coefficient (∼3.0 × 10−6/K) even at extreme temperatures. Here, we investigate the upconversion photoluminescence properties of Ho and Yb co-doped SiAlON (Ho/Yb-SiAlON) ceramics, prepared by hot-press method, at room temperature and at high temperature (298–1023 K). At room temperature, Ho/Yb-SiAlON ceramics show intense red upconversion emission under 980 nm excitation. At the high-temperature range, the spectral shift is absent in Ho/Yb-SiAlON indicating that the SiAlON ceramics have high-temperature spectral stability, which may be attributed to the low thermal expansion coefficient of SiAlON ceramics. Ho and Yb concentration-dependent spectra show an anomalous upconversion emission behavior. It is found that a higher sensitizer and low activator concentration combination is not always an ideal choice for sensitized luminescence in SiAlON ceramics which is in stark contrast to the common understanding of sensitized luminescence. The mechanism involving the dominant cross-relaxation process has been proposed to explain the observed anomalous upconversion behavior.

Synthesis and characterization of red-emitting Yb/Ho-CaSiO3 upconversion phosphors

Host material plays an important role in obtaining efficient photon upconversion and downshifting luminescence. Generally, fluoride and oxyfluoride glasses-based materials are used for high-efficiency photon upconversion. However, the poor thermal stability of fluoride glasses and the toxicity of fluorine ions limit their applications. In this report, Yb/Ho-doped CaSiO3 wollastonite phosphors have been demonstrated as efficient red-emitting upconversion phosphors. The phosphors have been synthesized by microwave hydrothermal process followed by heat treatment at 1050 ​°C in the air environment. 2M phase of the β-wollastonite has been confirmed by X-ray diffraction and Raman spectroscopy while the existence of the Yb and Ho dopants in the CaSiO3 lattice has been confirmed by X-ray photoelectron spectroscopy. The synthesized samples showed strong red upconversion emission centered at 662 ​nm and near-infrared downshifting emissions at 2016 ​nm upon 980 ​nm excitation. The emissions were found to depend significantly on the Ho concentration. Temporal evolution of the main emission bands was investigated to show that the energy transfer upconversion from Yb to Ho ions was responsible for the efficient upconversion and downshifting phenomenon.

Temperature sensing characteristics and first-principles simulation of Gd-doped NaY(WO4)2 phosphors

Up-conversion luminescence (UCL) materials have potential applications in many fields. In this work, NaY(WO4)2(NYW):1 mol%Ho3+/25 mol%Yb3+ up-conversion luminescent materials doped with different Gd3+ ion concentrations were synthesized by high-temperature solid-state method. X-ray diffraction and infrared absorption spectra show that the incorporation of Gd3+ ions increases the lattice parameters and changes the stretching vibration of O–W–O bonds. The properties of the obtained UCL materials were also studied under the excitation of a 980 nm laser. All the samples show strong green and red up-conversion emission peaks centered at 542 nm and 645 nm, which correspond to the Ho3+ ion 5F4/5S2→5I8 and 5F5→5I8 energy levels, respectively. The pumping power coefficient shows that the emission at 542 nm and 645 nm is a two-photon process. The temperature dependence of the red-green emission ratio under 980 nm excitation was studied in the range of 343 K–693 K. According to thermal quenching theory, the fluorescence intensity ratio (FIR) is a function of temperature, and the sensitivity of the obtained phosphor reaches its maximum value at 343 K. These results show that NYW:Ho/Yb/Gd UCL is worthy of further study due to its potential for applications in the fields of up-conversion lasers, bioluminescence labeling, and temperature sensing.

The downshifting and upconversion photoluminescence from NaBaSc2(PO4)3 for multicolor anti-counterfeiting

A novel langbeinite-type phosphate NaBaSc2(PO4)3 (NBSP) was synthesized by traditional solid-state reaction. The structure was refined on the basis of X-ray diffraction data. The Rietveld refinement result revealed that the structure of NBSP belongs to cubic system with space group P213 and cell parameters a = 10.0392 Å, b = 10.0392 Å, c = 10.0392 Å, α = β = γ = 90°, V = 1011.80 Å3, and Z = 4. The incorporations of Ce3+/Eu2+-Tb3+ into NBSP led to efficient downshifting emissions with adjustable color from blue to yellow. The luminescent properties, energy transfer and application in white light-emitting diode (WLED) were studied comprehensively. Moreover, blue, yellow and red upconversion emissions were also realized in NBSP by codoping Yb3+-Tm3+/Er3+/Ho3+, respectively. Finally, highly distinguishable luminescent images were obtained under the excitation of 254 nm, 365 nm or 980 nm, based on multiple NBSP phosphors with designed photoluminescence for the application in multicolor anti-counterfeiting. These results point out a practical approach for convenient and reliable anti-counterfeiting.

Achieving Photon Upconversion in Mononuclear Lanthanide Molecular Complexes at Room Temperature.

Photon upconversion luminescence at the molecule scale is a rarely observed phenomenon despite possessing colossal potential for basic research and reality applications. Here we show that the eight-coordinate erbium molecular complex composed of Er3+ ion, dibenzoylmethane, and 2,2'-bipyridine exhibits upconversion emission. Under direct excitation at the absorption band of Er3+ ion at 980 nm, the complex shows upconverted green emissions of Er3+ ion at 525 and 545 nm at room temperature. Noticeably, upon the introduction of fluoride ions into this complex, an additional upconverted red emission at 667 nm appears as well, and the luminescence intensities of both the green and red emissions increase by a factor of 13 at most. This study not only provides a strategy to adjust the green and red emissions in mononuclear erbium complexes but also broadens the horizons of designing lanthanide-based molecular upconversion systems.

Intense green photoluminescence and upconversion-based intrinsic optical bistability in Ho3+/Yb3+ codoped CaSc2O4 and upconversion in Ho3+/Yb3+ codoped CaY2O4 phosphors

The present study details the downshifting, frequency upconversion and intrinsic optical bistability in CaSc2O4:Ho3+/Yb3+ and upconversion in CaY2O4:Ho3+/Yb3+ phosphors synthesized by a complex-based precursor solution method. The X-ray powder diffraction confirms the formation of highly crystalline phosphors with pure orthorhombic phase. Fourier transform infrared studies of synthesized phosphors show vibrational bands due to the presence of Ca–O, Sc–O and Y–O groups. The diffuse reflectance spectra show a number of bands in the UV–vis–NIR regions due to presence of Ho3+ and Yb3+ ions. The values of optical band gap (Eg) are found to be 5.69 eV and 5.58 eV for the Ho3+/Yb3+ codoped CaSc2O4 and CaY2O4 phosphors, respectively. The Ho3+/Yb3+ codoped CaSc2O4 phosphors display intense green downshifting emission upon 454 nm excitations. The lifetime of green luminescence of CaSc2O4:Ho3+/Yb3+ phosphors with varied Yb3+ concentrations have also been measured. The decay curves show a non-exponential nature fitted to the Inokuti-Hirayama model and the energy transfer microscopic parameters have been also calculated. The Ho3+/Yb3+ co-doped CaSc2O4 and CaY2O4 phosphors display intense green alongwith weak blue, red, and near-infrared upconversion (UC) emissions upon 980 nm excitations. The spectral color purity (Sgr) of CaSc2O4:Ho3+(1%)/Yb3+(5%) phosphor is calculated to be 0.78. Importantly, the variation of pump power generates intrinsic optical bistability (IOB) in the CaSc2O4:Ho3+(1%)/Yb3+(5%) phosphor for the green emission, which is not observed in the CaY2O4:Ho3+(1%)/Yb3+(5%) phosphor. Therefore, the Ho3+/Yb3+ co-doped CaSc2O4 phosphor could potentially be used in UC-based devices, optical memory devices, optical gates, optical transistors and switching devices for optical communications.

Enhanced red up-conversion of β-NaYF4:Er3+,Tm3+ microcrystals for bio-imaging applications

Pure red fluorescence generated by near infrared (NIR) excitation is desired for bio-imaging with high biological penetration, but without cell damage or multi-photon photo-toxicity. In this work, β-NaYF4:Er3+ and β-NaYF4:Er3+,Tm3+ microcrystals were synthesized using a novel hydrothermal method. β-NaYF4:Er3+ microcrystals exhibited intense green and red up-conversion emissions under 976 nm laser diode excitation. The introduction of Tm3+ ions into β-NaYF4:Er3+ resulted in strong inhibition of green up-conversion emission (Er3+:2H11/2 /4S3/2→4I15/2) and, consequently, red up-conversion emission (Er3+:4F9/2→4I15/2) was significantly promoted. To simultaneously restrain green up-conversion emissions and enhance red up-conversion emissions, the optimal concentrations of Er3+ and Tm3+ ions were determined to be 20 mol% and 2.0 mol%, respectively. The red/green up-conversion emission intensity ratio increased from ~1.80 (β-NaYF4:20Er3+, mol%) to ~78 (β-NaYF4:20Er3+,2Tm3+, mol%) after the addition of 2 mol% Tm3+ ions to β-NaYF4:20Er3+ (mol%). The red up-conversion emission intensity of β-NaYF4:20Er3+,2Tm3+ was stronger than that for commercial red up-conversion phosphors (NaYF4 and ZnF2) under the same experimental conditions. Based on the results of the emission spectra, luminescence rise, and decay profiles, the mechanism of enhanced red up-conversion emission by Tm3+ ion co-doping in β-NaYF4:20Er3+,2Tm3+ (mol%) was proposed. The bio-fluorescence experiment under NIR excitation proves that β-NaYF4:Er3+,Tm3+ microcrystals have potential applications in bio-imaging.