Optical high temperature sensor based on enhanced green upconversion emissions in Er3+-Yb3+-Li+ codoped TiO2 powders

The Er3+-Yb3+-Li+ codoped TiO2 powders have been prepared by sol-gel method. The strong enhancement of green and red upconversion emissions were obtained for Er3+-Yb3+ codoped TiO2 by additional Li+ codoping and investigated using 976 nm semiconductor laser diode excitation. The enhanced upconversion emissions by the addition of Li+ resulted from the formation of Li compound with lower crystal field symmetry. The fluorescence intensity ratio (FIR) of green upconversion emissions from the transitions of 2H(11/2) --> 4I(15/2) and 4S(3/2) --> 4I(15/2) of Er3+ in the Er3+-Yb3+-Li+ codoped TiO2 has been studied as a function of temperature in the range of 300-925 K, and the maximum sensitivity was determined to be 0.0025 K(-1). Er3+-Yb3+-Li+ codoped TiO2 material with the highest operating temperature up to 925 K, has higher temperature sensitivity and fluorescence efficiency being a promising candidate for applications in optical high temperature sensor.     

An efficient and user-friendly method for the synthesis of hexagonal-phase NaYF(4):Yb, Er/Tm nanocrystals with controllable shape and upconversion fluorescence

Hexagonal-phase NaYF(4):Yb, Er/Tm nanocrystals are the best IR-to-visible upconverting materials to date, but user-friendly methods for making pure hexagonal-phase NaYF(4):Yb, Er/Tm nanocrystals with upconversion fluorescence are still lacking. Most of the methods reported so far require excess fluoride reactants in a high-temperature reaction which are very unfriendly to users and raise safety concerns. In this work, an efficient and user-friendly method was developed for the synthesis of uniform hexagonal-phase NaYF(4):Yb, Er/Tm nanocrystals with upconversion fluorescence, by forming small solid-state crystal nuclei and further growth and ripening of the nuclei. NaYF(4):Yb, Er/Tm nanoplates, nanospheres and nanoellipses were also selectively produced by varying the concentration of the surfactant. All the nanocrystals showed strong upconversion fluorescence, and fluorescence from the nanoplates was observed even when the laser power density was reduced to about 50?mW?cm(-2). These nanocrystals have great potential for use in biology and medicine as fluorescent labels or imaging probes.     

NaYF4:Er3+六棱柱的水热合成及上转换发光性能

以氟化钠、硝酸钇、硝酸铒为原料,利用水热法合成NaYF4:Er3+材料。利用X射线粉末衍射仪(XRD)、场扫描电子显微镜(SEM)、红外吸收(FT-IR)以及发光光谱等手段对产物的物相结构、形貌和荧光性能进行分析。结果表明,NaYF4:Er3+为六角棱柱晶体,属于六方晶系,具有P63/m(176)空间点群结构。在980nm光激发下,NaYF4:Er3+展现出强的上转换光,波长在520nm和539nm为绿光发射,对应为Er3+离子的2 H11/2→4/I15/2和4S3/2→4/I15/2跃迁发射,而652nm为红光发射,则对应于Er3+离子的4F9/2→4/I15/2跃迁发射。     

Controlled synthesis and up-conversion emission of rare-earth tri-doped NaYF4 nanocrystals under femtosecond-laser excitation

Cubic nanocrystal and hexagonal micro-rods NaYF4, with predictable size, shape and phase, have been successfully synthesized through hydrothermal reaction. The growth mechanism and the effect of mass transfer on the morphology of hexagonal micro-prism are both discussed in detail. The increase of tri-doping lanthanide ion concentration decreased the size of crystal particle, which was explained by the Arrhenius rate equation together combined with the Gibbs-Thomson relationship. Furthermore, the dopants did not only affect the sizes of tri-doped NaYF4 micro-rods, but also impacted upon fluorescence intensity. The fluorescence of tri-doped NaYF4: Nd3+/Yb3+/Er3+ system, excited by an 800 nm femtolaser, was intensified with the increase of doped lanthanide ions concentration. Nevertheless owing to the fluorescence quenching, the other two systems (NaYF4: Nd3+/Ho3+/Er3+ and NaYF4: Nd3+/Tm3+/Er3+) did not show the same phenomenon.     

纤维状中空结构NaYF4:Yb3+, Er3+上转换材料的 合成及防伪应用

采用水溶性聚合物聚乙烯亚胺(PEI)调介下的水热法,一步合成了具有纤维状中空结构的六方相NaYF4:Yb3+,Er3+上转换荧光材料,并将其作为荧光填料,通过流延成膜法制备了具有上转换荧光性能的壳聚糖/聚乙烯醇(CS/PVA)荧光复合薄膜.探究了PEI配体含量和反应体系pH值对合成的上转换材料的形貌、晶型和荧光性能的影响,以及壳聚糖/聚乙烯醇荧光复合薄膜中荧光填料的最佳掺杂量.研究结果表明,在PEI含量为0.3 g且反应体系pH=5的条件下,合成的产物为具有纤维状中空结构的六方相NaYF4:Yb3+,Er3+.荧光光谱表明,合成的NaYF4:Yb3+,Er3+上转换材料在980 nm激光激发下具有优异的荧光性能.当荧光填料的掺杂质量分数为3.0％时,制备的NaYF4:Yb3+,Er3+/(CS/PVA)荧光复合薄膜具有最佳的透明度和上转换荧光特性.     

Enhanced red upconversion luminescence in Er-Tm codoped NaYF4 phosphor

This paper presents a study on the enhanced red upconversion (UC) luminescence via efficient energy transfer (ET) between Er3+ and Tm3+ in Er-Tm codoped NaYF4 microtubes. Er doped and Er-Tm codoped NaYF4 UC hollow microtubes have been synthesized using a hydrothermal method. Under 1560 nm excitation from a diode laser, the Er doped NaYF4 microtubes emitted dominant green UC luminescence while the Er-Tm codoped NaYF4 microtubes emitted dominant red UC luminescence, which implies the energy transfer between Er3+ and Tm3+ plays a key role in the enhanced red UC emissions. The red UC luminescence is significantly enhanced compared with the green UC luminescence with the increase of Tm3+ doping concentration. In addition, our experimental results show that the UC luminescence properties under 980 nm excitation are almost identical with that under 1560 nm excitation. Furthermore, the possible ET mechanism was proposed on the basis of our experimental results.     

Synthesis of colloidal upconverting NaYF4: Er3+/Yb3+ and Tm3+/Yb3+ monodisperse nanocrystals

The synthesis, characterization, and spectroscopy of upconverting lanthanide-doped NaYF4 nanocrystals (NCs) is presented. The monodisperse cubic NaYF4 NCs were synthesized via a thermal decomposition reaction of trifluoroacetate precusors in a mixture of technical grade chemicals, octadecene and the coordinating ligand oleic acid. In this straightforward method, the dissolved precursors are added slowly to the reaction solution through a stainless-steel canula resulting in highly luminescent nanocrystals with an almost monodisperse particle size distribution. The NCs were characterized through the use of transmission electron microscopy, selected area electron diffraction, 1H NMR, powder X-ray diffraction, and high-resolution luminescence spectroscopy. The NaYF4 NCs are capable of being of dispersed in nonpolar organic solvents thus forming colloidally stable solutions. The colloids of the Er3+, Yb3+ and Tm3+, Yb3+ doped NCs exhibit green/red and blue upconversion luminescence, respectively, under 980 nm laser diode excitation with low power densities.     

NaYF_4∶Yb,Er/Tm上转换发光强度的影响因素

含有稀土离子的上转换发光材料因具有巨大的应用价值而受到人们的广泛研究,特别是六方相NaYF4已被公认为是迄今为止发光强度最大的上转换基质材料.以稀土离子Yb和Er或Yb和Tm的共掺杂NaYF4上转换材料为研究对象,讨论了几种不同因素对其上转换发光强度的影响,并对这种上转换材料的应用与研究前景提出了几点建议.     

Synthesis and luminescence properties of lanthanide (III)-doped YF3 nanoparticles

Synthesis process and luminescence properties of trivalent lanthanide ions (Ln3+) doped YF3 nanoparticles have been investigated. To synthesis Ln(3+)-doped YF3 nanoparticles, the mixture of (YCl3 x nH2O + LnCl3 x nH2O), and NH4F was hydrothermal treated at 180 degrees C in a Teflon-liner auto-clave or heated at higher temperatures (400 degrees C - 600 degrees C) in a stove. The XRD patterns showed that the Ln(3+)-doped orthorhombic YF3 nanoparticles with no second phase have been prepared. The solid solution Y(1-x)Eu(x)F3 (x = 0 - 0.4) nanoparticles have been synthesized. The luminescence concentration quenching resulted from resonance energy transfer between neighboring Eu3+ ions occurred at higher Eu3+ concentrations (30 mol%). The upconversion luminescence of Er(3+)-Yb3+ codoped YF3 nanoparticles under 980 nm excitation has also been observed. With increase of heated temperature, the size of the Er(3+)-Yb3+ codoped YF3 nanoparticles increased gradually, and upconversion luminescence intensity increased significantly.     

A strategy for simultaneously realizing the cubic-to-hexagonal phase transition and controlling the small size of NaYF4:Yb3+,Er3+ nanocrystals for in vitro cell imaging

Hexagonal-phase NaYF(4):Yb(3+),Er(3+) up-conversion nanocrystals (UCNCs) are synthesized by a combination of refluxing and hydrothermal treatment. This strategy leads to only a slight increase in particle size, from 4.5 to ca. 10 nm, during the cubic-to-hexagonal phase transition. The hexagonal UCNCs can be internalized by HeLa cells and exhibit visible emission in the cells under near-infrared excitation.     

K(+)-Na+ ion-exchanged waveguides in Er(3+)-Yb3+ codoped phosphate glasses using field-assisted annealing

Buried channel waveguides were fabricated in Er(3+)-Yb3+ codoped phosphate glasses with use of the thermal two-step ion-exhange method. The K(+)-Na+ ion-exchange process was first carried out in pure KNO3 molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The fluorescence properties of bulk glasses and channel waveguides were characterized, and the waveguide properties were studied. The formation mechanism of buried waveguides was analyzed, and the improved gain characteristics of the waveguide amplifiers with use of different FAA temperatures were measured. The maximum net gain of the waveguide amplifiers at 1.534-microns wavelength was measured to be approximately 3.4 +/- 0.1 dB/cm with 120-mW pump power at 975-nm wavelength.     

Study on NIR optical rewritable film based on dithienylethene and upconversion nanocrystals

Tm3+ and Yb3+ codoped NaYF4 upconversion (UC) nanoparticles (NPs) with intense ultraviolet (UV) fluorescence were synthesized using a solvothermal approach. NIR optical rewritable film incorporated with the UCNPs and dithienylethene (DTE) were performed for optical storage based on the photochromic reaction of DTE induced by the intense UV from themultiphoton UC fluorescence of NaYF4NPs. The photochromic DTE did not exhibit obvious fatigue after repetitious write/erase cycles using NIR/green irradiation.     

Upconversion emission from Yb3+ and Tm3+ codoped NaYF4 thin film prepared by thermal evaporation

Yb3+ and Tm3+ codoped fluoride thin film, with intense ultraviolet and visible upconversion emissions under 980 nm excitation, has been deposited on an Al2O3 ceramic substrate by thermal evaporation under high vacuum. NaY(0.835)Yb(0.15)Tm(0.015)F4 bulk material synthesized by high temperature solid-state reaction was used as target in preparing the thin film. Yb3+ and Tm3+ codoped system, which had been reported before, had been studied. Compared with the unannealed thin film, the annealed film showed better upconversion emission properties, especially in the ultraviolet region, given in the normalized upconversion emission spectra, due to the structure changed from amorphous to hexagonal NaYF4 (beta-NaYF4) during the annealing process. The upconversion mechanism of the thin film was also discussed in this paper.     

Sensitized upconversion emissions of Tb3+ by Tm3+ in YF3 and NaYF4 nanocrystals

Yb3+-Tm3+-Tb3+-codoped YF3 and NaYF4 nanocrystals (NCs) were synthesized using a simple hydrothermal method. Under 980 nm excitation, violet and ultraviolet upconversion (UC) emissions of 5D3 --> 7FJ (J = 6, 5, 4) and 5D4 --> 7FJ (J = 6, 5, 4, 3) of Tb3+ ions were observed with the fluoride NCs. In the Yb-Tm-Tb codoped NCs, energy transfer (ET) processes from Tm3+ to Tb3+ were proposed to be the main mechanisms for the UC emissions of Tb3+ ions. They are more efficient than the phonon assisted cooperative sensitization of the Yb3+ couple proposed previously for similar material system. The analysis of power dependence indicated that populating the 5D4 level of the Tb3+ ions was a four photon UC process, which demonstrated the existence of the two step ET process of Yb3+ --> Tm3+ --> Tb3+. It was also found that UC luminescence properties of Tb3+ ions were sensitive to crystal structures.     

色彩可调Yb, Er共掺杂NaYF4上转换荧光材料的 合成及包装防伪应用

采用阴离子表面活性剂柠檬酸三钠和阳离子表面活性剂氯代十六烷基吡啶共同调介下的水热法,合成Yb,Er共掺杂NaYF_4上转换荧光材料(NaYF_4:Yb~(3+),Er~(3+));研究敏化剂Yb~(3+)和激活剂Er~(3+)在NaYF_4中的掺杂原子百分数(掺杂量)对NaYF_4:Yb~(3+),Er~(3+)上转换材料荧光性能的影响;以合成的NaYF_4:Yb~(3+),Er~(3+)为荧光填料,聚乙烯醇(PVA)为基材,通过流延法,制备了一系列不同荧光特性的复合薄膜。研究结果表明:随着Yb~(3+)含量的提高,NaYF_4:Yb~(3+),Er~(3+)上转换材料发光强度先增大后减小,当Yb~(3+)掺杂量为30%时,荧光强度达到最大;另外,随着Yb~(3+)掺杂量从10%提高到98%,绿色荧光与红色荧光强度的比值(绿红比)逐渐减小,上转换材料发光颜色由绿色变为黄色再变为橙色。随着Er~(3+)掺杂量的增加,荧光强度同样先增大后减小,当Er~(3+)掺杂量为1%时,荧光强度达到最大;且Er~(3+)掺杂量的增加同样会使绿红比降低。所制备的复合薄膜透明性好,且具有易调控、易识别、难察觉的上转换荧光特性,预期在包装防伪领域有很好的应用前景。     

Direct imaging the upconversion nanocrystal core/shell structure at the subnanometer level: shell thickness dependence in upconverting optical properties

Lanthanide-doped upconversion nanoparticles have shown considerable promise in solid-state lasers, three-dimensional flat-panel displays, and solar cells and especially biological labeling and imaging. It has been demonstrated extensively that the epitaxial coating of upconversion (UC) core crystals with a lattice-matched shell can passivate the core and enhance the overall upconversion emission intensity of the materials. However, there are few papers that report a precise link between the shell thickness of core/shell nanoparticles and their optical properties. This is mainly because rare earth fluoride upconversion core/shell structures have only been inferred from indirect measurements to date. Herein, a reproducible method to grow a hexagonal NaGdF(4) shell on NaYF(4):Yb,Er nanocrystals with monolayer control thickness is demonstrated for the first time. On the basis of the cryo-transmission electron microscopy, rigorous electron energy loss spectroscopy, and high-angle annular dark-field investigations on the core/shell structure under a low operation temperature (96 K), direct imaging the NaYF(4):Yb,Er@NaGdF(4) nanocrystal core/shell structure at the subnanometer level was realized for the first time. Furthermore, a strong linear link between the NaGdF(4) shell thickness and the optical response of the hexagonal NaYF(4):Yb,Er@NaGdF(4) core/shell nanocrystals has been established. During the epitaxial growth of the NaGdF(4) shell layer by layer, surface defects of the nanocrystals can be gradually passivated by the homogeneous shell deposition process, which results in the obvious enhancement in overall UC emission intensity and lifetime and is more resistant to quenching by water molecules.     

六方相NaYF4∶Ln3+（Ln=Sm,Pr）微米棱柱的水热合成和发光性质

采用低温水热法合成了高质量的六方相NaYF4∶Ln3+(Ln=Sm,Pr)微米棱柱,其直径约为1μm,长约为2～5μm,端口呈规则六边形。利用X射线衍射、场发射扫描电镜、透射电子显微镜和高分辨透射电子显微镜对样品的物相、形貌和结构进行了分析与表征。室温下,采用稳态荧光光谱仪研究了六方相NaYF4∶Ln3+的光致发光性质,讨论了不同掺杂浓度对发光强度的影响。在400nm波长激发下,在500～720nm范围内观察到NaYF4∶Sm3+较强的橙色和红色发光,最强峰位于594nm;在444nm波长激发下获得了NaYF4∶Pr3+的蓝色和红色发光。     

Multicolor polystyrene nanospheres tagged with up-conversion fluorescent nanocrystals

Compared to conventional down-conversion fluorescent materials, NIR-to-visible up-conversion fluorescent materials which emit visible light upon near-infrared (NIR) excitation are better suited for biodetection/bioimaging due to their advantages such as minimum photo-damage to living organisms, weak background fluorescence, low signal-to-noise ratio and high detection sensitivity. Uniform hexagonal NaYF(4) (β-NaYF(4)) nanocrystals with up-conversion fluorescence were synthesized. Monodisperse polystyrene nanospheres with an average size of 400?nm in diameter, tagged with different color β-NaYF(4) nanocrystals, were prepared using a miniemulsion polymerization method. More than 20 β-NaYF(4) nanocrystals were encapsulated in a single polystyrene nanosphere. The nanospheres emit multicolor NIR-to-visible up-conversion fluorescence upon excitation at a wavelength of 980?nm. The nanospheres could be used for a variety of biological applications which require high-sensitivity detection of biomolecules.     

稀土掺杂上转换发光玻璃陶瓷的制备及性能

实验制备了一类含有SrF2∶Yb3+,Tm3+及SrF2∶Yb3+,Er3+的透明发光玻璃和玻璃陶瓷,对比研究了热处理工艺对玻璃陶瓷相组成、微观结构和光谱性能的影响规律。研究表明,玻璃陶瓷具有立方SrF2纳米晶相均匀分布于玻璃基体的复相结构,利用HRTEM可观测到SrF2纳米晶相的(111)晶面,其晶粒尺度在10~30nm之间,且该析晶相中富集有Yb3+/Tm3+和Yb3+/Er3+。基于此,玻璃陶瓷在980nm LD激光激发下的上转换发光强度较玻璃样品有较大提高。其上转换发光机制分别主要为Yb3+-Yb3+之间的合作上转换,Yb3+-Tm3+和Tm3+-Tm3+之间的交叉弛豫能量传递过程,以及Yb3+-Er3+之间的能量传递上转换。     

In-situ polymerization approach for preparation of rare earth fluoride phosphors coated with PAA

Up-conversion nanoparticles (UCNPs), which can convert a radiation from a longer wavelength to a shorter wavelength, have great potential uses as bio-labels in biological detection. However, these NPs usually cannot be used directly unless their surfaces are further modified. In this paper, NaYF4:Yb, Er nanoparticles (NPs) were coated with poly(acrylic acid) (PAA) by in situ polymerization for the first time. Accordingly, NaYF4:Yb, Er/NaYF4 NPs were synthesized before PAA coating to avoid the decay of optical intensity. The resulting UCNPs were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and up-conversion photoluminescence spectrometry. The XRD results indicated that the resultant UCNPs exhibited a pure hexagonal phase. The FT-IR spectra and TGA curves revealed that these NPs were coated successfully with PAA. Meanwhile, the TEM results showed that well-dispersed UCNPs with the best morphology and an average size of about 90 nm were obtained with 8.0 wt% acrylic acid content (the content percentage in the whole reaction system) at 0 degrees C within 130 min. Fluorescence tests showed that the UCNPs had a strong UC fluorescence intensity. Settlement tests revealed that PAA-coated NaYF4 UCNPs had more favorable dispersion stability than uncoated UCNPs in an aqueous system. These functionalized nanocomposites could be used for further bio-conjugation.