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.     

Er3+-Yb3+ ions doped fluoro-aluminosilicate glass-ceramics as a temperature-sensing material

The transparent Er³⁺-Yb³⁺-doped fluoro-aluminosilicate glass-ceramic (GC) was prepared by melt-quenching. The crystal phase, morphology, and up-conversion (UC) luminescence of as-produced GC were characterized by X-ray diffraction, scanning electron microscopy, and fluorescence spectrophotometry, respectively. The results show that BaYF₅ nanocrystals were uniformly distributed in the glass matrix of the as-produced GC. When the as-produced GC was subjected to heat treatment, the crystallinity was increased, but the crystal identity remains unchanged. Such heat-treatment doubled the intensity of the UC luminescence, and this enhancement was ascribed to the increased incorporation of both Er³⁺ and Yb³⁺ ions into the lower phonon energy environment of BaYF₅ nanocrystals. Furthermore, the heat-treated GC was stable against further crystallization, and consequently its UC luminescence was stable at the application temperature. The heat-treated GC was found to possess an outstanding temperature-sensing capability.     

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.     

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.     

Effect of Li+ Ions Doping on Microstructure and Upconversion Emission of Y2Ti2O7:Er3+/Yb3+ Nanophosphors Synthesized Via a Sol–Gel Method

Er³⁺/Yb³⁺/Li⁺‐tridoped Y₂Ti₂O₇ nanophosphors were synthesized via a facile sol–gel process. The samples were characterized by the inductively coupled plasma atomic emission spectrometer (ICP‐AES), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and infrared‐to‐visible upconversion (UC) luminescence spectra. XRD analysis showed that the crystallization temperature of pyrochore‐type Y₂Ti₂O₇ was reduced due to the flux effect of Li⁺ ions, whereas TEM measurements confirmed that the particles size of (Y_0.815Er_0.01Yb_0.075Li_0.10)₂Ti₂O₇ was about 30–40 nm when calcining at 800°C for 1.0 h. The calcining temperature and Li⁺ ion concentration dependence on UC luminescence spectra were investigated. It was found that, when incorporating 10.0 mol% Li⁺ ion, the UC red and green emission intensity was drastically increased by a factor of 18.6 and 8.3, respectively. The enhancement of UC emission may be mainly attributed to the modification of local symmetry around Er³⁺ ions by tridoping Li⁺ ions. And also, the pump power dependence of the emission intensity was investigated to understand the fundamental UC mechanism.     

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.     

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.     

Wide‐range thermometry based on green up‐conversion luminescence of K3LuF6:Yb3+/Er3+ bulk oxyfluoride glass ceramics

Yb³⁺/Er³⁺ ions codoped bulk glass ceramics (GC) with embedded monoclinic K₃LuF₆ nanocrystals are reported for potential temperature‐sensing application by using the fluorescence intensity ratio method. Such GC with good transparency and enhanced up‐conversion were prepared by the simple conversional melt‐quenching method and subsequent annealing process. Optical, structural, and temperature‐sensing up‐conversion properties were characterized systematically. Optical spectroscopy analysis confirms the incorporation of Yb³⁺/Er³⁺ into the K₃LuF₆ crystalline lattice, resulting in enhanced up‐conversion luminescence. Compared to other Er³⁺‐doped typical systems, Er³⁺ ions in K₃LuF₆ GC present large energy gap (870 cm^?1) and high relative sensitivity (37.6 × 10^?4 K^?1 at 625 K), revealing that K₃LuF₆:Yb³⁺/Er³⁺ GC can be excellent candidates for optical thermometers.     

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.     

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.     

Enhanced Up‐Conversion Luminescence in Er3+‐Doped 25GeS2·35Ga2S3·40CsCl Chalcogenide Glass–Ceramics

Enhanced luminescence in rare‐earth‐doped chalcogenide glass–ceramics is of great interest for the potential integrated optoelectronic devices. However, fundamental mechanism on the enhancement of luminescence upon crystallization remains largely unknown. We report the fabrication and characterization of wide transmission chalcogenide glass and glass–ceramics based on the 25GeS₂·35Ga₂S₃·40CsCl:0.3Er glass composition, and discuss the mechanism of enhanced luminescence. By monitoring the ⁴I_9/2–⁴I_15/2 of Er³⁺ transition, up‐conversion luminescence of 12 times higher was observed in glass–ceramics compared with that in base glass. Electron paramagnetic resonance (EPR) and Raman scattering spectroscopies were employed to obtain the information of selective environment of Er³⁺ ions and microstructural evolution with the crystallization progress. Both of them evidenced that the enhanced up‐conversion luminescence was mainly related to the local environmental evolution from a mixed chlorine‐sulfur coordination to a low phonon energy chlorine coordination in the residual glassy matrix of glass–ceramics.     

Nonlinear negative transmittance at a CW 980‐nm laser diodes pumping in Yb3+:CaF2 nanocrystals‐embedded glass ceramics

Cooperative upconversion luminescence (CUCL) occurs in spectral regions in which single ions do not have energy levels. However, all results reported so far are concentrated on luminescence properties from Yb³⁺ ions‐doped various hosts. Here, we report the observation of nonlinear negative transmittance (NNT) at continuous‐wavelength (CW) 980‐nm laser diodes (LDs) pumping in silicate oxyfluoride glass ceramics (GCs)‐containing CaF₂:Yb³⁺ nanocrystals. The unique optical nonlinearity is analyzed based on energy‐level transitions, dynamic evolution, rate equation, and power transmission equation, which can be explained as the cooperative optical absorption for the intense CUCL of Yb³⁺ ions. The NNT in the CaF₂:Yb³⁺ nanocrystals‐embedded GCs can be tailored with the power of a CW 980‐nm LDs, which possesses potential for the development of future optical limiters and switches.     

Transmission electron microscopic and optical spectroscopic studies of Ni2+/Yb3+/Er3+/Tm3+ doped dual‐phase glass‐ceramics

Ni²⁺/Yb³⁺/Er³⁺/Tm³⁺ codoped transparent glass‐ceramics (GCs) containing both hexagonal β‐YF₃ and spinel‐like γ‐Ga₂O₃ dual‐phase nanoparticles (NCs) are synthesized by melt‐quenching and subsequent heating procedures. Two techniques of transmission electron microscopy (TEM) nanoanalytics and optical spectroscopy are conjugated to understand the distribution of the rare‐earth ions (REs) and transition metals (TMs) in the nanostructured GCs. It is found that the REs are located predominantly in β‐YF₃, whereas the TMs in γ‐Ga₂O₃ NCs. As a result, energy transfer (ET) between the REs and TMs is considerably suppressed due to the large spatial separation (> 3 nm), but it is enhanced between the REs partitioned in the β‐YF₃ NCs. This has important implications for intended and demanding photoluminescence functions. For example, an ultrabroadband near‐infrared (NIR) emission in the wavelength region of 1000‐2000 nm covering the entire telecommunications window is observed for the first time. Meanwhile, intense upconversion (UC) emissions covering the 3 primary colors and locating in the first biological window can be also recorded under excitation by a single pump source at 980 nm.     

Ordering of fluorite-type phases in erbium-doped oxyfluoride glass ceramics

In this study, novel transparent Er³⁺ doped glass ceramics were prepared from melt-quenched oxyfluoride glasses with general composition of Na₂O-NaF-BaF₂-YbF₃-Al₂O₃-SiO₂. The crystallization of fluorite (BaF₂, BaF₂-YbF₃, NaF-BaF₂-YbF₃ and Na_0.5-xYb_0.5+xF_2+2x) and distorted fluorite (rhombohedral Ba₄Yb₃F₁₇ and tetragonal NaF-BaF₂-YbF₃) phases was analysed in glass ceramics with different BaF₂ and YbF₃ ratio. The phase composition and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Intense red upconversion luminescence (UCL) was detected under near-infrared excitation resulting from three photon upconversion followed by cross-relaxation between Er³⁺ and Yb³⁺ ions.The local environment of Er³⁺ ions in fluorite and distorted phases was analysed using site-selective spectroscopy. The Er³⁺ ions were found to act as nucleation centres in the glass ceramics containing BaF₂. The phase transition from metastable fluorite to rhombohedrally and tetragonally distorted fluorite phases was detected using Er³⁺ ions as a probe.     

Effect of crystalline fraction on upconversion luminescence in Er3+/Yb3+ Co-doped NaYF4 oxyfluoride glass-ceramics

The crystalline fraction were adjusted MgO concentration and the corresponding effect on upconversion (UC) luminescence in Er³⁺/Yb³⁺ co-doped NaYF₄ oxyfluorode glass-ceramics was investigated. With increase of MgO and the content of Na₂O reduced, the internal network structure of the glass became compact, which made the size of NaYF₄ nanocrystals unchanged, while the average distance between the nanocrystals increased significantly. Crystal growth is limited with the glass network, keeping the crystal size not changed. SNM-1 glass ceramics samples show a predominant red up-conversion emission under near infrared excitation at 980 nm, while a predominant green emission is observed in the SNM-3 samples. In this paper, it was indicated that it changed the effect of glass network modifier MgO in the glass structure. The possible mechanism responsible for the color variation of UC in Er³⁺/Yb³⁺ co-doped was discussed.     

Luminescent properties of doped amorphous and polycrystalline Y3Al5O12-Al2O3

Transparent polycrystalline ceramics (TPCs) are crystalline materials with single-crystal-like transparency, which, however, have to rely on fabrication processes with a relatively high cost. Here, we produced lab-scale TPCs based on the typical refractory Y₂O₃-Al₂O₃ system, through full congruent crystallization of the parent glass prepared by aerodynamic levitation melting method. Doping of the glass and TPCs by rare-earth (RE) ions (Ce³⁺, Tb³⁺, Nd³⁺, and Yb³⁺) and transition-metal (TM) ions (Cr³⁺) results in strong visible and near-infrared (NIR) photoluminescence with high quantum yield. The dominance of Stark splitting of the emission band for RE and TM ions in the TPCs as compared with that of the glass confirms crystallization of the parent glasses.     

Visible up-conversion luminescence of CaWO<Subscript>4</Subscript>: Er<Superscript>3+</Superscript>,Yb<Superscript>3+</Superscript> and emission enhancement by tri-doping of Li<Superscript>+</Superscript> ions

Er³⁺,Yb³⁺ co-doped CaWO₄ polycrystalline powders were prepared by a solid-state reaction and their up-conversion (UC) luminescence properties were investigated in detail. Under 980 nm laser excitation, CaWO₄: Er³⁺,Yb³⁺ powder exhibited green UC emission peaks at 530 and 550 nm, which were due to the transitions of Er³⁺ (²H_11/2)→Er³⁺ (⁴I_15/2) and Er³⁺ (⁴S_3/2)→Er³⁺ (⁴I_15/2), respectively. Effects of Li+ tri-doping into CaWO₄: Er³⁺,Yb³⁺ were investigated. The introduction of Li⁺ ions reduced the optimum calcinations temperature about 100 °C by a liquid-phase sintering process and the UC emission intensity was remarkably enhanced by Li⁺ ions, which could be attributed to the lowering of the symmetry of the crystal field around Er³⁺ ions.     

Color tunable up-conversion emission of ZrO2:Yb3+, Er3+ thin films prepared by chemical solution deposition

The color-tunable up-conversion (UC) emission was observed in ZrO₂:Yb³⁺, Er³⁺ thin films synthesized on fused silica substrates using a chemical solution deposition method. The crystal structure, surface morphology image and optical transmittance of ZrO₂:Yb³⁺, Er³⁺ thin films were detected in the matter of Yb³⁺/Er³⁺ doping content. Under excitation by 980?nm infrared light, intense UC emission can be obtained from ZrO₂:Yb³⁺, Er³⁺ thin films. Photoluminescence study shows that there are two emission bands centered at 548?nm and 660?nm in the UC luminescence spectra, which can be owing to (²H_11/2,⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ions, respectively. In addition, the color coordinate of UC emission between green-red can be tuned by properly adjusting the dopant concentration, because the composition of Yb³⁺/Er³⁺ affect the red/green ratio via the process of cross relaxation and energy back transfer. Our study suggests that ZrO₂:Yb³⁺, Er³⁺ thin films can be considered as promising materials for new photoluminescence devices.     

A Novel Multifunctional Upconversion Phosphor: Yb3+/Er3+ Codoped La2S3

Yb³⁺/Er³⁺ codoped La₂S₃ upconversion (UC) phosphors have been synthesized using high‐temperature solid‐state method. Under 971‐nm excitation, the maximum luminescence power can reach 0.64 mW at the excitation power density of 16 W/cm² and an absolute power yield of 0.36% was determined by an absolute method at the excitation power density of 3 W/cm², and the quantum yield of La₂S₃:Yb³⁺, Er³⁺ (green ~0.18%, red ~0.03%, integration ~0.21) was comparable to that of NaYF₄:Yb³⁺, Er³⁺ nanocrystals (integration ~0.005–0.30). Frequency upconverted emissions from two thermally coupled excited states of Er³⁺ were recorded in the temperature range 100–900 K. The maximum sensitivity of temperature sensing is 0.0075 K^?1. As the excitation power density increases, the temperature of host materials rapidly rises and the top temperature can reach to 600 K. Given the intense UC emission, high sensitivity, as well as good photothermal stability, La₂S₃:Yb³⁺/Er³⁺ phosphor can become a promising composite material for photothermal ablation of cancer cells possessing the functions of temperature sensing and in vivo imaging.     

Enhanced Sunlight Excited 1‐μm Emission in Cr3+–Yb3+ Codoped Transparent Glass‐Ceramics Containing Y3Al5O12 Nanocrystals

Cr³⁺–Yb³⁺ codoped transparent glass‐ceramics containing Y₃Al₅O₁₂ nanocrystals were prepared by heat treatment of as‐prepared glass sample and characterized by X‐ray diffraction and transmission electron microscopy. The efficient energy transfer from Cr³⁺ to Yb³⁺ ions through multi‐phonon‐assisted process was confirmed by the luminescence spectrum and fluorescent lifetime measurements. When excited by the lights from a solar simulator in the wavelength region of 400–800 nm, greatly enhanced near‐infrared emission around 1 μm was achieved from Cr³⁺–Yb³⁺ codoped glass ceramic compared with that from as‐prepared glass and Ce³⁺–Yb³⁺ codoped glass ceramic. These results demonstrate that the Cr³⁺–Yb³⁺ codoped glass ceramic is a promising material for enhancement of the efficiency of solar energy utilization.