Photoluminescence characteristics and energy transfer between Bi3+ and Eu3+ in Gd2O3: Eu3+, Bi3+ nanophosphors

Bi³⁺ and Eu³⁺ codoped cubic Gd₂O₃ nanocrystals were prepared by the Pechini sol-gel method. Their photoluminescent properties were investigated under ultraviolet light excitation. The introduction of Bi³⁺ ions broadened the excitation band of Eu³⁺ emission, of which a new strong band occurred ranging from 320 to 380 nm due to the 6s²→6s6p transition of Bi³⁺ ions, implying a very efficient energy transfer from Bi³⁺ ions to Eu³⁺ ions. Upon 325 and 355 nm light excitation, the luminescent intensity of Eu³⁺ ions was remarkably improved by the incorporation of Bi³⁺ ions. But a significant quenching of Eu³⁺ emission was observed under 266 nm light excitation when Bi³⁺ was codoped. The possible energy transfer processes between Bi³⁺ and Eu³⁺ were discussed. The decay curves of Eu³⁺ emission under the excitation of 266 nm pulsed laser were measured and gave further evidence for our discussion.     

壳聚糖/LaF3 ∶ Eu3+纳米复合粒子的简易合成及发光特性

在水溶液中采用化学共沉淀法制备了壳聚糖/LaF₃ ∶ Eu³⁺纳米复合粒子。通过透射电子显微镜(TEM),X射线衍射(XRD),傅立叶变换近红外(FT-IR)光谱对样品进行了表征。结果表明:所得纳米复合粒子大小在 20 nm左右,粒径均匀,表面包覆的壳聚糖使其易溶于水,并具备了与生物蛋白偶联的多个基团。测量了该纳米复合粒子的激发光谱与发射光谱,详细说明了各发光峰对应能级的跃迁及其发光机理,分析了不同掺杂浓度对其相对发光强度的影响。结果表明:当 Eu³⁺离子掺杂摩尔分数为 10%时,样品的相对发光强度达到最大值。最后介绍了壳聚糖/LaF₃ ∶ Eu³⁺纳米复合粒子与荧光蛋白 FITC偶联的方法,以表明其在生物学中潜在的应用价值。     

Optical Spectroscopy of Sm3+ and Dy3+ Doped ZnO Nanocrystals

Sm³⁺ and Dy³⁺ ions doped hexagonal wurtzite ZnO nanocrystals were fabricated by a sol-gel method. The obtained ZnO nanocrystals were characterized using the X-ray diffraction, transmission electron microscopy, ultraviolet-visible reflectance spectra, and low-temperature luminescence spectroscopy. Intense and well resolved emission lines for Sm³⁺ and Dy³⁺ ions can be achieved via an energy transfer process from ZnO host to the dopants. It was found that the host sensitized emissions were more efficient than that of direct excitation for Sm³⁺ and Dy³⁺ ions. Moreover, multiple sites of Sm³⁺ and Dy³⁺ ions in ZnO nanocrystals were identified based on the low-temperature photoluminescence spectra.     

Photoluminescence properties and energy transfer of Ca3WO6:Sm3+ co-doped Eu3+

A reddish-orange phosphor, Ca₃WO₆:Sm³⁺, was synthesized by the convenient solid-state reaction method and characterized by X-ray diffraction (XRD). Photoluminescence properties and concentration quenching of Ca₃WO₆:Sm³⁺ phosphor have been discussed in the excitation and emission spectra. Ca₃WO₆:Sm³⁺ phosphor is able to generate a strong excitation peak, which matches the emission wavelength from near-UV LEDs. Energy transfer from Sm³⁺ to Eu³⁺ in Ca₃WO₆ host is observed and investigated in detail. The chromaticity coordinates of Ca₃WO₆:Sm³⁺ can be regulated to approach the NTSC standard values of red phosphor by codoping Eu³⁺ ions. The photoluminescence properties suggest that novel Ca₃WO₆:Sm³⁺, Eu³⁺ phosphor might have a potential application for near-UV LEDs.     

Optical properties of Sm3+:CaO-Li2O-B2O3-BaO glass and codoped Sm3+:Eu3+:

Calibo glasses of Sm³⁺ with and without BaO have been synthesized and the optical properties (like absorption and photoluminescence) analyzed in terms of Judd–Ofelt theory. The fluorescence yield of Sm³⁺ enhances with BaO, and is optimum at 10% BaO. The effect of temperature on the energy transfer probability in (Sm³⁺:Eu³⁺) codoped glass has been reported and it can be inferred that the combination may serve as a temperature sensor. PACS 42.70.Ce     

Facile synthesis, structural and optical characterization of LnF3:Re nanocrystals by ionic liquid-based hydrothermal process

A novel and easy synthesis of highly luminescent rare-earth ion-doped LnF₃ (LnF₃:Re) nanocrystals by ionic liquid-based hydrothermal process was reported. Ionic liquids [bmim]BF₄ (1-butyl, 2-methylimidazolium tetrafluoroborate) acts as co-solvent, template and reactant. X-ray diffraction and field emission scanning electron microscopy were used to characterize the structural properties of the products. The luminescent properties of LaF₃:Re nanocrystals were evaluated under ultraviolet (397 nm for Eu³⁺, 254 nm for Ce³⁺, Tb³⁺) and (or) near-infrared (980 nm for Er³⁺) excitation. Under 980 nm laser excitation, intense green upconversion emissions were observed for LaF₃:Er(1%) samples in the solid state and dispersion in water and ethanol. The quantum efficiency of LaF₃:Ce(15%),Tb(5%) nanocrystals was about 34%. Our reports provide a facile method for the preparation of LnF₃:Re nanocrystals with excellent photoluminescent properties.     

Improving monochromaticity of upconversion luminescence by codoping Eu ions in Y2O3:Ho, Yb nanocrystals

Upconversion (UC) luminescence of Y₂O₃:Ho³⁺, Yb³⁺ nanocrystals codoped with different concentrations of Eu³⁺ ions were investigated to improve the monochromaticity of the UC emission. The results show that the monochromaticity, quantified by a parameter SR, increases as the concentration of Eu³⁺ ions becomes higher, which is due to the energy transfer between ⁵I₇ (Ho³⁺) and ⁷F₆ (Eu³⁺). The energy transfer accelerates the relaxation of Ho³⁺ ions from the ⁵I₇ to ⁵I₈ state and then quenches the red emission. The influence of the Eu³⁺ concentration on the pump power dependence of the red UC fluorescence in Y₂O₃:Ho³⁺, Yb³⁺, Eu³⁺ nanocrystals is verified using the steady-state rate equation theory.     

Structure and intense UV up-conversion emissions in RE3+-doped sol–gel glass-ceramics containing KYF4 nanocrystals

Transparent nano-glass-ceramics containing KYF₄ nanocrystals were successfully obtained by the sol–gel method, doped with Eu³⁺ and co-doped with Yb³⁺ and Tm³⁺ ions. Precipitation of cubic KYF₄ nanocrystals was confirmed by X-ray diffraction and high-resolution transmission electron microscope images. Excitation and emission spectra let us to discern between ions into KYF₄ nanocrystals and those remaining in a glassy environment, supplemented with time-resolved photoluminescence decays, that also clearly reveal differences between local environments. Unusual high-energy up-conversion emissions in the UV range were obtained in Yb³⁺–Tm³⁺ co-doped samples, and involved mechanisms were discussed. The intensity of these high-energy emissions was analyzed as a function of Yb³⁺ concentration, heat treatment temperature of precursor sol–gel glasses and pump power, determining the optimum values for potential optical applications as highly efficient UV up-conversion materials in UV solid-state lasers.     

Multiplesite structure and photoluminescence properties of Eu3+ doped MgO nanocrystals

MgO:Eu³⁺ nanocrystals with average diameter around 15 nm were prepared via a facile combustion method under a weak reductive atmosphere at temperature as low as 300°C. The photoluminescence spectra showed that the MgO:Eu³⁺ nanocrystals emit white light, the hypersensitive transition (⁵ D ₀→⁷ F _j of Eu³⁺) emission was prominent in the emission spectra resulting from the noinversion symmetry local site at which Eu³⁺ ions were located. Two kinds of luminescence sites of Eu³⁺ are identified by means of the fluorescence decay and site-selective spectroscopy. The excitation and absorption spectra indicated that the absorption of surface state decreased with the increase of Eu³⁺ concentration, meaning that the surface defect decreased through Eu³⁺ doping for some of them located at the disordered sites near the surface or absorbed at the surface of MgO host. Meanwhile, absorptivity and CIE chromaticity coordinates of all samples were measured; the results were in accordance with the excitation and absorption spectra and photoluminescence spectra, respectively.     

Luminescence and red long afterglow investigation of Eu3+–Sm3+ CO-doped CaWO4 phosphor

A series of Eu³⁺–Sm³⁺ co-doped CaWO₄ phosphors were synthesized by the high temperature solid-state method. The crystal structure of the obtained samples was identified by XRD, and the results showed that all the phases were indexed to scheelite structure. The effect of the doping concentration of Sm³⁺ on the luminescent properties of the obtained products was investigated, and the optimal Sm³⁺ concentration was experimentally determined to 0.5%. The photoluminescence properties indicate that there is an efficient energy transfers from Sm³⁺ to Eu³⁺. The energy-transfer process between Sm³⁺ and Eu³⁺ was also given. Red long afterglow originating from the ⁵D₀–⁷F_J (J=0, 1, 2, 3, 4) transitions of Eu³⁺ was observed after samples were excited by 254 nm, and the duration of the optimal sample can last more than 35 min in dark with naked eyes. The proposed explanation for the afterglow property was also discussed.     

Preparation and characterization of upconversion luminescent LaF3:Yb,Er/LaF3 core/shell nanocrystals

LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals were successfully synthesized using solvothermal method. The crystal structure, morphology and photoluminescence properties of as-prepared nanocrystals were investigated in detail. XRD patterns show that the obtained LaF₃:Yb³⁺,Er³⁺ core and LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals exhibit hexagonal structure. The average particle size is about 9.3 nm and 11.4 nm for core and core/shell nanocrystals, respectively. Compared with LaF₃:Yb³⁺,Er³⁺ nanocrystals, both the upconversion emission intensity and the lifetime increase in LaF₃:Yb³⁺,Er³⁺/LaF₃ core/shell nanocrystals. The enhancement can be attributed to the LaF₃ shell which can eliminate the nonradiative centers on the surface of LaF₃:Yb³⁺,Er³⁺ nanocrystals.     

The upconversion luminescence and magnetism in Yb<Superscript>3+</Superscript>/Ho<Superscript>3+</Superscript> co-doped LaF<Subscript>3</Subscript> nanocrystals for potential bimodal imaging

Biocompatible upconversion nanoparticles with multifunctional properties can serve as potential nanoprobes for multimodal imaging. Herein, we report an upconversion nanocrystal based on lanthanum fluoride which is developed to address the imaging modalities, upconversion luminescence imaging and magnetic resonance imaging (MRI). Lanthanide ions (Yb³⁺ and Ho³⁺) doped LaF₃ nanocrystals (LaF₃ Yb³⁺/Ho³⁺) are fabricated through a rapid microwave-assisted synthesis. The hexagonal phase LaF₃ nanocrystals exhibit nearly spherical morphology with average diameter of 9.8 nm. The inductively coupled plasma mass spectrometry (ICP-MS) analysis estimated the doping concentration of Yb³⁺ and Ho³⁺ as 3.99 and 0.41%, respectively. The nanocrystals show upconversion luminescence when irradiated with near-infrared (NIR) photons of wavelength 980 nm. The emission spectrum consists of bands centred at 542, 645 and 658 nm. The stronger green emission at 542 nm and the weak red emissions at 645 and 658 nm are assigned to ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. The pump power dependence of luminescence intensity confirmed the two-photon upconversion process. The nanocrystals exhibit paramagnetism due to the presence of lanthanide ion dopant Ho³⁺ and the magnetization is 19.81 emu/g at room temperature. The nanocrystals exhibit a longitudinal relaxivity (r ₁) of 0.12 s^?1 mM^?1 and transverse relaxivity (r ₂) of 28.18 s^?1 mM^?1, which makes the system suitable for developing T2 MRI contrast agents based on holmium. The LaF₃ Yb³⁺/Ho³⁺ nanocrystals are surface modified by PEGylation to improve biocompatibility and enhance further functionalisation. The PEGylated nanocrystals are found to be non-toxic up to 50 μg/mL for 48 h of incubation, which is confirmed by the MTT assay as well as morphological studies in HeLa cells. The upconversion luminescence and magnetism together with biocompatibility enables the adaptability of the present system as a nanoprobe for potential bimodal imaging.     

Yb3+/Er3+共掺锗碲酸盐玻璃上转换发光增强机理的研究

玻璃的最大声子能量决定稀土离子的上转换发光强度，但本研究发现：Yb³⁺/Er ³⁺共掺锗碲酸盐玻璃在980nm LD抽运下，上转换荧光强度随着Bi₂O ₃对PbO的取代和碱 金属离子半径的增大而明显增强.而Raman光谱显示基质玻璃的最大声子能量并不随Bi ₂O₃对PbO的取代和碱金属离子半径的增大而变化，但玻璃的最大声子密 度随着Bi₂O₃对PbO 取代和碱金属离子半径的增大而降低.从玻璃无辐射跃迁概率的角度，通过分析表明，最大 声子密度的降低是玻璃上转换发光强度增强的主要原因.     

Energy Back Transfer Induced Color Controllable Upconversion Emissions in La2MoO6:Er3+/Yb3+ Nanocrystals for Versatile Applications

Color controllable Er³⁺/Yb³⁺‐codoped La₂MoO₆ upconverting nanocrystals are successfully synthesized via a facile sol‐gel method. Under the irradiation of 980 nm light, the entire samples exhibit dazzling upconversion (UC) emissions arising from the intra‐4f transitions of Er³⁺ ions and the UC emission intensity is strongly dependent on the Yb³⁺ ion concentration. Moreover, by controlling the Yb³⁺ ion concentration, the emission color is changed from green to yellow and finally to red as a result of the energy back transfer from Er³⁺ to Yb³⁺ ions, which is further verified by the theoretically discussion based on the steady‐state rate expressions. The optical thermometric properties of the prepared nanocrystals based on the (²H_11/2,⁴S_3/2) thermally coupled levels of Er³⁺ ions are systematically studied by analyzing the temperature‐dependent green UC emission spectra in the range of 303–663 K. The maximum sensor sensitivity of resultant nanocrystals is determined to be 0.0083 K⁻¹ at 510 K. Furthermore, the emitting color of the synthesized nanocrystals relies on the temperature. In addition, the heating effect induced by the excitation pump power is also investigated and the host lattice temperature is enhanced from 319 to 404 K with raising the pump power from 159 to 757 mW.     

The luminescence enhancement of Eu3+ ion and SnO2 nanocrystal co-doped solben gel SiO2 films

SnO₂ nanocrystal and rare-earth Eu³⁺ ion co-doped SiO₂ thin films are prepared by sol-gel and spin coating methods. The formation of tetragonal rutile structure SnO₂ nanocrystals with a uniform distribution is confirmed by X-ray diffraction and transmission electron microscopy. Fourier transform infrared spectroscopy is used to investigate the densities of the hydroxyl groups, and it is found that the emission intensity from the ⁵D₀-⁷F₂ transitions of the Eu³⁺ ions is enhanced by two orders of magnitude due to energy transfer from the oxygen-vacancy-related defects of the SnO₂ nanocrystals to nearby Eu³⁺ ions. The influences of the amounts of Sn and the post-annealing temperatures are systematically evaluated to further understand the mechanism of energy transfer. The luminescence intensity ratio of Eu³⁺ ions from electric dipole transition and magnetic dipole transition indicate the different probable locations of Eu³⁺ ions in the sol-gel thin film, which are further discussed based on temperature-dependent photoluminescence measurements.     

Fluorescent properties of a green- to red-emitting Eu3+, Tb3+ codoped amorphous calcium silicate phosphor

A Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor was prepared by heating a Eu³⁺, Tb³⁺ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction for 30 min at 900 °C. The excitation peak wavelength of the resulting phosphor was 379 nm and the emission peak wavelengths were at 542 nm, attributed to the ⁵D₄→⁷F₅ transition of Tb³⁺, and at 613 mm, attributed to the ⁵D₀→⁷F₁ transition of Eu³⁺. The intensity ratio of the two peaks could be freely controlled by varying the Eu/Tb atomic ratio of the Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from green to red. It was clarified that electron transfer from Tb³⁺ to Eu³⁺ is occurring.     

Luminescence characteristics of energy transfer between Ce<Superscript>3+</Superscript> and Eu<Superscript>2+</Superscript> in NaMgPO<Subscript>4</Subscript> phosphor

Powder samples of NaMgPO₄ doped with Eu²⁺ and Ce³⁺ were prepared and their photoluminescence spectra were systemically studied. Energy transfer from Ce³⁺ to Eu²⁺ in NaMgPO₄ phosphor was observed by investigating the optical properties from photoluminescence spectra in Eu²⁺ or Ce³⁺ singly doped and Eu²⁺–Ce³⁺ codoped sodium magnesium orthophosphates, NaMgPO₄. The enhancement of UV excitation is attributed to energy transfer from Ce³⁺ to Eu²⁺, and Ce³⁺ plays a role as a sensitizer. Ce³⁺–Eu²⁺ codoped NaMgPO₄ phosphors in which Eu²⁺ can be efficiently excited by 390 nm are potential candidates for phosphor-converted LEDs.     

Luminescent properties of Eu3+ and Sm3+ activated M2SiO4 (M=Ba,Sr and Ca) red-emitting phosphors for WLEDs

Eu³⁺ and Sm³⁺ activated M₂SiO₄ (M=Ba, Sr and Ca) red-emitting phosphors were synthesized by a solid state reaction. The results of XRD and SEM measurements show that the samples are single phase and have irregular shape. The excitation and emission spectra indicate that these phosphors were effectively excited by ultraviolet (395 nm) and blue (466 nm) light and exhibited red performance. The charge compensator R⁺ (R⁺=Li⁺, Na⁺ and K⁺) injecting into the host efficiently enhanced the luminescence intensity of the M₂SiO₄: Eu³⁺ and M₂SiO₄: Sm³⁺ phosphors. The emission intensity of M₂SiO₄: Eu³⁺ and Sm³⁺ doping Li⁺ were higher than that of Na⁺ or K⁺.     

Ultra-small sized Y2O3:Eu3+ nanocrystals: One-step polyoxometalate-assisted synthesis and their photoluminescence properties

A novel hydrothermal approach for the preparation of europium(III)-doped yttrium oxide (Y₂O₃:Eu³⁺) nanocrystals was reported. The as-synthesized Y₂O₃:Eu³⁺ nanocrystals with diameter of about 5 nm are highly uniform and dispersed in water. The Y₂O₃:Eu³⁺ nanocrystals were characterized by high-resolution transmission electron microscopy and fluorescence spectroscopy. Due to their well dispersity in water, low toxicity, and good photoluminescence, the Y₂O₃:Eu³⁺ nanocrystals can potentially be used in high-definition displays and fluorescence probe in bioimaging.     

Jahn-Teller effect on Sm3+ ions in SmB6

Electron spin resonance on samarium ions with stabilized valence Sm³⁺ is investigated in the fluctuating-valence semiconductor SmB₆, both pure and doped with the rare-earth ions Eu²⁺, Er³⁺, and Gd₃₊. The dynamic and static Jahn-Teller effects have been observed for the first time on rare-earth ions. The relation between the Jahn-Teller effect in a fluctuating-valence semiconductor and the excitonic nature of the ground state of such a semiconductor is discussed. Zh. éksp. Teor. Fiz. 115, 1860–1871 (May 1999)