Red Upconversion Research Articles

Color thermal stability of white up-conversion luminescent emission in Eu3+/Yb3+ co-doped YOF phosphor

The Eu3+/Yb3+ co-doped YOF phosphor is synthesized via the hydrothermal method. The intense blue, green and red up-conversion emission of the phosphor can be radiated at 980 nm excitation. The up-conversion emission color adjustment from yellow to blue is realized through cooperative sensitization up-conversion process by altering the excitation power and pulse width of 980 nm pulse laser. Especially, the white up-conversion emission of the phosphor can be generated under 980 nm continuous laser and pulse laser irradiation. The corresponding energy transfer mechanism of Eu3+/Yb3+ co-doped YOF phosphor has been proposed in this work. The influence of temperature on up-conversion luminescent emission color is explored under the condition that can generate optimal white emission. The excellent color thermal stability of white up-conversion emission can be found at continuous high temperature. Strong white emission at high temperature and multicolor emissions of the phosphor make the phosphor suitable for using in high temperature environment for white light illuminating and color displaying application.

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.

Boosted thermometric performance in NaGdF4:Er3+/Yb3+ upconverting nanorods by Fe3+ ions doping for contactless nanothermometer based on thermally and non-thermally coupled levels

Series of Er3+/Yb3+/Fe3+-tridoped NaGdF4 nanorods with eminent upconversion (UC) emission performance were obtained at room temperature. The effect of Fe3+ ion content on the phase structure, morphology, thermal stability and luminescent behaviors of the resultant samples was systematically studied. Upon 980 nm light irradiation, dazzling green and red UC emissions originating from Er3+ ions were gained and their emission intensities were sharply improved with adding the Fe3+ ions. The relevant UC mechanism was verified to be the two-photon absorption process. Furthermore, through analyzing the responses of the emissions originating from the thermally (i.e., 2H11/2, 4S3/2) and non-thermally (i.e., 4F9/2, 4S3/2) coupled levels to temperature, the thermometric performance of the designed nanorods was investigated. Compared with those of the thermally coupled levels, the thermometric properties of the synthesized nanorods were significantly enhanced when the non-thermally coupled levels were employed. The maximum absolute and relative sensor sensitivities of the NaGdF4:Er3+/Yb3+/0.005Fe3+ nanorods were 0.0191 K-1 and 0.567% K−1, respectively, which were higher than those of the NaGdF4:Er3+/Yb3+ nanorods. Ultimately, the internal heating performance of the final products induced by the laser pump power was also explored. These results revealed that the Er3+/Yb3+/Fe3+-tridoped NaGdF4 nanorods were potential upconverting compounds for ratiometeric nanothermometers and optical heaters.

Optimized Multimetal Sensitized Phosphor for Enhanced Red Up-Conversion Luminescence by Machine Learning.

In this research, machine learning including the genetic algorithm (GA) and support vector machine (SVM) algorithm is used to solve the "low up-conversion luminescence (UCL) intensity" problem in order to find the optimal phosphor with enhanced red UCL emission using multielement K/Li/Mn metal modulation. Compared with the first generation of phosphors, the best phosphors' fluorescence intensity occurs in the third generation optimized by the GA, with a stronger brightness (4.91-fold), a higher relative quantum yield (6.40-fold), and an enhanced tissue penetration depth (by 5 mm). The single and multiple dopants effect on the upconversion intensity of K+Li+Mn sensitizers is also studied: the intensity increases first and then decreases with the increase of Yb/Er/K+Li+Mn content, and the optimized K+Li+Mn concentration is 6.03%. In order to confirm the stability of the brightness optimization by the GA, a batch of phosphors was synthesized with the same element proportion, and the similarity of fluorescence intensity of two batches of phosphors was evaluated by the SVM algorithm with the classification accuracy index. Finally, the optimized phosphor was used for bioimaging and phosphor-LED.

Enhanced red and near-infrared upconversion luminescence properties in CaSc2O4 microcrystals

CaSc2O4 co-doped with Yb/Er, Yb/Tm was successfully prepared through a high-temperature solid-state method. The enhanced red and near-infrared upconversion luminescence intensities in CaSc2O4 co-doped with Yb/Er, Yb/Tm were observed. As the increase of Ca2+ ion doping concentration, the crystal phase evolved from cubic Sc2O3 to orthorhombic CaSc2O4, then to cubic CaO. When Ca2+ ion doping concentration is about 40 mol%, CaSc2O4 co-doped with Yb/Er has the maximum red emission intensity (666 nm) which is increased by 39.5 times than that of Sc2O3 microcrystals (MCs). Similarly, the maximum near-infrared emission intensity (807 nm) in CaSc2O4 co-doped with Yb/Tm is enhanced by 9 times than that of Sc2O3 MCs when Ca2+ ion doping concentration is about 40 mol%. They are in accordance with that Er3+ ion and Tm3+ ion exhibit the longest luminescence lifetime in CaSc2O4 MCs than that in Sc2O3 and CaO MCs.

Efficiently visible and 2 μm infrared emission in K2YbF5: Ce3+/Ho3+ microcrystals

Ho3+/Ce3+ co-doped K2YbF5 microcrystals were synthesized by solvent-thermal method. Under excitation of 980 nm laser diode, effectively visible and 2 μm-infrared luminescence of Ho3+ ion were obtained in the microcrystals. By changing Ce3+-ion doping concentration, the luminescence properties of visible and 2 μm emission were effectively regulated. At low Ce3+-ion doping level, the red and green upconversion emission obviously increases and decreases respectively with the increase of Ce3+-ion amount in the samples, meanwhile the intensity of 2 μm fluorescence changes very little. At high Ce3+-ion doping level, the intensities of the red and green emission both decrease with the increase of Ce3+-ion concentration, while the 2 μm emission intensity increases obviously. In the sample doped with 16% Ce3+ ion, the intensity of 2 μm emission is about 4.5 times that of the sample without Ce3+ ion, and the corresponding quantum efficiency is about 78.3%. The result is attributed to the influence of the different cross relaxation between Ho3+ and Ce3+ ion in luminescence process at low and high Ce3+-ion doping concentration. The corresponding luminescence mechanism and energy transfer process were discussed in detail.

Near infrared broadband and visible upconversion emissions of erbium ions in oxyfluoride glasses for optical amplifier applications

Optical and visible upconversion properties of erbium (Er3+)-doped oxyfluoro-titania-phosphate glasses (PCfBfTiEr) with the chemical composition of P2O5–CaF2–BaF2–TiO2–Er2O3 have been explored. An intense emission at 1.53 µm of Er3+-doped PCfBfTiEr1.0 glass was obtained upon excitation of 980 nm diode laser. In addition, green and red visible upconversion emissions were obtained upon the optical excitation of Er3+ ions doped PCfBfTiEr glasses at 980 nm diode laser. Upconversion emission intensities and population densities of respective levels were tuned with the variation of Er3+ ion concentration. Fluorescence decay curves of the 4I13/2 level of PCfBfTiEr glasses were obtained upon 980 nm laser excitation in the pulsed mode and revealed a mono-exponential behavior. The stimulated emission cross-section (σem), full width at half maximum (FWHM) and gain bandwidth product (σem × FWHM) were found to be 9.3 × 10−21 cm2, 95.61 nm and 889.2 cm−2 nm for PCfBfTiEr2.0 glass, respectively. These results recommend that the Er3+ ions doped PCfBfTiEr glasses may possibly be worthy for the laser and optical amplification applications at 1.53 µm.

Small-sized red-emitting core/shell/shell nanoparticles through an efficient energy back transfer process

From the existing Er3+-based upconversion (UC) research, we can't judge whether the 980 nm near-infrared (NIR) emission photons from Er3+ in core or the 980 nm NIR excitation photons from laser were captured by Yb3+ in active shell, due to 980 nm excitation was the only excitationmode in the previous reports. In this work, the systematic research of small-sized (about 20 nm) NaErF4:0.5%Tm@NaYbF4@NaYF4 core/shell/shell nanoparticles with intense red UC luminescence under 808/980/1532 nm excitations is presented for the first time. We confirm that 980 nm NIR emissionphotons from Er3+ in core which would inevitably transfer to the external space, must be captured by Yb3+ in active shell. The coating of NaYbF4 active shell can maximize the capture of 980 nm NIR emissionphotons from Er3+ in core. Under theprotection of NaYF4 inert outer shell, the efficient energy back transfer from NaYbF4 active shell to Er3+ in core may induce the 4I15/2 → 4I11/2 and 4I13/2 → 4F9/2 transitions of Er3+, leading to the bright red UC luminescence with high purity under 808/980/1532 nm excitations. The small-sized red-emitting core/shell/shell nanoparticles under multiple excitations exhibit huge advantages in bioimaging and solar energy conversion.

Generalized synthesis of NaLn(MoO4)2 nano/microcrystals (Ln = La–Lu and Y): The effects of lanthanide contraction, structure, and down-/up-conversion luminescence

Systematic synthesis of scheelite-type double molybdate nano-/microcrystals NaLn (MoO4)2 (Ln = La–Lu lanthanides and Y) was successfully carried out via hydrothermal reaction at 180 °C without using any organic additive, and the intrinsic effects of lanthanide contraction on phase preference and crystallite morphology were unambiguously observed. It was shown that the products of larger Ln3+ (Ln = La–Dy) and smaller Ln3+ (Ln = Ho–Lu and Y) elements crystallized as tetragonal NaLn (MoO4)2 and new orthorhombic NaLnMo2O8·2H2O structures, respectively, with the latter being able to dehydrate to tetragonal NaLn (MoO4)2 upon calcination at ∼300 °C. Y was found to be a demarcation point for the two structures, and phase preference was shown to be influenced by solution pH and MoO42−/Y3+ molar ratio. With Eu3+ as a downconversion (DC) luminescence probe, the effects of Ln type (Ln = La, Gd, Y and Lu) and crystal structure were manifested. Furthermore, the upconversion (UC) luminescence of Yb3+/Ho3+ and Yb3+/Er3+ pairs in NaLu(MoO4)2 was studied for the first time, and strong red and green UC emissions were observed under the 978 nm laser excitation. Besides, the processes/mechanisms of UC were studied via varying the excitation power and were discussed with Yb3+-MoO42- dimer sensitizer.

Quantification of upconversion photon and thermosensitive feedback in Er3+/Yb3+ doped fluorotellurite glasses

Multi-photon-excited green and red upconversion (UC) emissions have been quantified and adaptive temperature sensing performance has been investigated in Er3+/Yb3+ doped fluorotellurite (BZLFT) glasses under the excitation of 978 nm laser. The net emission photon numbers of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transition emissions are identified to be 83.2 × 1012, 753.2 × 1012 and 136.8 × 1012cps in 0.4 wt% Er2O3-0.4 wt% Yb2O3 co-doping case under the excitation power density of 7.82 W/mm2. The quantum yields (QYs) and luminous fluxes for visible UC emissions of Er3+ are identified as a positive dependency with pumping powers. When the excitation power increases to 983 mW, the QYs for detected 526 and 545 nm green, and 658 nm red emissions are up to 0.155 × 10−4, 1.405 × 10−4 and 0.255 × 10−4, respectively. Low phonon-energy Er3+/Yb3+ doped BZLFT glasses with intense luminous efficiency can work steadily at medium temperature range in view of its fast responses and high stability, moreover, a maximum sensitivity reaches 4.77 × 10−3 K−1 at 587.5 K, which indicates that the BZLFT glass material has a good thermal sensitivity. The macroscopic quantization for UC photon generation and the exploration of maximum sensitivity value from Er3+ in high transparent fluorotellurite glasses provide a reliable reference for developing optical temperature-sensing materials.

Influence of silver nanoparticle on Raman and photoluminescence of heat treated Ag-Er codoped tungsten tellurite glasses

The glasses of composition TeO2-WO3-Li2O-Er2O3 with AgNO3 (TWLEOAG) were prepared by the high-temperature melting method. Surface Plasmon resonance (SPR) bands are observed in the range 520-535 nm and 640-650nm of without erbium-doped glass by absorption spectrum. The TEM measurement reveals the presence of spherical AgNPs in this glass. The DSC has also been analyzed for the stability of transition temperature Tg and on set crystallization T0 of this glass system. The Raman spectrum revealed that TWLEOAG glasses were mainly formed by the [TeO4], {TeO3}, [WO4] and [WO6] units and [TeO4] units convert to [TeO3] units with the addition of WO3 and Li2O content into tellurite glasses system. In particular TWLEOAG05 glass samples, heat-treated different time interval system reveal the enhancement in Raman peaks intensity with small shift in due to Ag nanoparticle. Also photoluminescence enhancement of red upconversion compare to green emission of TWLEOAG05 for different annealing duration has been observed under 808nm excitation. In this work optimum 0.5 mol% AgNP in TWEOAG glass would be useful for optical devices.

Enhanced red upconversion emission and energy transfer of Tm3+/Cr3+/Yb3+ tri-doped transparent fluorosilicate glass-ceramics

In this paper, the upconversion (UC) emission of Tm3+/Cr3+/Yb3+ tri-doped in SiO2–AlF3–BaF2–La2O3 transparent fluorosilicate glass-ceramics (SGC) containing Ba2LaF7 nanocrystals were investigated. The enhancement of red UC emission of Tm3+/Cr3+/Yb3+ tri-doped in SiO2–AlF3–BaF2–La2O3 transparent SGC were also investigated. Through the energy transfer processes between Cr3+ and Tm3+, and the combination of Cr3+-Tm3+ led to the red UC emission intensity of Tm3+/Cr3+/Yb3+ tri-doped transparent SGC bands centered at ~806 and ~858 nm has been significantly enhanced. The CIE chromaticity coordinates for UC emission of Tm3+/Cr3+/Yb3+ tri-doped in SGC-0.2Tm1Cr2Yb transparent SGC sample is calculated (x = 0.704; y = 0.296) and the UC emission of Tm3+/Cr3+/Yb3+ tri-doped in SiO2–AlF3–BaF2–La2O3 transparent SGC has been identified in red color region of CIE chromaticity coordinates. In addition, the ET processes between Cr3+ and Tm3+ are also proposed and discussed.

Tunable emission and optical trapping of upconverting LiYF4:Yb,Er nanocrystal

Present work investigates about hydrothermally prepared LiYF4:Yb,Er nanocrystals with novel tunable emission and optical trapping results. Optical trapping effect of LiYF4:Yb,Er upconversion nanocrystal under 980 nm diode laser interaction is reported and a single nanoparticle trapping is demonstrated. Optical trapping force of 10.8 fN is exerted on LiYF4:Yb,Er upconversion nanocrystal of size ~238 nm at laser power 50 mW and the result is in agreement with the reported values. At 980 nm excitation, tunable green to red upconversion emission is achieved by calcination. It depends on orthorhombic and tetragonal phases that confirmed by XRD. The nanocrystals calcined at 450 and 600 °C resulted in a decay time of 26 and 19 µs respectively and the upconversion emission mechanism is explained. FESEM and HRTEM examinations showed the hydrothermally synthesized nanocrystals are in trapezohedral, pyramidal, bi-pyramidal and tetragonal morphologies. The differential scanning calorimetry revealed the tetragonal phase formation temperature is lower at 450 °C compared to the reported value of 750 °C. The UV–VIS-NIR spectra of LiYF4:Yb,Er showed high absorption at 380, 480, 520 and 660 nm due to Er3+ ion and the broad absorption from 900 to 1000 nm centered at 980 nm is owing to Yb3+ ion. The crystalline phase, morphology, upconversion emission and optical trapping behaviors of LiYF4:Yb,Er nanocrystal are explained elaborately.

Effect of F/O ratio on up-conversion and down-conversion luminescence properties of Er3+/Yb3+ co-doped SiO2–Al2O3–AlF3-Gd2O3–Na2O glass

Er3+/Yb3+ co-doped SiO2–Al2O3–AlF3-Gd2O3–Na2O glass with different F/O ratios were prepared. The effect of F/O ratio on the structure and optical properties of the glasses was investigated. The amorphous nature of the glasses was determined by XRD, and the glass transition temperature (Tg) and the crystallization peak temperature (Tc) were obtained from the DSC curves. The change in the structure of the glass network was analyzed by infrared spectroscopy. The density and absorption spectra of the bulk glass was also investigated. The excitation spectrum, emission spectrum and 4S3/2 level fluorescence lifetime of Er3+ ions in the glasses were tested by a Xenon lamp with adjustable wavelength. The up-conversion luminescence spectrum of Er3+ ions was measured under excitation of a 980 nm laser. The results show that the green (2H11/2, 4S3/2) and red (4F9/2) up-conversion luminescence intensity is the highest when the F/O ratio is 0.214. According to the variation law of up-conversion luminescence intensity and pump power, it is confirmed that the up-conversion green (2H11/2 → 4I15/2, 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) light of Er3+ ions are the three-photon absorption process based on excited state absorption (ESA) and energy transfer up-conversion (ETU) mechanism. Finally, energy transfer transitions between Er3+ and Yb3+ are also analyzed. The CIE chromaticity coordinates were calculated from the emission spectrum obtained at two different excitation wavelengths of 378 nm and 980 nm.

Up-Conversion Nanoparticles/PMMA Composites for Light-Emitting Applications

In this paper, we prepared KMnF3:Yb[Formula: see text], Er[Formula: see text] nanoparticles (NPs)/polymethyl methacrylate (PMMA) composites, NaYF4:Yb[Formula: see text], Tm[Formula: see text]NPs/PMMA composites and NaYF4Yb[Formula: see text], Er[Formula: see text]NPs/PMMA composites by in situ polymerization, and these NPs/PMMA composites can emit red, blue and green up-conversion fluorescence excited by 980[Formula: see text]nm, respectively. The mixed white NPs/PMMA composites were obtained by adjusting the doping ratio of the above three NPs. These NPs/PMMA composites are transparent. We tested the up-conversion fluorescence spectra of all the NPs and NPs/PMMA composites, under 980[Formula: see text]nm excitation. The up-conversion luminescence spectra of the NPs and NPs/PMMA composites are consistent, which further indicates that the in situ polymerization has not changed the chemical structure of the NPs. Then, we prepared transparent bulk polymers by curing these NPs/PMMA composites. Under 980[Formula: see text]nm laser excitation, these NPs/PMMA bulk polymers can emit red, blue, green and white up-conversion emissions, respectively. The results indicate that the NPs/PMMA composites can be applied to three-dimensional (3D) display.

Effects of Ca2+ doping on upconversion luminescence intensity and crystal field asymmetry of β-NaYF4:Yb3+/Er3+ microcrystals

The green and red upconversion (UC) emissions of 35 mol% Ca2+ doped β-NaYF4:Yb3+/Er3+ microcrystals were improved to 102.8 and 43.1 times higher than those of Ca2+-free counterparts. X-ray powder diffraction spectra, scanning electron microscopy, Fourier transform infrared spectroscopy, upconversion luminescence (UCL) spectroscopy and Judd-Ofelt analysis were performed to investigate the mechanism of UCL intensity enhancement caused by Ca2+ doping. The hexagonal-to-cubic phase transition was observed when Ca2+ doping concentration exceeded 35 mol%. The incorporation of Ca2+ into hexagonal microcrystals lattice induced unit cell volume enhancement and microcrystals surface to volume ratio variation, which affected the UCL intensity by hindering the non-radiative energy loss process via Yb3+/Er3+ ions and changing the amount of surface quenchers adsorbed on the microcrystals surface. The hypersensitive transition related local crystal field asymmetry around Er3+ in the grown hexagonal microcrystals was also investigated by Judd-Ofelt analysis but unexpectedly found to be the less dominant factor for the UCL intensity variation. This study helps to realize the mechanism of impurity ion doping resulted UCL intensity enhancement, which is instructive for high-performance UC microcrystals synthesis.

Enhancing the red upconversion of water-soluble β-NaErF4:Yb nanocrystals through Ca2+ doping

With the aim of developing lanthanide fluoride-based nanoparticles (NPs) with enhanced upconversion (UC) luminescence properties, Ca2+-doped water-soluble β-NaErF4:Yb nanocrystals are prepared by a facile solvothermal reaction using ethylene glycol and water as the solvents and comb-like superplasticizer (SP), polyacrylic-co-methyl allyl polyoxyethylene ether copolymer as the capping agent. The SP-capped upconversion nanoparticles (UCNPs) are characterized by X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The results show that the capping agent has great influence on the morphology and colloidal solubility of the nanocrystals. The morphological and phase evolution of the nanocrystals is studied. Furthermore, bright red UC luminescence is observed when the sol is irradiated by a 980 nm laser, and the red UC photoluminescence (UCPL) emission is enhanced by eight times upon doping with 50 mol% Ca2+. The decay time and stability of the UCPL are also discussed. With the efficient stable red UC, the Ca2+ doped water-soluble NaErF4:Yb nanocrystals may find use in various optical applications.

Red blood cell membrane-coated upconversion nanoparticles for pretargeted multimodality imaging of triple-negative breast cancer.

Upconversion nanoparticles (UCNPs) have been widely employed for tumor imaging using magnetic resonance imaging (MRI) and upconversion luminescence (UCL) imaging. The short blood clearance time and immunogenicity of UCNPs have limited their further application in vivo. We have designed UCNPs camouflaged with an exterior red blood cell (RBC) membrane coating (RBC-UCNPs) to solve these problems. Moreover, because of some intrinsic disadvantages of MRI and UCL imaging, we investigated the use of pretargeted RBC-UCNPs for positron-emission tomography (PET) imaging to obtain more comprehensive information. Our data showed that RBC-UCNPs retained the immunity feature from the source cells and the superior optical and chemical features from the pristine UCNP cores. The tumor-targeting ability of RBC-UCNPs was enhanced by binding 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[folate(polyethylene glycol)-2000] (DSPE-PEG-FA) molecules onto the cell membranes. PET imaging with short half-life radionuclides to visualize the RBC-UCNPs was successfully realized by a combination of pre-targeting and in vivo click chemistry. Blood chemistry, hematology, and histologic analysis suggested good in vivo biocompatibility of the RBC-UCNPs. Our method provides a new potential biomedical application of biomimetic nanoparticles.

Doping-Site Dependence of Upconversion Emission of Ho3+ Ion in BaHfO3

We investigated the doping-site dependence of photoluminescence and upconversion (UC) emission characteristics of Ho3+ and Yb3+ co-doped BaHfO3 polycrystals. Under 980-nm near-infrared excitation, we detected sizable green (550 nm) and red (660 and 760 nm) UC emissions from Ho3+ ions based on the assistance of Yb3+ ions as sensitizers. These emission features showed strong temperature-dependent variation and became more intense with decreasing temperature. Notably, the UC emission from Ho3+ was stronger in the samples in which the Ho3+/Yb3+ dopants were substituted for Hf4+ ions rather than for Ba2+ ions. The results suggest that the asymmetry ratio of the crystal field surrounding the Ho3+ ions should be an important factor for the UC emission properties.

A novel upconversion core–multishell nanoplatform for a highly efficient photoswitch

Red upconversion emission is significantly increased on the premise of ensuring optimal UV-blue upconversion emission.