Upconversion Research Articles

Near-infrared photo-responsive Er3+-K+ co-doped Y2O3 phosphors with enhanced UC luminescence

Y2O3: x% Er3+ (x=5, 7, 10, 12, 15) and Y2O3: 10% Er3+，x% K+ (x=0, 1, 3, 5, 7, 10, 15) phosphors were successfully prepared by a low-temperature combustion method. The structure as well as the absorption/emission spectra of phosphors were investigated. The effect of doping concentration of K+ ions on the upconversion (UC) luminescence of Y2O3: 10% Er3+ phosphor was examined and the possible optical transitions were discussed. The results showed that K+ ion doping not only changed the microstructure and crystallinity of the phosphors, but also enhanced its UC luminescence intensity. The Y2O3: 10% Er3+, 7% K+ phosphor exhibit the strongest UC emission intensity. Compared with the Y2O3: 10% Er3+ phosphor, the UC luminescence intensity at 563 nm and 661 nm was enhanced by 67.8 and 27.3 times for the K-codoped samples, respectively. The phosphor with the optimal doping concentration was mixed with a polymer to form a composite film, which was employed for the fabrication of near-infrared (NIR) photo-responsive detection devices. The device exhibited strong photo-current response to NIR light at 980 nm, implying that our work could inspire new design strategy for the development of NIR photo-detection devices.

Simultaneous enhancement of upconversion luminescence and photoelectric effect in NaYF4:Yb3+/Tm3+/Zr4+ tri-doped microcrystals

Abstract β-NaYF4: Yb3+/Tm3+/Zr4+ microcrystals are synthesized by a facile hydrothermal route. The up-conversion (UC) luminescence and photoelectric effects are enhanced simultaneously with the introduction of Zr4+ ions under IR excitation. With the Tm3+ ions fluorescence spectra under 980 nm excitation, the blue and red emissions reached the highest intensity in the case of 4 mol% Zr4+ doping. Taking the advantage of Zr4+ stimulates the free electrons transfer from the valence band (VB) to the conduction band (CB) in β-NaYF4:Yb3+/Tm3+/Zr4+ microcrystals, the photocurrent reached the highest intensity in the case of 4 mol% Zr4+ doping at 780 nm excitation. The β-NaYF4: Yb3+/Tm3+/Zr4+ microcrystals with both enhanced UC luminescence and photoelectric properties are highly potential to contribute in photovoltaic and infrared sensors application.

Enhancing upconversion of Sc2Mo3O12:Yb/Ln (Ln = Er, Ho) phosphors by doping Ca2+ ions

Lanthanide doped Sc2(MoO4)3 upconversion (UC) crystals showed distinct negative thermal quenching effect, which show promising applications in the temperature sensing field. However, it is still valuable to develop novel routes to enhance the room temperature UC intensity. Herein, the UC intensity of the Yb/Er (Ho) doped Sc2Mo3O12 crystals were significantly enhanced by doping Ca2+ ions. The incorporation of Ca2+ ions can break the crystal symmetry around the activators and then promote the 4f-4f transitions followed by the enhanced UC. Especially, the UC intensity of the Sc2Mo3O12:20Yb/1Ho/20Ca was about 100 times than that of Sc2Mo3O12:20Yb/1Ho phosphors. Furthermore, the thermally couple levels based temperature sensing performance was improved after doping Ca2+ ions. Our results may promote the exploration of novel scheelite structural compounds with superior UC properties.

The exploration of upconversion luminescence nanoprobes for tobramycin detection based on Förster resonance energy transfer

Tobramycin (TOB) is a broad-spectrum aminoglycoside extensively used as veterinary medicine to treat bacterial infections. However, TOB residue in food and environment may cause great threat to human health. Here, we report, for the first time, the upconversion nanoparticles (UCNPs) for TOB detection based on Förster resonance energy transfer (FRET). The upconversion (UC) luminescence from core-shell structured UCNPs was effectively quenched after addition of MnO2 nanosheets due to the efficient energy transfer from the UCNPs to MnO2 nanosheets. When interacting with TOB, the UC luminescence was restored and increased as a result of coupling between TOB and aptamer, with MnO2 nanosheets dropping off the latter one. The UCNPs@SiO2-MnO2 aptasensors for TOB detection have a good selectivity and sensitivity in a linear range of 10–180 ng mL−1 with a limit of detection (LOD) of 3.04 ng mL−1. The sensors were tested for TOB detection in real samples of liquid milk and fishes and the results are very good and satisfactory. These type of aptasensors are promising for practical applications as they have a high sensitivity, selectivity, low LOD, and are autofluorescence-free.

Optimization of blue upconverted light emission based on fluoroindate glass matrix and subsequent chitosan functionalization.

Lanthanide-activated upconversion (UC) materials are fascinating for the new generation of biomedicine, because its unique ability to transform near-infrared light into visible light. Based on it, a set of fluoroindate glass matrices co-doped with Yb3+ and Tm3+ were prepared using the melting quenching technique. Then, the sample with the highest blue upconverted emission was milled and functionalized with chitosan (CS). The blue UC emission spectra of the glass increase with increasing Yb3+ concentration up to 1.8 mol%, then decreased. This could be attributed to the mechanism involved in the UC process, as confirmed by power dependence assessment. Also, according with the decay times the energy transfer efficacy of the samples is ∼50%. The milling of the glass resulted in particles of 44.8 ± 7.7 nm, which were successfully functionalized with CS. The CS-coated particles showed good cell viability, indicating that they could potentially be used in some biomedical applications.

Upconversion luminescence and temperature sensing performance of Er3+ ions doped self-activated KYb(MoO4)2 phosphors

In this work, a series of self-activated KYb(MoO4)2 phosphors with various x at% Er3+ doping concentrations (x = 0.5, 1, 3, 5, 8, 10, 15) was synthesized by the solid-state reaction method. The phase structure of the as-prepared samples was analyzed by X-ray diffraction (XRD), XRD Rietveld refinement and Fourier transform infrared (FT-IR) spectroscopy. The as-prepared samples retain the orthorhombic structure with space group of Pbcn even Er3+ doping concentration up to 15 at%. High-purity upconversion (UC) green emission with green to red intensity ratio of 55 is observed from the as-prepared samples upon the excitation of 980 nm semiconductor laser and the optimum doping concentration of Er3+ ions in the self-activated KYb(MoO4)2 host is revealed as 3 at%. The strong green UC emission is confirmed as a two-photon process based on the power-dependent UC spectra. In addition, the fluorescence intensity ratios (FIRs) of the two thermally-coupled energy levels, namely 2H11/2 and 4S3/2, of Er3+ ions were investigated in the temperature region 300–570 K to evaluate the optical temperature sensor behavior of the sample. The maximum relative sensitivity (SR) is determined to be 0.0069 K−1 at 300 K and the absolute sensitivity (SA) is determined to be 0.0126 K−1 at 300 K. The SA of self-activated KYb(MoO4)2:Er3+ is almost twice that of traditional KY(MoO4)2:Er3+/Yb3+ codoping phosphor. The results demonstrate that Er3+ ions doped self-activated KYb(MoO4)2 phosphor has promising application in visible display, trademark security and optical temperature sensors.

NIR emission and up-conversion spectral studies of Er/Yb co-doped B2O3-Bi2O3-NaF-BaF2 glasses for optical amplifiers and laser applications

Er-singly and Er/Yb-codoped oxyfluoride glasses were prepared by conventional melt-quenching technique. Structural, optical, steady-state and time-resolved spectroscopy studies were performed. The down-conversion (DC) and up-conversion (UC) mechanisms in the Er3+, Yb3+ (single) and Er3+/Yb3+ (co-doped) systems were investigated under 980 nm laser excitation and the energy transfers involved in the emissions of Er3+ and Yb3+ ions were discussed. Judd-Ofelt intensity and radiative factors such as absorption cross-section (σabs) and emission cross-sections (σemi) were evaluated and an enhanced σemi was observed for the co-doped (1 mol% Er/1 mol% Yb) glasses. Gain cross-section profiles were estimated based on σabs and σemi from McCumber theory for the 4I13/2 ↔ 4I15/2 transitions. At the same time, the studied glasses also exhibited intense UC transitions from the excited states of 2H11/2, 4S3/2, and 4F9/2 to the 4I15/2 ground state of Er3+ ions and covered the strong green and red spectral regions. Time-resolved spectroscopic measurements are extended for NIR and UC emission transitions were discussed in detail.

Large negative thermal quenching of upconversion luminescence through selective excitation in a YAG:Yb/Er transparent ceramics

This study has demonstrated a salient negative thermal quenching behavior of upconversion (UC) luminescence in YAG:Yb/Er transparent ceramic phosphor in a wide temperature range from 100 to 500 K. With the rise of temperature, UC luminescence deteriorates under the excitation at λex = 932 nm, whereas it intensifies remarkably at λex = 974 nm (i.e., the integrated emission intensity increased by nearly 30 folds from T = 100 K to T = 500 K), indicating a dynamic UC luminescence-temperature relation dependent upon the incident excitation energy. Our later photophysical analysis shows that the negative thermal quenching behavior was mainly ascribed to a phonon-assisted thermal excitation process which occurs at low excitation energy. More interestingly, it is found that despite a final decline at high temperature, the UC luminescence lifetime increases as the temperature rises before certain degrees. A modified equation for luminescence lifetime was proposed and used for our interpretation. This work not only demonstrates large negative thermal quenching of UC luminescence in the ceramic phosphor for the first time, but also provides a deep insight into the photophysical behavior of UC luminescence.

Stable and low-threshold photon upconversion in nondegassed water by organic crystals.

Photon upconversion (UC) is a technology that converts lower-energy photons (longer wavelength light) into higher-energy photons (shorter wavelength light), the opposite of fluorescence. Thus, UC is expected to open a vast domain of photonic applications that are not otherwise possible. Recently, UC by triplet-triplet annihilation (TTA) between organic molecules has been studied because of its applicability to low-intensity light, although the majority of such studies have focused on liquid samples in the form of organic solvent solutions. To broaden the range of applications, solid-state UC materials have been an active area of research. We recently developed air-stable, high-performance molecular UC crystals that utilize a stable solid-solution phase of bicomponent organic crystals. This article begins with a brief overview of previous challenges in developing and improving solid-state TTA-UC materials. Then, we briefly review and explain the concept as well as advantages of our molecular solid-solution UC crystals. We applied these organic crystals for the first time to a water environment. We observed blue UC emission upon photoexcitation at 542nm (green-yellow light) and then measured the excitation intensity dependence as well as the temporal stability of the UC emission in air-saturated water. In nondegassed water, these organic crystals were stable, functioned with a low excitation threshold intensity of a few milliwatts per square centimeter, and exhibited high photo-irradiation durability at least over 40h; indicating that the developed organic crystals are also viable for aqueous conditions. Therefore, the organic crystals presented in this report are expected to extend the domain of UC-based photonic applications in practical water systems including in vivo diagnostic, clinical, and therapeutic applications.

Cross Relaxation Channel Tailored Temperature Response in Er3+‐rich Upconversion Nanophosphor

AbstractRecently high doping of lanthanide ions (till 100 %) is realized unprecedentedly in nanostructured upconversion (UC) phosphors. However, oddly enough, this significant breakthrough did not result in a corresponding UC enhancement at ambient temperature, which hinders the otherwise very interesting applications of these materials in various fields. In this work, taking the Er3+‐rich UC nanosystem as an example, we confirm unambiguously that the phonon‐assisted cross relaxation (CR) is the culprit. More importantly, combining the theoretical modeling and experiments, the precise roles of different CR channels on UC energy loss are quantitatively revealed. As a result, lowering the temperature can exponentially enhance the relevant UC luminescence by more than two orders of magnitude. Our comprehension will play an important role in promoting the UC performance and further application of high doping rare earth materials. As a proof of concept, an Er3+‐rich core/multi‐shell nanophosphor is exploited which demonstrates the great potential of our finding in the field of ultra‐sensitive temperature sensing.

Cross Relaxation Channel Tailored Temperature Response in Er3+ -rich Upconversion Nanophosphor.

Recently high doping of lanthanide ions (till 100 %) is realized unprecedentedly in nanostructured upconversion (UC) phosphors. However, oddly enough, this significant breakthrough did not result in a corresponding UC enhancement at ambient temperature, which hinders the otherwise very interesting applications of these materials in various fields. In this work, taking the Er3+ -rich UC nanosystem as an example, we confirm unambiguously that the phonon-assisted cross relaxation (CR) is the culprit. More importantly, combining the theoretical modeling and experiments, the precise roles of different CR channels on UC energy loss are quantitatively revealed. As a result, lowering the temperature can exponentially enhance the relevant UC luminescence by more than two orders of magnitude. Our comprehension will play an important role in promoting the UC performance and further application of high doping rare earth materials. As a proof of concept, an Er3+ -rich core/multi-shell nanophosphor is exploited which demonstrates the great potential of our finding in the field of ultra-sensitive temperature sensing.

Variable ratio blue/red upconversion in Yb3+-Tm3+ codoped fluorosilicate glasses.

Simultaneous blue-red emission in a fiber pumped by a single wavelength source is perceived as a great challenge because of the large energy difference of the emitted photons. This Letter reports the dependence of the blue-to-red upconversion (UC) emission ratio in Yb3+-Tm3+ codoped fluorosilicate glasses (FSGs) under the excitation of a 980-nm laser on the host glass silica content. Photoluminescence spectra and SEM-EDS are used to clarify the UC mechanism, indicating that the probability of the cross-relaxation (CR) process 1G4 + 3F2→3H6 + 3F4 is key to the dominance of the blue or red emissions. This research can provide a new platform for variable UC luminescence.

Multiphonon assisted upconversion thermal enhancement for optical temperature sensing and high penetration depth

Thermal quenching that the upconversion (UC) luminescence intensities decrease with increasing temperature limits the application of UC luminescence materials in the field of optical temperature sensing. Herein, we report that Tm3+/Yb3+ doped Gd2O3 phosphors achieve thermal enhancement of UC luminescence with the multiphonon assisted process. Significantly, a possible mechanism of Yb3+ ions in thermal enhancement and multiphonon assisted UC luminescence process is proposed. Based on the luminescence intensity ratio technique of non-thermally coupled energy levels, research shows that thermal enhancement can effectively improve the optical temperature sensing absolute sensitivity. Owing to the near-infrared excitation and strong near-infrared emission, the UC luminescence of the Gd2O3: Tm3+/Yb3+ phosphors can penetrate 12 mm pork tissue and achieve UC thermal enhancement in 287-314 K after penetrating 6 mm pork tissue, which shows its potential in vivo application. The results not only provide a pathway to realize the thermal enhancement of UC luminescence and the improvement of the temperature sensing sensitivity, but also promote the understanding and utilization of the UC luminescence thermal enhancement.

Efficient upconversion and downshifting luminescence of CaIn2O4: Yb3+/Tm3+/RE3+ (RE=Er/Ho) phosphor: Temperature sensing performance in the visible and near-infrared range

In this paper, the upconversion(UC) and downshifting(DS) luminescence and thermal sensing performances of the CaIn2O4: RE3+/Tm3+/Yb3+(RE=Er, Ho) phosphors are investigated in detail. The bright UC emission bands from 450 to 900 nm and the DS luminescence bands from 1000 to 1700 nm are obtained under 980 nm excitation. The typical near infrared(NIR) emisson band(∼1550 nm) of Er3+ ions and the NIR emission band(∼1200 nm) of Ho3+ ions are gained. The temperature sensing features of CaIn2O4: RE3+/Tm3+/Yb3+ (RE=Er, Ho) phosphors are investigated with the luminescence intensity ratio (LIR) technique. The results indicate that the maximum relative sensitivity (SRmax) of CaIn2O4: Er3+/Tm3+/Yb3+ sample reaches 1.576% K-1 at 313 K, and the SRmax of CaIn2O4: Ho3+/Tm3+/Yb3+ sample reaches 0.797% K-1 at 353 K in UC range. In addition, the higher temperature measuring sensitivities have been obtained in NIR-II region with the DS luminescence of CaIn2O4: RE3+/Tm3+/Yb3+ phosphors. The above results indicate that the CaIn2O4: RE3+/Tm3+/Yb3+ phosphors show bright emission bands in visible and NIR-II, and higher sensitivity for temperature measurement. The CaIn2O4: RE3+/Tm3+/Yb3+ phosphors present great application in optical temperature measurement.

Phosphor-up-converted white LED materials assisted by 1-D photonic crystals

A series of lanthanide-doped aluminosilicate glasses have been prepared by sol-gel (SG) processing. The present work started by optimizing bulk aluminosilicate glasses doped with Er3+, Tm3+ and Yb3+ for white light generation (WLG) and possible application as white LED materials, using SG as a low-cost high purity processing alternative to conventional phosphor fabrication. Undoped alternating aluminosilicate glass and titania multilayers were also synthesized in the form of Bragg mirrors (BMs) and Fabry-Perot Microcavity (MC) 1-D photonic crystals to assist light emission, e.g. by enhancing it at selected wavelengths within the WLG spectrum. This work utilizes (anti-Stokes) up-conversion (UC) excited at 980 nm to generate white light, instead of conventional (Stokes) down-conversion. The materials prepared were characterized by X-ray diffraction (XRD), Spectroscopic Ellipsometry, FTIR, variable angle specular reflection and UC photoluminescence (UCPL) spectroscopies. The UC light emission was analyzed with the help of CIE chromaticity diagrams. Laser power dependence studies were also performed in order to assess the average number of infrared photons (980 nm) involved in the UC mechanism.

In-situ confinement of ultra-small NaYF4:Yb3+/Er3+ UCNPs as multifunctional ratiometric thermometer and optical heater

Lanthanide-doped upconversion (UC) nanomaterials with zero background auto-fluorescence are of extensive interest in optical fields. Here, a NaYF4:Yb3+/Er3+@Zr-UiO-66 nanostructure was prepared by simultaneously encapsulating NaYF4:Yb3+/Er3+ nanoparticles (NPs) and promoting the nucleation of UiO-66 under solvothermal condition. Traditional nanosize-surface induced defects and grain boundaries of NaYF4:Yb3+/Er3+ upconversion nanoparticles (UCNPs) were greatly minimized; enabling a highly efficient energy transfer confirmed by the prolonged fluorescence lifetime and enhanced UC emission of NaYF4:Yb3+/Er3+@Zr-UiO-66 nanocomposite. The optical thermometry property of nanocomposite was investigated based on the fluorescence intensity ratio (FIR) technique in the temperature range of 303–393 K. The maximum relative temperature sensitivities (Sr) were calculated to be 1.02 and 0.73% K−1 based on a pair of thermally coupled energy levels (TCLs) of (2H11/2 → 4I15/2, 4S3/2 → 4I15/2) and another pair of non-thermally coupled energy levels (NTCLs) of (2H11/2 → 4I15/2, 4F9/2 → 4I15/2), respectively. Besides, it exhibited effective internal optical heating property due to the thermal excitation between the two adjacent TCLs of 2H11/2 and 4S3/2 in NaYF4:Yb3+/Er3+ UCNPs as heated by increasing the laser power. This work demonstrates a feasible confined synthesis of NaYF4:Yb3+/Er3+@Zr-UiO-66 nanocomposite with extreme UC emitting performance, and it holds great potential acting as both ratiometric thermometer and optical heater for multifunctional applications.

Thermal enhancement of upconversion luminescence in Sc2Mo3O12: Nd/Yb/Er crystals for optical temperature sensing

Most thermometers suffer from thermal quenching resulting in low signal to noise ratio at elevated temperature. To overcome thermal quenching, we fabricated a negative thermal expansion (NTE) thermometer based on Sc2Mo3O12:Nd3+/Yb3+/Er3+ phosphor. Under 808 nm excitation, the upconversion (UC) emissions of Er3+ ions show the thermal enhancement and its 524 nm emission increases 10-fold as the temperature increased from 310 to 670 K. Furthermore, the temperature sensing behaviors are investigated. It is found fluorescence intensity ratio (FIR) of 524–559 nm is linear with temperature over a wide range of 310–790 K. The maximum relative sensitivity is 0.84% K−1 at 310 K. These results indicate such phosphor not only achieves a high sensitivity, but also exhibits excellent performances at high temperature due to its thermal enhancement.

The Upconversion Luminescence of Ca3Sc2Si3O12:Yb3+,Er3+ and Its Application in Thermometry.

To develop novel luminescent materials for optical temperature measurement, a series of Yb3+- and Er3+-doped Ca3Sc2Si3O12 (CSS) upconversion (UC) phosphors were synthesized by the sol-gel combustion method. The crystal structure, phase purity, and element distribution of the samples were characterized by powder X-ray diffraction and a transmission electron microscope (TEM). The detailed study of the photoluminescence emission spectra of the samples shows that the addition of Yb3+ can greatly enhance the emission of Er3+ by effective energy transfer. The prepared Yb3+ and Er3+ co-doped CSS phosphors exhibit green emission bands near 522 and 555 nm and red emission bands near 658 nm, which correspond to the 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions of Er3+, respectively. The temperature-dependent behavior of the CSS:0.2Yb3+,0.02Er3+ sample was carefully studied by the fluorescence intensity ratio (FIR) technique. The results indicate the excellent sensitivity of the sample, with a maximum absolute sensitivity of 0.67% K-1 at 500 K and a relative sensitivity of 1.34% K-1 at 300 K. We demonstrate here that the temperature measurement performance of FIR technology using the CSS:Yb3+,Er3+ phosphor is not inferior to that of infrared thermal imaging thermometers. Therefore, CSS:Yb3+,Er3+ phosphors have great potential applications in the field of optical thermometry.

Impacts of Ce3+ doping on temperature sensing characteristics of LuAG: Yb3+/Ho3+ up-conversion fluorescent materials

A series of LuAG: 5 mol%Yb3+/0.5 mol%Ho3+/x mol%Ce3+ (x = 0, 0.2, 0.5, 1.0, 2.0) up-conversion (UC) fluorescent materials were prepared using CO2 laser heating zone melting method. The X-ray diffraction (XRD) patterns confirm that the prepared samples have a pure LuAG crystal phase. Under excitation of a 980 nm laser, the up-conversion luminescence (UCL) spectra of the prepared samples were obtained. Two UCL emission bands of Ho3+ ions were observed, including green emission band of the 5F4/5S2→5I8 transition, and red emission band of the 5F5→5I8 transition. Due to the cross-relaxation (CR) processes between Ce3+ ions and Ho3+ ions, the doping of Ce3+ ions can effectively improve the UCL efficiency of Ho3+ ions, and the emission intensity reaches maximum when doped with 1 mol% Ce3+. The temperature sensing characteristics of LuAG: Yb3+/Ho3+ and LuAG: Yb3+/Ho3+/Ce3+ from 303 K to 723 K were studied by fluorescence intensity ratio (FIR) method. By doping 1 mol% Ce3+ ions, the absolute sensitivity of the sample has been significantly improved, with the maximal absolute sensitivity of 0.0951 K−1 (303 K), and the maximal relative sensitivity of 0.0103 K−1 (343 K). The high sensitivity and wide measurement range make LuAG: Yb3+/Ho3+/Ce3+ UCL material ideal for temperature sensing.

Upconversion 3D printing enhancement via silver sensitization to enable selective metallization

This work presents the synergistic effect of silver (Ag) ions to enhance the crosslinking of 3D printing photopolymers via near-infrared (NIR) to ultraviolet (UV)/visible light upconversion (UC) and allow selective metallization of multi-material parts. By incorporating a commercial micron-sized ytterbium-thulium co-doped sodium yttrium fluoride phosphor (NaYF4:Yb3+, Tm3+) into a photopolymer resin, a low-cost NIR laser is used to initiate curing exactly at the focal point in the depth of the vat, allowing voxel crosslinking in any point of the space at a depth in the centimeter scale, beyond the traditional layer-by-layer 2D scheme. This allows the printing with the presented resin on pre-existing printed parts, which is the basis for multi-material integration in 3D printing, beyond the 2D limitation. The addition of Ag(I) ions in the photopolymer enables improved photo-curing at 10-fold faster speeds, after printing through 10 mm of resin. In our approach, after the 3D printing process, the Ag(I) ions embedded into the photopolymer serve as seeding sites for electroless copper (Cu) plating, which allows the integration of metalized parts on different substrates, such as glass and 3D printed polymer. Finally, selective copper plating has enabled the manufacture of objects with metal (copper) and plastic (acrylic) surfaces.