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.     

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 and optical properties of novel double perovskite Ca_2MgTeO_6:Dy~(3+),Na~+ phosphors for white LEDs

Novel yellow-emitting phosphors of Dy~(3+)-doped double perovskite Ca_2 MgTeO_6 were synthesized by using a conventional high-temperature solid-state reaction.The phase purity,particle morphology,size distribution,elemental composition,luminescence properties,and luminescence decay curves of the resulting products were then analyzed in detail.The Ca2 MgTeO_6:Dy~(3+),Na~+ phosphors show three emission peaks after near-ultraviolet excitation at 350 nm,which correspond to ~4 F_(9/2)→~6 H_(11/2),~4 F_(9/2)→~6 H_(13/2),and ~4 F_(9/2)→~6 H_(13/2) transitions,respectively.Among them,the strongest peak is observed at 573 nm.The best doping content of Dy~(3+)in Ca_2 MgTeO_6:xDy~(3+),xNa~+ phosphors is x=5 mol%.The calculated critical distance of energy transfer between Dy~(3+) ions is 1.6 nm.Luminescence quenching is confirmed to be due to dipole-dipole interactions among Dy~(3+) ions.The phosphors show excellent thermal stability with high activation energy(0.27 eV).The Commission Internationale de l'Eclairage(CIE) chromaticity coordinates of the Ca_2 MgTeO_6 Dy~(3+),Na~+ phosphors are located in the yellow region.White light-emitting diodes(w-LEDs) were fabricated with a high color rendering index(R_a) of 88 and a good correlated color temperature(CCT) of 5440 K.All observed properties indicate that Ca_2 MgTeO_6:Dy~(3+),Na~+ phosphors have potential applications in display and photonic devices.     

Synthesis and optical characterization of Eu2+,Tb3+-codoped Sr3Y(PO4)3 green phosphors

A series of Eu~(2+),Tb~(3+)-codoped Sr_3 Y(PO_4)_3(SYP) green phosphors were synthesized by hightemperature solid-state reaction. Several techniques, such as X-ray diffraction, UV-vis spectrum,and photoluminescence spectrum, were used to investigate the obtained phosphors. The present study investigates in detail photoluminescence excitation and emission properties, energy transfer between the two dopants, and effects of doping ions on optical band gap. SYP:0.05 Eu2+ phosphor shows an intense and broad excitation band ranging from 220 to 400 nm and exhibits a bright green emission band with CIE chromaticity coordinates(0.189, 0.359) under 350 nm excitation. Green emission of SYP:0.03 Tb3+ is intensified by codoping with Eu~(2+), and energy transfer mechanism between them is demonstrated to be a dipole-dipole interaction. Upon 350 nm excitation, SYP:Eu~(2+),Tb~(3+) phosphors exhibits two dominating bands peaking at 466 and 545 nm, which are assigned to 4 f~65 d~1→4 f~7 transition of Eu~(2+) ions and ~5 D_4→~7 F_5 transition of Tb~(3+) ions, respectively. Optimal doping concentrations of Eu~(2+) and Tb~(3+) in the SYP host are 5 mol% and 15 mol%, respectively. Results indicate that SYP:Eu~(2+),Tb~(3+) phosphors are potentially used as green-emitting phosphors for white light-emitting diodes.     

Photoluminescence properties and thermal stability of Eu~(3+)-activated La_7Ta_3W_4O_(30) red-emitting phosphors for near-UV-excited w-LEDs

Novel trivalent europium(Eu~(3+))-activated La_7 Ta_3 W_4 O_(30):xEu~(3+)(x=0.5 mol%-40 mol%) red-emitting phosphors were synthesized by means of a high-temperature solid-state reaction.The structure,morphology,photoluminescence,thermal-stability properties,lifetime,and color-rendering of the prepared phosphors were investigated in detail.The La_7 Ta_3 W_4 O_(30):Eu~(3+) phosphors show five emission peaks under near-ultraviolet(n-UV) at 397 nm,and these peaks are ascribed to the transitions of ~5 D_0-~7 F_j(j=0,1,2,3 and 4) by Eu~(3+) ions.The optimal doping concentration of Eu~(3+) is 20 mol%,and the critical distance of the energy transfer between the Eu3+ions was calculated to be 1.768 nm.The quenching temperature(T_(0.5)) of La_7 Ta_3 W_4 O_(30):20 mol%Eu~(3+) is about 440 K.The quantum yield(QY) was measured to be 85.85%.The fabricated white-light-emitting diodes(w-LEDs) possess high color-rendering index(R_a) of 90,and high correlated color temperature(CCT) of 5810 K,respectively.The Commission Internationale de L'Eclairage(CIE) coordinates are(0.311,0.322).Therefore,the prepared phosphor has a promising application for w-LEDs.     

Highly sensitive up-conversion phosphor for optical thermometry:CaLaAl_3O_7:Er~(3+)/Yb~(3+)

Herein, we reported Er~(3+)/Yb~(3+) co-doped CaLaAl_3O_7 up-conversion phosphors synthesized via solid state reaction, which was further explored as a new optical thermometry. The luminescent properties of Er~(3+) or Er~(3+)/Yb~(3+) doped CaLaAl_3O_7 phosphor was studied in detail. The two-photon process for the green emissions of Er~(3+) were confirmed by the power-dependent luminescence. The up-conversion optical temperature sensing performances of the Er~(3+)/Yb~(3+)-codoped CaLaAl_3O_7 phosphor were investigated based on the FIR technique. The maximum sensitivity of this phosphor can reach about 0.00345 K~(-1) at 453 K, which reveals this phosphor can be a promising candidate for optical thermometry devices.     

Ultra-high sensitivity of rhodamine B sensing based on NaGdF4:Yb3+,Er3+@NaGdF4 core-shell upconversion nanoparticles

A series of mono-dispersed hexagon NaGdF_4:Yb~(3+),Er~(3+)@NaGdF_4 core-shell nanoparticles with different shell thickness were synthesized via a co-precipitation method. Nanoparticles with high upconversion fluorescent emissions result in large signal-to-noise ratio, which guarantees the accuracy of the sensitivity. Besides, the maximum sensitivity of these NPs as detection film increases first and then decreases with the shell thickness increasing. When the shell thickness is 2.3 nm(NaGdF_4-2), the maximum sensitivity(0.69959 ppm~(-1)) is reached. A large degree of overlap between the rhodamine B absorption band and the Er~(3+) green emission bands ensures that the NaGdF_4:Yb~(3+),Er~(3+)@NaGdF_4 nanoparticles can be used as fluorescent probe to detect the concentration of rhodamine B based on fluorescent intensity ratio technology. The linear relationship between the rhodamine B concentration and the intensity ratio(R) of green and red emission intensity(I_(S+H) and I_F) were studied systematically. The result shows that the maximum sensitivity can be obtained in low concentration rhodamine B(4 ppm), which is lower than the reported minimum detection concentration. Thus, the ultra-high sensitivity detection by NaGdF_4:Yb~(3+),Er~(3+)@NaGdF_4 core-shell upconversion nanoparticles in low concentration can be realized,which provides promising applications in bio-detection filed.     

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.     

Structural and optical properties of Dy~(3+):YAlO_3 phosphors for yellow light-emitting diode applications

The spectroscopic properties of a series of Dy~(3+) single-doped and Dy~(3+)/Nd~(3+),Dy~(3+)/Tb~(3+),and Dy~(3+)/Tm~(3+)co-doped YAlO_3(yttrium aluminum perovskite,YAP) phosphors were investigated and compared through the measurements of optical absorption,emission spectra,and fluorescence decay curves.For the Dy~(3+) ion single-doped samples,the intensity of each absorption band increases with an increment in Dy~(3+) ion doping concentration,and the identified strong absorption peak at 447 nm indicates that Dy~(3+):YAP phosphors are suitable to be pumped by a blue laser diode(LD).For all co-doped samples,absorption peaks of Dy~(3+) ion along with some of the absorption bands of Nd~(3+),Tb~(3+),and Tm~(3+) ions are observed.Under 351 and 447 nm excitation,a prominent emission peak at 572 nm was obtained in all the samples,corresponding to Dy~(3+):~4 F_(9/2)→~6 H_(13/2) transition.Here,2 at% Dy~(3+):YAP phosphor exhibits the highest yellow emission intensity under 447 nm pumping.Among the three kinds of Dy~(3+) co-doped phosphors,Dy~(3+)/Tb~(3+):YAP phosphor possesses the dominant yellow emission.The fluorescence decay curves show exponential behaviour and are fitted well.The Commission International de L'Eclairage(CIE)chromaticity coordinates were calculated following the respective emission spectra,and it is found that all the coordinates locate in the yellow region.The energy transfer(ET) processes were investigated and the concentration quenching mechanism was discussed.The obtained results suggest that Dy~(3+)-activated YAP phosphors are good candidates for yellow LED applications.     

First-principles study of electronic structure and optical properties of Er:Lu_2O_3

In the present computational study,we found that Er:Lu₂O₃materials have promise for application in laser applications.The crystal structure and the electronic and optical properties of Er:Lu₂O₃materials were studied using first-principle calculations under the framework of density functional theory.Based on the experimental and calculated results,the structure of Lu₂O₃was established.The calculated results show that doping by Er³⁺can effectively improve its absorption coefficient in the ultraviolet region and improve the static dielectric constant of Lu₂O₃.As the doping concentration of Er³⁺increases,the energy of the valence band electrons excited to the conduction band decreases,and the transition is more likely to occur.The absorption coefficient,reflectance,and electron energy loss spectroscopy are bathochromic shifted.The Lu_2-xEr_xO₃(02O₃.     

Preparation and luminescence properties of orange-red Ba3Y4O9:Sm3+ phosphors

A novel orange-red emitting Ba₃Y₄O₉:Sm³⁺ phosphors were prepared by a high temperature solid-state reaction in air. X-ray diffraction (XRD), photoluminescence spectra, fluorescence decay and temperature-dependent emission spectra were utilized to characterize the structure and luminescence properties. The results show that the excitation spectrum includes a series of linear peaks at 350, 367, 382, 410, 424, 445, 470 and 495 nm, respectively. Under 410 nm excitation, the emission peaks were located at 574 nm (⁴G_5/2—⁶H_5/2), 608 nm (⁴G_5/2—⁶H_7/2), 659 nm (⁴G_5/2—⁶H_9/2) and 722 nm (⁴G_5/2—⁶H_11/2), respectively. The concentration quenching occurs when x equals 0.08 for Ba₃Y_4–xO₉:xSm³⁺ phosphor and its mechanism is ascribed to the dipole–dipole interaction. The chromaticity coordinates of Ba₃Y_3.92O₉:0.08Sm³⁺ phosphor are in the orange-red region. The temperature-dependent study shows that this phosphor has excellent luminescence thermal-stability. And the luminescence intensity of Ba₃Y_3.92O₉:0.08Sm³⁺ phosphor at 473 K only declines by about 25.75% of its initial intensity. The experimental data indicate that Ba₃Y₄O₉:Sm³⁺ phosphor may be promising as an orange-red emitting phosphor for white light emitting diodes.     

Promising red phosphors LaNbO4:Eu3+,Bi3+ for LED solid-state lighting application

La0.75NbO4:Eu3+0.25 and La0.65NbO4:Eu3+0.25,Bi3+0.10 phosphors were synthesized by solid-state reaction method,and their photoluminescence properties were discussed in detail.With the increased incorporation of the co-activator Bi3+,the charge transfer(CT) bands of Nb5+→O2-and Eu3+→O2-(-280 nm) weakened and a new and significant broad band Bi3+-O2-(-330 nm) appeared,while the peaks at 395 and 466 nm assigned to f-f transitions of Eu3+ was slightly changed.Compared with the commercial phosphor Y2O2S:0.05Eu3+...     

Effect of Li+ ion concentration on upconversion emission and temperature sensing behavior of La2O3:Er3+ phosphors

The effects of Li~+ co-doping concentration on the structure, upconversion luminescence and temperature sensing behavior of Er~(3+):La_2O_3 phosphors were investigated. X-ray diffraction and scanning electron microscopy observations reveal that Li~+ ion co-doping can change the lattice parameter of La_2O_3 host and increase the particle size of the samples. The optical investigation shows that co-doping of Li~+ ions can enhance the upconversion emission of Er~(3+) ions in La_2O_3 matrix effectively. Most importantly, the temperature sensing sensitivity of the samples is found to be dependent on Li~+ co-doping concentration,when the emission intensity ratio of the(~2H_(11/2)→~4 I_(15/2)) and(~4 S_(3/2)→~4 I_(15/2)) transitions of Er~(3+) is chosen as the thermometric index. Both of the optimum upconversion luminescence and temperature sensing sensitivity are obtained for 7 mol% Li~+ co-doped sample. When the Li~+ concentration is beyond 7 mol%,both the quenching in upconversion intensity and the degradation of temperature sensitivity are observed, which may be due to the serious distortion in local crystal field around Er~(3+) ions caused by the excess Li~+ ions.     

Synthesis and luminescent properties of BaGd2O4:Eu3+ phosphors with highly efficient red-emitting

The BaGd_(2-2 x)Eu_(2 x)O_4(BG, x = 0.01-0.09) phosphors were successfully synthesized via the sol-gel method,and BaY_(2-2 y)Eu_(2 y)O_4(BY, y = 0.005-0.07) phosphors were included for comparison. The pure phase BG phosphors with the ordered CaFe_2 O_4-type structure are obtained by annealing at 1300℃ for5 h. The phosphors with uniform particle size of 120 nm and good dispersion display typical Eu~(3+)emission with the strongest peak at 613 nm(~5 D_0→~7 F_2 transition of Eu3+) under optimal excitation band at 262 nm(CTB band). The presence of Gd~(3+) excitation bands on the PLE spectra monitoring the Eu3+emission directly proves an evidence of Gd~(3+)-Eu~(3+) energy transfer. Owing to the concentration quenching, the optimum content of Eu3+ addition is 5 at%(x = 0.05), and the quenching mechanism is determined to be the exchange reaction between Eu3+. All the BG samples have similar color coordinates and temperature of(0.64 ± 0.02, 0.36 ± 0.01) and 2000 ± 100 K,respectively. The lifetime value of BaGd_(1.9)Eu_(0.1)O_4 for 613 nm is fitted to be 2.19 ± 0.01 ms, and the Eu~(3+) concentration does not change the lifetime significantly. Owing to the Gd~(3+)-Eu~(3+) energy transfer, the luminescent intensity of the BaGd_(1.9)Eu_(0.1)O_4 phosphor is better than BY system. The BG system served as a new type of phosphor is expected to be widely used in lighting and display areas.     

Influence of urea on microstructure and optical properties of YPO4：Eu^3＋ phosphors

YPO4:Eu 3+ phosphors were synthesized by solution coprecipitation method assisted by urea in the precursor reaction solution. X-ray diffraction spectral analysis showed that the samples synthesized with urea had smaller particle size and lower crystallinity than those samples synthesized without urea.Moreover,the calculated strain result indicated that the Eu 3+ site in the former exhibited a lower crystal field symmetry than that in the latter.Hence,the influence of crystal field symmetry dominated luminescence efficiency rather than crystallinity because the luminescence intensity observed in Eu0.05Y0.95PO4 synthesized with 1.0 g urea was six-fold higher than that of the as-synthesized sample.With increased concentration of Eu 3+ ion,the luminescence intensity initially increased,and then subsequently decreased as the concentration of Eu 3+ ion exceeded 12 mol.%due to concentration quenching.The optimal condition for YPO4:Eu 3+ phosphor was Eu0.12Y0.88PO4 with 1.0 g urea added in the precursor.The luminescence intensity of the optimal condition was again enhanced 1.6-fold relative to that of Eu0.05Y0.95PO4 synthesized with 1.0 g-urea.     

Oxalate-assisted morphological effect of NaYF4:Yb3+,Er3+ on photoelectrochemical performance for dye-sensitized solar cells

In this study. we have employed a facile oxalate-assisted hydrothermal approach to tailor the morphology of β-NaYF_4:Er~(3+),Yb~(3+)(NYFEY) powders through the variation of the molar ratio of oxalate ions(Oxa~(2-)) and rare earth ions(RE~(3+)) in the range of 0.5:1.1:1.2:1, 5:1. and 10:1. The obtained results show that the crystallinity, particle size and upconversion luminescence intensity of the as-synthesized NYFEY particles are gradually decreased as the Oxa~(2-):RE~(3+) molar ratio increases from 0.5:1 to 10:1. For the purpose of photoelectrochemical performance evaluation,the as-synthesized NYFEY particles with different morphologies are incorporated into the nanocrystalline TiO2 films to form the multifunctional nano-and sub-micro meter composite photoanodes of dye-sensitized solar cells(DSSCs). A short-circuit current density(Jsc) of 14.26 mA/cm~2 and power conversion efficiency(PCE) of 7.31% are obtained for DSSCs prepared with hexagonal rod-like NYFEY crystals,evidencing an increase of 29.8% compared with DSSCs prepared with only TiO_2 nanoparticles. The demonstrated synthesis approach for tailoring the morphology and size of NYFEY particles and enhancing the performance of DSSCs can also be applied for other types of solar cells.     

Tunable luminescence and energy transfer in Y_2BaAl_4SiO_(12):Tb~(3+),Eu~(3+)phosphors for solid-state lighting

Single-phase Y₂BaAl₄SiO₁₂:Tb³⁺,Eu³⁺phosphors with adjustable luminescence were successfully prepared by high-temperature solid-state reaction method.The structural,luminescent properties and ene rgy transfer(ET) process of Y₂BaAl₄SiO₁₂:Tb^(³⁺),Eu³⁺phosphors were syste matically analyzed with the help of X-ray diffraction(XRD),scanning electron microscopy(SEM),excitation spectra,emission spectra and photoluminescence decay curves.Tunable luminescence ranging from green through yellow and definitively to red can be achieved by elevating amounts of Eu³⁺ions in Tb³⁺,Eu³⁺co-doped samples.Besides,the ET mechanism and efficiency were also analyzed and the maximum ET efficiency is 67%.All the results show that Y₂BaAl₄SiO₁₂:Tb³⁺,Eu³⁺phosphors can be used in solid-state lighting.     

Synthesis,structure and optical properties of novel thermally robust Dy~(3+)-doped Ca_9Sc(PO_4)_7 phosphors for NUV-excited white LEDs

The powder samples of Ca₉Sc(PO₄)₇:xDy^{^(3+})white emitting phosphors were prepared via a solid state reaction technique.The Ca₉Sc(PO₄)₇:Dy³⁺samples were researched by using the GSAS Rietveld refinement and X-ray diffraction(XRD) methods,and SEM images and elemental maps were recorded.Under 350 nm excitatio n,the emission spectra of Ca₉Sc(PO₄)₇:xDy³⁺samples have two obvious peaks and one weak peak at 484,572 and660 nm,corresponding to the characteristic electron transitions of(⁴F_9/2→ ⁶H_15/2,blue),(⁴F_9/2→ ⁶H_13/2,yellow) and(⁴F_9/2→ 6 H11/2,red),respectively.The concentration quenching effect,decay lifetime and thermal quenching of the as-synthesized Ca₉Sc(PO₄)₇:Dy³⁺samples were researched systematically.The Ca₉Sc(PO₄)₇:0.02 Dy³⁺phosphor possesses a good thermal stability,of which the emission intensity at 423 K can maintain 79% of the initial value(273 K).In addition,through the study of the chro maticity coordinates of the Ca₉Sc(PO₄)₇:0.02 Dy³⁺phosphor,it is found that it is located in the white region,and the Commission Internationalede L’Eclairage(CIE) chromaticity coordinates are(0.339,0.389),The above results show that Ca₉Sc(PO₄)₇:xDy³⁺phosphors can be excellent candidate material for applications in NUV-excited white LEDs.     

Effect of gadolinium incorporation on optical characteristics of YNbO_4：Eu~（3＋） phosphors for lamp applications

Eu3+-doped (Y,Gd)NbO4 phosphor was synthesized by solid-state reaction for possible application in cold cathode fluorescent lamps. A broad absorption band with peak maximum at 272 nm was observed which was due to the charge transfer between Eu3+ ions and neighboring oxygen anions. A deep red emission at the peak wavelength of 612 nm was observed which could be attributed to the 5D0→7F2 transition in Eu3+ ions. The highest luminance for Y1-x-yGdyNbO4:Eux3+ under 254 nm excitation was achieved at Eu3+ concentration of 18 mol.% (x=0.18) and Gd3+ concentration of 8.2 mol.% (y=0.082). The luminance of Y0.738Gd0.082NbO4:Eu3+0.18 was higher than that of a typical commercial phosphor Y2O3:Eu3+ and the CIE chromaticity coordinate was (0.6490, 0.3506), which was deeper than that of Y2O3:Eu3+. The particle size of the synthesized phosphors was controlled by the NaCl flux and particle size as high as 8 μm with uniform size distribution of particles was obtained.