Effect of Yb concentration on the upconversion of Er ion doped La2O3 nanocrystals under 980 nm excitation

The effect of Yb³⁺ concentration on the upconversion of La₂O₃:Yb³⁺, Er³⁺ nanocrystals was reported. Green (about at 530 and 549 nm) and red (around at 672 nm) upconversion emissions under 980 nm excitation were observed at room temperature. It was found that the ratio of green to red upconversion emission intensity is considered as a function of Yb³⁺ ion concentration. Of the samples doped with varying Er³⁺ or constant Er³⁺ ion concentration, it can be observed that the intensity ratio drastically decreases with an Yb³⁺ ion concentration increase and the Yb³⁺ ions concentration is around 3 mol% as the emission intensity ratio of green to red upconversion is close to 1.     

Frequency upconversion properties of Er3+/Yb3+-codoped lead–germanium–bismuth oxide glasses

The frequency upconversion properties of Er³⁺/Yb³⁺-codoped heavy metal oxide lead–germanium–bismuth oxide glasses under 975 nm excitation are investigated. Intense green and red emission bands centered at 536, 556 and 672 nm, corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively, were simultaneously observed at room temperature. The influences of PbO on upconversion intensity for the green (536 and 556 nm) and red (672 nm) emissions were compared and discussed. The optimized rare earth doping ratio of Er³⁺ and Yb³⁺ is 1:5 for these glasses, which results in the stronger upconversion fluorescence intensities. The dependence of intensities of upconversion emission on excitation power and possible upconversion mechanisms were evaluated and analyzed. The structure of glass has been investigated by means of infrared (IR) spectral analysis. The results indicate that the Er³⁺/Yb³⁺-codoped heavy metal oxide lead–germanium–bismuth oxide glasses may be a potential materials for developing upconversion fiber optic devices.     

Ultraviolet upconversion luminescence in Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript>,Tm<Superscript>3+</Superscript> nanocrystals and its application in photocatalysis

Infrared-to-ultraviolet upconversion luminescence agent Y₂O₃:Yb³⁺,Tm³⁺ was prepared by a combustion method using citrate as a fuel/reductant. The prepared sample was characterized by X-ray diffraction, SEM, and fluorescence spectrophotometer. Two unusual ¹I₆ → ³H₆ (~297 nm) and ¹D₂ → ³H₆ (~363 nm) emissions from Tm³⁺ ions were observed at room temperature under 980-nm laser excitation. The change of upconversion emission intensity depending on the Yb³⁺ concentrations was discussed. The results showed that modest Yb³⁺ doping could make the upconversion emission of Tm³⁺ intense, and high Yb³⁺ concentrations might lead to fluorescence quenching. Moreover, the influence of ultraviolet upconversion luminescence on the photodegradation of methyl orange aqueous solution under solar light irradiation in the presence of TiO₂ catalyst doped with Y₂O₃:Yb³⁺,Tm³⁺ was also investigated. It was concluded from the experiment of this study that TiO₂/Y₂O₃:Yb³⁺,Tm³⁺ composite had higher photocatalytic activity than pure TiO₂ under solar light. This study would make TiO₂ utilize sunlight more efficiently and accelerate the practical application of photocatalytic technology in water treatment region.     

Microstructure and upconversion luminescence of Yb<Superscript>3+</Superscript> and Ho<Superscript>3+</Superscript> co-doped BST thick films

Ba_0.8Sr_0.2TiO₃ (BST) thick films co-doped with Yb³⁺ and Ho³⁺ were fabricated by the screen printing techniques on alumina substrates. The structure and morphology of the BST thick films were studied by XRD and SEM, respectively. After sintered at 1240 °C for 100 min the BST thick films are polycrystalline with a perovskite structure. The upconversion luminescence properties of the RE-doped BST thick films under 800 nm excitation at room temperature were investigated. The upconversion emission bands centered at 470 and 534 nm corresponding to ⁵F₁ → ⁵I₈ and ⁵F₄ → ⁵I₈ transition, respectively were observed, and the upconversion mechanisms were discussed. The dependence of the upconversion emission intensity upon the Ho³⁺ ions concentration was also examined; the emission intensity reaches a maximum value in the sample with 2 mol% Yb³⁺ and 0.250 mol% Ho³⁺ ions. All the results show that the BST thick films co-doped with Yb³⁺ and Ho³⁺ may have potential use for photoelectric devices.     

Bifunction in Er3+/Yb3+ co-doped BaTi2O5–Gd2O3 glasses prepared by aerodynamic levitation method

Novel Er³⁺/Yb³⁺ co-doped BaTi₂O₅–Gd₂O₃ spherical glasses have been fabricated by aerodynamic levitation method. The thermal stability, upconversion luminescence, and magnetic properties of the present glass have been studied. The glasses show high thermal stability with 763.3 °C of the onset temperature of the glass transition. Red and green emissions centered at 671 nm, 548 nm and 535 nm are obtained at 980 nm excitation. The upconversion is based on a two-photon process by energy transfer, excited-state absorption, and energy back transfer. Yb³⁺ ions are more than Er³⁺ ions in the glass, resulting in efficient energy back transfer from Er³⁺ to Yb³⁺. So the red emission is stronger than the green emissions. Magnetization curves indicate that magnetic rare earth ions are paramagnetic and the distribution is homogeneous and random in the glass matrix. Aerodynamic levitation method is an efficient way to prepare glasses with homogeneous rare earth ions.     

An experimental design approach for hydrothermal synthesis of NaYF4: Yb3+, Tm3+ upconversion microcrystal: UV emission optimization

Ultraviolet (UV) emissions of hydrothermally synthesized NaYF₄: Yb³⁺, Tm³⁺ upconversion crystals were optimized using the response surface methodology experimental design. In these experimental designs, 9 runs, two factors namely (1) Tm³⁺ ion concentration, and (2) pH value were investigated using 3 different ligands. Introducing UV upconversion emissions as responses, their intensity were separately maximized. Analytical methods such as XRD, SEM, and FTIR could be used to study crystal structure, morphology, and fluorescent spectroscopy in order to obtain luminescence properties. From the photo-luminescence spectra, emissions centered at 347, 364, 452, 478, 648 and 803 nm were observed. Some results show that increasing each DOE factor up to an optimum value resulted in an increase in emission intensity, followed by reduction. To optimize UV emission, as a final result to the UV emission optimization, each design had a suggestion.     

808 nm triggered multifunctional UCNPs@PDA nanocomposites for temperature sensing and photothermal conversion

Multifunctional nanoparticles with marvelous temperature sensing and photothermal conversion are preferable in photothermal therapeutic applications. Herein, nanocomposites of NaYF₄:Er³⁺/Yb³⁺@NaYF₄:Nd³⁺/Yb³⁺ core–shell upconversion nanoparticles (UCNPs) modified by polydopamine (PDA) were designed to achieve temperature sensing and efficiency of photothermal conversion. It has been found that under the irradiation of 808 nm laser, temperature sensing can be realized on the basis of the fluorescence intensity ratio of Er³⁺ green light emission. In addition, the photothermal conversion ability of the UCNPs modified by PDA is greatly improved. Moreover, Nd³⁺ as activator endows UNCPs excited by 808 nm, which can avoid overheating due to the absorption of the excitation band (980 nm) in water. Therefore, the multifunctional nanoparticles are expected to become an effective drug for cancer therapy in the future and this research will stimulate interest in designing multifunctional nanoparticles that are biocompatible, especially for in vivo tumor diagnosis.

     

Yellow lighting upconversion from Yb3+/Ho3+ co-doped CaMoO4

Yellow upconversion (UC) luminescence is observed in Ho³⁺/Yb³⁺ co-doped CaMoO₄ synthesized by complex citrate-gel method. Under 980 nm excitation, Ho³⁺/Yb³⁺ co-doped CaMoO₄ exhibited yellow emission based on green emission near 543 nm generated by ⁴F₄, ⁵S₂ → ⁵I8 transition and strong red emission around 656 nm generated by ⁵F₅ → ⁵I₈ transition, which are assigned to the intra 4f transitions of Ho³⁺ ions. The optimum doping concentration of Ho³⁺ and Yb³⁺ was investigated for highest upconversion luminescence. Based on pump power dependence, upconversion mechanism of Ho³⁺/Yb³⁺ co-doped CaMoO₄ was studied in detail.     

Yb3+ and Er3+ co-doped Y2Ce2O7 nanoparticles: synthesis and spectroscopic properties

Yb³⁺ and Er³⁺co-doped Y₂Ce₂O₇ nanoparticles sintered at different temperatures were prepared by homogeneous co-precipitation method. The products were characterized by X-ray powder diffraction (XRD), energy-dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The results indicated that the particle sizes and morphologies of the samples were heavily influenced by the sintering temperature. As temperature increased, the particle sizes became gradually larger and more agglomerate. The emissions including green and red upconversion emissions were investigated under 980 nm excitation. The emission intensities of the samples also depended on the sintering temperature. Two photon processes were mainly responsible for green and red upconversion emissions.     

Luminescence properties of Ca0.65La0.35F2.35:Yb, Er with enhanced red emission via upconversion

Yb³⁺/Er³⁺ codoped Ca_0.65La_0.35F_2.35 materials with intense red emission via upconversion were prepared by a high temperature solid-state method. Based on the upconversion luminescence properties investigations, it was found that, under 980 nm excitation, Ca_0.65La_0.35F_2.35:20 mol.%Yb³⁺, xEr³⁺ showed intense red upconversion luminescence, which was ascribed to ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺, although both green and red emissions could be detected. It was also found that the green and red emissions originated the two photon processes, and the ground-state absorption (GSA), excited-state absorption (ESA) and energy transfer (ET) processes between Er³⁺/Yb³⁺ ions and Er³⁺/Er³ ions were involved in the enhanced red emission mechanism.     

Investigation of near-infrared-to-ultraviolet upconversion luminescence of Tm doped NaYF4 phosphors by Yb codoping

2 mol% Tm³⁺ doped NaYF₄ phosphors with 0–98 mol% Yb³⁺ codoping were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single cubic phase of Tm³⁺–Yb³⁺:NaYF₄ phosphors was investigated with increasing of Yb³⁺ concentration. Near-infrared, red, blue, violet and ultraviolet upconversion emissions of Tm³⁺ were observed from the Tm³⁺–Yb³⁺:NaYF₄ phosphors under 976 nm laser diode excitation, with the strongest near-infrared to ultraviolet emissions at 20 mol% Yb³⁺ codoping. The violet and blue emissions for the ¹D₂ → ³F₄ and ¹G₄ → ³H₆ transitions of Tm³⁺ can be tuned by varying Yb³⁺ codoping concentration, which was elucidated using steady-state equations. The intensity ratio of red emissions for the ³F₂ → ³H₆ and ³F₃→³H₆ transitions of Tm³⁺ was strongly related to the Yb³⁺ codoping concentration and temperature, implying a potential application of Tm³⁺–Yb³⁺:NaYF₄ phosphors for optical temperature sensing.     

Upconversion luminescence of holmium and ytterbium co-doped yttrium oxysulfide phosphor

The upconversion optical characteristic and brightness of Y₂O₂S:Yb,Ho phosphors are investigated. It is shown that Y₂O₂S:Yb,Ho exhibits NIR, red, green, blue and even ultraviolet-blue emission bands under 980 nm pumping. Moreover, Y₂O₂S:Yb,Ho shows excellent upconversion luminescence and its upconversion brightness is 2.2 times to that of the commercial Y₂O₂S:Yb,Er phosphors. The upconversion mechanism is discussed: the green and NIR emissions are due to two consecutive energy transfer from Yb³⁺ to Ho³⁺, and the blue and red emissions can be explained as a loop-like mechanism.     

ZnAl2O4 co-doped with Yb3+/Er3+ prepared by combustion reaction: evaluation of photophysical properties

Zinc aluminate co-doped with Yb³⁺/Er³⁺ for potential application in upconversion lasers was prepared in proportions of 2:1, 5:1, and 10:1 by combustion reaction. The samples were characterized by X-ray diffraction, transmission electron microscopy, and optical spectroscopy. The results reveal the formation of crystalline ZnAl₂O₄ primary phase and trace amounts of ZnO and Yb₃Al₅O₁₂ secondary phases. They also indicate that increasing the Yb³⁺/Er³⁺ ratio favored the increase of secondary phases. The optical spectroscopy analysis revealed that red emission predominated over green, and that emission intensity was directly influenced by the infrared intensity of the diode laser. These results indicate that ZnAl₂O₄ doped with rare earth ions may also be an interesting material for luminescence obtained by energy upconversion.     

Upconversion emission in antimony–germanate double-clad optical fiber co-doped with Yb3+/Tm3+ ions

In the paper upconversion luminescence properties in Yb³⁺/Tm³⁺ co-doped antimony–germanate glass and double-clad optical fiber were studied. The concentration of lanthanides, which has shown the highest upconversion emission intensity at 478 nm (¹G₄ → ³H₆) and 650 nm (¹G₄ → ³F₄), is 1Yb₂O₃/0.1Tm₂O₃ (mol%) as a result of exciting with a laser diode (976 nm). The lifetime of ²F_5/2 (Yb³⁺) level decreases from 781 μs to 71 μs in the presence of Tm³⁺ 0.1–0.75 mol% respectively. Luminescence decay curve of glass co-doped with 1Yb₂O₃/0.75Tm₂O₃ suggests donor–donor fast migration followed by Tm³⁺ → Yb³⁺ energy transfer. Glass characterized by highest intensity of upconversion luminescence (1Yb₂O₃/0.1Tm₂O₃ mol%) was used as core of double-clad optical fiber made by modified rod-in-tube method. Mechanisms influencing differences in upconversion amplified spontaneous emission of the fabricated optical fiber and bulk glass were discussed. Reabsorption of the amplified spontaneous emission signal along the fibre resulting from Tm³⁺:³H₆ → ¹G₄, transition was observed.     

Frequency upconversion fluorescence studies of Er/Yb-codoped KNbO3 phosphors

Different concentrations of Er³⁺ and Yb³⁺ ions-doped potassium niobate (K_0.9NbO₃:Yb_(x)Er_{(0.1 − x)} for x = 0, 0.01, 0.05, 0.09 and 0.1) polycrystalline powder phosphors were prepared by the conventional solid state reaction method and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Energy transfer and upconversion fluorescence properties of the Yb³⁺ and Er³⁺-codoped phosphors have been discussed. The XRD data has shown mono-phase for pure KNbO₃ while the doped samples represented additional phase formation. The SEM micrographs represented the rectangular crystal growth habit for the KNbO₃ phosphors when doped with 0.1 mol of Er³⁺ ions. An intense green emission at 557 nm along with a red emission at 674 nm was observed when the doped samples were excited with 975 nm IR radiation. The upconversion mechanism has been discussed based on the excited state absorption and energy transfer mechanisms.     

Effect of CdF2 addition on thermal stability and upconversion luminescence properties in Tm–Yb codoped oxyfluoride silicate glasses

Tm³⁺–Yb³⁺ codoped oxyfluoride silicate glasses suitable for upconversion laser has been fabricated. In this paper, effect of CdF₂ addition on thermal stability and upconversion luminescence properties in Tm³⁺–Yb³⁺ codoped oxyfluoride silicate glasses have been systematically investigated. The experimental results indicate that, with the substitution CdF₂ for PbF₂, the glass thermal stability increases and the UV cutoff edge moves to short-wave band slightly. With increasing CdF₂ content, the blue and red upconversion luminescence intensity increases slightly at first, and then increases rapidly. While the near infrared (NIR) upconversion emission intensity increases notably at first and then increases slightly. However, the blue and NIR luminescence intensity are much stronger than that of red, indicating these oxyfluoride silicate glasses are more preferable for blue and NIR emissions than red emission. The possible upconversion mechanisms for the blue, red and NIR fluorescence are also estimated and evaluated.     

Infrared to visible upconversion emissions of La1-0.02-xEr0.02YbxOF nanorods

In this work, well-crystallized La_1-0.02-xEr_0.02Yb_xOF (x = 0, 0.08, 0.06, 0.04, 0.02) nanorods were prepared by annealing the hydrothermally synthesized La_1-0.02-xEr_0.02Yb_xF₃ nanocrystals at ambient condition. The structural properties of these samples were characterized by XRD and TEM (HRTEM), the results show that the nanorods have a pure tetragonal structure and a preferential growth along the (1 0 1) direction, the average length and cross-section radius are 340 nm and 65 nm. Excited by 984 nm continuous wave laser, strong red (643-677 nm), strong green (515-557 nm) and weak violet (382-417 nm) upconversion emissions have been observed in these nanorods. The influence of Yb³⁺ concentration on the upconversion emission intensity has been discussed. The possible upconversion mechanisms were proposed through research on pump power dependence of upconversion integrated intensity. Especially, both two-photon process and three-photon process were contributed to the green upconversion in lower Yb³⁺ concentration doped nanorods.     

Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca,Sr; B = W,Mo) phosphors

ABO₄ (A = Ca, Sr; B = W, Mo):Er³⁺/Yb³⁺/Li⁺ phosphors tri-doped with different concentrations of Li⁺ ion ranging from 0 to 22.5 mol% were prepared by using a solid-state reaction method. And their upconversion (UC) luminescence properties were in estimated under a 975 nm laser-diode excitation. The four kinds of phosphors (CaWO₄, CaMoO₄, SrWO₄, and SrMoO₄) tri-doped with Er³⁺, Yb³⁺ and Li⁺ ions showed strong green UC emission peaks at 530 nm and 550 nm and weak red UC emission. The intensity of green UC emission of Li⁺ doped samples was several higher than that of Li⁺ un-doped samples due to the reduction of lattice constant and the local crystal field distortion around rare-earth ions. The optimum doping concentration of Li⁺ ions was investigated and the effects of Li⁺ concentration for UC emission intensity were studied in detail.     

Synthesis of LaF3: Yb3+ ,Ln3+ nanoparticles with improved upconversion luminescence

Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ co-doped LaF₃ nanoparticles with upconversion luminescence properties were prepared via the co-precipitation method, followed by heat treatment at different temperatures in the range of 180°C to 600°C. We investigated the influence of heat treatment temperatures on the size, morphology, and upconversion luminescence intensity of the nanoparticles. Significant increases of the particle size and upconversion luminescence intensity of the nanoparticles were observed with increasing heat treatment temperature. The upconversion mechanism of the LaF₃:Yb³⁺,Er³⁺ and LaF₃:Yb³⁺,Tm³⁺ nanoparticles was also discussed.     

Intense frequency upconversion luminescence in Yb<Superscript>3+</Superscript>/Tm<Superscript>3+</Superscript>-codoped oxychloride germanate glasses

Yb³⁺/Tm³⁺-codoped oxychloride germanate glasses for developing potential upconversion lasers have been fabricated and characterized. Structural properties were obtained based on the Raman spectra analysis, indicating that PbCl₂ plays an important role in the formation of glass network and has an important influence on the maximum phonon energies of host glasses. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions ¹G₄ → ³H₆ and ¹G₄ → ³H₄, respectively, were observed at room temperature. With increasing PbCl₂ content, the intensity of blue (477 nm) emission increases significantly, while the red (650 nm) emission increases slowly. The results indicate that PbCl₂ has more influence on the blue emissions than the red emission in oxychloride germanate glasses. The possible upconversion mechanisms are discussed and estimated. Intense blue upconversion luminescence indicates that these oxychloride germanate glasses can be used as potential host material for upconversion lasers.