Er3+/Yb3+/Li+/Zn2+: Gd2(MoO4)3 upconverting nanophosphors in optical thermometry

Er~(3+)-Yb~(3+)-Li~+:Gd_2(MoO_4)_3 and Er~(3+)-Yb~(3+)-Zn~(2+):Gd_2(MoO_4)_3 nanophosphors, synthesized by chemical co-precipitation technique were characterized through XRD,FESEM,dynamic light scattering(DLS),diffuse reflectance, photoluminescence, photometric and decay time analysis. The enhancement of about～28, ～149 and ～351 times in the green upconversion emission band is observed for the optimized Er~(3+)-Yb~(3+),Er~(3+)-Yb~(3+)-Li~+ and Er~(3+)-Yb~(3+)-Zn~(2+):Gd_2(MoO_4)_3 nanophosphors in comparison to the singly Er~(3+) doped nanophosphors. The electric dipole-dipole interaction is found to be responsible for the concentration quenching. The temperature dependent behaviour of the two green thermally coupled levels of the Er~(3+) ions based on the fluorescence intensity ratio technique was studied. The maximum sensor sensitivity ～38.7 × 10~(-3) K~(-1) at 473 K for optimized Er~(3+)-Yb~(3+)-Zn~(2+) codoped Gd_2(MoO_4)_3 nanophosphors is reported with maximum population redistribution ability～88% among the ~2H_(11/2) and ~4S_(3/2) levels.     

Structure Property and Visible Upconversion of Er^3＋ Doped Gd2O3 Nanocrystals

Gd2O3:Er nanoparticles were prepared by a simple sol-gel method, The structure properties ot Gd2O3:Er were studied by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The visible up-converted luminescence spectra of Er^3 were investigated under excitation to ^4I9/2 level by 785nm laser. Laser power, Er^3 ion concentration and temperature dependences of the upconverted emissions were investigated to understand the upconversion mechanisms. Excited state absorption and energy transfer process are discussed as the possible mechanisms for the upconversion.     

Boosting single-band red upconversion luminescence in colloidal NaErF_4 nanocrystals:Effects of doping and inert shell

Single-band red upconversion luminescence(UCL) is vital to in vivo bioimaging as well as "see and treat" biomedicines. Herein, starting with the previously reported β-NaErF4:Tm UCNPs, we examined the effects of both Yb~(3+) doping and inert shell coating on the red-to-green(R/G) ratio for Er3+ based UCL. The doping of Yb~(3+) into the β-NaErF4:Tm3+ core not only enhances the whole UCL intensity, but also raises the R/G ratio by 1.25 times. In addition, the coating of an inert NaYF4 shell, which is usually adopted for the enhancement of UCL intensity, further boosts the R/G value up to as high as 77.92. This work may benefit the potential bioimaging application of single-band red UCL.     

Optical properties of NaY(MoO4)2:Eu3+ nanophosphors prepared by molten salt method

We synthesized NaY(MoO_4)_2:Eu~(3+)phosphors of different doping concentrations by a molten salt method.This facile way possesses advantages such as simple process,lower calcination temperature(350℃) and small particle size(70 nm).The crystal system is tetragonal phase and crystal lattice is body centered.The photo luminescence measurements including emission spectra,excitation spectra and fluorescence decay curves were carried out,elucidating that NaY(MoO_4)_2:Eu~(3+)can be effectively excited by near UV and blue light.Moreover,it can be concluded that Eu3+energy transfer type is exchange interaction.Huang-Rhys factor and the critical energy transfer distance(Rc) were calculated to be 0.043 and 0.995 nm,respectively.Auzel's model was used to obtain the intrinsic radiative transition lifetime of~5 D_0 level(τ_0=0.923 ms).Furthermore,a calculation method was used to calculate refractive index n of nontransparent NaY(MoO_4)_2:1 mol% Eu3+phosphor,and n was obtained to be 1.86.     

Multicolor luminescence of hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals with tunable morphology under 940 nm excitation for temperature-responsive anti-counterfeiting

In this study, the hexagonal NaYF₄:Yb³⁺/Ho³⁺/Ce³⁺ microcrystals were synthesized controllably, and upconversion luminescence excited at 940 nm and its application in temperature-responsive anti-counterfeiting are reported. It is clarified that the Ln³⁺ (Ln = Y + Yb + Ho + Ce) density ratio of bottom plane to side plane in the unit cell can be regulated by Ce³⁺ doping. It is also proved that the energy transfer of Yb³⁺ to Ho³⁺ is responsible for the activation of Ho³⁺ under 940 nm excitation, while the cross relaxation between Ho³⁺ and Ce³⁺ participates in the redistribution of electron population of ⁵S₂/⁵F₄ and ⁵F₅ levels. Both theory and experiment confirm that the intensity ratio of red to green emission (I_R/I_G) as a function of temperature as an independent variable has good linear characteristics in the temperature range of 300–500 K. Due to the good responsiveness of multicolor luminescence to temperature, the hexagonal NaYF₄:Yb³⁺/Ho³⁺/Ce³⁺ with tunable morphology is a promising candidate for advanced temperature-responsive upconversion anti-counterfeiting. Our results provide a new pathway for the controllable synthesis of hexagonal NaYF₄ microcrystals as well as the regulation of upconversion luminescence that is excited by wavelengths other than 980 nm and its application in anti-counterfeiting.     

Upconversion luminescence of Y2O3:Er3+, Yb3+ nanoparticles prepared by a homogeneous precipitation method

Y2O3: Er3 , Yb3 nanoparticles were synthesized by a homogeneous precipitation method without and with different concentrations of EDTA 2Na. Upconversion luminescence spectra of the samples were studied under 980 nm laser excitation. The results of XRD showed that the obtained Y2O3:Er3 ,Yb3 nanoparticles were of a cubic structure. The average crystallite sizes calculated were in the range of 28-40 nm. Green and red upconversion emission were observed, and attributed to 2H11/ 2,4S3/ 2→4I15/ 2 and 4F9/ 2→4I15/2 transitions of the Er3 ion, respectively. The ratio of the intensity of green emission to that of red emission drastically changed with a change in the EDTA 2Na concentration. In the sample synthesized without EDTA, the relative intensity of the green emission was weaker than that of the red emission. The relative intensities of green emission increased with the increased amount of EDTA 2Na used. The possible upconversion luminescence mechanisms were discussed.     

Improving luminescence and thermometric performance of Ba2CaWO6:Er3+ by tri-doping with Yb3+ and Na+

The Er³⁺doped double perovskite Ba₂ CaWO₆ crystal is a promising ratiometric thermometer based on the fluorescence intensity ratio(FIR) of transitions from ² H_11/2 and ⁴ S_3/2to the lowered ⁴ I_15/2 level.However,the Ca²⁺vacancy defect caused by the charge difference between rare-earth ions and the substituted alkaline-earth ions gives rise to the non-radiative probability and limits the t...     

Effects of Bi3+ on down-/up-conversion luminescence,temperature sensing and optical transition properties of Bi3+/Er3+ co-doped YNbO4 phosphors

A series of YNbO₄:Bi³⁺ and YNbO₄:Bi³⁺/Er³⁺ phosphors were prepared by a conventional high temperature solid–state reaction method. The results of XRD and Rietveld refinement confirm that monoclinic phase YNbO₄ samples are achieved. The down-/up-conversion luminescence of Er³⁺ ions was investigated under the excitation of ultraviolet light (327 nm) and near infrared light (980 nm). Under 327 nm excitation, broad visible emission band from Bi³⁺ ions and characteristic green emission peaks from Er³⁺ ions are simultaneously observed, while only strong green emissions from Er³⁺ ions are detected upon excitation of 980 nm. Remarkable emission enhancement is observed in down-/up-conversion luminescence processes by introducing Bi³⁺ ions into Er³⁺-doped YNbO₄ phosphors. Pumped current versus up-conversion emission intensity study shows that two-photon processes are responsible for both the green and the red up-conversion emissions of Er³⁺ ion. Through the study of the temperature sensing property of Er³⁺ ion, it is affirmed that the temperature sensitivity is sensitive to the doping concentration of Bi³⁺ ions. By comparing the experimental values of the radiative transition rate ratio of the two green emission levels of Er³⁺ ions and the theoretical values calculated by Judd-Ofelt (J-O) theory, it is concluded that the temperature sensing property of Er³⁺ ions is greatly affected by the energy level splitting.     

Synthesis and characterization of NaYF_4:Yb~(3+),Er~(3+)@silica-N=folic acid nanocomplex for bioimaginable detecting MCF-7 breast cancer cells

Upconversion nanophosphors are new promising nanomaterials to be used as biolabels for detection and imaging of cancer cells.These nanophosphors absorb long-wavelength excitation radiation in the infrared or near infrared region and emit shorter wavelength,higher energy radiation from ultraviolet to infrared.In this paper,we studied the hydrothermal method and optical properties of the functionalized NaYF_4:Yb~(3+),Er~(3+)for biomedical application.After synthesis,these NaYF_4:Yb~(3+),Er~(3+)nanophosphors were functionalized with aminosilanes and folic acid.Folic acid binds to the folate receptor on the surface of MCF-7 breast cancer cells and this binding promotes internalization of the nanophosphors via endocytosis.The sizes of the functionalized NaYF_4:Yb~(3+),Er~(3+)@silica-N=FA(folic acid) nanophosphors can be controlled with length of the rod about 300-800 nm and diameter of the rod about 100-200 nm.Phase structure of NaYF_4:Yb~(3+),Er~(3+)is in hexagonal crystal system.The photo luminescence(PL) spectra of the functionalized NaYF_4:Yb~(3+),Er~(3+)@silica-N=FA nanophosphors were measured.These nanophosphors emit in red color with the strongest band at 650 nm under 980 nm excitation.This result can provide NaYF_4:Er~(3+),Yb~(3+)@silica-N=FA complex for developing fluorescence label and image tool in cancer biology and medicine.     

Thermally stable photoluminescence and long persistent luminescence of Ca3Ga4O9:Tb3+/Zn2+

A green long persistent luminescence (LPL) phosphor Ca₃Ga₄O₉:Tb³⁺/Zn²⁺ was prepared. Ca₃Ga₄O₉ matrix exhibits blue self-activated LPL due to the creation of intrinsic traps. When Tb³⁺ is doped, the photoluminescence (PL) and LPL colors change from blue to green with their intensities significantly enhanced. The doping of Zn²⁺ evidently improves the PL and LPL performances of the Ca₃Ga₄O₉ matrix and Ca₃Ga₄O₉:Tb³⁺. The thermoluminescence (TL) spectra show that a successive trap distribution is formed by multiple intrinsic traps with different depths in the Ca₃Ga₄O₉ matrix, and the incorporation of Tb³⁺ and Zn²⁺ effectively increases the densities of these intrinsic traps. The existence of a successive trap distribution makes the Ca₃Ga₄O₉:Tb³⁺/Zn²⁺ phosphor exhibit thermally stable PL and LPL. It is indicated that this phosphor shows great promise for the application such as high-temperature LPL phosphor.     

Synthesis,upconversion luminescence and optical heating of hexagonal NaGdF4:Yb3+,Er3+

An optical heater based on hexagonal NaGdF_4:Yb~(3+)/Er~(3+) is reported. XRD, SEM and EDS characterization results show that F~-/Ln~(3+) can not only control the phase composition, particle size and morphology, but also affect the effective doping concentration of Yb~(3+) and Er~(3+).When F~-/Ln~(3+) is 12/1, the strongest upconversion luminescence is obtained. Based on the luminescent temperature sensing behavior of Er~(3+),the photo-thermal conversion performance was investigated. The results indicate that the temperature of irradiation spot is linearly dependent on the power density, and the photo-thermal responsivity is determined to be 3.3K·cm~2/W. Also, it is found that the photo-thermal conversion efficiency can be regulated by changing the Yb~(3+) doping concentration. Compared with the nano-gold, copper sulfide and carbon nanotubes, the NaGdF_4:Yb~(3+)/Er~(3+) has the triple functions of upconversion luminescence, temperature sensing, and photo-thermal conversion, and may therefore be a promising optical heater for photo-thermal therapy of tumors.     

Structural and photoluminescence properties of La~(3+) doped CdO nanophosphors synthesised via green mediated solution combustion route

This paper reports a facile environmentally safe green combustion synthesis method of La~(3+) doped CdO nanoparticles(NPs) using different concentrations of Aloe vera(A.V) gel as combustible material. The crystallite size obtained using PXRD pattern is found to be 12-22 nm. The structural analysis was further analysed by Rietveld refinement technique. The X-ray photoelectron spectroscopy(XPS) analysis reveals the presence of two electronic states of CdO around 400 and 411 eV attributed to Cd ~3 d_(3/2) and ~3 d_(3/2),respectively. The photoluminescence(PL) studies were carried upon 350 nm excitation wavelength. The PL emission spectra are found to have two prominent peaks centered at 410 and 510 nm and are attributed to ~5D_3→~7F_1 and ~5D_1→~7F_0 transitions, respectively. The Commission International de l'Eclairage(CIE) chromaticity diagram reveals blue-green color emission. The correlation color temperature(CCT) value is found to be around ～9793 K. The obtained results suggest that the product is quite useful for cool light emitting diode applications when doping concentration is 2 mol%.     

Synthesis of NaYF4:20% Yb3+,2% Er3+,2% Ce3+@NaYF4 nanorods and their size dependent uptake efficiency under flow condition

Lanthanide doped fluorescent nanoparticles have gained considerable attention in biomedical applications. However, the low uptake efficiency of nanoparticles by cells has limited their applications. In this work, we demonstrate how the uptake efficiency is affected by the size of nanoparticles under flow conditions. Using the same size NaYF₄:20% Yb³⁺,2% Er³⁺,2% Ce³⁺ (the contents of rare earths elements are in molar fraction) nanoparticles as core, NaYF₄:20% Yb³⁺,2% Er³⁺,2% Ce³⁺@NaYF₄ core–shell structured nanorods (NRs) with different sizes of 60–224 nm were synthesized by thermal decomposition and hot injection method. Under excitation at 980 nm, a strong upconversion green emission (541 nm, ²H_11/2 → ⁴I_15/2 of Er³⁺) is observed for all samples. The emission intensity for each size nanorod was calibrated and is found to depend on the width of NRs. Under flow conditions, the nanorods with 96 nm show a maximum uptake efficiency by endothelial cells. This work demonstrates the importance of optimizing the size for improving the uptake efficiency of lanthanide-doped nanoparticles.     

Plasmonic enhancement of upconversion emission in Ag@NaYF4:Er3+/Yb3+ phosphor

In this article upconversion luminescence of silver nanoparticles(AgNPs) coated NaYF_4:Er~(3+)/Yb~(3+)phosphor nano-particles was investigated.The prepared samples were characterized through various techniques.The surface plasmon band is observed for prepared AgNPs by analyzing UV-vis measurements and is used to enhance the upconversion emission.From the upconversion measurement the emission bands are observed at 522,546,and 656 nm corresponding to the ~2 H_(11/2)→ 4~1_(15/2),~4 S_(3/2)→~4 I_(15/2)and ~4 F_(9/2)→~4 I_(15/2) levels,respectively.The upconversion emission intensity of the above bands is found to enhance for sample containing 1 mmol AgNPs.Decay time of ~4 S_(3/2) and 4~F_(9/2) levels is found to decrease on coating of AgNPs and hence intensity enhancement is assumed due to the surface plasmon resonance(SPR) effect.     

Synthesis and enhanced photoluminescence properties of red emitting divalent ion (Ca2+) doped Eu:Y2O3 nanophosphors for optoelectronic applications

This work presents the synthesis of Y₂O₃:Eu³⁺,xCa²⁺ (x = 0 mol%, 1 mol%, 3 mol%, 5 mol%, 7 mol%, 9 mol%, 11 mol%) nanophosphors with enhanced photoluminescence properties through a facile solution combustion method for optoelectronic, display, and lighting applications. The X-ray diffraction (XRD) patterns of the proposed nanophosphor reveal its structural properties and crystalline nature. The transmission electron microscope (TEM) results confirm the change in the shape of the particle and aggregation of particles after co-doping with Ca²⁺. Fourier transform infrared spectroscopy (FTIR) and Raman vibrations also confirm the presence of Y–O vibration and subsequently explain the crystalline nature, structural properties, and purity of the samples. All the synthesized nanophosphors samples emit intense red emission at 613 nm (⁵D₀→⁷F₂) under excitation with 235, 394 and 466 nm wavelengths of Eu³⁺ ions. The photoluminescence (PL) emission spectra excited with 235 nm illustrate the highest emission peak with two other emission peaks excited with 466 and 394 nm that is 1.4 times higher than 466 nm and 1.9 times enhanced by 394 nm wavelength, respectively. The emission intensity of Y₂O₃:Eu³⁺,xCa²⁺ (5 mol%) is increased 8-fold as compared to Eu:Y₂O₃. Doping with Ca²⁺ ions enhances the emission intensity of Eu:Y₂O₃ nanophosphors due to an increase in energy transfer in Ca²⁺→Eu³⁺ through asymmetry in the crystal field and by introduction of radiative defect centers through oxygen vacancies in the yttria matrix. It is also observed that the optical band gap and the lifetime of the ⁵D₀ level of Eu³⁺ ions in Y₂O₃:Eu³⁺,xCa²⁺ nanophosphor sample gets changed with a doping concentration of Ca²⁺ ions. Nanophosphor also reveals high thermal stability and quantum yield as estimating activation energy of 0.25 eV and 81%, respectively. CIE, CCT, and color purity values (>98%) show an improved red-emitting nanophosphor in the warm region of light, which makes this material superior with a specific potential application for UV-based white LEDs with security ink, display devices, and various other optoelectronics devices.     

Effect of BaF2 addition on luminescence properties of Er3+/Yb3+ co-doped phosphate glasses

Er~(3+)/Yb~(3+) co-doped phosphate glasses(P_2O_5-Al_2O_3-BaO-BaF_2-K_2O-Er_2O_3-Yb_2O_3) with varying BaF_2 content,were prepared by a conventional melt quenching technique and their spectroscopic properties were examined through the Raman, absorption, emission and decay measurements. Raman spectra(350-1400 cm~(-1)) of the Er~(3+)/Yb~(3+) co-doped phosphate glasses with varying BaF_2 content, were recorded upon laser excitation at 785 nm. Near infrared luminescence spectra were measured in the1400-1600 nm region under 970 nm diode laser excitation and characteristic band was observed at1533 nm corresponding to ~4Ⅰ_(13/2)→~4Ⅰ_(15/2) transition of Er~(3+) ion. The decay curves for the ~4Ⅰ_(13/2) level of Er~(3+)ion, were measured and the lifetime is found to decrease from 7.94 to 7.70 ms when BaF_2 content increases from 0 to 8 mol% and then increases up to 7.83 ms with further increase in BaF_2 content(12 mol%). The emission cross-section.lifetime and figure of merit for the ~4Ⅰ_(13/2)→~4Ⅰ_(15/2) transition of Er~(3+) ion were evaluated and compared to the other host matrices. The upconversion luminescence was measured and intense red emission was observed for all the studied samples.     

Multi-color luminescence in Eu3+/Tb3+ co-doped ZnAl amorphous materials and their annealed samples

Multi-color luminescence basing on amorphous Eu~(3+)/Tb~(3+) co-doped Zn-Al hydroxides and their annealed samples were studied in detail. Results suggest that excellent red emissions due to Eu~(3+) and green emissions attributed to Tb~(3+) appear under the excitation of favorable wavelength for all the asprepared amorphous samples. Moreover, the emission intensity depends on the Eu~(3+)/Tb~(3+) molar ratio. The samples annealed at 300, 500, and 700 ℃ still exhibit amorphous state,and multi-color luminescence kept in the samples annealed at 300 ℃, while luminescence quenched for the samples annealed at 500 and 700 ℃. However, a broad emission ranging from 450 to 650 nm occurs in some samples annealed at 900 ℃. Further, the fluorescence decay and lifetimes for the as-prepared samples and the samples annealed at 300 ℃ were investigated. It is found that all the decay curves of emissions due to Tb~(3+) and Eu~(3+) present characteristic double exponential function despite their different lifetimes.The present work may be a good example for developing new multi-color even white light emitting materials.     

Growth and property enhancement of Er3+-doped 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystal

Er~(3+)-modified 0.68 Pb(Mg_(1/3)Nb_(2/3))O_3-0.32 PbTiO_3(PMN-32 PT) single crystals were grown by using the flux method. The growth mechanism of the crystal and influences of Er~(3+) ions on phase structure,electrical and optical properties were investigated. Results reveal that the crystals are still pure perovskite structure with Er3+ ions doping, but lattice enlarges slightly. The coercive electric field is increased from 4.83 to 6.37 kV/cm for [100]-oriented crystals comparing to undoped PMN-32 PT single crystals.Moreover, the crystal exhibits upconversion emission properties. Green(531 and 552 nm) and red(670 nm) emission bands are recorded under the excitation of 980 nm diode laser, which correspond to the ~2 H_(11/2)→~4 I_(15/2), ~4 S_(3/2)→~4 I_(15/2) and ~4 F_(9/2)→~4 I_(15/2) transitions of Er~(3+) ions. Our results show the feasibility of using this crystal in photoelectric multifunctional devices.     

Synthesis and upconversion luminescence properties of NaYF4：yb^3＋/Er^3＋ microspheres

Cubic NaYF4:yb3+(20%)/Er3+(1%) microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980 nm exci-tation, ultraviolet (4G11/2→4I15/2), violet (2H9/2→4I15/2), grogn (4F7/2→4I15/2, 2H11/2→4I15/2, and 4S3/2→4I15/2), and red (4F9/2→4I15/2) upconversion fluorescence were observed. The number of laser photons absorbed in one upconversion excitation process, n, was determined to be 3.89, 1.61, 2.55, and 1.09 for the ultraviolet, violet, green, and red emissions, respectively. Obviously, n=3.89 indicated that a four-photon process was involved in populating the 4G11/2 state, and n=2.55 indicated that a three-photon process was involved in populating the 4F7/2/2H<11/2>4S3/2 levels. For the violet and red emissions, the population of the states 2H9/2 and 4F9/2 separately came from three-photon and two-photon proc-esses. The decrease of n was well explained by the mechanism of competition between linear decay and upconversion processes for the de-pletion of the intermediate excited states.     

Long afterglow yellow luminescence from Pr3+ doped SrSc2O4

A yellow emitting long afterglow luminescence material SrSc₂O₄:Pr³⁺ was successfully prepared by solid state reaction method. SrSc₂O₄:Pr³⁺ phosphor shows a long afterglow luminescence peak at about 495, 545, 621, 630 and 657 nm, respectively, corresponding to the f–f transitions of Pr³⁺. The afterglow chromaticity coordinates of SrSc₂O₄:1 at%Pr³⁺ were calculated to be (0.35, 0.41), indicating that the afterglow emission is close to the light of yellow region. And, the afterglow luminescence of the optimal sample doped by 1 at%Pr³⁺ can persist for over 3 h. The thermoluminescence results suggest that there are three types of traps with depth of 0.61, 0.69 and 0.78 eV exiting for all the samples, which are produced by the addition of Pr³⁺ ions. The trap density of SrSc₂O₄:1 at%Pr³⁺ is the maximum when the incorporation of Pr³⁺ ions reaches 1 at%, which thus results in the longest afterglow luminescence. All the results indicate that SrSc₂O₄:Pr³⁺ can be a potential candidate of novel long afterglow phosphors.