Er3+/Yb3+ codoped oxyfluoride borosilicate glass ceramic containing NaYF4 nanocrystals for amorphous silicon solar cells

Transparent oxyfluoride borosilicate glass ceramic containing cubic NaYF₄ nanocrystals were successfully fabricated. The cubic NaYF₄ nanocrystals with average size of 30 nm were precipitated in the glass matrix, which was confirmed by the X-ray diffraction and TEM results. In comparison with the as-made glass, significant enhancement of upconversion luminescence is observed in the Er³⁺/Yb³⁺ codoped transparent glass ceramic, which may be due to the variation of coordination environment of Er³⁺ and Yb³⁺ ions after crystallization. The high transparency, intense upconversion luminescence and the simple, low-cost fabrication process make this material exhibiting potential applications in the fields of amorphous silicon solar cells.     

Multi-photon luminescence in Er/Yb:SrO⋅TiO2 glass ceramic

Er³⁺/Yb³⁺ doped strontium titanate borosilicate glass was prepared. Glass ceramic was prepared by controlled heat treatment (at 955 °C) of glass. Ti₁₀O₁₉ and Sr₃Ti₂O₇ were found as major crystalline phases. The emission spectra of glass and glass ceramic samples were investigated under 976 nm laser excitation. In glass ceramic, the intensity of the emitted radiation was much higher (≈50 times for green and ≈10 times for red emission) than in the glass. A new three photon process was found to be responsible for emission at low power which is not yet observed in Er³⁺/Yb³⁺:SrO⋅TiO₂ glass ceramic system to the best of our knowledge. The details of upconversion mechanisms e.g. Energy Transfer (ET) and Excited State Absorption (ESA) were studied by power-intensity log dependence. It is expected that Er³⁺/Yb³⁺ doped nanocrystalline (?10 nm) Sr₃Ti₂O₇ phase was responsible for the observed upconversion phenomenon in glass ceramic.     

Effects of aging time on phase,morphology, and luminescence by two-photon processes of BiPO4:Er3+, Yb3+ in the solvothermal synthesis

BiPO₄:Er³⁺, Yb³⁺ phosphors were synthesized by the solvothermal process. The phase transformation, morphology, and UC luminescent property were characterized by different analytical techniques. The aging time has obvious influence on the phase, morphology, and luminescence of the samples. With the extension of aging time, the phase of BiPO₄:Er³⁺, Yb³⁺ phosphors changes from hexagon to monocline. The morphology changes from nanorods through nanorugbies to microoctahedra. Under the excitation at 980 nm, BiPO₄:Er³⁺, Yb³⁺ phosphors show green and red UC emissions, which originate from the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions. The green and red UC emission intensities increase gradually with the increase of pumping power. On the basis of the luminescent properties, one can conclude that the two-photon process is involved in green and red UC emissions.     

New transparent Er-doped oxyfluoride tellurite glass ceramic with improved near infrared and up-conversion fluorescence properties

Transparent glass ceramics have been obtained by nucleation and growth of Y₂Te₆O₁₅ or Er₂Te₅O₁₃ cubic phase in a new Er³⁺-doped oxyfluoride tellurite glass. Effect of heat treatment on absorption spectra, luminescence and up-conversion properties in the oxyfluoride tellurite glass has been investigated. With heat treatment the ultraviolet absorption edge red shifted evidently for the oxyfluoride telluride glass. The near infrared emission that corresponds to Er³⁺:⁴I_13/2 → ⁴I_15/2 can be significantly enhanced after heat treatment. Under 980 nm LD pumping, red and green up-conversion intensity of Er³⁺ in the glass ceramic can be observed much stronger than that in the base glass.     

The 1.53 μm spectroscopic properties of Er3+/Ce3+/Yb3+ tri-doped tellurite glasses containing silver nanoparticles

The metallic silver nanoparticles (NPs) was introduced into the Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses with composition TeO₂–ZnO–La₂O₃ to improve the 1.53 μm band fluorescence. The UV/Vis/NIR absorption spectra, 1.53 μm band fluorescence spectra, fluorescence lifetimes, X-ray diffraction (XRD) curves, differential scanning calorimeter (DSC) curves and transmission electron microscopy (TEM) image of tri-doped tellurite glasses were measured, together with the Judd–Ofelt intensity parameters, emission cross-sections, absorption cross-sections and radiative quantum efficiencies were calculated to investigate the effects of silver NPs on the 1.53 μm band spectroscopic properties of Er³⁺ ions, structural nature and thermal stability of glass hosts. It is shown that Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses can emit intense 1.53 μm band fluorescence through the combined energy transfer (ET) processes from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions under the 980 nm excitation. At the same time, the introduction of an appropriate amount of silver NPs can further improve the 1.53 μm band fluorescence owing to the enhanced local electric field effect induced by localized surface Plasmon resonance (LSPR) of silver NPs and the possible energy transfer from silver NPs to Er³⁺ ions, and an improvement by about 120% of fluorescence intensity is found in the studied Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass containing 0.5 mol% amount of silver NPs with average diameter of ∼15 nm. The energy transfer mechanisms from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions were also quantitatively investigated by calculating energy transfer microparameters and phonon contribution ratios. Furthermore, the thermal stability of glass host increases slightly with the introduction of silver NPs while the glass structure maintains the amorphous nature. The results indicate that the prepared Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass with an appropriate amount of silver NPs is an excellent gain medium applied for 1.53 μm band EDFA pumped with a 980 nm laser diode (LD).     

Transparent glass ceramic containing Er:CaF2 nano-crystals prepared by sol-gel method

Er³⁺ doped SiO₂-CaF₂ transparent glass ceramic was prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared spectra (IR). After heat-treatment at 900 °C, the Si-OH bonds and other organic groups were basically eliminated. The CaF₂ crystallites in the sample heat-treated at 900 °C are 10-20 nm in size, distributed homogeneously among the amorphous silica matrix. The efficient upconversion emission for Er^3+.4F_9/2 → ⁴I_15/2 transition was recorded under 980 nm excitation, which could be ascribed to the incorporation of Er³⁺ ions into the CaF₂ nano-crystals with low phonon energy.     

Influences of Er3+ content on structure and upconversion emission of oxyfluoride glass ceramics containing CaF2 nanocrystals

Transparent 45SiO₂–25Al₂O₃–5CaO–10NaF–15CaF₂ glass ceramics doped with different content of erbium ion (Er³⁺) were prepared. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses evidenced the spherical CaF₂ nanocrystals homogeneously embedded among the glassy matrix. With increasing of Er³⁺ content, the size of CaF₂ nanocrystals decreased while the number density increased. The crystallization kinetics studies revealed that CaF₂ crystallization was a diffusion-controlled growth process from small dimensions with decreasing nucleation rate. Er³⁺ could act as nucleating agent to lower down crystallization temperature, while some of them may stay at the crystal surfaces to retard the growth of crystal. Intense red and weak green upconversion emissions were recorded for glass ceramics and their intensities increased with the increasing of Er³⁺ content under 980 nm excitation. However, the concentration quenching effect appeared when Er³⁺ doping reached 2 mol%. These results could be attributed to the change of ligand field of Er³⁺ ions due to the incorporation of Er³⁺ ions into precipitated fluoride nanocrystals.     

Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca,Sr; B = W,Mo) phosphors

ABO₄ (A = Ca, Sr; B = W, Mo):Er³⁺/Yb³⁺/Li⁺ phosphors tri-doped with different concentrations of Li⁺ ion ranging from 0 to 22.5 mol% were prepared by using a solid-state reaction method. And their upconversion (UC) luminescence properties were in estimated under a 975 nm laser-diode excitation. The four kinds of phosphors (CaWO₄, CaMoO₄, SrWO₄, and SrMoO₄) tri-doped with Er³⁺, Yb³⁺ and Li⁺ ions showed strong green UC emission peaks at 530 nm and 550 nm and weak red UC emission. The intensity of green UC emission of Li⁺ doped samples was several higher than that of Li⁺ un-doped samples due to the reduction of lattice constant and the local crystal field distortion around rare-earth ions. The optimum doping concentration of Li⁺ ions was investigated and the effects of Li⁺ concentration for UC emission intensity were studied in detail.     

Bifunction in Er3+/Yb3+ co-doped BaTi2O5–Gd2O3 glasses prepared by aerodynamic levitation method

Novel Er³⁺/Yb³⁺ co-doped BaTi₂O₅–Gd₂O₃ spherical glasses have been fabricated by aerodynamic levitation method. The thermal stability, upconversion luminescence, and magnetic properties of the present glass have been studied. The glasses show high thermal stability with 763.3 °C of the onset temperature of the glass transition. Red and green emissions centered at 671 nm, 548 nm and 535 nm are obtained at 980 nm excitation. The upconversion is based on a two-photon process by energy transfer, excited-state absorption, and energy back transfer. Yb³⁺ ions are more than Er³⁺ ions in the glass, resulting in efficient energy back transfer from Er³⁺ to Yb³⁺. So the red emission is stronger than the green emissions. Magnetization curves indicate that magnetic rare earth ions are paramagnetic and the distribution is homogeneous and random in the glass matrix. Aerodynamic levitation method is an efficient way to prepare glasses with homogeneous rare earth ions.     

NIR luminescent Er/Yb co-doped SiO2–ZrO2 nanostructured planar and channel waveguides: Optical and structural properties

Optical and structural properties of planar and channel waveguides based on sol–gel Er³⁺ and Yb³⁺ co-doped SiO₂–ZrO₂ are reported. Microstructured channels with high homogeneous surface profile were written onto the surface of multilayered densified films deposited on SiO₂/Si substrates by a femtosecond laser etching technique. The densification of the planar waveguides was evaluated from changes in the refractive index and thickness, with full densification being achieved at 900 °C after annealing from 23 up to 500 min, depending on the ZrO₂ content. Crystal nucleation and growth took place together with densification, thereby producing transparent glass ceramic planar waveguides containing rare earth-doped ZrO₂ nanocrystals dispersed in a silica-based glassy host. Low roughness and crack-free surface as well as high confinement coefficient were achieved for all the compositions. Enhanced NIR luminescence of the Er³⁺ ions was observed for the Yb³⁺-codoped planar waveguides, denoting an efficient energy transfer from the Yb³⁺ to the Er³⁺ ion.     

Ag nanoparticles enhanced near-IR emission from Er3+ ions doped glasses

Vitreous materials containing rare-earth (RE) ions and metallic nanoparticles (NPs) attract considerable interest because the presence of the NPs may lead to an intensification of luminescence. In this work, the characteristics of 1.54 μm luminescence for the Er³⁺ ions doped bismuthate glasses containing Ag NPs were studied under 980 nm excitation. The surface plasmon resonance (SPR) band of Ag NPs appears from 500 to 1500 nm. Transmission electron microscopic (TEM) image reveals that the Ag NPs are dispersed homogeneously with the size from 2 to 7 nm. The strength parameters Ω_t(t = 2, 4, 6), spontaneous emission probability (A), radiative lifetime (τ) and stimulated emission section (σ_em) of Er³⁺ ions were calculated by the Judd–Ofelt theory. When the glass contains 0.2 wt% AgCl, the 1.54 μm fluorescence intensity of Er³⁺ reaches a maximum value, which is 7.2 times higher than that of glass without Ag NPs. The Ag NPs embedded glasses show significantly fluorescence enhancement of Er³⁺ ions by local field enhancement from SPR.     

Strong upconversion luminescence in LiYMo2O8:Er,Yb towards efficiency enhancement of dye-sensitized solar cells

Strong green upconversion (UC) emissions induced by a 940 nm laser have been observed in a series of Er³⁺, Yb³⁺ co-doped LiYMo₂O₈ samples. The diffuse reflectance absorption spectra show that the co-doped samples can absorb the photons in the range of 900–1050 nm efficiently, demonstrating the excellent sensitization effect of Yb³⁺ ions. Based on power switched UC luminescence, UC mechanisms are discussed, and the emission spectra excited by 490 nm are also measured to give additional evidence. The results reveal that application of these phosphors may improve the conversion efficiency of DSSCs.     

Preparation of novel transparent glass–ceramics containing fluoride crystals

Transparent glass–ceramics containing YLiF₄ nano-crystals were synthesized by controlled heat-treatments of LiFYF₃Al₂O₃SiO₂ glass. The crystallite size estimated to be about 8 nm was much less than the wavelength of the visible light. The transmittance of the glass–ceramic with a thickness of 2 mm was more than 85% at 400 nm and as high as 95% in the infrared region. The fluorescence centered around 1000 nm was hardly observed from Er³⁺-doped precursor glass under 800 nm excitation, while the emission with the Stark spitting was clearly observed for the Er³⁺-doped glass–ceramic. The phonon sideband of the ⁷F₀ → ⁵D₂ excitation spectra of Eu³⁺ reveals that Eu³⁺ doped in precursor glass is in silicate network while Eu³⁺ doped in the glass–ceramic is in not only silicate framework but also fluoride framework. These results indicate that rare-earth ions such as Er³⁺ and Eu³⁺ could be successfully incorporated into YLiF₄ nano-crystals in the glass–ceramics.     

Near-infrared quantum cutting platform in transparent oxyfluoride glass–ceramics for solar sells

This study investigated the photoluminescent properties of Er³⁺/Yb³⁺ and Ce³⁺/Er³⁺/Yb³⁺ -doped oxyfluoride glass–ceramics. The transparent oxyfluoride glass–ceramics were prepared by high temperature melting method and subsequent heat treatment. Effect of heat treatment schedules on crystallization behavior and microstructure were analyzed by differential scanning caborimetry, X-ray diffraction, infrared spectrum and scanning electron microscopy. The structure of fluoride nanocrystals indicates that the main phase in the oxyfluoride glass ceramics is CaF₂ nanocrystal sized at 25 nm at the optimal crystallization temperature 600 °C for 8 h. The Ce³⁺/Er³⁺/Yb³⁺ tri-doped oxyfluoride glass–ceramics shows wider absorption bands comparing with Er³⁺/Yb³⁺ co-doped oxyfluoride glass–ceramics. The effective energy transfer processes from Ce³⁺ to Yb³⁺, Er³⁺ to Yb³⁺ and Ce³⁺ to Er³⁺ all can take place simultaneously. The idea of using Ce³⁺ together with Er³⁺ and Yb³⁺ ions could enhance the ultraviolet visible light absorption and the 960–1040 nm near infrared emission. Results of this study demonstrate that the tri-activator Ce³⁺/Er³⁺/Yb³⁺ materials are promising for practical application to enhance the energy efficiency of crystalline Si solar cells via spectrum modification.     

Multicolor upconversion of Er3+/Tm3+/Yb3+ doped oxyfluoride glass ceramics

Er³⁺/Tm³⁺/Yb³⁺ tridoped oxyfluoride glass ceramics was synthesized in a general way. Under 980 nm LD pumping, intense red, green and blue upconversion was obtained. And with those primary colors, multicolor luminescence was observed in oxyfluoride glass ceramics with various dopant concentrations. The red and green upconversion is consistent with ⁴F_9/2 → ⁴I_15/2 and ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ respectively. While the blue upconversion originates from ¹G₄ → ³H₆ transition of Tm³⁺. This is similar to that in Er³⁺/Yb³⁺ and/or Tm³⁺/Yb³⁺ codoped glass ceramics. However the upconversion of Tm³⁺ is enhanced by the energy transfer between Er³⁺ and Tm³⁺.     

Influence of Yb3+ content on microstructure and fluorescence of oxyfluoride glass ceramics containing LaF3 nano-crystals

The influence of Yb³⁺ content on structural evolution and fluorescence properties of oxyfluoride glass ceramics containing LaF₃ nano-crystals were systematically investigated. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) experiments indicated that Yb³⁺ ions acted as nucleating agent to facilitate LaF₃ crystallization. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) results verified the incorporation of Yb³⁺ into LaF₃ nano-crystal lattice. The absorption, emission spectra and fluorescence decays were measured. The infrared emission intensity of ⁴F_5/2 → ⁴F_7/2 transition under 980 nm excitation enhanced, while the measured lifetime reduced due to the increase of non-radiative transition probability, with the increase of Yb³⁺ content in glass ceramic. However, when Yb³⁺ doping reached 4.0 mol% the concentration quenching effect appeared.     

Investigation on broadband near-infrared emission and energy transfer in Er-Tm codoped germanate glasses

Broadband near-infrared emission has been investigated in a new type host composition of Er³⁺-Tm³⁺ codoped germanate glass. A broadband emission extend from 1350 to 1675 nm with the full width at half maximum (FWHM) around 138 nm is obtained in the germanate glass which codoped with 0.2 wt.% Er₂O₃ and 0.8 wt.% Tm₂O₃. The energy transfer between Er³⁺ and Tm³⁺ plays an important role in the emission mechanism, which is evidenced by the visible upconversion and the lifetime of Er³⁺:⁴I_13/2 level effected by the addition of Tm₂O₃. And energy transfer efficiency from Er³⁺ to Tm³⁺ reaches 76% for the highest Tm³⁺ concentration of 0.8 wt.%. These results suggest that this glass would be a promising material for broadband light source and broadband amplifier for the wavelength division multiplexing transmission systems.     

Effects of Li+ ions on the enhancement of up-conversion emission in Ho3+-Yb3+ co-doped transparent glass–ceramics containing Ba2LaF7 nanocrystals

The up-conversion (UC) emission of Ho³⁺-Yb³⁺ and Li⁺ co-doped transparent glass ceramics 45SiO₂-15Al₂O₃-12Na₂CO₃-21BaF₂-7LaF₃-0.2HoF₃-1YbF₃-xLi₂CO₃ (x = 0, 0.5, 1, 2, 4 and 6 mol%) containing Ba₂LaF₇ nanocrystals were investigated. These glass ceramics samples were prepared using the conventional quenching techniques. The Ba₂LaF₇ nanocrystals precipitated from the glass matrix was confirmed by X-ray diffraction (XRD). Compared with the glass ceramics sample without Li⁺, the UC emission intensity of glass ceramics samples with Li⁺ were enhanced. It can be proved that the Li⁺ can affect the enhancement up-conversion (UC) emission. Particularly, the green UC emission intensity band centered at 546 nm was strongly increased twice with the concentration of Li⁺ increasing up to 4 mol%. Through the comparison and analysis of the energy graph, it was shown that the ⁵F₄/⁵S₂→⁵I₈ transition of Ho³⁺ ion obtained the green (546 nm) light. There are two weak fluorescences in the red (657 nm) region and near infrared (753 nm) region of spectrum, which is the ⁵F₄/⁵S₂→⁵I₇ and ⁵F₅→⁵I₈ transition of Ho³⁺. Therefore, the emission results showed that the incorporation of Li⁺ ions into the Ba₂LaF₇:Eu³⁺ lattice could induce a remarkable change of the emission intensity in red region (R = I_ED/I_MD) with 393 nm excitation wavelength. It was indicated that the symmetry of the lattice was destroyed by Li⁺ in glass ceramics. The possible mechanism responsible for the enhancement of UC emission in Ho-Yb co-doped was discussed.     

Effect of Yb concentration on the upconversion of Er ion doped La2O3 nanocrystals under 980 nm excitation

The effect of Yb³⁺ concentration on the upconversion of La₂O₃:Yb³⁺, Er³⁺ nanocrystals was reported. Green (about at 530 and 549 nm) and red (around at 672 nm) upconversion emissions under 980 nm excitation were observed at room temperature. It was found that the ratio of green to red upconversion emission intensity is considered as a function of Yb³⁺ ion concentration. Of the samples doped with varying Er³⁺ or constant Er³⁺ ion concentration, it can be observed that the intensity ratio drastically decreases with an Yb³⁺ ion concentration increase and the Yb³⁺ ions concentration is around 3 mol% as the emission intensity ratio of green to red upconversion is close to 1.     

The influence of Yb doping on the upconversion luminescence of Pr in aluminum oxide based powders prepared by combustion synthesis

The effect of ytterbium (Yb³⁺) doping on the upconversion (UC) emission of praseodymium (Pr³⁺) doped in aluminum oxide based powders prepared by combustion synthesis is reported for near-infrared excitation (λ = 980 nm). Our experimental results show that the crystalline structure and the UC emission changes with the Yb³⁺ concentration. The sample containing only Pr³⁺ (1.0 wt.%) did not show any UC signal and the UC emission profiles of the samples containing Pr³⁺ (1.0 wt.%) and Yb³⁺ (0.5, 2.0 wt.%) are quite different. The sample containing 0.5 wt.% of Yb³⁺ has five emission lines in the visible range associated to Pr³⁺ 4f–4f transitions, ³P₀ → ³H₄ (497 nm), ³P₀ → ³H₅ (525 and 550 nm), ³P₀ → ³H₆ (620 nm) and ³P₀ → ³F₂ (650 nm). We believe that the UC process has its origin in energy transfer from Yb³⁺ ions to Pr³⁺ ions in Pr_0.83Al_11.83O₁₉ phase. The sample containing 2.0 wt.% of Yb³⁺ has only one emission line in the visible range peaked at 507 nm which we believe has its origin in cooperative UC emission due to excited Yb³⁺ pairs in YbAlO₃ phase. The samples containing Yb³⁺ also present UC emission lines in the near-infrared which are assigned to intrinsic lattice defects.