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.     

Quantitative relationship between microstructure and optical properties of Er3+-Yb3+ co-doped glass-ceramics containing pyrochlore-type crystalline phases

Er³⁺-Yb³⁺ co-doped transparent glass-ceramics (GCs) containing Y₂Ti₂O₇ crystalline phase were prepared by the melting crystallization method. The qualitative relationship between light transmittance and three-dimensional structure parameters of glass-ceramic was studied according to the principle of stereology under different heat treatment conditions. As indicated by the research results, the light transmittance decreased with the increase in the equivalent spherical diameter (D_3S) and specific surface area per unit volume of the grains (S_V). However, the light transmittance increased linearly with the increase in the discrete grains (S_VP) and the mean free distance (λ). The up-conversion luminescence intensity was found to be most vigorous under Er³⁺ and Yb³⁺ doping concentrations of 0.5% and 0.9%, respectively, at 980 nm excitation. Considering the correlation between pump power and up-conversion emission intensity, the up-conversion luminescence mechanism was explored. As revealed by the results of color purity and chromaticity coordinates, Er³⁺-Yb³⁺ co-doped GCs containing Y₂Ti₂O₇ have promising applications in green up-conversion luminescence.     

Enhanced up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped Sr10(PO4)6O transparent glass-ceramics

In this paper, the Yb³⁺/Er³⁺ co-doped parent glass (PG) with composition (in mol%) of 30P₂O₅-10B₂O₃-38SrO-22K₂O and transparent glass-ceramics (GCs) containing hexagonal Sr₁₀(PO₄)₆O nanocrystals (NCs) were synthesized for the first time by melt-quenching method and subsequent heating treatment in air. Under 980 nm laser prompting, the GCs samples showed intense red and green up-conversion emissions compared to those characteristics for the PG sample. The emission intensities varied with Er³⁺ concentration and heat treatment conditions. Furthermore, in Yb³⁺/Er³⁺ co-doped GCs specimens, the optical thermometry was researched by means of fluorescence intensity ratio (FIR) of ⁴S_3/2 and ²H_11/2 levels. The GC sample heated at 620°C for 5 hours possessed a high relative temperature sensitivity (S_r) of 0.769% K⁻¹ at 303 K and the maximal absolute temperature sensitivity (S_a) of 5.951 × 10⁻³ K⁻¹ at 663 K, respectively. It is expected that the as-fabricated GC materials with Sr₁₀(PO₄)₆O NCs are promising efficient up-conversion materials for optical temperature sensor.     

Boroaluminosilicate glass-ceramics containing mullite-type Cr3+:Al4B2O9 nanocrystals with broadband near-infrared luminescence

Multi-component silicate glass is an ideal matrix for fabricating glass-ceramics because of its excellent physical-chemical stability and high optical transmittance. In this paper, a series of Cr³⁺ doped multi-component silicate glasses were designed for the preparation of glass-ceramics that crystalizes mullite-type Cr³⁺:Al₄B₂O₉ nanocrystals. When excited at 450 nm, the obtained GCs exhibit a broadband NIR luminescence band covering a spectral region from 650 to 1200 nm. Two different crystallographic sites of Cr³⁺ in Al₄B₂O₉ nanocrystal are considered to account for the observed broadband luminescence. Due to the controllable size and uniformly dispersion of precipitated nanoparticles, this boroaluminosilicate glass-ceramic could find potential applications as monolithic near-infrared light sources in solid-state light emitting devices.     

The LSPR regulation of TiO2: W nanocrystals and its application in enhanced upconversion luminescence

The localized surface plasmon resonance (LSPR) absorption peaks of semiconductor nanocrystals are mainly concentrated in the infrared band, and the absorption characteristics can be controlled by the amount of element doping. The coupling of upconversion nanocrystals (UCNPs) and semiconductor nanocrystals can improve the upconversion luminescence (UCL) of rare-earth ions. Here, the LSPR absorption and morphology of the semiconductor nanocrystalline TiO₂: W were adjusted by using ammonium fluoride during synthesis. Significant absorption enhancement of TiO₂: W in the near-infrared region was obtained to enhance the UCL of NaYF₄: Yb³⁺, Er³⁺. The Glass/NaYF₄: Yb³⁺, Er³⁺/TiO₂: W@SiO₂ layered structure films were fabricated through spin coating. Compared with Glass/NaYF₄: Yb³⁺, Er³⁺, the green and red lights of the Glass/NaYF₄: Yb³⁺, Er³⁺/TiO₂: W@SiO₂ films were enhanced by 15.9 and 17.8 times, respectively. The UCL enhancement of Glass/NaYF₄: Yb³⁺, Er³⁺/TiO₂: W@SiO₂ was derived from the LSPR property of TiO₂: W through the enhancement of the excitation. The present work is important for possible applications of these layered structures as biomarkers, photocatalysts, flexible materials, and photoluminescence display panels.     

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.     

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.     

Au nanoparticles embedded inverse opal photonic crystals as substrates for upconversion emission enhancement

Photonic crystals (PCs) with periodic dielectric structures are capable to control the propagation of photons when photon energy is in the region of photonic band gap. The upconversion luminescence (UCL) of nanocrystals coated on the PCs surface can be enhanced by the PCs effects. While surface plasmon resonance (SPR) of noble metal nanoparticles (NPs) is being extensively applied to enhance the UCL properties of nanocrystals. However, the PCs or SPR effect are developed separately for the UCL enhancement. In this work, we present a facile preparation method of the Au NPs embedded inverse opals, which was used as substrates to improve the UCL properties of NaYF₄:Yb³⁺, Er³⁺ NPs. The significant luminescence enhancement of NaYF₄:Yb³⁺, Er³⁺ upconverting NPs was obtained by the coupling between the SPR of Au NPs and PCs effects from Au NPs embedded inverse opals substrates. The finding demonstrated that the Au NPs embedded inverse opals as substrates may be useful for the enhanced UCL of other phosphors, producing novel photonic devices.     

Thermal enhancement of up-conversion luminescence in Lu2W2.5Mo0.5O12: Er3+, Yb3+ phosphors

Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors were synthesized through high temperature solid state method. Under 980 nm laser excitation, the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ compounds show thermal enhancement of up-conversion luminescence (UCL), which is attributed to the lattice contraction and distortion from negative thermal expansion (NTE) of Lu₂W_2.5Mo_0.5O₁₂ host enhancing the energy transfer of Yb³⁺ to Er³⁺, eliminating the energy transfer of Er³⁺ to Er³⁺ through Er³⁺ single-doped Lu₂W_2.5Mo_0.5O₁₂ phosphors without thermal enhancement of UCL. The green luminescence intensities at 693 K of the Lu_1.98-xW_2.5Mo_0.5O₁₂: 0.02Er³⁺, xYb³⁺ (x = 0.2, 0.3, 0.4) samples are 4.6, 4.3 and 7.0 times as that of 302 K, respectively. And through fluorescence intensity ratio (FIR) technique, the corresponding maximum absolute sensitivities are 0.00741, 0.00744 and 0.00723, respectively. The green monochromaticity of UCL spectra in Er³⁺/Yb³⁺ co-doped samples increase with the increasing of temperature, and the possible UCL mechanism with temperature was discussed. The results indicate that the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors can be applied at a high temperature as optical thermometer with a good green monochromaticity.     

Enhanced up-conversion luminescence of Tb3+-Yb3+ doped glass ceramics by doping Li+

Tb³⁺-Yb³⁺ co-doped transparent glass ceramics (GCs) containing Y₂Ti₂O₇ crystal phases were synthesized by the melt crystallization. The light transmittance of GCs in the visible region reached 78%, and the average grain size was 278 nm under the optimal heat treatment conditions (720 °C/2 h). The GCs exhibited greater up-conversion luminescence intensity than precursor glass, and the reason for this result was explained in accordance with the Judd-Ofelt theory. Moreover, the introduction of Li⁺ did not change the crystalline phase of GCs. The emission intensity of the green light of the 8% Li ⁺ doped GCs was significantly enhanced by nearly 4.48 times under 980 nm excitation. The XRD refinement results suggest that the enhanced luminescence intensity is correlated with the change of the Y₂Ti₂O₇ crystal lattice caused by Li⁺ doping. The relevant luminescence mechanism was elucidated. The results suggest that Li⁺ doped transparent GCs open novel avenues for green UC applications.     

Enhanced visible emissions of Pr3+-doped oxyfluoride transparent glass-ceramics containing SrF2 nanocrystals

The Pr³⁺-doped oxyfluoride transparent glass and glass-ceramic (GC) with the composition of 41SiO₂ + 10Al₂O₃ + 25.5LiF + 23SrF₂ + 0.5Pr₂O₃ were prepared and investigated their optical and luminescence properties. The formation of SrF₂ nanocrystals in GC has been confirmed by X-ray diffraction (XRD) and transmission electron micrographs (TEM). The Fourier transform infrared spectroscopy (FT-IR) studies were used to examine the network structure characteristics of silicates in the glass matrices. The XRD and TEM results suggest that the Pr³⁺ ions are progressively incorporated into the SrF₂ nanocrystals in the GC with increase in time of thermal treatment at 650 °C, corresponding to the first crystallization temperature of the glass. The obtained visible emissions of Pr³⁺-doped GC are several times enhanced than that in the glass and the lifetime of the ³P₀ level of the Pr³⁺ ions in glass and GC are found to be 7 and 12 μs, respectively. Therefore, the enhanced visible emission and lifetimes in GC are due to the incorporation of Pr³⁺ ions into the lower phonon energy of SrF₂ nanocrystals in the GCs. Moreover, the smaller difference in ionic radius between the added trivalent ions (Pr³⁺) and Sr²⁺ induces the larger enhancement of luminescence intensity in the GC. Hence, these enhanced visible luminescence properties indicate that the present glass and GC could be useful for photonic device applications.     

Energy transfer and up-conversion luminescence in Er/Yb co-doped transparent glass ceramic containing YF3 nano-crystals

Transparent glass ceramics containing YF₃ nano-crystals were fabricated by heat treatment of the SiO₂–Al₂O₃–NaF–YF₃–LnF₃ (Ln = Er, Yb) glasses. X-ray diffraction and transmission electron microscopy analyses evidenced the homogeneous distribution of spherical YF₃ nano-crystals sized 25–30 nm among the glassy matrix. Energy dispersive X-ray spectroscopy measurement, combined with the Stark splitting of the absorption and emission bands, verified the incorporation of Er³⁺ and Yb³⁺ ions into YF₃ nano-crystals. The infrared to visible up-conversion emission of Er³⁺ intensified with the increasing of Yb³⁺ concentration, ascribing to the increase of the efficiency of non-radiative energy transfer from Yb³⁺ to Er³⁺ which exceeded 45% for the 0.5Er³⁺/1.0Yb³⁺ co-doped sample. The up-conversion luminescence at 545 and 660 nm were affirmed coming from two-photon excitation process.     

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.     

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.     

Two-wavelength excitation–driven robust operation of green up-conversion emission in Er3+-doped oxyfluoride glass–ceramic

Efficient optical modulation enables a significant improvement of optical conversion efficiency and regulation of optical response rate, showing great potential for optoelectronics applications. However, the weak interaction between photons poses a strong obstacle for manipulating photon–photon interactivity. Here, upon simultaneous excitation of 850 and 1550 nm, a fast–slow optical modulation of green up-conversion (UC) luminescence in oxyfluoride glass ceramics containing NaYF₄:Er³⁺ nanocrystals can be achieved. Compared with the sum of luminescence intensity excited by the two single-wavelengths, green UC luminescence excited by simultaneous two-wavelength presents an obvious increase by approximate six times. Interestingly, the response rate of green UC luminescence relies on the pump strategy of two-wavelength excitation, showing as high as two times of the fast–slow response difference. The fast–slow optical modulation of green UC luminescence under two-wavelength excitation is promising for emerging applications in all-optical switching.     

Up-conversion luminescence and temperature-sensing properties of Li+, K+ doped Na2Zn3Si2O8: Er3+ phosphors

In this work we detail the preparation of new luminescent Li⁺ and K⁺ doped Na₂Zn₃Si₂O₈: Er³⁺ up-conversion phosphors using the high-temperature solid-phase method. We investigate the phosphors phase structure, elemental distribution, up-conversion luminescence characteristics and temperature sensing properties. Our fabricated samples were found to be homogeneous and when excited using 980 nm light, they emitted wavelengths in the green and red visible wavelength bands, which correspond to two major emission bands of Er³⁺. Doping with Li⁺ and K⁺ increased the luminescence intensity of the Na₂Zn₃Si₂O₈: Er³⁺ phosphor at 661 nm by 36 and 21 times respectively. The highest relative temperature sensitivity (Sa) of the fabricated phosphor reached a value of 19.69% K^?1 and the highest absolute temperature sensitivity (Sr) reached 1.20% K^?1. These values are superior to other materials which utilize up-conversion by Er³⁺ ions as a tool for temperature sensing. We anticipate that these new phosphors will find significant application as components in optical temperature measurement systems.     

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.     

Evolution of Strong Red Upconversion Luminescence in Er3+‐Containing Oxy‐Fluoride Glass and Glass‐Ceramics

The Er³⁺ concentration dependencies of upconversion luminescence in oxy‐fluoride glass and glass‐ceramics containing PbF₂ nanocrystals were investigated. Strong red emission from the ⁴F_9/2 → ⁴I_15/2 transition was observed with the addition of ~0.8 mol% Er³⁺ ions, whereas ~10 mol% of Er³⁺ is required to achieve such emission in several other crystalline hosts. Intensities of red emission further increased with the formation of nanocrystals through heat treatment. The Er³⁺ ions enriched in glass and segregated preferentially inside the PbF₂ nanocrystals that decreased the distance among Er³⁺ ions and thereby facilitated energy transfer.     

Effects of Er3+ and/or Cr3+ doping on crystallization activation energy and fluorescence properties of transparent ZnGa2O4 glass-ceramics

Er³⁺ and/or Cr³⁺ doped transparent ZnGa₂O₄ glass-ceramics were successfully obtained by one-step heat treatment. The results showed that Er³⁺ ions can enrich around ZnGa₂O₄ crystal to reduce the crystallization activation energy and promote the growth of ZnGa₂O₄ crystal. Cr³⁺ ions may successfully occupy the Ga³⁺ sites in the ZnGa₂O₄ lattice but will increase crystallization activation energy and inhibit the growth of the ZnGa₂O₄ crystal. Before and after crystallization, the coordination-field intensity of Cr³⁺ ions increased from 2.17 to 2.86, resulting in the peak position of its emission spectra moving from 850 to 688 nm. By excitation at 378 nm, the precursor glass co-doped with Er³⁺ and Cr³⁺ ions only showed the characteristic emission peaks belonging to Er³⁺ ions. After heat treatment, the characteristic emission peaks belonging to Er³⁺ and Cr³⁺ ions existed simultaneously, and the emission color changed from green to yellow. By excitation at 980 nm, there were only characteristic emission peaks belonging to Er³⁺ ions of the Er³⁺/Cr³⁺ co-doped glasses before and after heat treatment. The results showed that the Er³⁺ and/or Cr³⁺ doped ZnGa₂O₄ glass-ceramics have adjustable luminescence ability and show potential application value in the field of luminescence display.     

Enhanced up-conversion luminescence of Er:LaOF oxyfluoride borosilicate glass ceramics

Er³⁺-doped transparent oxyfluoride borosilicate glass ceramics containing LaOF nanocrystals have been obtained by the high temperature melt-quenching and subsequent heat treatment method. The formation of LaOF nanocrystals in the glass matrix was confirmed by XRD and TEM results. In comparison with the precursor glass, Er³⁺-doped transparent oxyfluoride glass ceramics containing LaOF nanocrystals exhibited efficient up-conversion luminescence. Especially, the green emission intensity was greatly enhanced about nearly 200 times and its up-conversion mechanism can be ascribed to a two-photon absorption process.