Synthesis of monodisperse NaYF4:Yb, Tm@SiO2 nanoparticles with intense ultraviolet upconversion luminescence

In this paper, the NaYF4:Yb, Tm upconversion (UC) nanoparticles (NPs) were synthesized using a solvothermal approach, and the core-shell structured NaYF4:Yb, Tm@SiO2 NPs, coated with a thin layer of SiO2 on the surface of the NaYF4:Yb, Tm NPs, were prepared by a typical St?ber method. X-ray diffraction (XRD), transmission electron microscopy (TEM), and luminescence spectroscopy were applied to characterize these samples. The obtained core-shell structured NaYF4:Yb, Tm@SiO2 NPs exhibited a perfect spherical morphology with narrow size distribution and smooth surfaces. Under 980 nm excitation, NaYF4:Yb, Tm and NaYF4:Yb, Tm@SiO2 samples showed intense ultraviolet UC luminescence, which originated from the 1D2 --> 3H6, 1I6 --> 3F4 transitions of Tm3+. These NPs have great potential for applications as fluorescent labels, imaging probes, optical storage, photodynamic therapy (PDT) in deep tissue, and solid-state lasers.     

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.     

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.     

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.     

Yb3+、Ho3+掺杂浓度及晶粒尺寸对NaYF4∶Yb3+,Ho3+上转换材料发光的影响

EDTA作为络合剂,在pH值为5的条件下,采用水热法制备了NaYF4∶Yb3+,Ho3+微米棱柱上转换材料,研究了掺杂浓度和晶粒尺寸对上转换发光特性的影响。实验发现材料发光主要以绿光为主,最佳的Yb3+、Ho3+掺杂浓度分别为25%和1%,高于此值均会出现浓度猝灭效应。通过改变溶液中NaF/NH4HF2摩尔比来调控晶粒的尺寸,发现随着晶体颗粒尺寸的增大,材料中Yb3+浓度增加,从而使上转换发光增强。     

Core/shell structured NaYF4:Yb3+/Er3+/Gd+3 nanorods with Au nanoparticles or shells for flexible amorphous silicon solar cells

A simple approach for preparing near-infrared (NIR) to visible upconversion (UC) NaYF4:Yb/Er/Gd nanorods in combination with gold nanostructures has been reported. The grown UC nanomaterials with Au nanostructures have been applied to flexible amorphous silicon solar cells on the steel substrates to investigate their responses to sub-bandgap infrared irradiation. Photocurrent–voltage measurements were performed on the solar cells. It was demonstrated that UC of NIR light led to a 16-fold to 72-fold improvement of the short-circuit current under 980 nm illumination compared to a cell without upconverters. A maximum current of 1.16 mA was obtained for the cell using UC nanorods coated with Au nanoparticles under 980 nm laser illumination. This result corresponds to an external quantum efficiency of 0.14% of the solar cell. Mechanisms of erbium luminescence in the grown UC nanorods were analyzed and discussed.     

上转换荧光光子晶体薄膜的制备及包装防伪应用

以热分解法制备的蓝色荧光NaYF4:Yb3+,Tm3+上转换纳米颗粒为核,外延生长一层具有钝化表面缺陷、增强荧光效果的NaYF4壳层,制备得到核壳上转换纳米颗粒(CSNPs).利用反相微乳液法在CSNPs上包覆一层修饰3-(三甲氧基甲硅基)甲基丙烯酸丙酯(MPS)的SiO2,实现上转换纳米颗粒亲水改性的同时,赋予其可参与加成聚合的双键.将CSNPs@SiO2-MPS与苯乙烯单体通过乳液聚合共聚形成镧系掺杂NaYF4/PS复合微球.通过垂直沉积法,利用镧系掺杂NaYF4/PS复合微球自组装构建上转换荧光光子晶体(UCPC)薄膜,并探讨其在包装防伪中的应用.结果表明:该上转换荧光光子晶体薄膜,在可见光下从特定角度可以观察到明显的粉色结构色,在980 nm激光照射下可观察到蓝色荧光,这两种模态下的光学特性可隐藏信息,预期在信息保护、包装防伪等领域有广阔的应用前景.     

Size dependent ultraviolet upconversion in single YF3:Yb3+/Tm3+ particles

Yb3+-Tm3+ codoped YF3 bulk material was synthesized through a facile high-temperature calcinations method. By grinding and selecting, the particles with different desired sizes in microns were obtained. Under 980 nm excitation, optical upconversion (UC) from near-infrared (NIR) to ultraviolet (UV) was studied for each group of particles for the effect of their size on UC. Comparing with the bulk sample, the micro-size particles exhibited strong ability for NIR-to-UV UC. With the particle size decreasing from 800 microm to 20 microm, their UV emission intensities increased rapidly. Two possible mechanisms were proposed and discussed for clarifying the small size effect (SSE).     

Synthesis and characterization of lanthanide-doped sodium holmium fluoride nanoparticles for potential application in photothermal therapy

Upconversion nanoparticles (UC NPs) in combination with plasmonic materials have great potential for cancer photothermal therapy. Recently, sodium holmium fluoride (NaHoF₄) is being investigated for luminescence and magnetic resonance imaging (MRI) contrast agent. Here, we present successful synthesis of excellent quality doped NaHoF₄ NPs for possible UC luminescence application and coated for possible photothermal therapy application. Synthesized NaHoF₄ nanocrystals were doped with Yb/Er and coated with gold, gold/silica, silver and polypyrrole (PPy). XRD, XPS and TEM were used to determine structure and morphology of the NPs. Strong UC photoluminescence (PL) emission spectra were obtained from the NPs when excited by near-infrared (NIR) light at 980 nm. Cell viability and toxicity of the NPs were characterized using pancreatic and ovarian cancer cells with results showing that gold/silica coating produced least toxicity followed by gold coating.     

Upconversion fluorescent nanoparticles as a potential tool for in-depth imaging

Upconversion nanoparticles (UCNs) are nanoparticles that are excited in the near infrared (NIR) region with emission in the visible or NIR regions. This makes these particles attractive for use in biological imaging as the NIR light can penetrate the tissue better with minimal absorption/scattering. This paper discusses the study of the depth to which cells can be imaged using these nanoparticles. UCNs with NaYF(4) nanocrystals doped with Yb(3+), Er(3+) (visible emission)/Yb(3+), Tm(3+) (NIR emission) were synthesized and modified with silica enabling their dispersion in water and conjugation of biomolecules to their surface. The size of the sample was characterized using transmission electron microscopy and the fluorescence measured using a fluorescence spectrometer at an excitation of 980 nm. Tissue phantoms were prepared by reported methods to mimic skin/muscle tissue and it was observed that the cells could be imaged up to a depth of 3 mm using the NIR emitting UCNs. Further, the depth of detection was evaluated for UCNs targeted to gap junctions formed between cardiac cells.     

Synthesis and characterization of upconverting NaYF4:Er3+, Yb3+ nanocrystals via thermal decomposition of stearate precursor

Er3+-Yb3+ codoped hexagonal NaYF4 nanocrystals were prepared via a method of thermal decomposition of stearate precursor. Their crystal structure, morphologies and photoluminescence (PL) properties were characterized by XRD, SEM, and fluorescence spectra. The hexagonal NaYF4:Er3+, Yb3+ nanocrystals could be well dispersed in cyclohexane to form a clear solution. Under 980 nm excitation, the solution of Er3+-Yb3+ codoped NaYF4 nanocrystals emits bright green upconversion fluorescence.     

Unexpected luminescence enhancement of upconverting nanocrystals by cation exchange with well retained small particle size

Highly luminescent upconversion nanoparticles （UCNPs） with small sizes are highly desirable for bioapplications. A facile in situ cation exchange strategy has been developed to greatly enhance the UC luminescence of hexagonal phase NaYF4 NPs while maintaining their small particle size and shape. Via a cation exchange treatment by hot-injecting Gd3＋ precursors into the as-prepared NPs solution without pre-separation, the naked-eye visible UC emission of the NPs was enhanced about 29 times under 980 nm near infrared （NIR） excitation with unchanged particle size. The cation exchange process was further demonstrated for the case of NaYF4 nanorods （NRs）. After the cation exchange, the nanorod was broken into two NPs with stronger emission. The cation exchanged hydrophobic UCNPs were further encapSulated with poly（amino acid） and successfully applied for targeted cancer cell UC luminescence imaging.     

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.     

Up-conversion fluorescence of Tm3+ and Gd3+ in Yb3+/Tm3+ co-doped Gd2O3 nanocrystals

Through a co-precipitation method Gd(OH)3:20%Yb3+, 1%Tm3+ nanorods were synthesized. After sintered at 900 degrees C for 1 h in air, the as-prepared Gd(OH)3:20%Yb3+, 1%Tm3+ nanorods were converted into Gd2O3:20%Yb3+, 1% Tm3+ nanocrystals. Crystalline phases, sizes, and morphologies of the two samples were characterized by X-ray diffraction and field emission scanning electron microscope. The up-conversion (UC) fluorescence spectra of the Gd2O3:20%Yb3+, 1%Tm3+ nanocrystals were recorded by using a fluorescence spectrophotometer with a 980 nm continuous wave laser diode as excitation source. The nanocrystals not only present characteristic blue and ultraviolet (UV) UC emissions of activated Tm3+, but also show UV UC emissions of host Gd3+. The experimental study suggests that the excitation power has great effects on UC fluorescence properties and the energy transfer from Tm3+ to Gd3+ is very efficient.     

The 1.53 μm spectroscopic properties of Er3+/Ce3+/Yb3+ tri-doped tellurite glasses containing silver nanoparticles

The metallic silver nanoparticles (NPs) was introduced into the Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses with composition TeO₂–ZnO–La₂O₃ to improve the 1.53 μm band fluorescence. The UV/Vis/NIR absorption spectra, 1.53 μm band fluorescence spectra, fluorescence lifetimes, X-ray diffraction (XRD) curves, differential scanning calorimeter (DSC) curves and transmission electron microscopy (TEM) image of tri-doped tellurite glasses were measured, together with the Judd–Ofelt intensity parameters, emission cross-sections, absorption cross-sections and radiative quantum efficiencies were calculated to investigate the effects of silver NPs on the 1.53 μm band spectroscopic properties of Er³⁺ ions, structural nature and thermal stability of glass hosts. It is shown that Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses can emit intense 1.53 μm band fluorescence through the combined energy transfer (ET) processes from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions under the 980 nm excitation. At the same time, the introduction of an appropriate amount of silver NPs can further improve the 1.53 μm band fluorescence owing to the enhanced local electric field effect induced by localized surface Plasmon resonance (LSPR) of silver NPs and the possible energy transfer from silver NPs to Er³⁺ ions, and an improvement by about 120% of fluorescence intensity is found in the studied Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass containing 0.5 mol% amount of silver NPs with average diameter of ∼15 nm. The energy transfer mechanisms from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions were also quantitatively investigated by calculating energy transfer microparameters and phonon contribution ratios. Furthermore, the thermal stability of glass host increases slightly with the introduction of silver NPs while the glass structure maintains the amorphous nature. The results indicate that the prepared Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass with an appropriate amount of silver NPs is an excellent gain medium applied for 1.53 μm band EDFA pumped with a 980 nm laser diode (LD).     

Broadband downconversion in Bi3+, Yb3+-Co-doped YVO4 phosphor

Yttrium vanadate phosphors co-doped with Bi3+ and Yb3+ ions have been prepared via the solid-state reaction. The phosphors were characterized by various methods including X-ray diffraction, photoluminescence excitation and photoluminescence spectra. Upon ultraviolet (UV) light excitation, an intense near-infrared (NIR) emission of Yb3+ corresponding to the transition of 2F(5/2) --> 2F(7/2) peaking at 985 nm was observed as a result of energy transfer from O2(-)-V5+ or Bi3+-V5+ charge transfer state (CTS) to Yb3+. A broad excitation band ranging from 250 to 375 nm was recorded when the Yb3+ emission was monitored, which suggests an efficient energy transfer from CTS to Yb3+ ions. The dependence of Yb3+ doping concentration on the visible emission, the NIR emission and decay lifetime has been investigated. The results of visible and NIR spectral evolution with temperature indicate that the mechanism for the NIR-emission is mainly phonon-assisted energy transfer at room temperature, while the mechanism is mainly cooperative energy transfer at low temperature. The YVO4:Bi3+, Yb3+ phosphor has prospects for realizing high efficiency crystalline Si solar cells by converting broadband UV energy into NIR light.     

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.     

Upconversion in NaYF(4):Yb, Er nanoparticles amplified by metal nanostructures

Upconversion (UC) fluorescence in NaYF(4):Yb, Er nanoparticles amplified by metal nanostructures was compared in two nanostructure geometries: gold nanoshells surrounding nanoparticles and silver nanostructures adjacent to the nanoparticles, both placed on a dielectric silica surface. Enhanced UC luminescence signals and modified lifetimes induced by these two metals were observed in our study. The UC luminescence intensities of green and red emissions were enhanced by Ag nanostructures by a factor of approximately 4.4 and 3.5, respectively. The corresponding UC lifetimes were reduced ～ 1.7-fold and ～ 2.4-fold. In NaYF(4):Yb, Er nanoparticles encapsulated in gold nanoshells, higher luminescence enhancement factors were obtained (～9.1-fold for the green emission and ～ 6.7-fold for the red emission). However, the Au shell coating extended the red emission by a factor of 1.5 and did not obviously change the lifetime of green emission. The responsible mechanisms such as plasmonic enhancement and surface effects are discussed.     

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.     

Efficient dual mode multicolor luminescence in a lanthanide doped hybrid nanostructure: a multifunctional material

The present work deals with inorganic-organic hybrid nanostructures capable of producing intense visible emission via upconversion (UC), downconversion (DC), and energy transfer (ET) processes which show the potential of the material as a luminescent solar collector (LSC), particularly to improve the efficiency of silicon solar cells. To achieve this, Gd2O3:Yb3+/Er3+ phosphor (average particle size～35 nm) and a Eu(DBM)3Phen organic complex have been synthesized separately and then the hybrid structure has been developed using a simple mixing procedure. Intense UC emission (in the red, green, and blue regions) due to Er3+ is observed on near infrared (976 nm) excitation which shows color tunability with input pump power. In contrast, intense red emission of Eu3+ is observed on ultaviolet (UV) (355 nm) excitation. The feasibility of energy transfer from Er3+ ions to Eu3+ ions has also been noted. These excellent optical properties are retained even if the particles of the hybrid nanostructure are dispersed in liquid medium, which also makes it suitable for security ink purposes.