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.     

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.     

Color‐Tunable Up‐Conversion Emission and Infrared Photoluminescence and Dielectric Relaxation of Er3+/Yb3+ Co‐Doped Bi2Ti2O7 Pyrochlore Thin Films

The color‐tunable up‐conversion (UC) emission and infrared photoluminescence and dielectric relaxation of Er³⁺/Yb³⁺ co‐doped Bi₂Ti₂O₇ pyrochlore thin films prepared by a chemical solution deposition method have been investigated. The pyrochlore phase structure of Bi₂Ti₂O₇ can be stabilized by Er³⁺/Yb³⁺ co‐doping. Intense color‐tunable UC emission and infrared photoluminescence can be detected on the thin films excited by a 980 nm diode laser. Two UC emission bands centered at 548 and 660 nm in the spectra can be assigned to ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. A Stokes infrared emission centered at 1530 nm is due to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions. The dependence of UC emission intensity on pumping power indicates that the UC emission of the thin films is a two‐photon process. The thin films also exhibit a relatively high dielectric constant and a low dissipation factor as well as a good bias voltage stability. Temperature‐ and frequency‐dependent dielectric relaxation has been confirmed. This study suggests that Er³⁺/Yb³⁺ co‐doped Bi₂Ti₂O₇ thin films can be applied to new multifunctional photoluminescence dielectric thin‐film devices.     

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.     

Upconversion emission of ZrO2 nanoparticles doped with erbium (Er3+) and ytterbium (Yb3+), synthesized by hydrothermal route

This paper reports the luminescent response upconversion of zirconium oxide (ZrO₂) nanoparticles doped with erbium (Er³⁺) and ytterbium (Yb³⁺) ions, synthesized by hydrothermal route. X ray diffraction (DRX) showed that the synthesized material presents the face centered cubic (FCC) structure. High resolution transmission electron microscopy (HRTEM) showed the presence of crystals size smaller than 10 nm. The photoluminescent analysis allowed to observe an intense upconversion luminescence emission of the samples doped with both ions Er³⁺ and Yb³⁺, when these are excited with 910 nm laser source, showing the electronic transitions ⁴F_9/2 → ⁴I_5/2; ²H_11/2 → ⁴I_5/2; ⁴S_3/2 → ⁴I_15/2 of Er³⁺. Two decay times were observed, whose behavior can be associated to the average distance between erbium ions within the nanocrystals.     

Down-conversion from Er3+-Yb3+ codoped CaMoO4 phosphor: A spectral conversion to improve solar cell efficiency

The present paper focuses on near infrared (NIR) down-conversion photoluminescence (PL) properties by studying the energy transfer mechanism between Er³⁺ and Yb³⁺ in CaMoO₄:Er³⁺, Yb³⁺ phosphors. We have successfully synthesized a series of Er³⁺ doped and Yb³⁺ codoped CaMoO₄ phosphors by hydrothermal method. The down-conversion of Er³⁺-Yb³⁺ combination with CaMoO₄ phosphor is designed to overcome the energy losses due to spectral mismatch when a high energy photon is incident on the Si-solar cell. The XRD, FESEM, EDX, PL, UV–Vis, Lifetime measurements were carried out to characterize the prepared down-converting phosphors. The crystallinity and surface morphology were studied by X-ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) techniques. The down-conversion PL spectra have been studied using 380 nm excitation wavelength. The Er³⁺ doped phosphors exhibit hypersensitive emission at 555 nm in the visible region due to ⁴S_3/2→⁴I_15/2 transition. The addition of Yb³⁺ into Er³⁺ doped CaMoO₄ attribute an emission at 980 nm due to ²F_5/2→²F_7/2 transition. The decrease in emission intensity in visible region and increase in NIR region reveals the energy transfer from Er³⁺ to Yb³⁺ through cross relaxation. The UV–Vis–NIR spectra shows the strong absorption peak around 1000 nm due to Yb³⁺ ion. The lifetime measurement also reveals the energy transfer from Er³⁺ to Yb³⁺ ions. The maximum value of energy transfer efficiency (ETE) and corresponding theoretical internal quantum efficiency are estimated as 74% and 174% respectively.     

Upconversion luminescence properties of NaBi(MoO4)2:Ln3+, Yb3+ (Ln = Er,Ho) nanomaterials synthesized at room temperature

Studies on lanthanide ions doped upconversion nanomaterials are increasing exponentially due to their widespread applications in various fields such as diagnosis, therapy, bio-imaging, anti-counterfeiting, photocatalysis, solar cells and sensors, etc. Here, we are reporting upconversion luminescence properties of NaBi(MoO₄)₂:Ln³⁺, Yb³⁺ (Ln = Er, Ho) nanomaterials synthesized at room temperature by simple co-precipitation method. Diffraction and spectroscopic studies revealed that these nanomaterials are effectively doped with Ln³⁺ ions in the scheelite lattice. DR UV–vis spectra of these materials exhibit two broad bands in the range of 200–350 nm correspond to MoO₄²⁻ charge transfer, s-p transition of Bi³⁺ ions and sharp peaks due to f-f transition of Ln³⁺ ions. Upconversion luminescence properties of these nanomaterials are investigated under 980 nm excitation. Doping concentration of Er³⁺ and Yb³⁺ ions is optimized to obtain best upconversion photoluminescence in NaBi(MoO₄)₂ nanomaterials and is found to be 5, 10 mol % for Er³⁺, Yb³⁺, respectively. NaBi(MoO₄)₂ nanomaterials co-doped with Er³⁺, Yb³⁺ exhibit strong green upconversion luminescence, whereas Ho³⁺, Yb³⁺ co-doped materials show strong red emission. Power dependent photoluminescence studies demonstrate that emission intensity increases with increasing pump power. Fluorescence intensity ratio (FIR) and population redistribution ability (PRA) of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ increases with increasing the Yb³⁺ concentration. Also, these values increase linearly with increasing the pump power up to 2 W. It reveal that these thermally coupled energy levels are effectively redistributed in co-doped samples due to local heating caused by Yb³⁺.     

Thermal effects of Er3+/Yb3+‐doped NaYF4 phosphor induced by 980/1510 nm laser diode irradiation

The thermal effects of Er/Yb‐doped NaYF₄ phosphor induced by 980/1510 nm laser diode irradiation were intuitively and contrastively investigated using an infrared thermal imaging technology with real‐time online monitoring. The Yb³⁺/Er³⁺ codoped materials have strong thermal effects and high‐temperature elevation under 980 nm irradiation. However, the severe thermal effects of materials with higher Er³⁺ ion doping concentration are remarkably attributed to the cross relaxation between the Er³⁺ ions under 980 nm irradiation. The energy transfer between Er³⁺ and Yb³⁺ ions in Er³⁺/Yb³⁺‐codoped materials also contributes to the thermal effects under 1510 nm laser diode irradiation. Under the same testing conditions, the temperature elevation ?T of samples induced by 1510 nm laser diode irradiation is lower than that induced by 980 nm laser diode irradiation. The temperature rising rate and elevation ?T value of samples depend on the ion doping concentration and power density of the laser diode excitation. The internal temperature of the samples exhibits deep temperature gradient under 980/1510 nm laser diode irradiation. By comparing the two kinds of thermometry methods, the temperature value calculated by fluorescence intensity ratio is almost similar to that obtained through infrared thermal imaging technology under higher excitation power pumping.     

Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

ZnO-TiO₂ composites co-doped with Er³⁺ and Yb³⁺ ions were successfully synthesized by powder-solution mixing method and their upconversion (UC) luminescence was evaluated. The effect of firing temperature, ZnO/TiO₂ mixing ratio, and dopant concentration ranges on structural and UC luminescence properties was investigated. The crystal structure of the product was studied and calculated in detail by means of X-ray diffraction (XRD). Also, the site preference of Er³⁺ and Yb³⁺ ions in the host material was considered and analyzed based on XRD results and UC luminescence characteristics. Brightest UC luminescence was observed in the ZnO-TiO₂:Er³⁺,Yb³⁺ phosphor fired at 1300 °C in which the system consisted of mixed phases; Zn₂TiO₄, TiO₂, RE₂Ti₂O₇ and RE₂TiO₅ (RE = Er³⁺ and/or Yb³⁺). Under the excitation of a 980 nm laser, the two emission bands were detected in the UC emission spectrum, weak green band centered at 544 and 559 nm, and strong red band centered at 657 and 675 nm wavelengths in accordance with ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The simple chemical formula equations, for explaining the site preference of Er³⁺ and Yb³⁺ ions in host crystal matrix, were generated by considering the Zn₂TiO₄ crystal structure, its crystal properties, and the effect of Er³⁺ and Yb³⁺ ions to the host crystal matrix. The UC emission intensity of the products was changed by varying ZnO/TiO₂ mixing ratios, and Er³⁺ and Yb³⁺ concentrations. The best suitable condition for emitting the brightest UC emission was 1ZnO:1TiO₂ doped with 3 mol% Er³⁺, 9 mol% Yb³⁺ fired at 1300 °C for 1 h.     

Effect of ligand field symmetry on upconversion luminescence in heat‐treated LaBGeO5:Yb3+, Er3+ glass

In this paper, we report upconversion (UC) luminescence enhancement in LaBGeO₅:Yb³⁺, Er³⁺ glass‐ceramics (GCs), surface crystallized glass‐ceramics (SCGCs) and ceramics compared with the as‐melt glass fabricated by the conventional melt‐quenching technique. Based on structural investigations, we find that the nucleation and crystallization of trigonal stillwellite LaBGeO₅:Yb³⁺, Er³⁺ nanocrystals occur first at the glass surface before the following volume crystallization. The local site symmetry around rare earth (RE) ions which was evaluated using the Eu³⁺ ions as a probe together with Judd‐Ofelt theory calculations exhibits a clear increase with the devitrification of the glass. Consequently, complete crystallization of the glass leads to largest enhancement in the UC emissions of the LaBGeO₅:Yb³⁺, Er³⁺ ceramics. We ascribe the enhancement of UC luminescence in the LaBGeO₅:Yb³⁺, Er³⁺ GCs, SCGCs, and ceramics to the structural ordering and the improvement of site symmetry surrounding RE ions that minimizes the rate of nonradiative relaxation process.     

Effect of BaF2 Content on Luminescence of Rare‐Earth Ions in Borate and Germanate Glasses

Rare‐earth‐doped oxyfluoride germanate and borate glasses were synthesized and next studied using spectroscopic methods. Influence of fluoride modifier on luminescence properties of rare earths in different glass hosts was examined. The excitation and emission spectra of Pr³⁺ and Er³⁺ ions in the studied glasses were registered. The emission spectra of Pr³⁺ ions in germanate and borate glasses are quite different and depend strongly on the glass host. In samples doped with Er³⁺ ions emission bands located around 1530 nm corresponding to the main ⁴I_13/2→⁴I_15/2 laser transition were registered, independently of the glass host. Quite long‐lived near‐infrared luminescence of Er³⁺ ions was observed for germanate glasses with low BaF₂ content, while in borate glass systems influence of barium fluoride on luminescence lifetimes is not so evident. The Judd–Ofelt calculations were used in order to determine quantum efficiencies of excited states of rare‐earth ions in germanate and borate glasses.     

REVO4‐Based Nanomaterials (RE = Y,La, Gd,and Lu) as Hosts for Yb3+/Ho3+, Yb3+/Er3+, and Yb3+/Tm3+ Ions: Structural and Up‐Conversion Luminescence Studies

Rare‐earth vanadates of the form REVO₄ (RE = Y, La, Gd, and Lu) doped by Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺_, or Yb³⁺/Tm³⁺ lanthanide ions were successfully synthesized using the sol–gel method and annealing at 600°C in an air atmosphere. The structure and morphology of the prepared nanocrystals were investigated by X‐ray diffraction, thermogravimetric analysis, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. All prepared materials were homogenous and had nanosized dimensions. Their elemental compositions were confirmed by optical emission spectrometry. Spectroscopic analysis of the materials was carried out by measuring excitation and emission spectra, luminescence decays, and dependence between the intensity of the luminescence and the laser energy. Following effective excitation by NIR radiation, Ln³⁺ co‐doped vanadate matrices exhibited a strong up‐conversion (UC) luminescence. Differences in spectroscopic properties between monoclinic LaVO₄ and tetragonal YVO₄, GdVO₄, or LuVO₄ doped by Ln³⁺ ions were observed, indicating the influence of the crystal structure on the UC emission. Drawing conclusions from these spectroscopic investigations, the UC mechanisms were proposed, including energy‐transfer processes between Yb³⁺ ions and emitting ions.     

Synthesis and Up‐conversion Luminescence of Yb3+/Ln3+ (Ln = Er,Tm, Ho) Co‐Doped Strontium Cerate by Pechini Method

Novel up‐conversion (UC) luminescent nanopowders, Sr₂CeO₄:Yb³⁺,Ln³⁺ (Ln = Er, Tm, Ho) were prepared with Pechini method. The Sr₂CeO₄:Yb³⁺,Ln³⁺ (Ln = Er, Tm, Ho) nanopowders had an orthorhombic crystal structure, and showed olive‐like morphology with the length of about 260 nm and width of about 130 nm. Under 980 nm lazer excitation, the Sr₂CeO₄:Yb³⁺/Er³⁺, Sr₂CeO₄:Yb³⁺/Tm³⁺, and Sr₂CeO₄:Yb³⁺/Ho³⁺ nanophosphors exhibit strong green, blue, and green UC luminescence, respectively. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.     

Efficient dual‐mode up‐conversion and down‐shifting emission in β‐NaYF4:Yb3+,Er3+ microcrystals via ion exchange

We report efficient dual‐mode up‐conversion (UC) and down‐shifting (DS) emission in a single Yb³⁺/Er³⁺‐co‐doped β‐NaYF₄ microcrystals with controlled morphology and size via a simple Na⁺ ion‐exchange modification (IEM) method. IEM well preserves the crystal structure and monodispersed morphology of hydrothermal‐synthesized β‐NaYF₄. Meanwhile, IEM gives rise to the significant enhancement of UC emission intensity up to 3800 times and strongly enhanced DS emission intensity of Er³⁺ and Yb³⁺ by several times in β‐NaYF₄:Yb³⁺,Er³⁺ microcrystals. IEM also strongly prolongs the DS emission lifetimes of Er³⁺ and Yb³⁺ in visible and near‐infrared region. The enhanced UC and DS emission intensities and prolonged lifetimes in β‐NaYF₄:Yb³⁺,Er³⁺ are mainly ascribed to the dispersing of localized Yb³⁺ and Er³⁺ clusters during IEM.     

Enhancement of up-conversion emission and emerging cool white light emission in co-doped Yb3+/ Er3+: Li2O-LiF-B2O3-ZnO glasses for photonic applications

A series of co-doped (Yb³⁺/Er³⁺): Li₂O-LiF-B₂O₃-ZnO glasses were prepared by standard melt quenching technique. Structural and morphological studies were carried out by XRD and FESEM. Phonon energy dynamics have been clearly elucidated by Laser Raman analysis. The pertinent absorption bands were observed in optical absorption spectra of singly doped and co-doped Yb³⁺/Er³⁺: LBZ glasses. We have been observed a strong up-conversion red emission pertaining to Er³⁺ ions at 1.0 mol% under the excitation of 980 nm. However, the up-conversion and down conversion (1.53 µm) emission intensities were remarkably enhanced with the addition of Yb³⁺ ions to Er³⁺: LBZ glasses due to energy transfer from Yb³⁺ to Er³⁺. Up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses exhibits three strong emissions at 480 nm, 541 nm and 610 nm which are assigned with corresponding electronic transitions of ²H_9/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 respectively. Consequently, the green to red ratio values (G/R) also supports the strong up-conversion emission. The Commission International de E′clairage coordinates and correlated color temperatures (CCT) were calculated from their up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses. The obtained chromaticity coordinates for optimized glass (0.332, 0.337) with CCT value at 5520 K are very close to the standard white colorimetric point in cool white region. These results could be suggested that the obtained co-doped (Yb³⁺/Er³⁺): LBZ glasses are promising candidates for w-LEDs applications.     

Enhanced and Long‐Lived Near‐Infrared Luminescence of Er3+ Ions in Lead Borate Glass‐Ceramics Containing PbWO4 Nanocrystals

Lead tungstate PbWO₄ nanocrystals in transparent lead borate glass‐ceramics containing Er³⁺ ions were fabricated. Luminescence spectra at about 1530 nm due to main ⁴I_13/2–⁴I_15/2 laser transition of Er³⁺ ions were examined for glass samples before and after heat treatment. Near‐infrared luminescence of Er³⁺ ions in glass‐ceramics is enhanced and long‐lived in comparison to precursor glasses. It suggests that the Er³⁺ ions are partially incorporated into PbWO₄ crystalline phase.     

Stannum-(II) dopant effects on morphology evolution and upconversion performance of Yb3+/Er3+:NaGdF4 crystals

Hexagonal Yb³⁺/Er³⁺:NaGdF₄ nanocrystals codoped with Sn²⁺ ions were prepared via a modulated solvothermal method. Upon introducing 25mol% Sn²⁺ ions into the host lattice by substituting Gd³⁺ ions, a portion of Yb³⁺/Er³⁺:NaGdF₄ nanocrystals was converted into nanorods. Meanwhile, the upconversion (UC) luminescence intensity of 542 nm and 652 nm were intensified by 24 and 33 times respectively, when compared with samples without Sn²⁺ ions doping. The effect of Sn²⁺ ions doping content on the morphology and UC emission performances of Yb³⁺/Er³⁺:NaGdF₄ nanocrystals were discussed in detail. The enhancement of UC luminescence intensity could be attributed to the growth of UC nanocrystals and the low crystal local symmetry around Yb³⁺/Er³⁺ ion pairs. This study may be beneficial for fabricating high-performance UC materials and realizing their practical applications.     

Effect of Yb3+ concentration on the green-yellow upconversion emission of SrGe4O9:Er3+ phosphors

This work presents the structural, morphological and luminescent, properties of SrGe₄O₉ (SGO):Er³⁺,Yb³⁺ phosphors. These phosphors were synthesized by simple combustion synthesis and subsequently annealed at 1100 °C. The XRD patterns revealed that all the SGO samples doped with Yb³⁺ concentrations from 2 to 10 at.% presented a trigonal pure phase (the Er³⁺ concentration was fixed to 1 at.%). The morphology of the SGO samples was analyzed by scanning electron microscopy and found that they are formed by microparticles with irregular shapes and average sizes in the range of 0.2 μm–3 μm. The luminescence measurements of the SGO:Er³⁺,Yb³⁺ samples showed the presence of two main emission bands at 551 nm (green) and at 662 nm (red) under excitation at 980 nm, which are associated to Er³⁺ transitions. For Yb concentration of 2 and 3 at.% the green band dominated, but the red band became more intense for Yb concentrations above 5 at.%. As result, the CIE coordinate changed from the green to the yellow region. The increase for the Yb content from 2 to 10 at.% also enhanced of the NIR emission of Er³⁺ ≈5 times and the maximum upconversion emission was observed for 8% of Yb concentration. Further, the surface of the SGO samples was analyzed by the FTIR technique in order to find OH groups which are common luminescent quenching centers, but these groups were not detected on the samples. Since the SGO samples presented tunable emission, absence of OH groups on their surface and stable crystalline structure for high Yb dopant concentrations, they could be good candidates as phosphors for solid state lighting or displays applications.