Preparation and characterization of upconversion nanocomposite for β-NaYF_4:Yb~(3+),Er~(3+)-supported TiO_2 nanobelts

Hexagonal NaYF4:Yb3+,Er3+ (β-NaYF4:Yb3+,Er3+) nanoparticles supported on TiO2 nanobelts were prepared using two-step pro- cedures of ion-exchangeable process and hydrothermal treatment: layered titanate nanobelts were first ion-exchanged with Y3+, Yb3+ and Er3+ cations to produce titanate nanobelts with these cations, and then, the product nanobelts in NaY solution were treated under hydrothermal con- dition to transform into anatase TiO2 nanobelts supported with β-NaYF4:Yb3+,Er3+ nanoparticles. The final products were characterized by various measurement techniques. Many preparation conditions, e.g., the content of added precursors, reaction time and pH value, were optimized to obtain nanocomposites including relatively uniform nanoparticles with strong upconversion (UC) emission intensity. The measurement results showed that the anatase TiO2 nanobelts, supported by the large number of β-NaYF4:Yb3+,Er3+ nanoparticles with 30–70 diameter range and very strong UC emission, could be achieved under suitable preparation condition. The UC emission intensity of the nanoparticles with the concentrations of Yb3+ and Er3+ closer to 20 mol.% and 1 mol.% was the strongest.     

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.     

Controlled synthesis and luminance properties of lanthanide-doped β-NaYF4 microcrystals

Hexagonal Yb3+,Er3+-doped Na YF4 crystals with different morphologies were synthesized by a facile hydrothermal method at 140–200 oC. Their shape and size could be controlled by varying hydrothermal temperature, time and doping effect of Ce3+ ions. Interestingly, the products displayed hexagonal sheet, plate, crown-like prism to tube changes in the temperature range from 140 to 200 oC. A combination of "diffusion-controlled growth" and "selective adsorption" was proposed to understand the formation of the Na YF4 crystals with well-defined shapes. Na YF4:Yb3+/Er3+ crystals exhibited shape-dependent up-conversion(UC) luminescence under excitation of 980 nm, and their luminescence property could be tuned by doping Ce3+ ions.     

Hydrothermal Preparation and Luminescence of LaF3：Eu^3＋ Nanoparticle

Pure andEu^3＋-doped LaF3 nanoparticle were prepared by a hydrothermal process at a low temperature and characterized by X-ray diffraction （XRD）, transmission electron microscopy （TEM） and fluorescence spectra. Well-dispersed nanopartiele with an average size of 30 nm and a hexagonal shape were synthesized. The effects of temperature and reaction time on the preparation of nanoparticle were investigated, and the growth mechanism of nanoparticle was briefly discussed. The effects of Eu^3＋-doped concentration, the calcination time and temperature were also investigated. An optimal Eu^3 ＋-doped concentration of 5% is found. Calcination temperature and prolonged of the nanoparticle at high time greatly reduce the fluorescence intensity. The sample calcinated at 600 ℃ for 6 h emits the strongest fluorescence.     

Energy transfer and frequency upconversion in Er~（3＋）-Yb~（3＋） codoped oxy-fluoro-tungstosilicate glasses

Er3+-Yb3+ codoped oxy-fluoro-tungstosilicate glasses with infrared-to-visible frequency upconversion luminescence were prepared by melting quenching in air.The effects of Er3+ doping on the optical properties of the samples were measured by means of techniques such as optical absorption spectra and photoluminescence spectra.The results showed that intense green and red signals centered at 546 and 665 nm,corresponding to the 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ by a multiphoton stepwise phonon-assisted excited-state absorption process,respectively,were simultaneously observed by exciting the samples with a diode laser operating at 980 nm at room temperature.The upconversion process was found very sensitive to Er3+ content at a constant Yb2O3 content of 5 mol.%.With the increase of Er3+ content from 0.5% to 1.5%,the upconversion intensity increased gradually.Further increasing of Er3+ content to 3.0% resulted in a significant fluorescence quenching.Moreover,the possible upconversion mechanisms were discussed based on the energy-matching conditions and the quadratic dependence on excitation power.     

Pressure and temperature dependent up-conversion properties of Yb3＋-Er3＋ co-doped BaBi4Ti4015 ferroelectric ceramics

Yb^3＋ and Er^3＋ co-doped BaBi4Ti4015 （BBT） ceramic samples showed brighter up-conversion photoluminescence （UC-PL） under excitation of 980 nm. The monotonous increase of fluorescence intensity ratio （FIR） from 525 to 550 nm with temperature showed that this material could be used for temperature sensing with the maximum sensitivity to be 0.0046 KI and the energy dif- ference was 700 cm-1. Moreover, the sudden change of red and green emissions around 400 ℃might imply a phase transition. With increasing pressure up to 4 GPa, the PL intensity decreased but was still strong enough. These results illustrated the wide applications of BBT in high temperature and high pressure conditions.     

Effect of Yb203 Additive on Transformation Behavior of Anatase for TiO2/（ O＇ ＋ β＇ ）-Sialon Multi phase Ceramics

TiO2/（O＇ ＋ β＇）-Sialon multiphase ceramics were prepared with nano TiO2 （anatase） powder and （O＇ ＋ β＇）-Sialon powder as raw materials. Effect of Yb2O3 additive on transformation behavior of anatase for TiO2/（O＇ ＋ β＇）-Sialon multi phase ceramic was investigated and its influence mechanism was discussed. XRD was employed for the analysis of phase composition and lattice parameters. The results show that even though Yb2O3 has no obvious influence on starting temperature of phase transformation, it significantly accelerates the transformation process, which displays a weakened effect with more Yb2O3 addition. There exist two forms of the added Yb2O3 ： some enters TiO2 lattice and the other deposits on the surface of TiO2. The function of Yb2O3 on phase transformation of anatase can be attributed to the coaction of active and negative influence mechanisms as follows： some Yb^n＋ enter TiO2 lattice and replace Ti^4＋ , as well as the redox reaction between Yb^3＋ and TiO2, which promote the transformation, whereas other Yb2O3 deposits on the surface of TiO2, and Ti- O-Yb bond is formed by the coaction of Yb^3＋ and TiO2, which inhibit the process.     

Pressure and temperature dependent up-conversion properties of Yb~(3+)-Er~(3+) co-doped BaBi_4Ti_4O_(15) ferroelectric ceramics

Yb3+ and Er3+ co-doped BaBi4Ti4O15(BBT) ceramic samples showed brighter up-conversion photoluminescence(UC-PL) under excitation of 980 nm. The monotonous increase of fluorescence intensity ratio(FIR) from 525 to 550 nm with temperature showed that this material could be used for temperature sensing with the maximum sensitivity to be 0.0046 K–1 and the energy difference was 700 cm–1. Moreover, the sudden change of red and green emissions around 400 oC might imply a phase transition. With increasing pressure up to 4 GPa, the PL intensity decreased but was still strong enough. These results illustrated the wide applications of BBT in high temperature and high pressure conditions.     

Up-conversion of Er3＋/yb3＋ co-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals

The up-conversion of Er3+/Yb3+co-doped transparent glass-ceramics 50SiO2-10AlF3-5TiO2-30BaF2-4LaF3-0.5ErF3-0.5YbF3 containing Ba2 LaF7 nanocrystals under the changing of heat treatment temperature and time were investigated.The Ba2 LaF7 nanocrystals precipitated from the glass matrix was confirmed by X-ray diffraction(XRD).The structural investigation carried out by XRD and transmission electron microscopy(TEM) evidenced the formation of cubic Ba2 LaF7 nanocrystals with crystal size of about 14 nm.Comparing with the samples before heat treatment,the high efficiency up-conversion emission of Er3+/Yb3+co-doped samples was observed in the glass-ceramics under 980 nm laser diode excitation.The increase in red emission intensity bands was stronger than the green bands when the crystal size increased.The mechanism for the up-conversion process in the glass-ceramics and the reasons for the increase of Er3+/Yb3+co-doped up-conversion intensity after heat treatment were discussed.     

Synthesis of Spherical （Y, Gd） BO3 ： Eu^3＋ Phosphor Using W/O Emulsion System

Spherical （Y, Gd）BO3：Eu^3＋ phosphor particles with a narrow size distribution（2 -4 μm） was obtained by firing the Y-Gd-Eu-BO3 precursor prepared in a W/O style emulsion system. In the W/O emulsion system, kerosene, used as oil phase, was mixed with Span 80 and Tween 80 compounds which were employed as the emulsifier with an HLB （hydrophile-lipophile balance） value of 5.2- 5.3. Both rare earths （Y, Gd and Eu） nitrate and boric acid solution or ammonia solution were used as aqueous phase. The synthesis conditions, such as emulsion composition, emulsifying style, precipitation reaction process, reaction temperature, morphology control, and so on, were investigated, and the optimum synthesis conditions for preparing spherical （Y, Gd）BO3：Eu^3＋ phosphor was obtained. The phosphor was characterized by XRD, SEM, laser particle size analysis, emission and excitation spectrum under vacuum ultraviolet （VUV）, and so on. The phosphor synthesized using the water-in-oil emulsion method with median diameter （D50） of 2 - 4 μm shows agreeable photoluminescence （PL） property and sphericity. The main emission peak appears at about 593 nm, which corresponds to ^5D0→^7F1 transition （magnetic-dipole transition） of the Eu^3＋ ion. The cell parameters and powder diffraction data were indexed. The structure of the phosphor belongs to the hexagonal system with space group P63/m.     

NaYF4:Yb,Er材料的制备及其上转换发光性能

通过沉淀法制备Yb3+和Er3+共掺的NaYF4上转换发光材料,考察了主要制备工艺条件对材料上转换发光性能的影响,并探讨了其上转换发光的机理。利用x射线衍射(XRD)、透射电镜(TEM)、荧光光谱仪对样品进行了表征。结果表明,Yb3+和Er3+的掺杂浓度分别15%(摩尔分数,下同)、3%,焙烧温度为600℃时可得到发光性能较好的六方相NaYF4:Yb,Er上转换材料,其主要上转换途径是Yb3+和Er3+之间的能量转移,且Er3+的红、绿光发射均为双光子过程。     

Up-Conversion Emission of Er3+ Ions in Yb3+ Sensitized Oxide Crystals

Most of the up-conversion lasers operated at room temperature are realized with heavy metal fluorides, In this paper the Judd-Ofelt parameters Ωλ ( λ = 2,4,6 ) were calculated for Er3+ ions in Yb3 + sensitized LiNbO3 and YVO4 crystals at room temperature, together with the radiative transition probabilities, non-radiative transition probabilities and resonant transition probabilities of Er3+ ions. Taking into account the energy transfer from Yb3 + to Er3 +, the rate equations are given for Er3 + ions. We obtained from a solution of the rate equations that Yb3 + sensitized YVO4 crystal is more efficient than Yb3 + sensitized LiNbO3 crystal in the up-conversion of 550 nm of Er3+ emission, which is consistent with our observation.     

NaYF4: Yb,Er@SiO2的高温相变及上转换

通过固-液热分解法合成平均尺寸为27 nm的六方相NaYF₄: Yb, Er上转换纳米晶。制备了NaYF₄: Yb, Er@SiO₂核壳纳米粒子, 壳层厚度约为8 nm。将NaYF₄: Yb, Er@SiO₂分别在不同高温下焙烧3 h, 发现600 ℃下产物物相未发生变化, 但向非晶态过渡; 700、800 ℃下NaYF₄内核与SiO₂壳层发生化学反应, 产物物相均转变为NaYSiO₄。样品均用980 nm红外激光进行激发, 显示出强烈的可见光上转换发射。以初始NaYF₄: Yb, Er@SiO₂的上转换发光强度为对比样, 随着焙烧温度的升高, 样品的发光强度依次提高。这与理论上低声子能量的氟化物发光强度更高不符, 可能原因是高温处理后样品去除了部分晶格缺陷和有机杂质。      

Size dependence of up-conversion luminescence of NaYF₄:Yb³⁺/Tm³⁺

Tm3+/Yb3+ codoped NaYF4 microcrystals were synthesized using a hydrothermal method.The bright upconversion light was observed under 980 nm excitation.The upconversion luminescence was systematically investigated at different Yb3+ concentrations and different reaction temperatures and time.The sample with 60% Yb3+ concentration and reacting at 180 oC for 24 h possessed the highest luminescent efficiency.The higher luminescent efficiency was contributed to a large surface area.The large surface area induced the large vibration mode by absorbed H2O and CO2.The larger vibration mode could enhance the energy transfer efficiency from the excited Yb3+ to Tm3+ by the process of phonon assisted energy transfer.     

Upconversion luminescence ofNaYF4：Yb,Er nanocrystals with high uniformity

The NaYF4：Yb,Er nanocrystals were synthesized via the thermal decomposition ot metal oleate precursors, lhe nanocrys- tals in hexagonal structure were highly uniform and in size of 25 nm. The bright upconversion luminescence was observed under the excitation of 980 nm laser and the upeonversion emission spectra were investigated at different pump powers. The emission intensity ratio of red light to green light linearly increased with pump power increasing. This result indicated that there existed a large threshold power of saturation pump for the first excitation state in NaYFa：Yb,Er nanocrystals comparing to that in bulk material.     

Luminescence properties of Tm^3＋/Yb^3＋, Er^3＋/Yb^3＋ and Ho^3＋/Yb^3＋ activated calcium tungstate

CaWO4 phosphor activated by the Tm3+/Yb3+,Er3+/Yb3+ and Ho3+/Yb3+ ions were synthesized by a traditional high-temperature solid-state method.The crystal structures and morphologies of the products were characterized by X-ray powders diffraction method(XRD) ,infrared spectra(FT-IR) and scanning electron microscopy(SEM) .The samples were found to show up-conversion luminescence properties.CaWO4 doped with Tm3+/Yb3+ showed blue luminescence characteristic of Tm(III) ion in the range of 460-485 nm,corresponding to the 1G4→3H6 electronic transition.CaWO4 doped with Er3+/Yb3+ showed strong green luminescence at 510-565 nm(2H11/2,4S3/2→4I15/2) and weak red luminescence at 640-685 nm(4F9/2→4I15/2) of Er(III) ion.CaWO4 doped with Ho3+/Yb3+ phosphor emitted green luminescence at 525-560 nm(5S2,5F4→5I8) and red luminescence at 630-670 nm(5F5→5I8) and at 730-770 nm(5S2,5F4→5I7) ,which is the characteristic of Ho(III) ion.     

水溶性NaYF4：Yb，Er上转换发光纳米粒子的可控合成与发光性能研究

采用溶剂热法合成出尺寸小于30 nm的NaYF4：Yb, Er上转换纳米晶,用柠檬酸（CA）做表面活性剂使得产物具有良好的水溶性。实验探索得出NaOH或者CA的加入量能够调节溶液中能与稀土离子络合的有效配位基团-COO-的浓度,进而影响NaYF4从立方相到六方相的相变过程、实现产物形貌的控制和上转换发光性能的调变。逐渐提高NaOH的加入量,产物从30 nm的α-NaYF4：Yb, Er纳米粒子转变为尺寸300 nm 的β-NaYF4：Yb, Er 微球。提高CA 的加入量同样会促进β-NaYF4：Yb, Er微球的形成。还研究上述不同合成条件下,不同形貌结构的产物的上转换发光性能。     

Controlled synthesis and tunable luminescence of NaYF₄:Eu³⁺

High quality NaYF4:Eu3+ luminescent materials were successfully synthesized via a facile template technique by hydrothermal method.The samples were characterized by X-ray powder diffraction(XRD),transmission electron microscopy(TEM) and fluorescence spectroscopy(FS).The incorporating of Eu3+ ions into NaYF4 crystal lattice influenced the symmetry types of NaYF4 crystals,resulting in phase transformation of NaYF4 crystals between α and β phase.The pure hexagonal phase of branched NaYF4:Eu3+ was obtained as the Eu3+ concentration reached 15 mol.%.In addition,the luminescence color was tuned by changing the doping concentration of Eu3+ ions.     

Polarized spectral analysis of Er3+ ions in Er3+/yb3+:LiLa(MoO4)2 crystal

Optical characteristics and upconversion dynamics of Er3+ in Er3+/Yb3+:LiLa(MoO4)2 crystals were investigated. The absorption spectra, fluorescence spectra and the fluorescence decay curves were analyzed at room temperature. The infrared emission at 1538 nm and visible emissions at 520–569 and 640–670 nm, corresponding to 2H11/2,4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ ions, were simultaneously observed in Er3+/Yb3+:LiLa(MoO4)2 crystals under 976 nm excitation at room temperature. The maximal emission cross-section near 1530 nm for π-polarization was 0.63×10–20 cm2 and the measured lifetime of 4I13/2 was 4.88 ms. The upconversion process was involved in sequential two-phonon processes, either the energy transfer from Yb3+ ions or by the excited state absorption.