Luminescence properties of Yb:Er:KY3F10 nanophosphor and thermal treatment effects

In this work, we present the spectroscopic properties of KY₃F₁₀ nanocrystals activated with erbium and codoped with ytterbium ions. The most important processes that lead to the erbium upconversion of green and red emissions of Er³⁺ were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays of ⁴S_3/2 and ⁴F_9/2 excited levels of Er³⁺ and to determine the upconversion processes and the luminescence efficiencies of erbium in the visible. Analysis of the luminescence kinetics in Yb:Er:KY₃F₁₀ shows a rapid upconversion (Up₁) for the green emission with a time constant of 0.31 μs after pulsed laser excitation at 972 nm for as synthesized nanocrystals, which is faster than the time constant measured for the bulk crystal (23 μs). In addition, it is observed a second upconversion process (non-resonant) (Up₂) responsible for the red emission (Er³⁺), which competes with Up₁ process. However, the luminescence efficiency of the green emission (⁴S_3/2) is observed to be very low (1.6%) for the as synthesized nanocrystal (25 °C). Nevertheless, it increases with the nanopowder heat treatment reaching an efficiency of 99% (T = 550 °C) relative to the bulk crystal. Similar luminescence behavior was observed for the ⁴F_9/2 level (Er³⁺) that emits red emission. X-ray diffraction analysis of nanopowder by Rietveld method reveled that the mean crystallite size remains unchanged (8.3–12.3 nm) after thermal treatments with T ∼ 400 °C, while the ⁴S_3/2 luminescence efficiency strongly increases to 20%. The luminescence dynamics indicates that Er³⁺ ions distribution plays a determinant role in the luminescence efficiency of green and red emissions of Er³⁺ besides also the strong influence on the upconversions processes. The observed luminescence effect is caused by the non-uniform Er³⁺ (and Yb³⁺) ions distribution due to the nanocrystal grown, which introduces a concentration gradient that increases towards the nanoparticle surface. This concentration effect produces strong (Er × Er) cross-relaxations depleting the excited states populations of ⁴S_3/2 and ⁴F_9/2 levels and their luminescence efficiencies in KY₃F₁₀ nanocrystals. The concentration gradient is very accentuated in the as synthesized nanocrystal and gradually decreases with the thermal treatments where the dopant ions can migrate through the lattice towards the nanocrystal’s interior to get a more uniform and random distribution, which is reached after heat treatment to T = 550 °C.     

Polarized spectral properties and 1.5–1.6 μm laser operation of Er:Sr3Yb2(BO3)4 crystal

Undoped and Er³⁺-doped Sr₃Yb₂(BO₃)₄ crystals were grown by the Czochralski method. Room temperature polarized spectral properties of the Er:Sr₃Yb₂(BO₃)₄ crystal were investigated. The efficiency of the energy transfer from Yb³⁺ to Er³⁺ ions in this crystal was calculated to be about 95%. End-pumped by a diode laser at 970 nm in a hemispherical cavity, a 0.75 W quasi-CW laser at 1.5–1.6 μm with a slope efficiency of 7% and an absorbed pump threshold of 3.8 W was achieved in a 0.5-mm-thick Z-cut crystal glued on a 5-mm-thick pure YAG crystal with UV-curable adhesive.     

Frequency upconversion properties of Er3+/Yb3+-codoped lead–germanium–bismuth oxide glasses

The frequency upconversion properties of Er³⁺/Yb³⁺-codoped heavy metal oxide lead–germanium–bismuth oxide glasses under 975 nm excitation are investigated. Intense green and red emission bands centered at 536, 556 and 672 nm, corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively, were simultaneously observed at room temperature. The influences of PbO on upconversion intensity for the green (536 and 556 nm) and red (672 nm) emissions were compared and discussed. The optimized rare earth doping ratio of Er³⁺ and Yb³⁺ is 1:5 for these glasses, which results in the stronger upconversion fluorescence intensities. The dependence of intensities of upconversion emission on excitation power and possible upconversion mechanisms were evaluated and analyzed. The structure of glass has been investigated by means of infrared (IR) spectral analysis. The results indicate that the Er³⁺/Yb³⁺-codoped heavy metal oxide lead–germanium–bismuth oxide glasses may be a potential materials for developing upconversion fiber optic devices.     

Synthesis and two-color emission properties of BaGd2(MoO4)4:Eu3+,Er3+,Yb3+ phosphors

Eu³⁺, Er³⁺ and Yb³⁺ co-doped BaGd₂(MoO₄)₄ two-color emission phosphor was synthesized by the high temperature solid-state method. The structure of the sample was characterized by XRD, and its luminescence properties were investigated in detail. Under the excitation of 395 nm ultraviolet light, the BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ phosphor emitted an intense red light at 595 and 614 nm, which can be attributed to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺, respectively. The phosphor will also show bright green light under 980 nm infrared light excitation. The green emission peaks centred at 529 and 552 nm, were attributed to ⁴H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺, respectively. It indicated that the two-color emission can be achieved from the same BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ host system based on the different pumping source, 395 nm UV light and 980 nm infrared light, respectively. The obtained results showed that this kind of phosphor may be potential in the field of multi-color fluorescence imaging and anti-counterfeiting.     

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.     

Luminescence and scintillation properties of rare-earth-doped LuF3 scintillation crystals

The Nd-doped and Er-doped LuF₃ single crystals were grown by the micro-pulling-down method to study their scintillation properties in the vacuum-ultraviolet (VUV) region. The doubly Nd–Er codoped single crystal was grown to study possibility of scintillation performance improvement by energy transfer from Er³⁺ to Nd³⁺ ions. The LiF flux was to avoid phase transition below melting temperature. The 1%Nd-doped sample showed the highest overall scintillation efficiency under X-ray excitation which was 7 times as high as that of the LaF₃:Nd 8% standard. The leading Nd³⁺ 5d–4f emission was situated at 176 nm, while the Er³⁺ 5d–4f emission for Er-doped samples was observed at 163 nm, which better matches the sensitivity of some VUV-sensitive photodetectors. The optimum Er concentration was determined to be around 1–3 mol%. No Er³⁺ 5d–4f emission was observed for the doubly Er,Nd-codoped sample due to energy transfer from the Er³⁺ to Nd³⁺ ions. Slight improvement of the light yield was observed in the doubly-doped sample with respect to the Nd-only doped one.     

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.     

Up-conversion luminescence properties of Y2O2S:Yb3+,Er3+ nanophosphors

Up-converting yttrium oxysulfide nanomaterials doped with ytterbium and erbium (Y₂O₂S:Yb³⁺,Er³⁺) were prepared with the flux method. The precursor oxide materials were prepared using the combustion synthesis. The morphology of the oxysulfides was characterized with transmission electron microscopy (TEM). The particle size distribution was 10–110 nm, depending on the heating temperature. According to the X-ray powder diffraction (XPD), the crystal structure was found hexagonal and the particle sizes estimated with the Scherrer equation agreeded with the TEM images. Upon the 970 nm infrared (IR) laser excitation, the materials yield moderate green ((²H_11/2, ⁴S_3/2) → ⁴I_15/2 transition) and strong red (⁴F_9/2 → ⁴I_15/2) luminescence. The green luminescence was enhanced with respect to the red one by an increase in both the crystallite size and erbium concentration due to the cross-relaxation (CR) processes. The most intense up-conversion luminescence was achieved with x_Yb and x_Er equal to 0.10 and 0.005, respectively. Above these concentrations, concentration quenching occurred.     

Up-conversion luminescence of the NaYF4:Yb3+,Er3+ nanomaterials prepared with the solvothermal synthesis

Up-converting NaYF₄:Yb³⁺,Er³⁺ (x_Yb: 0.20, x_Er: 0.02) nanomaterials were prepared with a microwave assisted solvothermal synthesis to study how the synthesis parameters affect the structure and up-conversion luminescence of the materials and thus their usability as labels in biomedical applications. The purity of the materials was studied with Fourier transform infra-red (FT-IR) spectroscopy and the particle size and morphology with transmission electron microscopy (TEM). The crystal structure was characterized with X-ray powder diffraction (XPD) and the crystallite sizes were calculated with the Scherrer formula. Up-conversion luminescence and luminescence decays were studied with near infra-red (NIR) laser excitation at 970 nm.The presence of the oleic acid was observed in the FT-IR spectra. The TEM images showed small quasi-spherical nanoparticles as well as long nanorods. The XPD measurements revealed that both cubic and hexagonal forms of NaYF₄ were present in the materials. The crystallite sizes ranged from ca. 20 to over 150 nm for the cubic and hexagonal phases, respectively. The characteristic up-conversion luminescence of Er³⁺ in red (640–685 nm; ⁴F_9/2 → ⁴I_15/2) and green (515–560 nm; ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions) wavelengths was observed. The most intense luminescence and the longest luminescence emission lifetime were obtained with the material annealed for 12 h at 177 °C with 1.8 MPa pressure due to the predominance of the well-crystallized hexagonal form of NaRF₄ (R: Y, Yb, Er).     

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.     

Enhanced emission of 2.7 μm from Er3+/Nd3+-codoped LiYF4 single crystals

An Er³⁺/Nd³⁺ co-doped LiYF₄ single crystal of ～Φ 12 mm × 95 mm size with high quality was grown by a Bridgman method. The luminescent properties of the crystals with different Er³⁺ and Nd³⁺ concentrations were studied. Compared with the Er³⁺ single-doped LiYF₄ crystal extremely enhanced emission at 2.7 μm from the Er³⁺/Nd³⁺ co-doped LiYF₄ was observed upon excitation of an 800 nm laser diode. Meanwhile, the green up-conversion emission and near infrared emission at 1.5 μm from Er³⁺ in the co-doped crystals were effectively restricted. The luminescent mechanisms for the Er³⁺/Nd³⁺ co-doped crystals were analyzed and the possible energy transfer processes were proposed. The energy transfer efficiencies for (Er³⁺:⁴I_13/2, Nd³⁺:⁴I_9/2) → (Er³⁺:⁴I_15/2, Nd³⁺:⁴I_15/2) and (Nd³⁺:⁴F_3/2, Er³⁺:⁴I_15/2) → (Nd³⁺:⁴I_9/2, Er³⁺:⁴I_9/2) were calculated. It was found that Er³⁺/Nd³⁺ co-doped single crystal may be a potential host for 2.7 μm lasers.     

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⁺.     

Effect of vapor transport equilibration and Mg doped on the luminescence of Er:LiNbO3

Luminescence properties of the congruent and vapor transport equilibration (VTE) treated Er:LiNbO₃ and Er:Mg:LiNbO₃ crystals were recorded at room temperature. It is observed that VTE treatment could enhance the emission intensity of Er³⁺ ions and doping with MgO would weaken it in the visible spectra. As a result, the luminescence intensity of Er³⁺ ions in the VTE treated Er:Mg:LiNbO₃ crystal increased up to 2.2 times than that in the congruent Er:LiNbO₃ crystal. In addition, both VTE treatment and doping with MgO result in some changes of the relative emission intensity of some peaks in the visible emission spectra. In the infrared emission spectra, the luminescence peak at 1540 nm of Er³⁺ ions shifts towards the larger wavelength when the Er:LiNbO₃ crystals were treated using VTE or doped with MgO. The changes in crystalline environment of Er³⁺ ions due to VTE treatment or doping with Mg²⁺ play a key role in these phenomena.     

Upconversion luminescence of Ba(MoO4)h(WO4)1−h:Yb3+/Er3+ nanocrystals synthesized through hydrothermal method

A series of Yb³⁺/Er³⁺ co-doped Ba(MoO₄)_h(WO₄)_1−h upconversion nanocrystals (UCNCs) were prepared via hydrothermal method. The effects of different concentration ratios of Yb³⁺/Er³⁺ and Mo₄O²⁻/WO₄²⁻ on the upconversion luminescence were investigated, and the optimum doping concentrations of Yb³⁺ and Er³⁺ in the Ba(MoO₄)_0.5(WO₄)_0.5 host were found to be 3 mol% and 1 mol%, respectively. Structure of Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺ was identified as the tetragonal in the X-ray diffraction (XRD) results and the particle size observed in the scanning electron microscope (SEM) was about 40 nm. Under excitation of 980 nm semiconductor laser, three emission bands centered at 528, 550 and 660 nm, originating from ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively, were observed for Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺. The pump power dependence research suggested that these bands arise due to two-photon absorption. The variation of CIE coordinate at different excitation powers was observed.     

Effect of Ag nanoparticles on the radiative properties of tellurite glasses doped with Er3+, Yb3+ and Tm3+ ions

The present work is devoted to the characterization of the thermal and spectroscopic properties of tellurite glasses, codoped with Er³⁺, Yb³⁺ and Tm³⁺ rare-earth ions and silver nanoparticles (NPs). The techniques used for this investigation were UV–visible and infrared absorption, time-resolved luminescence and thermal lens. Time-resolved luminescence studies indicate efficient Yb³⁺ → Er³⁺ and Yb³⁺ → Tm³⁺ energy transfers and intense Er³⁺ and Tm³⁺ mid-infrared emissions around 1550 nm and 1860 nm, respectively. The presence NPs is found to increase the thermal diffusivity of the materials and to shorten the mid-infrared emission lifetime of both the Er³⁺ and Tm³⁺ ions.     

Efficient up-conversion emission and energy transfer in LaAlO3 doped with Er3+, Ho3+, and Yb3+ ions

The spectroscopic properties of LaAlO₃ polycrystals doped with Er³⁺, Ho³⁺ and Yb³⁺ ions have been investigated. Very efficient up-conversion emission occurs upon IR excitation. The strongest luminescence has been observed for a sample doped with Er³⁺, Ho³⁺, and Yb³⁺ ions simultaneously and annealed at 1500 °C. An efficient energy transfer to Yb³⁺ ions is observed when Er³⁺ or Ho³⁺ ions are excited. The energy transfer mechanisms are proposed.     

Phase transition of Er3+-doped Al2O3 powders prepared by the non-aqueous sol–gel method

The Er³⁺-doped Al₂O₃ powders have been prepared by the non-aqueous sol–gel method using the aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium nitrate, as dopant under isopropanol environment. The phase structure and phase transition of the Er³⁺-doped Al₂O₃ powders were investigated by using thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The phase contents diagram for the Er-doped Al–O system with the doping concentration up to 5 mol% was described at the sintering temperature from 550 to 1250 °C. There were the three crystalline types of Er³⁺-doped Al₂O₃ phases, γ-, θ- and α-(Al, Er)₂O₃, and the two relative stoichiometric compounds composed of Al, Er, and O, ErAlO₃ and Al₁₀Er₆O₂₄ phases in the Er–Al–O phase contents diagram. The Er³⁺ doping suppressed crystallization of the γ and θ phases and delayed phase transition of the γ → θ and θ → α. The increased Er³⁺ doping concentration and the elevated sintering temperature enhanced the precipitation of the ErAlO₃ and Al₁₀Er₆O₂₄ phases. The preparation procedure for the Er³⁺-doped Al₂O₃ powders in the non-aqueous sol–gel process, including chelating, hydrolysis, peptization, doping and gelation, has a significant effect on the phase formation and its transition for the Er³⁺-doped Al₂O₃ powders.     

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).     

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.     

Infrared to visible up conversion energy transfer confined at ordered micro-ring structures

Infrared to visible up-conversion energy transfer processes are demonstrated in two dimensional ordered arrays of fluorescent Er³⁺:Yb³⁺:CaNb₂O₆ micro-rings embedded in LiNbO₃ optical substrate. Spatially resolved confocal spectroscopy reveals that Yb³⁺ → Er³⁺ energy transfer process only takes place at these confined ring spatial regions. Thus, the formation of rare earth based co-doped micro-rings can be used to generate green and red visible fluorescent ring emitters upon infrared diode laser excitation on wide spatial areas. The results allow to envisage diode pumped multi-color integrated photonic devices for high quality shadow-free phosphor displays.