Tunable structure and intensive upconversion photoluminescence for Ho3+-Yb3+ codoped bismuth titanate composite synthesized by sol-gel-combustion (SGC) method

Ho³⁺ and Yb³⁺ codoped bismuth titanate (BTO) composite powders with infrared to visible upconversion luminescence (UCL) function were prepared by SGC method. The effects of Ho³⁺ and Yb³⁺ doping content on the structure and property were investigated for BTO: xHo, 0.2 Yb (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) and BTO: 0.02Ho, yYb (y = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) samples. All the samples include three bismuth titanate phases (Bi₄Ti₃O₁₂, Bi₂Ti₂O₇, and Bi₂₀TiO₃₂), and the phase proportion can be tuned by changing Ho³⁺ and Yb³⁺ doping content. These powders are well crystalized with honeycomb-like microscopic structure, and with good absorption for 233 nm, 310 nm and 975 nm wavelength. The band gap can be tuned from 3.53 eV to 4.03 eV when increasing Yb³⁺ content from y = 0 to y = 0.9. A strong 530–580 nm green emission band and a relative weak 630–690 nm red one corresponding to Ho³⁺: ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions appear in the UCL spectra for all the BTO: Ho, Yb samples when pumped at 980 nm. The emission intensities can well be tuned with various Ho³⁺ and Yb³⁺ content. The optimal UCL was obtained in BTO: 0.02Ho, 0.5 Yb for all the prepared samples. The energy transfer mechanism is analyzed by building a two-photon energy transfer model, which is proved by the relationship between emission intensities and pumping power measurement. The concentration quenching of Ho³⁺ is caused by cross relaxation of CR₁ and CR₂ (Ho: ⁵F₄, ⁵S₂ + ⁵I₈ → ⁵I₄ + ⁵I₇) and by CR₃ (Ho: ⁵F₄, ⁵S₂ + Yb: ²F_7/2 → Ho: ⁵I₆ + Yb: ²F_5/2) for Yb³⁺ quenching. The mean luminescence lifetime (τ_m) from Ho: ⁵S₂ decreases monotonously with the increase of Ho³⁺ and Yb³⁺ content.     

Influence of Ce3+ and Gd3+ co-doping on the structure and upconversion emission in hexagonal Ho3+ doped NaYbF4 phosphors

Hexagonal Ho³⁺ doped NaYbF₄ phosphors are synthesized via a hydrothermal method. The influence of Gd³⁺ and Ce³⁺ content on the phase structure and upconversion (UC) emission of NaYbF₄ phosphors is investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. The results of XRD and TEM indicate that the solubility of Ce³⁺ in hexagonal NaYbF₄ is low due to the large difference of ionic radius between Ce³⁺ and Yb³⁺. With help of Gd³⁺ co-doping (15 mol%), pure hexagonal NaYbF₄ phosphors with high doping concentration of Ce³⁺ (15 mol%) and small crystal size are obtained. When excited by a 980 nm laser diode, Ho³⁺ doped hexagonal NaYb_0.85Gd_0.15F₄ phosphors exhibit strong green UC emission at 540 nm and weak red one at 646 nm. UC luminescence tuning from green emission to red emission is observed in hexagonal Ho³⁺ doped NaYb_0.85Gd_0.15F₄ phosphors by co-doping with Ce³⁺ ions. The UC luminescence tuning phenomenon is attributed to two resonant energy transfer processes of ⁵S₂/⁵F₄(Ho³⁺)+²F_5/2(Ce³⁺)→⁵F₅(Ho³⁺)+⁵F_7/2(Ce³⁺) and ⁵I₆(Ho³⁺)+²F_5/2(Ce³⁺)→⁵I₇(Ho³⁺⁾+⁵F_7/2(Ce³⁺) between Ho³⁺ and Ce³⁺, which suppress the green emission at 540 nm, while promote the red one at 646 nm.     

Preparation and infrared to visible upconversion luminescence of Yb2O3:Ho3+ nanocrystalline powders

Yb₂O₃:Ho³⁺ nanocrystalline powders were synthesized through a solid state reaction method. X-ray diffraction analysis and field emission scanning electron microscopy were used to analyze the phase composition and morphology of the powders. Then under the 980 nm excitation of laser diode, the fluorescence of the crystals was studied via a fluorescence spectrometer. The green and red emissions centered on 551 and 668 nm were observed, and the green band dominated the emission spectrum. The effect of the concentration of Ho³⁺ on the upconversion luminescence intensity was discussed and the possible upconversion emission mechanism was explained. It indicates that like other metal oxide nanoparticles, Yb₂O₃ could also be a potential host material for doping to prepare the upconversion phosphor.     

Abnormal upconversion luminescence induced by defects and Er3+–Yb3+ distance change in Cs3GdGe3O9:Er3+/Yb3+ phosphors

A series of Er³⁺/Yb³⁺ co-doped Cs₃GdGe₃O₉ (CGG) phosphors were prepared by solid-phase sintering method, and the microstructure and upconversion luminescence (UCL) properties were tested by variable-temperature X-ray diffractometry and variable-temperature spectrometer. Abnormal UCL phenomena were found, which include UCL intensity continuously increasing under 980 nm laser continuous irradiation and UCL thermal enhancement. After 10 min of continuous irradiation by 980 nm laser at 513 K, the UCL intensity increased 2.91 times compared with the initial UCL intensity. The phenomenon is due to the electron releasing of host defects. The green UCL intensity of CGG:0.1Er³⁺/0.2Yb³⁺ decreases at 303–423 K and increases at 423–723 K, which reaches 13.23 times compared with that at 423 K. The phenomenon is due to Er³⁺–Yb³⁺ distance change by temperature and phonon-assisted transitions. In addition, the absolute temperature sensitivities of samples are calculated by luminescence intensity ratio technology, the maximum absolute sensitivity of CGG:0.1Er³⁺/0.4Yb³⁺ is 0.00691 K⁻¹ at 546 K, and the maximum relative sensitivity of CGG:0.1Er³⁺/0.1Yb³⁺ is 0.01224 K⁻¹ at 303 K. These results indicate that CGG:Er³⁺/Yb³⁺ phosphors can be used as a high-temperature optical thermometer.     

Optimization of sensitizer concentration for upconversion photoluminescence of Yb3+/Er3+: La10W22O81 nanophosphor rods

Yb³⁺/Er³⁺codoped La₁₀W₂₂O₈₁ (LWO) nanophosphor rods have been successfully synthesized by a facile hydrothermal assisted solid state reaction method, and their upconversion photoluminescence properties were systematically studied. X-ray diffraction patterns revealed that the nanophosphors have an orthorhombic structure with space group Pbcn (60). A microflowers-like morphology with irregular hexagonal nanorods was observed using field emission scanning electron microscopy for the Yb³⁺(2 mol%)/Er³⁺(2 mol%):LWO nanophosphor. The shape and size of the nanophosphor and the elements along with their ionic states in the material were confirmed by TEM and XPS studies, respectively. A green upconversion emission was observed in the Er³⁺: LWO nanophosphors under 980 nm laser excitation. A significant improvement in upconversion emission has been observed in the Er³⁺: LWO nanophosphors by increasing the Er³⁺ ion concentration. A decrease in the upconversion emission occurred due to concentration quenching when the doping concentration of Er³⁺ ions was greater than 2 mol%. An optimized Er³⁺(2 mol%): LWO nanophosphor exhibited a strong near infrared emission at 1.53 μm by 980 nm excitation. The green upconversion emission of Er³⁺(2 mol%): LWO was remarkably enhanced by co-doping with Yb³⁺ ions under 980 nm excitation because of energy transfer from Yb³⁺ to Er³⁺. The naked eye observed this upconversion emission when co-doping with 2 mol% Yb³⁺. In order to obtain the high upconversion green emission, the optimized sensitizer concentration of Yb³⁺ ions was found to be 2 mol%. The upconversion emission trends were studied as a function of stimulating laser power for an optimized sample. Moreover, the NIR emission intensity has also been enhanced by co-doping with Yb³⁺ ions due to energy transfer from Yb³⁺ to Er³⁺. The energy transfer dynamics were systematically elucidated by energy level scheme. Colorimetric coordinates were determined for Er³⁺ and Yb³⁺/Er³⁺: LWO nanophosphors. The energy transfer mechanism was well explained and substantiated by several fluorescence dynamics of upconversion emission spectra and CIE coordinates. The results demonstrated that the co-doped Yb³⁺(2 mol%)/Er³⁺(2 mol%): LWO nanophosphor material is found to be a suitable candidate for the novel upconversion photonic devices.     

Intense upconversion luminescence and energy‐transfer mechanism of Ho3+/Yb3+ co‐doped SrLu2O4 phosphor

A series of novel SrLu₂O₄: x Ho³⁺, y Yb³⁺ phosphors (x=0.005‐0.05, y=0.1‐0.6) were synthesized by a simple solid‐state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu₂O₄:Ho³⁺, Yb³⁺ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm (⁵F₄,⁵S₂→⁵I₈) and weak red emission peaked at 673 nm (⁵F₅→⁵I₈). Under different pump‐power excitation, the UC luminescence can be finely tuned from yellow‐green to green light region to some extent. Based on energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the analysis of pump‐power dependence and luminescence decay curves. The energy‐transfer mechanisms for green and red UC emissions can be determined to be two‐photon absorption processes. Compared with commercial NaYF₄:Er³⁺, Yb³⁺ and common Y₂O₃:Ho³⁺, Yb³⁺ phosphors, the SrLu_1.49Ho_0.01Yb_0.5O₄ sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu₂O₄:Ho³⁺, Yb³⁺ can be a good candidate for green UC material in display fields.     

Tunable multicolor upconversion luminescence of Yb3+ sensitized Na3La(VO4)2 crystals

Multicolor upconversion luminescence materials show significantly applications in materials science. In this paper, the novel Yb³⁺-sensitized Na₃La(VO₄)₂ upconversion luminescence crystals are synthesized by the solid-state reaction method. Three primary colors upconversion luminescence are successfully achieved in Na₃La(VO₄)₂:Yb³⁺,Tm³⁺, Na₃La(VO₄)₂:Yb³⁺,Er³⁺, and Na₃La(VO₄)₂:Yb³⁺,Ho³⁺ crystals excited by the single 980 nm LD. Multicolor upconversion luminescence can be obtained by simply adjusting the combination ratios of these three samples. Luminescence mechanisms of the Yb³⁺-sensitized system are discussed in detail. In the Na₃La(VO₄)₂ host material, the Yb³⁺/Ho³⁺ codoped system exhibits unusual red upconversion luminescence based on the short decay time of Ho³⁺ ion ⁵I₆ level, which provides the possibility of three primary color luminescence under 980 nm excitation.     

Enhanced 2.0 μm Emission and Lowered Upconversion Emission in Fluorogermanate Glass‐Ceramic Containing LaF3:Ho3+/Yb3+ by Codoping Ce3+ Ions

Intense 2.0 μm emission of Ho³⁺ has been achieved through Yb³⁺ sensitization in fluorogermanate glass‐ceramic (GC) containing LaF₃ pumped with 980 nm laser diode (LD). The observation of concurrent emissions at 538, 650, and 1192 nm points to the additional deexcitation routes based on infrared‐to‐visible upconversion processes and Ho³⁺:⁵I₆ → ⁵I₈ radiative transition. Comparative investigations of photoluminescent spectra and decay curves have indicated the effective role of Ce³⁺ ions in enhancing the 2.0 μm fluorescence along with suppressing the occurrence of these concurrent emissions. This would offer a promising approach to develop compact and efficient 2.0‐μm laser systems.     

Temperature dependent upconversion properties of Yb3+:Ho3+ co-doped Gd2O3 nanoparticles prepared by pulsed laser ablation in water

Yb³⁺:Ho³⁺ co-doped Gd₂O₃ nanoparticles were successfully synthesized by pulsed laser ablation in water under different laser energy. The phase structure, morphology, crystallization and upconversion photoluminescence properties of obtained samples were investigated using X-Ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and photoluminescence spectra. The mechanism of the upconversion process was discussed based on the energy level diagram and power dependent upconversion emission. Upconversion mechanisms and thermal effects caused by absorption of excitation laser were discussed. Temperature dependent green and red emissions of Yb³⁺:Ho³⁺ co-doped Gd₂O₃ nanoparticles under the excitation of 980 nm were investigated in the low temperature range of 130 K–280 K. Non-radiative decay rate theory was used to explain the difference of quenching rates of green and red emissions. A further study on temperature sensing properties based on fluorescence intensity ratio (FIR) of green and red emissions was carried out. The FIR as a function of temperature can be well fitted by the model based on the thermal quenching theory. The relative sensitivity reaches its maximum value of 0.804% K⁻¹ at 216 K.     

Intense emission at 2.9 μm from Yb3+/Ho3+ co-doped TeO2–Ga2O3–ZnO tellurite glasses

Mid-infrared lasers have important applications in infrared countermeasures, sensing, environmental monitoring, biomedicine, and many military and civilian fields. In this work, an intense emission at 2.9 μm from Yb³⁺/Ho³⁺ co-doped TeO₂-Ga₂O₃-ZnO (TGZ) glass was reported. The 2 μm, 1.2 μm and visible emissions were also performed to understand the competitive luminescent mechanism. With the increase in Yb³⁺ concentration, all the emissions of Ho³⁺ increased, whereas the emission of Yb³⁺ decreased due to the phonon-assisted energy transfer from Yb³⁺ to Ho³⁺. The lifetimes of optimized 3 mol% Yb₂O₃ and 1 mol% Ho₂O₃ co-doped TGZ glass, which has the maximum emission intensity, are 548 μs and 1.7 ms at 2.9 and 2 μm, respectively. The Judd–Ofelt intensity parameters, absorption, and emission cross sections were calculated to evaluate the mid-infrared fluorescence properties of this new glass matrix material. The gain coefficients show that the 2 and 2.9 μm laser gain can be realized by small pump energy, indicating that this glass is a promising medium for the mid-infrared optical fiber laser.     

Designing and temperature sensing characteristics of upconversion luminescence core-shell structures with negative and positive thermal expansion

Generally, lanthanum ions doped positive expansion and negative expansion materials exhibit thermal quenching and enhancement of upconversion luminescence (UCL), respectively. Combining the UCL characteristics of positive expansion and negative expansion lattices is of importance for developing efficient temperature sensing systems. Here, positive expansion TiO₂:Yb³⁺, Er³⁺ three dimensionally ordered macroporous film was prepared by the template-assisted approach, and the Yb₂W₃O₁₂: Er³⁺ solution was filled into the TiO₂: Yb³⁺, Er³⁺ three dimensionally ordered macroporous film. After secondary sintering, the shell of negative expansion Yb₂W₃O₁₂: Er³⁺was formed on the surface of TiO₂:Yb³⁺/Er³⁺ core. Under 980 nm excitation, the red and green UCL is predominate for the spectra of TiO₂:Yb³⁺/Er³⁺ core and Yb₂W₃O₁₂: Er³⁺ shell, respectively. With the measurement temperature increasing, the green UCL from negative expansion Yb₂W₃O₁₂: Er³⁺ shell increases, while the red UCL from positive expansion TiO₂:Yb³⁺, Er³⁺ core decreases. The performance of temperature sensing was characterized by the monitoring the UCL intensity ratio between 525 nm and 660 nm. The temperature sensitivity is about 1.12% K^?1, which is larger than that of thermally coupled FIR technology. We believed that the present work is instructive for developing new generation temperature sensor.     

Yb3+/Er3+ co-doped Gd2Te4O11 nanosheets with intrinsic polarity: One-step hydrothermal synthesis and upconverted optical temperature measuring ability

The digadolinium tellurite phosphors of Gd₂Te₄O₁₁(GTO):Yb³⁺/Er³⁺ have been successfully synthesized as upconversion luminescence (UCL) materials via one-step hydrothermal method. The crystal structure, morphology, and upconversion luminescence property were systematically characterize by XRD, SEM, and spectroscopy techniques. The Rietveld refinements of crystal structure were carried out on the XRD patterns and the feature of crystal structure was analyzed. Under the 980 nm NIR excitation, these materials showed very bright upconverted emissions. The concentrations of Yb³⁺ and Er³⁺ were optimized and the strongest upconverted emissions were achieved in the GTO:15%Yb³⁺/1%Er³⁺. The possible energy transfer mechanism of UCL was proposed based upon the analysis of power-dependent UCL and fluorescence kinetics. Furthermore, the fluorescence intensity ratio (FIR) derive from the two thermally coupled energy levels (²H_11/2 and ⁴S_3/2) of Er³⁺ was employed as indicator for temperature measurement. The maximum absolute sensitivity can be achieved to be 7.34 × 10^?3 K^?1 at 501 K. This material exhibited good reliability and repeatability in optical temperature measurement, which could be a novel promising candidate for noncontact temperature sensors.     

Facile precipitation synthesis of green-emitting BaY2F8:Yb3+, Ho3+ upconverting phosphor

Monoclinic phase of BaY₂F₈:0.2Yb³⁺, xHo³⁺ (x = 0.005, 0.01, 0.02, 0.05 and 0.1 mol) and BaY₂F₈:yYb³⁺, 0.02Ho³⁺ (y = 0.01, 0.02, 0.05, 0.1 and 0.2 mol) phosphors were prepared by a facile precipitation method. Rietveld analysis was employed to carry out the structural refinement. The upconversion (UC) luminescence properties of BaY₂F₈:Yb³⁺, Ho³⁺ phosphor were investigated under near infra-red (NIR) excitation and the influence of dopant concentration on their spectra was also accessed. The possible UC processes and the energy transfer mechanisms between Yb³⁺ and Ho³⁺ were discussed on the basis of UC decay curves and the UC spectra obtained from a laser-pump power variation. The thermal stability of the BaY₂F₈:0.2Yb³⁺, 0.02Ho³⁺ phosphor was studied in the range of 30–300 °C. The color coordinates of the phosphor were located in the green region with very high color purity.     

Effect of Ba nonstoichiometry on the phase composition,microstructure, chemical stability and electrical conductivity of BaxCe0.7Zr0.1Y0.1Yb0.1O3−δ (0.9≤x≤1.1) proton conductors

The perovskite proton conductors Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x=0.9, 0.94, 0.98, 1.0, 1.03, 1.06, and 1.1) have been successfully prepared by the conventional solid state reaction route. X-ray diffraction (XRD) patterns of the samples indicate that Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x≥1.0) samples possess a single phase orthorhombic structure, but a secondary phase (Y,Ce)O_2−δ exists in Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x<1.0) samples. SEM photographs show that the grain size of Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ increases and the porosity decreases with Ba²⁺ content varying from x=0.9 to 1.1. Because of ZnO addition as sintering aid, the sintering temperature of the samples reduces from 1550 °C to 1250 °C. The chemical stability of the samples against CO₂ decreases with the increase in Ba content from x=0.9 to 1.1. All the samples show a excellent stability against water vapor at 850 °C. The conductivities of the samples increase and the activation energies reduce with the increase in Ba content. The present results suggest that it is very important to control the stoichiometry of cations to obtain desired perovskite type high temperature proton conductors.     

Near infrared light-induced photocurrent in NaYF4:Yb3+, Er3+/WO2.72 composite film

Spectral conversion technology based on NaYF₄:Yb³⁺, Er³⁺ upconversion nanoparticles was extensively used to improve photovoltaic conversion efficiency of solar cells. However, the response mismatch between absorption of semiconductors and upconversion luminescence (UCL) limits the application of spectral conversion technology. Nonstoichiometric WO_2.72 nanoparticles display the broad absorption from visible to near-infrared region due to the presence of oxygen vacancy, which is overlapped with the UCL of NaYF₄:Yb³⁺, Er³⁺ nanoparticles. Thus, the combination between NaYF₄:Yb³⁺, Er³⁺ nanoparticles, and nonstoichiometric WO_2.72 provides a possibility for designing a novel UCL spectral converted solar cells. In this work, composite film consisted of NaYF₄:Yb³⁺, Er³⁺ nanoparticles, and WO_2.72 nanofibers was prepared. The UCL of NaYF₄:Yb³⁺, Er³⁺/WO_2.72 film was decreased in contrast to pure NaYF₄:Yb³⁺, Er³⁺ nanoparticles due to energy transfer from NaYF₄:Yb³⁺, Er³⁺ nanoparticles to WO_2.72 nanofibers. The NaYF₄:Yb³⁺, E³⁺/WO_2.72film exhibits the photocurrent generation upon the 980 nm excitation. This novel UCL spectral converted solar cells based on the broad absorption of defects in the WO_2.72 host will provide a novel view for photovoltaic devices.     

Color-tunable up-conversion emission from Yb3+/Er3+/Tm3+/Ho3+ codoped KY(MoO4)2 microcrystals based on energy transfer

Tunable up-conversion luminescent material KY(MoO₄)₂: Yb³⁺, Ln³⁺ (Ln=Er, Tm, Ho) has been synthesized by a typical hydrothermal process. Under 980 nm laser diode (LD) excitation, the emission intensity and the corresponding luminescence colors of KY(MoO₄)₂: Yb³⁺, Ln³⁺ (Ln=Er, Tm, Ho) have been investigated in detail. The energy transfer from the Yb³⁺ sensitizer to Ho³⁺, Er³⁺ and Tm³⁺ activators plays an important role in the development of color-tunable single- phased phosphors. The emission intensity keep balance through control of the Ho³⁺ co-doping concentrations, white light was experimentally shown at KY(MoO₄)₂: 20 mol% Yb³⁺, 0.8 mol% Er³⁺, 0.5 mol% Tm³⁺, 1.0 mol% Ho³⁺ phosphor with further calcination at 800 °C for 4 h under 980 nm laser excitation. The color tunability, high quality of white light and high intensity of the emitted signal make these up-conversion (UC) phosphors excellent candidates for applications in solid-state lighting.     

High sensitive Ln3+/Tm3+/Yb3+ (Ln3+ = Ho3+, Er3+) tri-doped Ba3Y4O9 upconverting optical thermometric materials based on diverse thermal response from non-thermally coupled energy levels

Upconversion (UC) optical thermometers using the fluorescence intensity ratio (FIR) technique arising from the thermally coupled energy levels (TCLs) are still suffering from low sensitivity owing to the restriction of small energy gap. In the present study, a strategy to strive for superior temperature sensitivity and signal discriminability is employed with the help of non-thermally coupled energy levels (NTCLs). A novel tri-doped Ba₃Y₄O₉: Ho³⁺/Tm³⁺/Yb³⁺ phosphor with rhombohedral symmetry was successfully prepared via a solid-state reaction method, and the temperature sensing performance was evaluated by analyzing temperature-dependent upconversion emission spectra. The emission intensities of both Ho³⁺ and Tm³⁺ activators can be almost completely restored to their original values when the temperature of the sample is cooled to room temperature. The temperature-dependent FIR between NTCLs can be fitted well by a derived three-term equation with the correlation coefficient above 99.6%, and the FIR of NTCLs exhibits high temperature sensitivity over a wide temperature range owing to the different temperature responses of the NTCLs. The maximum absolute sensitivity S_A and relative sensitivity S_R values reaches as high as 0.0552?K^?1 and 1.49% K^?1, respectively, which are much higher than those of the previously reported bulk UC optical temperature sensing materials. Moreover, the emission bands of NTCLs are well separated, which endows the material a good signal discriminability for temperature detection. Excellent temperature sensing performance is also demonstrated in Er³⁺/Tm³⁺/Yb³⁺ tri-doped Ba₃Y₄O₉, evidencing the validity of this strategy. These results indicate that the present UC materials can be potential candidates for optical temperature sensors, and the present strategy will provide a thought for developing other innovative UC temperature sensing materials.     

Up-conversion emission color tuning in NaLa(MoO4)2: Nd3+/Yb3+/Ho3+ excited at 808 nm

As alternatives to Yb³⁺-sensitized up-conversion (UC) materials excited at 980 nm, Nd³⁺-sensitized UC phosphors irradiated by 808 nm have been used to decrease the absorption of water and alleviate the overheating effect in vivo biological application. Intense red and green UC emissions from ⁵F₅→⁵I₈ and ⁵F₄/⁵S₂→⁵I₈ transitions of Ho³⁺ appeared in Nd³⁺/Yb³⁺/Ho³⁺ tri-doped NaLa(MoO₄)₂ through successive energy transfer Nd³⁺→Yb³⁺→Ho³⁺ under 808 nm excitation, in which Yb³⁺ ions were proven to be the energy transfer bridge between Nd³⁺ and Ho³⁺ by lifetime measurement. The variable emission color and intensity ratios of red to green emissions were realized by adjusting the doping concentration of Yb³⁺, pulse width of the excitation laser and the addition of Ce³⁺ ion, which depends on the different population pathways to the green and red emitting states of Ho³⁺. The chromaticity modulation mechanisms of these approaches were proposed, which provides a feasible strategy to tune the UC emission color.     

Optical property evaluation of thoria doped with heavier rare-earth oxides LnO1.5 (Ln = Er3+, Ho3+, Tm3+, and Yb3+)

To address the existing gap in the literature of upconversion studies involving thoria, samples of thoria doped with Ho³⁺-Yb³⁺, Er³⁺-Yb³⁺, and Tm³⁺-Yb³⁺ were synthesized following epoxide gel method and were characterized. Fluorite structure of the doped samples was evident for the calcined samples from the corresponding xerogels as noticed in their powder X-ray diffraction patterns. The presence of a sharp band near 460 cm⁻¹ in the Raman spectra of all these samples supported the results from diffraction experiments. Intraconfigurational f-f transitions of Ho³⁺, Er³⁺, and Tm³⁺ were present in the UV-visible absorbance spectra of these samples. Both normal emission and upconversion emission from these samples have been studied. Emissions in all three basic colors were observed in the upconversion emission spectra. Two-photon process was found to be responsible for the upconversion in these systems. Decay time measurements for these emissions were analyzed. This synthetic process was further extended to determine the extent of dissolution of heavier lanthanides and found that up to 50 mol% of Ho³⁺, Er³⁺ and 60 mol% of Tm³⁺, Yb³⁺ could be dissolved retaining fluorite structure. Creation of oxygen vacancies for these heavily doped specimens was quite encouraging and it could be useful as solid electrolytes in fuel cells.     

Blue Upconversion Emission in Germanate Glass Co-Doped with Yb3+/Tm3+ Ions

In the paper, the upconversion luminescence of 70GeO₂–30[Ga₂O₃–BaO–Na₂O] glass system co-doped with Yb³⁺/Tm³⁺ ions was investigated. Strong blue emission at 478 nm corresponding to the transition ¹G₄ → ³H₆ in thulium ions was measured under the excitation of 976-nm diode laser. The dependence of the upconversion emission upon the thulium ion concentration was studied to determine the optimal conditions of energy transfer between energy levels of active dopants. The most effective energy transfer Yb³⁺ → Tm³⁺ was obtained in glass co-doped with molar ratio of dopant 0.7 Yb₂O₃/0.07 Tm₂O₃. The increase in thulium concentration more than 0.07 mol% results in the reverse energy transfer from Tm³⁺ → Yb³⁺, which leads to rapid quenching of the luminescence line at the wavelength 478 nm. In germanate glass co-doped with 0.7Yb₂O₃/0.07Tm₂O₃, the longest lifetime of ¹G₄ level equal 278 μs was achieved. The presented results indicate that elaborated germanate glass co-doped with Yb³⁺/Tm³⁺ ions is a promising material that can be used to produce fiber lasers and superluminescent fiber sources generating radiation in the visible spectrum.