Fabrication and Upconversion Luminescent Properties of Er3+‐Doped and Er3+/Yb3+ Codoped La2O2CN2 Nanofibers

La₂O₂CN₂:Er³⁺and La₂O₂CN₂:Er³⁺/Yb³⁺ upconversion (UC) luminescence nanofibers were successfully fabricated via cyanamidation of the respective relevant La₂O₃:Er³⁺ and La₂O₃:Er³⁺/Yb³⁺ nanofibers which were obtained by calcining the electrospun composite nanofibers. The morphologies, structures, and properties of the nanofibers are investigated. The mean diameters of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are 179.46 ± 12.58 nm and 198.85 ± 17.07 nm, respectively. It is found that intense green and weak red emissions around 524, 542, and 658 nm corresponding to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_l5/2 energy levels transitions of Er³⁺ ions are observed for La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers under the excitation of a 980‐nm diode laser. Moreover, the emitting colors of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are all located in the green region. The upconversion luminescent mechanism and formation mechanism of the nanofibers are also proposed.     

An Upconversion Niobium Pentoxide Bulk Glass Codoped with Er3+/Yb3+ Fabricated by Aerodynamic Levitation Method

(82?x)NbO_2.5–17.4LaO_1.5–xZrO₂(NLZ) (x = 7.5, 10, 12.5, 15, 17.5, and 20) bulk glasses codoped with Er³⁺/Yb³⁺ were successfully fabricated by aerodynamic levitation method for the first time. The structure, thermal stability, and luminescent properties of the samples were investigated systemically by XRD, differential scanning calorimetry, and upconversion spectra. Under 980 nm laser excitation, all samples exhibited green and red upconversion emissions centered at 531, 546, and 674 nm. Results showed that the sample with 15 mol% ZrO₂ obtained the most efficient upconversion luminescence and good thermal stability with the glass‐transition temperature as high as 743 °C. The effect of the addition of ZrO₂ on the structure behavior and the phonon density in the glass was investigated by Raman spectra, which are the key factors for the upconversion luminescence intensity.     

Highly Sensitive Optical Thermometry of Yb3+‐Er3+ Codoped AgLa(MoO4)2 Green Upconversion Phosphor

Er³⁺–Yb³⁺ codoped AgLa(MoO₄)₂ phosphors with intense green emission from ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions and negligible red emission from ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ were synthesized by sol–gel process. Its temperature sensing performance was evaluated based on the temperature dependence of fluorescence intensity ratio (FIR) of two green emission bands in the range 300–510 K. The maximum sensitivity of AgLa(MoO₄)₂: 0.02Er³⁺/0.4Yb³⁺ is approximately 0.018 K^?1 at 480 K, which is much higher than those of reported samples based on green emissions of Er³⁺. Result suggests that AgLa(MoO₄)₂: Er³⁺/Yb³⁺ has a great potential application in optical temperature sensors.     

Multifunctional optical thermometry based on the stark sublevels of Er3+ in CaO-Y2O3: Yb3+/Er3+

A conventional high temperature solid state method was utilized to prepare CaO-Y₂O₃, which is a potential candidate for manufacturing crucible material to melt titanium and titanium alloys with low cost. Meanwhile, Yb³⁺ ions and Er³⁺ ions were selected as the sensitizers and activators respectively to dope into CaO-Y₂O₃, aimed at providing real-time optical thermometry during the preparation process of titanium alloys realized using fluorescence intensity ratio (FIR) technology. The results reveal that a high measurement precision can be acquired by using the Stark sublevels of Er^{3+ 4}F_9/2 to measure the temperature with a maximum absolute error of only about 3 K. In addition, by analyzing the dependence of ⁴I_13/2 → ⁴I_15/2 transition on pump power of 980 nm excitation wavelength, it was found that the laser-induced thermal effect has almost no influence on the temperature measurement conducted by using the FIR of the Stark sublevels of Er^{3+ 4}I_13/2, which means that a high excitation pump power can be used to obtain strong NIR emission and good signal-to-noise ratio for optical thermometry without the influence of the laser-induced thermal effect. All the results reveal that CaO-Y₂O₃: Yb³⁺/Er³⁺ is an excellent temperature sensing material with high measurement precision.     

The upconverted luminescence and chemical stability of morphologically controllable La2O3:Yb3+/Er3+ nanoparticles

Morphology and size controls are critical for the research of luminescent nanomaterials. In this work, La₂O₃:18%Yb³⁺/2%Er³⁺ nanoparticles were synthesized by a urea-assisted coprecipitation process, where the morphology and size of nanoparticles could be precisely controlled by adjusting the doping concentration, urea dosage and reaction time. With increasing Yb³⁺ doping concentration and reaction time, morphological evolution processes from nanosheets to nanospheres to nanofibers were observed. The experimental results revealed that the nanospheres could only be synthesized when 18%Yb³⁺ and 2%Er³⁺ were doped into the La₂O₃ host, where the size of the nanospheres could be precisely controlled by adjusting the urea dosage. The effects of the particle morphology and size on the upconversion luminescence of La₂O₃:18%Yb³⁺/2%Er³⁺ nanoparticles were investigated. In addition, the chemical stability of La₂O₃:18%Yb³⁺/2%Er³⁺ nanospheres in air was investigated by recording XRD and upconversion luminescence spectra after exposure to air for different periods. The experimental conclusions were useful for further probing the effects of the particle morphology and size on the upconversion emission of Er³⁺.     

Upconversion Luminescence and Energy‐Transfer Mechanism of NaGd(MoO4)2: Yb3+/Er3+ Microcrystals

Uniform and well‐crystallized NaGd(MoO₄)₂: Yb³⁺/Er^{3 +} microcrystals with tetragonal plate morphology were synthesized by a facile hydrothermal method. The structure and phase purity of the samples were identified by powder XRD analysis. The steady‐state and transient luminescence spectra were measured and analyzed. Under 980 nm excitation, intense green luminescence at 531 and 553 nm, and red luminescence at 657 and 670 nm were observed. The optimum doping concentrations for Yb³⁺ and Er³⁺ are determined to be 20% and 1% in NaGd(MoO₄)₂ tetragonal plate microcrystals. With increasing Yb³⁺ doping concentrations, the total integral emission intensities increase first and then decrease. The red/green intensity ratio of NaGd(MoO₄)₂: Yb³⁺/Er³⁺ microcrystals increases from 0.4 to 1.0 with the increase in Yb³⁺ concentrations. Based on the energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the double logarithmic plot of upconversion intensities versus pump powers. The energy‐transfer mechanisms for green and red upconversion luminescence are ascribed to two‐photon processes at lower Yb³⁺ concentrations, and involve high‐Yb³⁺‐induced one‐photon processes at higher Yb³⁺ concentrations. For the red upconversion luminescence, energy back‐transfer process, that is, ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺), is dominant at higher Yb³⁺ concentrations. Theoretical model of the energy‐transfer mechanisms based on rate equations is established, which agrees well with the experimental results.     

A Novel Scheme to Obtain Y2O2S:Er3+ Upconversion Luminescent Hollow Nanofibers via Precursor Templating

Y₂O₃:Er³⁺ hollow nanofibers were prepared by calcination of the monoaxial electrospinning‐derived PVP/[Y(NO₃)₃+Er(NO₃)₃] composite nanofibers, and then Y₂O₂S:Er³⁺ hollow nanofibers were synthesized by sulfurization of the as‐obtained Y₂O₃:Er³⁺ hollow nanofibers via a double‐crucible method using sulfur powders as sulfur source. X‐ray diffraction (XRD) analysis shows that the Y₂O₂S:Er³⁺ hollow nanofibers are pure hexagonal phase with the space group of . Scanning electron microscope (SEM) observation indicates that the Y₂O₂S:Er³⁺ hollow nanofibers are obvious hollow‐centered with the outer diameter of 176 ± 25 nm. Upconversion emission spectrum analysis manifests that Y₂O₂S:Er³⁺ hollow nanofibers emit strong green and weak red upconversion emissions centering at 526, 546, and 667 nm, respectively. The green emissions and the red emission are, respectively, originated from ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_l5/2 energy levels transitions of the Er³⁺ ions. The emitting colors of Y₂O₂S:Er³⁺ hollow nanofibers are located in the green region in CIE chromaticity coordinates diagram. The formation mechanism of the Y₂O₂S:Er³⁺ hollow nanofibers is also advanced. This preparation technique can be applied to prepare other rare‐earth oxysulfides hollow nanofibers.     

Preparation,Structure, and Up‐Conversion Luminescence of Yb3+/Er3+ Codoped SrIn2O4 Phosphors

SrIn₂O₄, which shows lower phonon energy than CaIn₂O₄, is not only a good photocatalyst but also can be an excellent up‐conversion (UC) host to exhibits UC luminescence. In this work, Yb³⁺ and/or Er³⁺ doped SrIn₂O₄ phosphors were synthesized, and their UC luminescence properties were studied and compared with those in the CaIn₂O₄ host. The structure of SrIn₂O₄: 0.01Er³⁺ and SrIn₂O₄: 0.1Yb³⁺/0.01Er³⁺ samples were refined by the Rietveld method and found to that SrIn₂O₄: 0.1Yb³⁺/0.01Er³⁺ showed increasing unit cell parameters and cell volume, indicating In³⁺ sites were substituted successfully by Yb³⁺ and/or Er³⁺ ions. From the UC luminescence spectra and diffuse reflection spectra, Er³⁺‐doped SrIn₂O₄ showed very weak luminescence due to ground state absorption of Er³⁺; Yb³⁺/Er³⁺ codoped SrIn₂O₄ presented strong green (550 nm) and red (663 nm) UC emissions which were assigned to energy transfer from Yb³⁺ transition ²F_7/2→²F_5/2 to the Er³⁺ transition ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2. Comparing with CaIn₂O₄, Yb³⁺/Er³⁺ codoped SrIn₂O₄ showed obvious advantages with higher UC luminescent intensity. The pumping powers study showed that UC emissions in Yb³⁺/Er³⁺ codoped SrIn₂O₄ were attributed to energy transfer of Yb³⁺→Er³⁺ with a two‐photon process. The possible UC luminescent mechanism of Yb³⁺/Er³⁺‐doped SrIn₂O₄ was discussed.     

Er3+和Yb3+共掺锗硅酸盐玻璃的上转换发光性能

采用熔融-淬火法制备了基体为30SiO2-20GeO2-15Al2O3-5B2O3-30CaF2的Er3+和Yb3+共掺锗硅酸盐玻璃.采用X射线衍射仪、荧光光谱仪和热分析仪对样品进行了表征.结果表明:Er2O3含量从0.5％(摩尔分数,下同)增加到2.0％时,玻璃转变温度Tg和Tx-Tg(Tx为第一析晶温度)数值均有明显下降.Yb2O3含量从1.5％增加到4.5％时,玻璃上转换红光发光强度数量级由103提高到105,对其中1个样品在740℃热处理12h后发现,微晶化处理并未对Er的上转换发光产生有益影响.     

Microwave Sol–Gel Synthesis of CaGd2(MoO4)4:Er3+/Yb3+ Phosphors and Their Upconversion Photoluminescence Properties

CaGd₂(MoO₄)₄:Er³⁺/Yb³⁺ phosphors with the doping concentrations of Er³⁺ and Yb³⁺ (x = Er³⁺ + Yb³⁺, Er³⁺ = 0.05, 0.1, 0.2, and Yb³⁺ = 0.2, 0.45) have been successfully synthesized by the microwave sol–gel method, and the crystal structure refinement and upconversion photoluminescence properties have been investigated. The synthesized particles, being formed after heat‐treatment at 900°C for 16 h, showed a well‐crystallized morphology. Under the excitation at 980 nm, CaGd₂(MoO₄)₄:Er³⁺/Yb³⁺ particles exhibited strong 525 and 550‐nm emission bands in the green region and a weak 655‐nm emission band in the red region. The Raman spectrum of undoped CaGd₂(MoO₄)₄ revealed about 15 narrow lines. The strongest band observed at 903 cm^?1 was assigned to the ν₁ symmetric stretching vibration of MoO₄ tetrahedrons. The spectra of the samples doped with Er and Yb obtained under 514.5 nm excitation were dominated by Er³⁺ luminescence preventing the recording Raman spectra of these samples. Concentration quenching of the erbium luminescence at ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions in the CaGd₂(MoO₄)₄:Er³⁺/Yb³⁺ crystal structure was established to be approximately at the 10 at.% doping level.     

Optical Temperature Sensing Behavior of High‐Efficiency Upconversion: Er3+–Yb3+ Co‐Doped NaY(MoO4)2 Phosphor

A class of Yb³⁺/Er³⁺ co‐doped NaY(MoO₄)₂ upconversion (UC) phosphors have been successfully synthesized by a facile hydrothermal route with further calcination. The structural properties and the phase composition of the samples were characterized by X‐ray diffraction (XRD). The UC luminescence properties of Yb³⁺/Er³⁺ co‐doped NaY(MoO₄)₂ were investigated in detail. Concentration‐dependent studies revealed that the optimal composition was realized for a 2% Er³⁺ and 10% Yb³⁺‐doping concentration. Two‐photon excitation UC mechanism further illustrated that the green enhancement arised from a novel energy‐transfer (ET) pathway which entailed a strong ground‐state absorption of Yb³⁺ ions and the excited state absorption of Yb³⁺–MoO₄^2? dimers, followed by an effective energy transfer to the high‐energy state of Er³⁺ ions. We have also studied the thermal properties of UC emissions between 303 and 523 K for the optical thermometry behavior under a 980 nm laser diode excitation for the first time. The higher sensitivity for temperature measurement could be obtained compared to the previous reported rare‐earth ions fluorescence based optical temperature sensors. These results indicated that the present sample was a promising candidate for optical temperature sensors with high sensitivity.     

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.     

Visible green upconversion luminescence of Er3+/Yb3+/Li+ co-doped CaWO4 particles

The upconversion (UC) luminescence of Li⁺/Er³⁺/Yb³⁺ co-doped CaWO₄ phosphors is investigated in detail. Single crystallized CaWO₄:Li⁺/Er³⁺/Yb³⁺ phosphor can be obtained, co-doped up to 25.0/5.0/20.0 mol% (Li⁺/Er³⁺/Yb³⁺) by solid-state reaction. Under 980 nm excitation, CaWO₄:Li⁺/Er³⁺/Yb³⁺ phosphor exhibited strong green UC emissions visible to the naked eye at 530 and 550 nm induced by the intra-4f transitions of Er³⁺ (²H_11/2,⁴S_3/2→⁴I_15/2). The optimum doping concentrations of Yb³⁺/Li⁺ for the highest UC luminescence were verified to be 10/15 mol%, and a possible UC mechanism that depends on the pumping power is discussed in detail.     

Infrared to Visible Up‐Conversion Emission in Er3+/Yb3+ Codoped Fluoro–Phosphate Glass–Ceramics

Erbium Er³⁺ and ytterbium Yb³⁺ codoped fluoro‐phosphate glasses belonging to the system NaPO₃–YF₃–BaF₂–CaF₂ have been prepared by the classical melt‐quenching technique. Glasses containing up to 10 wt% of erbium and ytterbium fluorides have been obtained and characterized using differential scanning calorimetry (DSC) and UV–visible and near‐infrared spectroscopy. Transparent and homogeneous glass–ceramics have been then reproducibly synthetized by appropriate heat treatment above glass transition temperature of a selected parent glass. Structural investigations of the crystallization performed through X‐ray diffractometry (XRD) and scanning electron microscopy (SEM) have evidenced the formation of fluorite‐type cubic crystals based during the devitrification process. Finally, infrared to visible up‐conversion emission upon excitation at 975 nm has been studied on the Er³⁺ and Yb³⁺ codoped glass–ceramics as a function of thermal treatment time. A large enhancement of intensity of the up‐conversion emissions–about 150 times‐ has been observed in the glass–ceramics if compared to the parent glass one, suggesting an incorporation of the rare‐earth ions (REI) into the crystalline phase.     

BaCl2:Er3+—A High Efficient Upconversion Phosphor for Broadband Near‐Infrared Photoresponsive Devices

Utilization of photons with subband‐gap energy, mostly near‐infrared (NIR) photons, is highly desirable for photovoltaic cells; which can be achieved by adding an upconversion layer at the rear face of photovoltaic cells. Here, we study the upconversion luminescence properties of BaCl₂:Er³⁺ phosphors and hexagonal NaYF₄:Er³⁺ phosphors upon excitation of incoherent NIR sunlight with wavelength λ > 800 nm. Higher efficacious upconversion emissions of BaCl₂:Er³⁺ phosphors have been observed in comparison with the well‐known hexagonal NaYF₄:Er³⁺ phosphors. We demonstrate that the photocurrent response from the thin‐film‐hydrogenated amorphous silicon solar cell attached with the BaCl₂:Er³⁺ phosphor is notably enhanced under irradiation of incoherent NIR sunlight with wavelength λ > 800 nm. This judicious design may be envisioned to shorten the distance for the remarkable improvement of the power conversion efficiency of the next‐generation photovoltaic cells and suggests a promising application for other NIR photoresponsive devices.     

Optical properties of Er3+ and Yb3+/Er3+-doped NaF-Na2SO4-Al(PO3)3 fluoro-sulfo-phosphate glasses

Fluoro-sulfo-phosphate (FPS) glass is of current interest as potential material for laser application due to its good glass-forming ability, thermal, and chemical stability as well as the complicated local environment for incorporated species. Herein, the physical and luminescent properties of Er³⁺ and Yb³⁺/Er³⁺-doped FPS glasses vs S/F ratio are investigated comprehensively. The low melting temperature (750°C) leads to fewer ingredients evaporation and easier operation. The sulfate addition depolymerizes the structure of FPS glasses, leading to either monotonic or nonmonotonic variations of physical properties, while no deterioration in thermal and limited one in chemical stability is caused. The addition of sulfate also modifies the local structure around optical active species and thus, leading to higher emission cross section (1.52 × 10⁻²⁰ cm²), effective linewidth (68.4 nm), figure of merit (5.61 × 10⁻²³ s cm²), gain bandwidth (102.44 × 10⁻²⁷ cm³), and energy-transfer microparameters (51.87 × 10⁻³⁹ cm⁶/s), implying high possibility to serve as 1.5 μm laser application.     

Yb~(3+)/Er~(3+)共掺锗酸盐氧氟微晶玻璃的白光输出

对组成为50GeO_2-20Al_2O_3-15CaF_2-15LiF稀土离子锗酸盐氧氟玻璃在550℃微晶化热处理24h,得到了透明的微晶玻璃。X射线衍射表明:玻璃中析出了CaF_2纳米晶粒,晶粒尺寸在17 nm左右。在980 nm泵浦光的激发下,Yb~(3+)/Er~(3+)双掺微晶玻璃产生了蓝绿红上转换荧光。随着玻璃中Yb~(3+)的掺杂浓度的增加蓝光和红光荧光强度增大,其中5%Yb~(3+)/1%Er~(3+)(摩尔分数,下同)的微晶玻璃样品的上转换发光已经出现白光效果。通过研究一系列高Yb~(3+)/Er~(3+)浓度比的共掺微晶玻璃样品,实现了对上转换红光绿光与蓝光的荧光强度比例的调整,当Yb~(3+)掺杂浓度为12%、Er~(3+)掺杂浓度为0.01%时,微晶玻璃的上转化发光接近白光。     

Power dependent upconversion in Er3+/Yb3+ co-doped BiNbO4 phosphors

In the present article, optical properties and energy upconversion in Er³⁺/Yb³⁺ co-doped BiNbO₄ matrix were investigated. The BiNbO₄ matrix was prepared using the solid-state reaction method. X-ray diffraction of the matrix shows that the crystal structure is consistent with ICSD code 74338. The grain distribution and the behavior of doping with Er³⁺ and Yb³⁺ on the sample surface were obtained by scanning electron microscope. Raman spectral characterization was carried out to examine the behavior of the vibrational modes of the samples. Upconversion emissions in the visible region at 484.5, 522, 541.5 and 670.5 nm in the matrices BiNbO₄:Er,Yb and BiNbO₄:Er were observed and analyzed as a function of 980 nm laser excitation power and rare-earth doping concentration. The results show that BiNbO₄ is a promising host material for efficient upconversion phosphors.     

Tuning size and up-conversion luminescence of CeO2:Yb3+/Er3+ nanoparticles through lanthanide doping

Particle size is a critical parameter in up-conversion luminescence tuning and application research. In this study, CeO₂:Yb³⁺/Er³⁺ nanospheres were synthesized via coprecipitation. The average size of these nanospheres gradually decreased as the Yb³⁺ doping concentration increased, which might be attributed to the influence of Yb³⁺ doping on the growth rate of nanospheres by surface charge repulsion. Upon exciting these nanospheres using a 980-nm laser, the corresponding up-conversion red-green emission intensity ratio gradually increased as the Yb³⁺ doping concentration increased, which might be ascribed to two reasons: the strengthened ⁴S_3/2 → ⁴F_9/2 nonradiative relaxation process and the enhanced Er³⁺ → Yb³⁺ energy back-transfer process. To assess the influence of the nonradiative relaxation process on the up-conversion emission red-green ratios, the down-conversion emission spectra and decay curves of CeO₂:x%Yb³⁺/2%Er³⁺ nanospheres that were excited by a 520 nm laser were investigated. To validate how the particle size affects the up-conversion emission, CeO₂:x%Lu³⁺/2%Yb³⁺/2%Er³⁺ nanospheres of various sizes were synthesized by substituting optically active Yb³⁺ using optically inert Lu³⁺. The corresponding up-conversion emission spectra and decay curves were investigated. The experimental results enhanced our understanding of how lanthanide doping affects the up-conversion luminescence tuning of Er³⁺, offering great potential to regulate the morphology and optical properties of the up-conversion luminescence nanoparticles.