Yb3+敏化的Er3+/Ho3+共掺碲酸盐玻璃的上转换发光研究

用高温熔融法制备了系列Er³⁺/Yb³⁺共掺，Ho³⁺/Yb³⁺共掺，和Er³⁺/Yb³⁺/Ho³⁺三掺碲酸盐玻璃，在975nm激光抽运下三种掺杂玻璃中都出现了较强的绿光和红光上转换.研究了Yb³⁺离子对Er³⁺和Ho³⁺离子上转换发光强度的影响以及Yb³⁺→Er关键词： 3+/Yb³⁺/Ho³⁺共掺')" href="#">Er³⁺/Yb³⁺/Ho³⁺共掺 碲酸盐玻璃 光谱性质 上转换     

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.     

Synthesis and up-conversion luminescence properties of Ho3+, Yb3+ co-doped BaLa2ZnO5

Up-conversion phosphors BaLa₂ZnO₅ co-doped with Ho³⁺/Yb³⁺ were synthesized by high temperature solid-state reaction method. The phase composition of the phosphors was characterized by X-ray diffraction (XRD). The structure of BaLa₂ZnO₅: 0.75% Ho/15% Yb phosphor was refined by the Rietveld method and results showed the decreased unit cell parameters and cell volume after doping Ho³⁺ and Yb³⁺, indicating Ho³⁺ and Yb³⁺ have successfully replaced La³⁺. Under the excitation of 980 nm diode laser, the strong green and weak red up-conversion emissions centered at 548 nm, 664 nm and 758 nm were observed, which originating from ⁵S₂, ⁵F₂→⁵I₈, ⁵F₄→⁵I₈ and ⁵S₂, ⁵F₂→⁵I₇ transitions of Ho³⁺ ions, respectively. The optimum doping concentrations of Ho³⁺ and Yb³⁺ were determined to be 0.75% and 15%, and the corresponding Commission International de L'Eclairage (CIE) coordinates are calculated to be x=0.298 and y=0.692. The related UC mechanism of Ho³⁺/Yb³⁺ co-doped BaLa₂ZnO₅ depending on pump power was studied in detail. The results indicate that BaLa₂ZnO₅: Ho³⁺/Yb³⁺ can be an effective candidate for up-conversion yellowish-green light emitter.     

应用于白光显示的Tm3+/Ho3+/Yb3+共掺碲酸盐玻璃上转换发光特性研究

用高温熔融法制备了Tm³⁺/Ho³⁺/Yb³⁺共掺碲酸盐玻璃(TeO₂-ZnO-La₂O₃)样品,测试了玻璃样品的吸收光谱和上转换发光光谱,分析了上转换发光机理.结果发现:在975 nm波长激光二极管(LD)激励下,制备的碲酸盐玻璃样品可以观察到强烈的红光(662 nm)、绿光(546 nm)和蓝光(480 nm)三基色上转换发光,红光对应于Tm³⁺离子 关键词： 碲酸盐玻璃 上转换发光 白光 3+/Ho³⁺/Yb³⁺共掺')" href="#">Tm³⁺/Ho³⁺/Yb³⁺共掺     

Experimental investigation on the upconversion mechanism of 754 nm NIR luminescence of Ho/Yb:Y2O3, Gd2O3 under 976 nm diode laserexcitation

Upconversion (UC) spectra of Ho³⁺/Yb³⁺ codoped Y₂O₃, Gd₂O₃ bulk ceramics were obtained under the excitation of a 976 nm diode laser. Systematic experimental studies, including power dependence, luminescence lifetime, and the intensity ratio σ for the green to NIR emissions, were carried out in order to confirm the UC mechanism of Ho³⁺ ions. Our results demonstrated that the NIR emission was associated with the ⁵F₄/⁵S₂→⁵I₇ transition of Ho³⁺ ions without the contribution of the ⁵I₄→⁵I₈ transition for Ho³⁺/Yb³⁺ codoped Y₂O₃ and Gd₂O₃ bulk ceramics. Additionally, population saturation in the ⁵I₇ energy level had been observed in Ho³⁺/Yb³⁺ codoped Y₂O₃, Gd₂O₃ bulk ceramics. All experimental observations can be well explained by the steady-state rate equations.     

Infrared-to-visible frequency upconversion in SrAl2O4 powders containing Er3+ and Yb3+

The phenomenon of frequency upconversion (UC) is observed in Er³⁺:Yb³⁺:SrAl₂O₄ powders prepared by combustion synthesis. Strong UC emission at the green (bands peaked at 521, 538, 547, and 562 nm) and weak UC emission at the red (bands peaked at 659 and 682 nm) corresponding to 4f–4f transitions of Er³⁺ were observed when the samples were irradiated with near-infrared laser excitation at ～980 nm. Saturation of UC emission is observed for concentrations of 1.5 wt.% of Er³⁺ and 1.5 wt.% of Yb³⁺. The green-to-red intensity ratio, on the other hand, increases linearly with Er³⁺ concentration (Er³⁺ concentration varying from 0.5 to 1.5 wt.%) while keeping Yb³⁺ concentration fixed (at 1.5 wt.%). The green UC decay time was measured and Er³⁺–Er³⁺ interaction was suggested as a possible mechanism to explain the luminescence quenching effect observed.     

Microwave sol-gel process of KGd(WO4)2:Ho3+/Yb3 phosphors and their upconversion photoluminescence properties

Double tungstate KGd_1−x(WO₄)₂:Ho³⁺/Yb³⁺ phosphors with doping concentrations of Ho³⁺ and Yb³⁺ (x=Ho³⁺+Yb³⁺, Ho³⁺=0.05, 0.1, 0.2 and Yb³⁺=0.2, 0.45) were successfully synthesized by the microwave sol–gel method, and the upconversion mechanisms were investigated in detail. The synthesized particles formed after heat-treatment at 900 °C for 16 h showed a well crystallized morphology with particle sizes of 2–5 μm. Under excitation at 980 nm, the UC intensities of KGd_0.7(WO₄)₂:Ho_0.1Yb_0.2 and KGd_0.5(WO₄)₂Ho_0.05Yb_0.45 particles exhibited yellow emissions based on a strong 550-nm emission band in the green region and a strong 655-nm emission band in the red region, which were assigned to the ⁵S₂/⁵F₄→⁵I₈ and ⁵F₅→⁵I₈ transitions, respectively. The Raman spectra of the doped particles indicated the presence of strong peaks at higher frequencies of 764, 812, 904, 984, 1050, 1106, 1250 and 1340 cm⁻¹ induced by the disorder of the [WO₄]²⁻ groups with the incorporation of the Ho³⁺ and Yb³⁺ elements into the crystal lattice or by a new phase formation.     

Towards efficient upconversion and downconversion of NaYF4:Ho3+,Yb3+ phosphors

NaYF₄ microcrystals co-doped with Ho³⁺ and Yb³⁺ were prepared by a facile hydrothermal synthesis. The products were characterized by X-ray diffractometer, scanning electron microscopy, and photoluminescence spectroscopy. Upon excitation with a 980 nm laser diode, the sample shows an intense green upconversion emission centered at 540 nm corresponding to the ⁵S₂→⁵I₈ transition of Ho³⁺. The quadratic dependence of the green emission intensity on the excitation power reveals a two-phonon upconversion process. On the contrary, upon excitation with 448 nm, both visible and near-infrared emissions peaked at 483, 540, 644, 749, and 978 nm are simultaneously observed, which could be assigned to the electronic transitions of Ho³⁺: ⁵F₃→⁵I₈, ⁵S₂→⁵I₈, ⁵F₅→⁵I₈, ⁵S₂→⁵I₇, and Yb³⁺: ²F_5/2→²F_7/2, respectively. The energy transfer processes between Ho³⁺ and Yb³⁺ ions and the involved mechanisms have been investigated and discussed.     

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.     

Spectral Investigations on Ho3+ Doped Mixed Alkali Chloroborate Glasses

The optical absorption and emission spectra of two different Ho³⁺ doped mixed alkali chloroborate glasses have been studied in the ultraviolet-visible near-infrared regions. Various spectroscopic parameters like Racah (E¹, E², and E³), spin orbit (ξ_4f), and configuration interaction (α) parameters have been calculated. From the measured spectral intensities of the various absorption bands of Ho³⁺ ion, the Judd–Ofelt intensity parameters (Ω₂, Ω₄, and Ω₆) have been evaluated and covalency was studied as a function of x in the glass matrices. Using these parameters, radiative transition probabilities, radiative lifetimes, branching ratios, and integrated absorption cross-sections have been calculated and reported for certain excited states of Ho³⁺ ion. From the emission spectra, stimulated emission cross-sections are determined for the emission transitions, ⁵F₄, ⁵S₂?→?⁵I₈, and ⁵F₅?→?⁵I₈ in these two mixed alkali chloroborate glasses. An attempt has been made to throw some light on the environment of Ho³⁺ ions in these glass systems by studying the variation in various spectroscopic parameters.     

Conversion of infrared radiation into visible emission in NaGd(WO4)2:Yb3+, Ho3+ crystals

NaGd(WO₄)₂:Yb³⁺, Ho³⁺ single crystals have been grown by the Czochralski technique along the (0 0 1) orientation. Conversion of the infrared (IR) radiation at 980 nm into the visible emission in NaGd(WO₄)₂ crystals containing several different concentrations of Yb³⁺ and Ho³⁺ has been investigated. The NaGd(WO₄)₂: 8 at. % Yb³⁺, 4 at. % Ho³⁺ system exhibits intense red upconverted emission originating from the ⁵F₅ level. The upconversion mechanism in a Ho³⁺-Yb³⁺ system under near infrared excitation is discussed. It is concluded that the green emission is excited by energy transfers from Yb³⁺ to Ho³⁺, whereas excited state absorption is involved in the excitation of red emission. The emission cross-section of the ⁵F₅→⁵I₈ transition at about 660 nm was estimated by using the Füchtbauer–Ladengurg formula. PACS 78.55.Hx; 78.20.-e     

Infrared-to-visible up-conversion luminescence and energy transfer of RE3+/Yb3+(RE = Ho,Tm) co-doped SrIn2O4

Near-infrared excited up-conversion phosphors of RE³⁺/Yb³⁺(RE = Ho, Tm) co-doped SrIn₂O₄ were synthesized by a solid-state reaction method. X-ray diffraction analysis revealed the phase composition of those samples, and the up-conversion spectroscopic properties were studied in terms of up-conversion emission spectra. Under 980 nm near-infrared laser excitation, strong green emission with the peak at 546 nm was observed in SrIn₂O₄: Ho³⁺/Yb³⁺, which can be assigned to the characteristic ⁵S₂(⁵F₄) → ⁵I₈ transition of Ho³⁺. Furthermore, SrIn₂O₄: Tm³⁺/Yb³⁺ showed bright blue emission with the peak at 486 nm, which is associated with the ¹G₄ → ³H₆ transition of Tm³⁺. The UC power studies indicated that the luminescence of SrIn₂O₄: Ho³⁺/Yb³⁺ and SrIn₂O₄: Tm³⁺/Yb³⁺ are attributed to two-photon and three-photon process, respectively. The possible UC luminescence mechanism and energy transfer in SrIn₂O₄: RE³⁺/Yb³⁺ were discussed.     

A comparison of selected organic tracers for quantitative scalar imaging in the gas phase via laser-induced fluorescence

Single crystal of La₃Ga_5.5Ta_0.5O₁₄ (LGT) containing intentionally 0.5 % of Ho³⁺ and 1 % of Yb³⁺ was grown by the Czochralski method. Examination of chemical composition of the grown crystal revealed that luminescent holmium and ytterbium ions are preferably retained in the melt and their actual concentrations are 0.12 and 0.24 %, respectively. Spectroscopic investigation performed encompassed IR host absorption spectra and Raman spectra at room temperature, optical absorption and luminescence spectra of Ho³⁺ and Yb³⁺ at room temperature and at 5 K, and luminescence decay curves at room temperature. It was found that all spectral bands recorded show important inhomogeneous line broadening. This feature was attributed to structural disorder inherent to the crystal lattice in which pentavalent Ta⁵⁺ ions occupy octahedral Ga(1) sites together with trivalent Ga³⁺ ions. Despite small concentrations of luminescent ions, the occurrence of nonradiative interaction that feeds the ⁵I₆ and ⁵I₇ levels of Ho³⁺ ions by transfer of an excitation from the ²F_5/2 level of Yb³⁺ ions was evidenced. Based on examination of spectroscopic parameters evaluated, it was concluded that LGT:Ho, Yb may be considered as a potential intermediate-gain laser active material able to emit infrared radiation from the ⁵I₇ → ⁵I₈ transition of Ho³⁺ around 2 micrometres upon laser-diode pumping into Yb³⁺ absorption band.     

Single color upconversion emission in Ho/Yb and Tm/Yb doped P2O5-MgO2-Sb2O3-MnO2-AgO glasses

The Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ doped P₂O₅-MgO₂-Sb₂O₃-MnO₂-AgO glasses were prepared by high temperature melting method. Under a 975 nm laser diode (LD) excitation, the single red and single blue upconversion (UC) emissions were observed in Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ doped samples, respectively. By studying the spontaneous radiative and multiphonon relaxation probabilities, we find that the multiphonon relaxation probability of ⁵I₆ (Ho³⁺) state is very large (1.39 × 10⁶ s^− 1), which is helpful to the population of ⁵I₇ state. The multiphonon relaxation probability of ³H₅ and ³F_2,3 (Tm³⁺) is also very large, which results in lots of population in ³F₄ and ³H₄ states. The results are that the red UC emission of Ho³⁺ and the blue UC emission of Tm³⁺ are stronger.     

Local symmetry distortion-induced enhancement of upconversion luminescence in Gd2O3:Ho3+/Yb3+/Zn2+ nanoparticles for solid-state lighting and bioimaging

The Ho³⁺/Yb³⁺/Zn²⁺-tridoped Gd₂O₃ nanoparticles were prepared by a simple urea-based homogeneous precipitation method. Under near-infrared (NIR) light excitation, all the synthesized nanoparticles exhibit bright green and red upconversion (UC) emissions corresponding to the intra-4f transitions of Ho³⁺ ions and the UC mechanism is found to be a two-photon process. With the introduction of Zn²⁺ ions, not only the local symmetry surrounding the dopants is decreased, but also the UC emission intensity is also enhanced, which is further verified by the Judd-Ofelt theory. The temperature-dependent UC emission spectra were recorded to examine the thermal stability of the final products. From theoretical calculations, the activation energy is found to be about 0.18 eV. A novel green light-emitting diode device, which consists of the resultant nanoparticles and a NIR chip, was fabricated to examine their suitability for solid-state lighting. Meanwhile, the synthesized nanoparticles exhibit low cytotoxicity in various cell lines, suggesting their potential applications in in vivo UC luminescence imaging. Additionally, the applicability of the Ho³⁺/Yb³⁺/Zn²⁺-tridoped Gd₂O₃ nanoparticles for in vivo bioimaging applications was also analyzed.     

Multi-phonon-assisted relaxation and Yb3+ sensitized bright red-dominant upconversion luminescence of Ho3+ in YF3-BaF2-Ba(PO3)2 glass

Unusual bright red-dominant upconversion light was observed in Ho³⁺/Yb³⁺ co-doped YF₃-BaF₂-Ba(PO₃)₂ glasses excited by the 980-nm laser diode at room temperature. The integral intensity ratios of the red upconversion emission to the green one reached about 10:1 in optimized 0.125Ho³⁺-15Yb³⁺ co-doped sample. In order to find out its behind-the-scene mechanism, the optical properties and the phonon-assisted relaxations on the excited levels of Ho³⁺ in our samples were investigated. Additionally, the effects of the concentrations of the doping ions, excitation pump power, and temperature on the upconversion emissions were also systematically studied. These results revealed that the proper phonon frequency of fluorophosphate glasses, the efficient phonon-assisted relaxations from ⁵I₆ to ⁵I₇ levels (4,960 s^?1), and the long lifetime of the ⁵I₇ (about 2.8 ms) levels should be responsible for bright red upconversion emission at a much greater concentration ratio of C _Yb ³⁺ /C _Ho ³⁺ .     

Sensitizer-dependent up-conversion of Ho<Superscript>3+</Superscript> in nanocrystalline Y<Subscript>2</Subscript>O<Subscript>3</Subscript>

Ho³⁺–Yb³⁺ co-doped Y₂O₃ nanocrystals were synthesized by firing hydroxy carbonate precursors. Yb³⁺-concentration-dependent up-conversion properties of Ho³⁺ in Y₂O₃ nanocrystals have been investigated. The relative intensity of up-converted red emission increases more quickly than that of the green and the near-infrared ones with the enhancement of the concentration of Yb³⁺. It is believed that the energy process ⁵ S ₂ (⁵F₄) (Ho) + ⁵ I ₇ (Ho) →⁵ I ₆ (Ho)+⁵ F ₅ (Ho) plays an important role in the population of the ⁵ F ₅ level of Ho³⁺. The result indicates that the intensity ratio of the green emission to the red one can be tuned by changing the sensitizer concentration. PACS 78.55.-m     

Laser induced optical heating from Yb3+/Ho3+:Ca12Al14O33 and its applicability as a thermal probe

Yb³⁺/Ho³⁺ co-doped calcium aluminate phosphor has been synthesized using solution combustion process. Multicolored (blue, green and red) strong upconversion emission (λ_exc=980 nm) due to Ho³⁺ ion is observed which shows a color tunability (from green to red) with a variation in input laser power. The color tunability has been attributed to be due to the induced heating in the local volume of the sample and the temperature produced has been estimated using the fluorescence intensity ratio (FIR) method. The sample shows temperature sensing behavior and more importantly the temperature could be sensed through two pairs of thermally coupled levels, one lying in the green region (⁵F₄/⁵S₂→⁵I₈) and the other in the blue region (⁵G₄/⁵G₅→⁵I₈). The temperature sensing through the blue pair of levels is novel in itself. The material thus prepared serves as temperature sensor as well as a source for the production of heat in a localized volume.     

Downconversion for Solar Cells in YF3:Pr3+, Yb3+

ABSTRACT

Energy losses in solar cells caused by the spectral mismatch can be reduced by adapting the solar spectrum using a downconversion material where one higher energy visible photon is ‘cut' into two lower energy near-infrared photons that both can be absorbed by the solar cell. Downconversion with the (Pr³⁺, Yb³⁺) couple in YF₃ is investigated. Based on analysis of luminescence and diffuse reflectance spectra it is evident that two-step energy transfer takes place from the ³P₀ level of Pr³⁺ (around 490 nm) exciting two Yb³⁺ to the ²F_5/2 level giving emission around 980 nm. The transfer efficiency increases with Yb³⁺ concentration and is 86% for YF₃ doped with 0.5% Pr³⁺ and 30% Yb³⁺. Due to concentration quenching the intensity of emission from Yb³⁺ is strongly reduced and the ²F_5/2 emission intensity reaches a maximum for the sample with 0.5% Pr³⁺ and 2–5% Yb³⁺ at 300 K. Temperature dependent measurements reveal the role of the Pr^{3+ 1}G₄ level in the energy transfer between Pr³⁺ and Yb³⁺. Back-transfer of excitation energy from the Yb^{3+ 2}F_5/2 level to the ¹G₄ level of Pr³⁺ occurs and quenches the Yb³⁺ emission. The quenching is shown to become more efficient between 4 and 50 K due to faster phonon-assisted energy transfer between the Yb³⁺ donors. Upon raising the temperature from 50 to 300 K, the luminescence life time of the Yb³⁺ emission increases again because the small energy difference between the Pr³⁺ (¹G₄) level and the Yb³⁺ (²F_5/2) level (～300 cm^?1) which makes the ¹G₄ less efficient as a trap for the excitation energy. The present results give insight into factors involved in the concentration quenching in downconversion materials based on the (Pr³⁺, Yb³⁺) couple.     

A novel scheme to acquire enhanced up-conversion emissions of Ho3+ and Yb3+ co-doped Sc2O3

A detailed investigation about the effect of Sc₂O₃: 1 mol%Ho³⁺/5 mol%Yb³⁺ co-doped with Ce⁴⁺ ions prepared by sol-gel methods was performed systematically. Under the excitation of 980 nm laser diode, both green emission (553 nm, ⁵F₄/⁵S₂→⁵I₈) and red emission (672 nm, ⁵F₅→⁵I₈) were both observed in the emission spectra of the samples, which were found to be two-photon process and sensitized by Yb³⁺ ions. With the increasing of Ce⁴⁺ ions, the up-conversion green emission intensity are increased by 6.52, 8.69, 10.85, 13.92 and 16.66 fold, corresponding to the Ce⁴⁺ ions concentrations from 5 mol% to 13 mol%, respectively. The number of photons are necessary to populate the upper emitting state decreases to 2 and the infrared absorption coefficient is reduced, when the Ce⁴⁺ ions concentration increase to 13 mol%. Ce⁴⁺ ions play an important role in tailoring the local crystal field around Ho³⁺ ions, lowering the highest phonon cut-off energy of matrix and reducing the infrared absorption coefficient, thus hindering the non-radiative processes, which contribute to the increased emission intensity. The excellent enhancement makes it a promising multifunctional optical luminescence material.