Er~(3+)-Yb~(3+)-Na~+:ZnWO_4 phosphors for enhanced visible upconversion and temperature sensing applications

The crystal structure and surface morphology of the Er³⁺/Yb³⁺/Na+:ZnWO₄ phosphors synthesized by solid state reaction method were analyzed by X-ray diffraction(XRD) and field emission scanning electron microscopy(FESEM) analysis.The frequency upconversion(UC) emission study in the developed phosphors was investigated by using 980 nm laser diode excitation.The effect of codoping in the Er³⁺:ZnWO₄ phosphors on the UC emission intensity was studied.The UC emission bands that are exhibited in the blue(490 nm),green(530,552 nm),red(668 nm) and NIR(800 nm) region correspond to the ⁴F_7/2→⁴I_15/2.²H_11/2,⁴S_3/2→⁴I_15/2,⁴F_9/2→⁴I_15/2 and ⁴I9/2→⁴I_15/2 transitions,respectively.The temperature sensing performance of the Er³⁺-Yb³⁺-Na+:ZnWO₄ phosphors was investigated based on the 2 H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 thermally coupled transitions of the Er³⁺ions.The photometric study was also carried out for the developed phosphors.     

Synthesis of well oil-dispersible BaYF5:Yb3+/Er3+ nanocrystals with green upconversion fluorescence

Nearly monodisperse, regular-shaped and well oil-dispersible tetragonal BaYF₅:0.2Yb³⁺/0.02Er³⁺ nanocrystals (NCs) were synthesized in water-ethanol-oleic acid-sodium oleate system. The as-obtained NCs exhibited bright upconversion (UC) fluorescence under the 980 nm excitation. Blue (²H_9/2-⁴I_15/2), green ((²H_11/2, ⁴S_3/2)-⁴I_15/2) and red (⁴F_9/2-⁴I_15/2) transitions were observed. The results indicated that the relative intensity of green to red increased gradually with increasing power density, which were seldom in the previous work. Therefore, the UC properties and mechanism were studied in detail.     

Upconversion emission of SrYbF5：Er3＋nanosheets modified by Tm3＋ions

Through a hydrothermal route, the Er3＋and Tm3＋co-doped SrYbF5 nanosheets were synthesized. The resulting samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and luminescence spec-tra. Under the excitation of 980 nm laser irradiation, the upconversion emissions of Tm3＋ions centered at 474 nm （1G4→3H6）, 679 nm （3F2→3H6）, 699 nm （3F3→3H6）, 803 nm （3H4→3H6） and emissions of Er3＋ions centered at 522 nm （2H11/2→4I15/2）, 543 nm （4S3/2→4I15/2）, 654 nm （4F9/2→4I15/2） were observed. The upconversion emissions of Er3＋ions were adjusted by the concentration of Tm3＋ions. The energy transfer mechanisms among Er3＋-Yb3＋-Tm3＋in SrYbF5 nanosheets were discussed.     

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.     

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.     

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.     

Energy transfer processes from Yb3＋ to Ln3＋ （Ln=Er or Tm） in heavy metal glasses

Heavy metal glasses doubly doped with Yb3+ and Ln3+ ions(Ln=Er or Tm) were studied. Glass host matrices were limited to lead borate glass and lead germanate glass. Efficient resonant(Yb3+-Er3+) and non-resonant(Yb3+-Tm3+) energy transfer was observed for the studied systems. Near-infrared luminescence spectra at 1.53 μm(Er3+) and 1.9 μm(Tm3+) were detected under excitation of Yb3+ by 975 nm diode laser line. They corresponded to 4I13/2→4I15/2(Er3+) and 3F4→3H6(Tm3+) transitions of rare earth ions, respectively. The unusual large spectral linewidth nearly close to 110 nm for 4I13/2→4I15/2 transition of Er3+ ions in lead borate glass was obtained, whereas long-lived near-infrared luminescence at 1.53 μm was detected in lead germanate glass. Quite different situation was observed for Yb3+-Tm3+ doubly doped glasses. In contrast to lead borate glass, near-infrared(3F4→3H6) luminescence spectra were registered for Tm3+ ions in lead germanate glasses, only. These phenomena strongly depended on stretching vibrations of glass host, which was confirmed by FT-IR spectroscopy.     

Structure,Upconversion and Fluorescence Properties of Er3+-Doped TeO2-TiO2-La2O3 Tellurite Glass

Tellurite glasses were generally applied in rare earth optical materials due to their excellent physical and chemical properties. In this study, novel tellurite glasses composed of TeO₂-TiO₂-La₂O₃ were prepared by conventional melting-quenching method. Some basic physical parameters such as density, refractive indices, transition temperature and crystalline temperature were measured. The structure was analyzed by Raman spectra. The absorption, upconversion and fluorescence spectra were measured by UV-Vis-NIR spectrophotometer and spectrofluorimeter. Under 980 nm laser excitation, upconversion luminescence centered at 531, 545 and 657 nm corresponding to the transition ⁴H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 respectively, were observed. The effects of TiO₂ concentration on structure and upconversion luminescence intensity were discussed. The result indicated that the upconversion intensity increased as the phonon concentration decreased. The fluorescence properties of Er³⁺ doped glass were also studied. The dominant peak centered at 1531 nm and full width at half maximum (FWHM) was 64 nm. The Er³⁺ stimulated emission cross-section was calculated on the basis of McCumber theory. The possible mechanism of upconvesion and fluorescence were proposed.     

Luminescence properties of calcium tungstate activated by lanthanide(Ⅲ) ions

Calcium tungstate phosphors activated by the Ln3+ ions(Ln=Pr, Nd, Tb, Yb) were synthesized by a traditional high-temperature solid-state method. The crystal structures and morphologies of the products were characterized by scanning electron microscopy(SEM), X-ray powders diffraction(XRD) and infrared spectra(FT-IR). The samples were found to show luminescence properties(down-conversion, DC, at excitation wavelength 254 nm and up-conversion, UC, at excitation wavelength 980 nm). CaWO4 doped with Tb3+/Yb3+ showed green DC and UC luminescence characteristic of Tb(III) ion in the range of 470–660 nm, corresponding to the 5D4→7F6,5,4,3,2 electronic transition. CaWO4 doped with Pr3+/Yb3+ showed week blue, green and red(DC and UC) luminescence of Pr(III) ion, in the wavelength region of 450–700 nm. Emission peaks were ascribed to the 3P1→3H4,5,6, 3P0→3H4,5,6, 3P1→3F2 and 3P0→3F2 transitions, respectively. CaWO4 doped with Nd3+/Yb3+ phosphor emitted orange UC luminescence at 450–690 nm(2P3/2→4I15/2, 4G7/2→4I9/2,11/2,13/2) and strong near-infrared UC luminescence at 720–900 nm(4F7/2+4S3/2→4I9/2, 4F5/2+2H3/2→4I9/2, 4F3/2→4I9/2) which is the characteristic of Nd(III) ion.     

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.     

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.     

Local microstructure and photoluminescence of Er-doped 12CaO·7Al2O3 powder

Er-doped 12Ca0.7Al2O3 （C12A7：Er） powders were prepared using the sol-gel method followed by annealing inorganic precursors. X-ray diffraction （XRD）, Raman and absorption spectra revealed that Er ions existed and substituted Ca^2＋ lattice site in C12A7. The photoluminescence of C12A7：Er at room temperature was observed in the visible and infrared region using 488 nm （2.54 eV） Ar^＋ line as excitation source, respectively. The sharp and intense green emission bands with multi-peaks around 520 nm and 550 nm correspond to the transitions from the excited states ^2H11/2 and ^4S3/2 tO the ground state ^4I15/2, respectively. Furthermore, red emission band around 650 nm was also observed. It was attributed to the electronic transition from excited states ^4F9/2 to the ground state ^4I15/2 inside 4f-shell of Er^3＋ ions. The intensive infrared emission at 1.54 μm was attributed to the transition from the fast excited states of ^4I13/2 to the ground state （^4I15/2）. The temperature dependent photoluminescence of infrared emission showed that the integrated intensity reached a maximum value at near room temperature. The forbidden transitions of intra-4f shell electrons in free Er^3＋ ions were allowed in C12A7 owing to lack of the inversion symmetry in the Er^3＋ position in C12A7 crystal field. Our results suggested that C12A7：Er was a candidate for applications in Er-doped laser materials, and full color display.     

White up-conversion luminescence and highly-sensitive optical temperature sensing in Na3La(VO4)2:Yb,Er,Tm,Ho phosphors

In this work,tunable white up-conversion luminescence was achieved in the Yb³⁺,Er³⁺,Tm³⁺,Ho³⁺ codoped Na₃La(VO₄)₂ phosphors under 980 nm excitation.The emissions of three primary colors are mainly attributed to the ²H_11/2/⁴S_3/2→⁴I_15/2 transitions of Er³⁺,¹G₄→³H₆ transition of Tm³⁺,and_5...     

Flexible double narrowband near-infrared photodetector based on PMMA/core-shell upconversion nanoparticle composites

Flexible narrowband near infrared(NIR)photodetectors(PDs)are urgently in demand in the fastdeveloping era of flexible electronics,due to their crucial roles in various innovative applications.Hence,we designed and synthesized the core-shell structured NaYF4:Yb³⁺,Er³⁺@NaYF4:Nd³⁺upconversion nanoparticles(UCNPs),which can be pumped by the 808 and 980 nm lights.The upconversion luminescence(UCL)are significantly enhanced after being assembled with the opal photonic crystals(OPCs)due to the photonic crystal effect,with 55 and 48 folds of enhancement factors under illuminations of 808 and 980 nm lights,respectively.Based on this hybrid,the flexible narrowband PDs were successfully fabricated on the PET substrate with the structure of OPCs/NaYF4:Yb³⁺,Er³⁺@Nd³⁺/MAPbl3,which displays excellent detection performance to double narrowband NIR light(808 and 980 nm)benefiting from the amplified UCL,with responsivity of 8.79 and 7.39 A/W,detectivity of 3.01×10¹¹and2.68×10¹¹cm·Hz^1/2/W for 808 and 980 nm lights detection respectively,along with short response time in the range of 120-160 ms.Furthermore,the OPCs/NaYF4:Yb³⁺,Er³⁺@NaYF4:Nd³⁺/MAPbI3 double narrowband PDs display low photodetection power threshold(0.05 W/cm2),outstanding flexibility,prominent moisture resistance,and good long-time stability.This work displays a new concept of narrowband NIR PDs,which open a new field for specific NIR light detections.     

Saturation effect of up-conversion luminescence from erbium-doped,silicatitania sol-gel powders

The intensity of the visible up-conversion luminescence could be limited by a saturation effect produced by increased pump power. Visible up-conversion luminescence was obtained in erbium doped,silica-titania sol gel powders under dynamical pumping at 1532 nm. The saturation effect was studied for erbium radiative transitions 2H9/2→4I15/2 (410 nm),2H11/2→4I15/2 (530 nm),4S3/2→4I15/2 (550 nm),2H9/2→4I13/2 (567 nm) and 4F9/2→4I15/2 (675 nm). The recorded up-conversion luminescence decreased when increasing ex...     

Intense single-band red upconversion emission in BiOCl:Er~(3+) layered semiconductor via co-doping Ho~(3+)

Exploring a new tuning way to facilely realize single-band red emission in trivalent rare-earth ions(RE~(3+)) doped upconversion(UC) materials is still desirable.In this work,the intense single-band red emission is achieved by co-doping only Ho~(3+)in the BiOCl:Er~(3+) under 1550 nm excitation.In the BiOCl layered host,co-doping Ho~(3+)can further enhance the red emission and simultaneously suppress the green emission of Er~(3+),and thus obviously improve the red-to-green(R/G) ratio.It is found that Ho~(3+)does not se rve as ene rgy trapping through the ~5 I_6 state as in traditional UC materials but acts as ET bridge(~4 S_(3/2),~2 H_(11/2)(Er~(3+))→~5 F_4,~5 S_2(Ho~(3+))→~4 F_(9/2)(Er3+)).The tuning mechanism of Ho3+is discussed in detail and further confirms through a comparative experiment.Our research gives an unusual perspective to tune the UC behavior of Er3+through co-doping Ho~(3+),which might be inspiring for achievement of single-band red UC emission.     

Upconversion luminescence of Gd2O3:Er3+ and Gd2O3:Er3+/silica nanophosphors fabricated by EDTA combustion method

Gd_2O_3:Er~(3+) nanophosphors were fabricated by the combustion method in presence of Na_2 ethylene diamine tetra acetic acid(EDTA-Na_2) as fuel at not high temperature(≤350℃) within a very short time of 5 min.The added concentration of Er~(3+)ions in Gd_2O_3 matrix was changed from 0.5 mol% to 5.0 mol%.The X-ray diffraction pattern of samples indicates the monoclinic structure of Gd_2O_3:Er3+.The morphology and chemical composition analysis of the Gd_2O_3:Er~(3+) samples are characterized by a field emission scanning electron microscope(FESEM) and a Fourier-transform infrared spectrometer(FTIR).The photoluminescence(PL),photo luminescence excitation(PLE) and upconversion(UC) at room temperature of the prepared materials with different concentrations of Er~(3+) were investigated.The PL of Gd_2O_3:Er~(3+)nanomaterials are shown in visible at 545,594,623,648,688 nm under excitation at 275 nm.The emission bands from transitions of Er~(3+) from ~2P_(3/2) to ~4F_(9/2) are observed,UC luminescent spectra of the Gd_2O_3:Er~(3+)/silica nanocomposites under 976 nm excitation show the bands at 548 and 670 nm.The influence of excitation power at 980 nm for transitions were measured and calculated.The results indicate that the upconversion process of Gd_2O_3:Er~(3+)/silica is two photons absorption mechanism.The low temperature dependence of UC luminescent intensities of the main bands of Gd_2O_3:Er~(3+)was investigated towards development of a nanotemperature sensor in the range of 10-300 K.     

Photoluminescence properties of Sm3＋-doped LiY（Mo04）2 red phosphors

A series of novel Sm3＋-doped LiY（MoO4）2 red phosphors under the UV excitation were synthesized by solid state reaction at 800 ℃ for 7 h. The data measured by X-ray diffraction （XRD） indicated that the samples were all pure phases of LiY（MoO4）2. Their excitation spectra had a broad band ranging from 250 to 350 nm and several sharp peaks. The centers of the peaks were located at about 365 nm （6H5/2→4D3/2）, 378 nm （6H5/2→rp7/2）, 406 nm （6H5/2→4FT/2）, 420 nm （6H5/2→6ps/2）, 442 nm （6H5/2→4Gg/2）, 471 nm （6H5/2→4I13/2） and 482 nm （6H5/2→419/2）, respectively. The strongest emission was excited by 406 nm, and the main emissions were located at 568 nm （4G5/2→6Hs/2）, 610 nm （4Gs/2→rH7/2）, 649 nm （4G5/2→6H9/2） and 710 nm （4Gs/2→6HII/2）. Photoluminescence prop- erties were determined for various concentrations of Sm3＋-doped LiY（MoO4）2 host, and the luminescence intensity had the best value when x=0.02 in LiYix（MoO4）2：xSm3＋.     

Optimization of synthesis of upconversion luminescence material NaYF4：Er^3＋,yb^3＋ nanometer-phosphor by low-temperature combustion synthesis method

A kind of Er3+-Yb3+ co-doped natrium yttrium fluoride nanometer-phosphor sensitive to 980 nm was synthesized by the low-temperature combustion synthesis method, which expanded the application range of the low-temperature combustion synthesis (LCS) method which is always used in the synthesis of oxides and compound oxides. The synthesis conditions were optimized with orthogonal experiments and the optimum technological parameters were obtained. Intense upconversion emissions at 522, 540 and 653 nm corresponding to the 2H11/2, 4S3/2, and 4F9/2 transitions to the 4I15/2 ground state were observed when excited by continuous wavelength (CW) laser radiation at 980 nm. The effect of the carbamide amount on the phase formation and the luminescence intensity was analyzed. The average particle size of the sample was 30-40 nm.     

Synthesis of well oil-dispersible BaYF₅:Yb³⁺/Er³⁺ nanocrystals with green upconversion fluorescence

Nearly monodisperse,regular-shaped and well oil-dispersible tetragonal BaYF5:0.2Yb3+/0.02Er3+ nanocrystals(NCs) were synthesized in water-ethanol-oleic acid-sodium oleate system.The as-obtained NCs exhibited bright upconversion(UC) fluorescence under the 980 nm excitation.Blue(2H9/2-4I15/2),green((2H11/2,4S3/2)-4I15/2) and red(4F9/2-4I15/2) transitions were observed.The results indicated that the relative intensity of green to red increased gradually with increasing power density,which were seldom in the previous work.Therefore,the UC properties and mechanism were studied in detail.