Upconversion Luminescence Properties of Ho^3＋, Tm^3＋, Yb^3＋ Co-Doped Nanocrystal NaYF4 Synthesized by Hydrothermal Method

Nanocrystal of upconversion （UC） phosphor Ho^3＋, Tm^3＋ , and Yb^3＋ co-doped NaYF4 was prepared by the hydrothermal method in the presence of the complexing agent EDTA. Under 980 nm diode laser excitation, the impact of different concentrations of Ho^3＋ ion on the UC luminescence intensity was discussed. The law of luminescence intensity versus pump power shows that the 474 nm blue emission, 538 nm green emission, and 642 nm red emission are all due to the two-photon process, while the 450 nm blue emission is a three-photon process. The UC mechanism and processes were also analyzed. The sample was characterized by transmission electron microscopy （TEM） and X-ray diffraction （XRD）. The result shows that Ho^3＋ ,Tm^3＋ , and Yb^3＋ co-doped NaYF4 prepared by the hydrothermal method exhibits a hexagonal nanocrystal.     

Synthesis and Upconversion Luminescence of LaF3 ： Yb^3＋ , Er^＋/SiO2 Core/Shell Microcrystals

LaF3：Yb^3＋ , Er^＋ microcrystals were synthesized by a hydrothermal method, and then, the LaF3： Yb^3＋ , Er^＋ microcrystals were coated with silica. Phase identification of LaF3： Yb^3＋ , Er^＋ and LaF3： Yb^3＋ , Er^＋/SiO2 was performed via XRD. The TEM image showed that the size of LaF3： Yb^3＋ , Er^＋ was 150 nm and LaF3： Yb^3＋ , Er^＋/SiO2 presented clearly a core/shell structure with 20 nm shell thickness. The upconversion spectra of LaF3： Yb^3＋ , Er^＋ and LaF3： Yb^3＋ , Er^＋/SiO2 in solid state and in ethanol were studied with a 980 nm diode laser as the excitation source. The upconversion spectra showed that the silica shell had little effect on the properties of fluorescence of the LaF3：Yb^3＋ , Er^＋ microcrystals. At the same time, the green luminescence photo of LaF3： Yb3＋, Er3＋/SiO2 in the PBS buffer was obtained, which indicated that the LaF3： Yb^3＋ , Er^＋/SiO2 could be used in biological applications.     

Three-color upconversion luminescence of rare earth codoped oxyhalide tellurite glasses

The red, green, and blue upconversion properties of Er^3＋/Tm^3＋/Yb^3＋-codoped oxyhalide tellurite glasses were studied under 980 nm LD excitation. The intense red （657 nm）, green （530 and 545 nm）, and blue （476 nm） emissions were simultaneously observed at room temperature. The results showed that the mixed halide modified tellurite glass （TZFCB） had strong upconversion emissions. The effect of halide on upconversion intensity was observed and discussed, and possible upconversion mechanisms were evaluated. The intense red, green, and blue upconversion luminescence of Er^3＋/Tm^3＋/Yb^3＋-codoped oxyhalide tellurite glasses might be a potentially useful material for developing three-dimensional displays applications.     

Influence of B2O3 on Spectroscopic Properties of Er^3＋/Yb^3＋ Co-Doped Tungsten-Tellurite Glasses

A series of novel Er^3＋/Yb^3＋ co-doped （85- x ） TeO2-15WO3-xB2O3 （TWB;x=2%,5%,8%（mole fraction） ） glasses were prepared. Influence of B203 on the spectroscopic properties of Er^3＋/Yb^3＋ co-doped tungsten-tellurite glasses were investigated. It is found that the intensity of 1.5μm fluorescence, lifetime of the ^4I13/2 level and upconversion fluorescence all decrease with the increase of B2O3 content. The product of full width at half maximum （FWHM） and stimulated emission cross-section （σe^peak） of Er^3＋ ：^4I13/2→^4I15/2 transition has an optimum when B203 is 5% （mole fraction）. The emission spectra of Er^3＋ ： ^4I13/2→^4I15/2 transition was analyzed using peak-fit routine, and an equivalent four-level system was proposed to estimate the stark splitting for the 411512 and ^4I13/2 levels of Er^3＋ ions in TWB glasses at room temperature.     

Optical Spectroscopy of Er^3＋ and Er^3＋/Yb^3＋ Co-doped Bi2O3-GeO2-B2O3-ZnO Glasses

The （60 - x）Bi2O3 - xGeO2-30B2O3-10ZnO （x = 5, 10, 20, 30 molar percent） glasses doped with Er^3＋ and Er^3＋/Yb^3＋ were fabricated using the melting method. The thermal stability of the glasses was studied with their DTA curves. The results show that the difference between the glass transition temperature and the crystallization onset temperature increases with the increase of GeO2 content, indicating that the thermal stability of the glass has become better. The absorption spectra were recorded and the stimulated emission cross sections were calculated using the McCumber theory. The Ω2, O4, and Ω6 parameters,the transition probability, the radiative lifetime, and the fluorescence branch ratio of Er^3＋ for optical transition were calculated from their absorption spectra in terms of reduced matrix U^（t）（λ = 2, 4, 6） character for optical transitions. The infrared emission of Er^3＋ was measured upon excitation with 970 nm light and the full width at half-maximum （FWHM） was estimated from the emission spectra. The pumping efficiency and the intensity of the emission at the 1.54 μm band of Er^3＋ were enhanced considerably by co-doping Yb^3＋ .     

Spectroscopic Properties and Effect of Radiation Trapping of a New Er^3＋/Yb^3＋ Co-Doped Tellurite-Silicate Glasses

A new serials of Er^3＋/Yb^3＋ co-doped tellurite-silicate glasses were prepared by the technique of high-temperature mehing. The thermal stability, absorption spectra, emission spectra and upconversion spectra were measured and investigated. It is found that these kinds of glasses have good thermal stability, broad FWHM and large stimulated emission cross-section. The three upconversion emission at 525, 546, 658 nm, corresponding to the ^2H11/2→^4Ⅰ15/2, ^4S3/2→4^Ⅰ15/2 and ^F9/2→^4Ⅰ15/2 transitions of Dr^3＋ ions,     

Up-conversion luminescence properties of Yb^3＋ and Ho^3＋ co-doped Bi3.84W0.16O6.24 powder synthesized by hydrothermal method

Using polyethylene glycol (PEG) as the surfactant, Bi3.84W0.16O6.24 up-conversion luminescence nano-crystal co-doped with Yb3+ and Ho3+ ions was synthesized by the hydrothermal method. The structure and properties of luminescence powder were studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). It was of cubic system when the sample was synthesized at a low temperature and the particle size was about 30 nm. The results showed that adding surfactants was useful to improve the powder agglomeration and the grain crystal was spherical. The green emission peak at 546 nm and red emission peak at 655 nm, corresponding to the ( 5F4, 5S2)→ 5 I 8 and the 5 F 5 → 5 I 8 transitions of Ho 3+ , respectively, were simultaneously observed at room temperature under excitation of 980 nm semiconductor laser. The up-conversion luminescence intensity was the strongest when the concentration ratio of Yb3+ /Ho3+ was 6:1 and the concentration of Ho 3+ ion was 1.5 mol.%. The up-conversion mechanism was also studied. The green and red emission peaks were the two-photon absorption according to the relationship between the pump power and the luminescence intensity.     

Spectroscopic investigations on Er3+/Yb3+-doped oxyfluoride glass ceramics containing YOF nanocrystals

Spectroscopic properties of Er3+/Yb3+-doped transparent oxyfluoride borosilicate glass ceramics containing YOF nanocrystals were systematically investigated. X-ray diffraction (XRD) confirmed the formation of YOF nanocrystals in the glassy matrix. Based on the Judd-Ofelt theory, the intensity parameters Ωi (i=2, 4, 6), spontaneous emission probability, radiative lifetime, radiative quantum efficiency and the effective emission bandwidth were investigated. The upconversion luminescence intensity of Er3+ ions in the glass ceramics increased significantly with the increasing crystallization temperature. The transition mechanisms of the green and red upconversion luminescence were ascribed to a two-photon process, and the blue upconversion luminescence was a three-photon absorption process.     

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.     

Three-photon-excited fluorescence of Tb-doped CaO-Al2O3-SiO2 glass by femtosecond laser irradiation

A near infrared to visible blue, green, and red upconversion luminescence in a Tb^3＋-doped CaO-Al2O3-SiO2 glass was studied, which was excited using 800 nm femtosecond laser irradiation. The upconversion luminescence was attributed to ^5D3→^7F5, ^5D3→^7F4, ^5D3→^7F3, ^5D4→^7F6, ^5D4→^7F5, ^5D4→^7F4, and ^5D4→^7F3 transitions of Tb^3＋. The relationship between upconversion luminescence intensity and the pump power indicated that a three-photon simultaneous absorption process was dominant in this upconversion luminescence. The intense red, green, and blue upconversion luminescence of Tb^3＋-doped CaO-Al2O3-SiO2 glass may be potentially useful in developing three-dimensional display applications.     

Up-conversion luminescence of Er~（3＋）-doped and Yb~（3＋）/Er~（3＋） co-doped 12CaO·7Al_2O_3 poly-crystals

The optical properties of Er3+-doped and Yb3+/Er3+ co-doped 12CaO·7Al2O3 (C12A7) poly-crystals, synthesized by high temperature solid state method, were investigated in detail. For Er3+-doped and Yb3+/Er3+ co-doped C12A7 poly-crystals, two main emission bands centered around 530/550 nm (green) and 660 nm (red) were observed under 980 nm diode laser excitation via an up-conversion process. The intensity of green up-conversion emission had a strong increase in Er3+ (1.0 mol.%, 1.5 mol.%, 3.0 mol.%), and the intensity ratio of red to green up-conversion emission had an increase in Yb3+ (1.0 mol.%, 2.0 mol.%, 10. 0 mol.%)/Er3+ (fixed at 1.0 mol.%). This detailed study of the up-conversion processes allowed us to identify the dominant up-conversion mechanisms in Er3+-doped and Yb3+/Er3+ co-doped C12A7 poly-crystals.     

Intense upconversion luminescence of Er3+/Yb3+ codoped oxyfluoride borosilicate glass ceramics containing Ba2GdF7 nanocrystals

Er3+/Yb3+-codoped transparent oxyfluoride borosilicate glass ceramics containing Ba2GdF7 nanocrystals were prepared and spectroscopic properties of rare earth ions were investigated.Fluoride nanocrystals Ba2GdF7 were successfully precipitated in glass matrix,which was confirmed by X-ray diffraction(XRD)and transmission electron microscopy(TEM)results.In comparison with the as-made precursor,significant enhancement ofupconversion luminescence was observed in the Er3+/Yb3+codoped oxyfluoride glass ceramics,which may be due to the variation of coordination environment around Er3+and Yb3+ions after crystallization.The transition mechanisms of the green and red upconversion luminescence were ascribed to a two-photon process,and that of the blue upconversion luminescence was a three-photon process.     

Characterization of Y2O2S： Eu^3＋, Mg^2＋, Ti^4＋ Long-Lasting Phosphor Synthesized by Flux Method

Long-lasting phosphor Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ was synthesized by a flux method and their luminescence properties were investigated. The result indicates that the unit cell parameter c is linearly increased with the increase of Eu₂O₃ content in Y₂O₂S: Eu_x³⁺ (0.01 ≤ x ≤ 0.10). On the other hand, the change of unit cell parameter a is not linear dependence. In the Y₂O₂S: Eu³⁺ crystal structure, Eu³⁺ ions only replaced Y³⁺ ions' places in which it posited center position of c axis. With the increase of Eu₂O₃ content, the position of the strongest emission peak changed from 540 nm (⁵D₁→⁷F₂ transition) to 626 nm (⁵D₀→⁷F₂ transition), and the maximum intensity was obtained when x = 0.09 in Y₂O₂S: Eu_x³⁺ (0.01 ≤ x ≤ 0.10). This is due to the environment of trivalent europium in the crystal structure of Y₂O₂S. Doping with Mg²⁺ or Ti⁴⁺ ions alone cannot get the good long-lasting afterglow effect, whereas co-doping with Mg²⁺ and Ti⁴⁺ ions and excited with 365 nm ultraviolet light, a strong thermoluminesence peak appeared, red and orange long-lasting phosphorescence (LLP) was also observed and the phosphorescence lasted nearly 3 h in the light perception of the dark-adapted human eye (0.32 mcd · m⁻²). Thus the LLP mechanism was analyzed.     

Green,orange, and red upconversion luminescence in Nd^3＋/Yb^3＋： Cs2NaGdCl6 powders under 785 nm laser excitation

Nd^3＋：Cs2NaGdCl6 and Nd^3＋, Yb^3＋：Cs2NaGdCl6 polycrystalline powder samples were prepared by Morss method E. Under 785 nm semiconductor laser pumping, the upconversion luminescence of Nd^3＋ ions in Cs2NaGdCl6 was investigated at room temperature, and three upconversion emissions near 538 nm （Green）, 603 nm （Orange）, and 675 nm （Red） were observed and assigned to ^4G7/2→^4I9/2, （^4G7/2→^4I11/2; ^4G5/2→^4I9/2）, and （^4G7/2→^4I13/2; ^4G5/2→^4I11/2 ）, respectively. The dependences of these upconverted emissions on laser power and Nd^3＋ ion concentration were investigated, to explore the upconversion mechanism. The effect of doping Yb^3＋ ions on the upconversion luminescence of Nd^3＋ in Cs2NaGdCl6 was also studied under 785 nm laser excitation. The energy transfer processes were discussed as the possible mechanism for the above upconversion emissions.     

Vacuum ultraviolet excited photoluminescence properties of Gd2O2CO3：Eu^3＋ phosphor

The Gd2O2CO3：Eu^3＋ with type-Ⅱ structure phosphor was successfully synthesized via flux method at 400 ℃ and their photoluminescence properties in vacuum ultraviolet （VUV） region were examined. The broad and strong excitation bands in the range of 153-205 nm owing to the CO3^2- host absorption and charge transfer （CT） of Gd^3＋-O2^- were observed for Gd2O2CO3：Eu^3＋. Under 172 nm excitation, Gd2O2CO3：Eu^3＋ exhibited strong red emission with good color purity, indicating Eu^3＋ ions located at low symmetry sites and the chromaticity coordination of luminescence for Gd2O2CO3：Eu^3＋ was （x=0.652, y=0.345）. The photoluminescence quenching concentration of Eu^3＋ excited by 172 nm for Gd2O2CO3：Eu^3＋ was about 5%. Gd2O2CO3：Eu^3＋ would be a potential VUV-excited red phosphor applied in mercury-free fluorescent lamps.     

Spectral Properties and Upconversion Luminescence of Er^3＋, yb^3＋： BaWO4 Crystal

The optical quality of Er^3＋, yb^3＋： BaWO4 crystal was gown by Czochralski method. Absorption spectra were measured and energy levels were assigned. According to Judd-Ofelt theory, the spectral strength parameters of Er^3＋ ion were fitted to beΩ2 =0.3926 x 10^-20 cm^2, Ω4 =0.0721×10^-20 cm^2, Ω6 =0.0028 ×10.20 cm^2. Emission peaks centered at around 523,544 and 670 nm were observed under 334 nm He-Cd laser excitation and emission peaks centered at 1001 and 1534 nm were detected under 976 nm laser excitation. Strong green emission was also observed when the crystal was pumped with 808 nm and 976 nm laser. The mechanisms of frequency upconversion and sensitization were analyzed.     

Preparation and Characterization Upconversion Luminescence Material of Yttrium Aluminate Doped with Rare Earth

Low phonon energy yttrium aluminate was adopted as matrix and the upconversion materials yttrium aluminate co-doped with Yb³⁺ and Er³⁺ was synthesized by solid-state reaction method. The X-ray diffraction spectra and the upconversion emission spectra of various samples were measured and the effect of Er³⁺ concentration, sensitization, fluxing agent and calcining temperature on the luminescence properties was studied. Research results showed that the sample could emit green light at 543 and 570 nm when excited 980 nm laser; the optimum concentration of Er³⁺ should be 1.0%; sensitization of Yb³⁺ could enhance luminescent effects of Er³⁺ obviously; the optimum content of F₃BO₃ fluxing agent should be 8% and the optimum calcining temperature should be 1500 °C.     

Luminescent properties of Ca2SiO4：Eu^3＋ red phosphor for trichromatic white light emitting diodes

The luminescent properties of Eu^3＋doped Ca2SiO4 red phosphors synthesized by the flux fusion reaction method were investigated. It was found that the excitation spectrum included two regions： the weak excitation band below 325 nm and strong narrow peaks above 325 nm. The main peak of the excitation band was located at 400 nm. The peaks located at 290 nm were assigned to the combination of the charge transfer transition of O-Eu, peaks above 325 nm （325, 385, 400, 470, 511, and 539 nm） were assigned to the f-f transitions of Eu^3＋. The emission spectrum was dominated by the red peak located at 612 nm due to the electric dipole transition of ^5D0-^7F2. In addition, the effects of the Eu^3＋ content and charge compensators of Li^＋, Na^＋, K^＋, and Cl^- ions on the emission intensity were investigated. The experiment results suggested that the strongest emission was obtained when the concentration of the Eu^3＋ ions was 0.3 mol^-1, and Li^＋ ions gave the best improvement to enhance the emission intensity. Ca2SiO4：Eu^3＋, Li^＋ was thus suitable for low-cost trichromatic white light emitting diodes （WLED） based on UV InGaN chip.     

Colour modulation and enhancement of upconversion emissions in K2NaScF6:Yb/Ln (Ln = Er,Ho, Tm) nanocrystals

Effective colour modulation of upconversion emissions in lanthanide-doped nanomaterials becomes even more important for fundamental and applied research. Herein, on the one hand, by raising the content of doped Yb³⁺ from 10 mol% to 50 mol%, a significant increase of the red/green emission ratio from 4.0 to 68.2 is observed in K₂NaScF₆:Yb/Er nanocrystals. This yellow to red colour change is attributed to the increased cross relaxation between Er³⁺ and Yb³⁺ caused by the increased Yb³⁺ amount, ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺). On the other hand, the upconversion green and red emission of K₂NaScF₆:Yb/Er (20/2 mol%) nanocrystals are intensified 10.6 and 8.8 folds, respectively, after an active shell (K₂NaScF₆:Yb) is epitaxially grown, which are more effective than the 7.4- and 6.4-fold enhancement from an inert shell (K₂NaScF₆) growth. Moreover, the shell thickness from 2.85 to 9.5 nm through controlling the molar ratio of shell-precursor to core from 1:2 to 3:1 can be easily realized. This study will provide more opportunities for the application of K₂NaScF₆:Yb/Ln nanoparticles in varied fields such as theranostics, photovoltaics, and photocatalysis.