Upconversion Luminescence in Mononuclear Yb/Sm Co-crystal Assemblies at Room Temperature

Metal-based upconversion luminescence transforming high-energy photons into low-energy photons is an attractive anti-Stokes shift process for fundamental research and promising applications. In this work, we developed the upconversion luminescence in co-crystal assemblies consisting of discrete mononuclear Yb and Sm complexes. The characteristic visible emissions of Sm³⁺ were observed under the excitation of absorption band of Yb³⁺ at 980 nm. A series of co-crystal assemblies were investigated based on mononuclear Yb and Sm complexes, and the strongest luminescence was obtained when the molar concentration between Yb³⁺ and Sm³⁺ is equivalent. The crystal structure was fully characterized by the single crystal X-ray diffraction and upconverting energy transfer mechanisms were verified as cooperative sensitization upconversion and energy transfer upconversion. This is the first example of Sm³⁺-based upconverting luminescence in discrete lanthanide complexes which present as co-crystal assemblies at room temperature.     

Upconversion Luminescence through Cooperative and Energy-Transfer Mechanisms in Yb3+-Metal-Organic Frameworks

Lanthanide-doped metal–organic frameworks (Ln-MOFs) have versatile luminescence properties, however it is challenging to achieve lanthanide-based upconversion luminescence in these materials. Here, 1,3,5-benzenetricarboxylic acid (BTC) and trivalent Yb³⁺ ions were used to generate crystalline Yb-BTC MOF 1D-microrods with upconversion luminescence under near infrared excitation via cooperative luminescence. Subsequently, the Yb-BTC MOFs were doped with a variety of different lanthanides to evaluate the potential for Yb³⁺-based upconversion and energy transfer. Yb-BTC MOFs doped with Er³⁺, Ho³⁺, Tb³⁺, and Eu³⁺ ions exhibit both the cooperative luminescence from Yb³⁺ and the characteristic emission bands of these ions under 980 nm irradiation. In contrast, only the 497 nm upconversion emission band from Yb³⁺ is observed in the MOFs doped with Tm³⁺, Pr³⁺, Sm³⁺, and Dy³⁺. The effects of different dopants on the efficiency of cooperative luminescence were established and will provide guidance for the exploitation of Ln-MOFs exhibiting upconversion.     

Synthesis and Upconversion Luminescence in LaF3:Yb3+, Ho3+, GdF3: Yb3+, Tm3+ and YF3:Yb3+, Er3+ obtained from Sulfide Precursors

Rare earth fluorides are mainly obtained from aqueous solutions of oxygen‐containing precursors. Probably, this method is simple and efficient, however, oxygen may partially be retained in the fluoride structure. We offer an alternative method: obtaining fluorides and solid solutions based on them from an oxygen‐free precursor. As starting materials, we choose sulfides of rare‐earth elements and solid solutions based on them. The fluorination is carried out by exposure to hydrofluoric acid of various concentrations. The transmission electron microscopy images revealed the different morphologies of the products, which depend on the concentration of the fluorinating component (HF) and the host element. The solid solution particle size varied from 30–35 nm in the case of GdF₃:Yb³⁺, Tm³⁺ (4 % HF) to larger structures with dimensions exceeding 200 nm, such as that for LaF₃:Yb³⁺, Ho³⁺ (40 % HF). The thermal characteristics, such as the temperatures of the transitions and melting and enthalpies, were determined for the solid solutions and simple fluorides. Applicability of the materials obtained as biological luminescent markers was tested on the example of upconversion luminescence, and good upconversion properties were detected.     

Er3+-Tm3+-Yb3+ tri-doped CaMoO4 upconverting phosphors in optical devices applications

The structural and optical properties of the Er³⁺-Tm³⁺-Yb³⁺codoped CaMoO₄ phosphors prepared by chemical route have been explored. The crystalline structures of the prepared phosphors have been investigated with the help of X-ray diffraction analysis. The presence of different vibrational modes and absorption bands arising due to the transitions from the ground state to different excited states of rare earth ions have been identified using the Raman and UV-VIS-NIR absorption spectra of the developed phosphor, respectively. The concentration quenching effect on the luminescence property of the prepared materials has been explained in detail. The upconversion luminescence property of the Er³⁺-Tm³⁺-Yb³⁺codoped CaMoO₄ phosphor annealed at different temperatures under 980 nm and 808 nm excitations have been reported. The energy transfer Er³⁺ → Tm³⁺, Yb³⁺ → Er³⁺ and Tm³⁺ has been found to be responsible for efficient UC emission. The dipole-dipole interaction is observed to be responsible for the concentration quenching of the luminescence intensity. The effect of annealing temperature on the upconversion luminescence property has been explained in detail. The results suggest that the developed tri-doped phosphor may be suitable in making the efficient NIR to visible upconverter and lighting based optical devices.     

Nd3+/Yb3+离子共掺的氧化钛基上转换发光玻璃的热学和力学性能

利用气悬浮无容器技术制备出了Nd³⁺/Yb³⁺稀土离子共掺杂的TiO₂-La₂O₃-ZrO₂(TLZ)发光玻璃. 利用差热分析(DTA)技术研究了该类新型稀土掺杂TiO₂基上转换发光玻璃的热稳定性,主要包括玻璃化转变温度、析晶起始温度以及析晶峰值温度. 并采用两种热分析动力学计算方法得到TLZ玻璃的析晶活化能值和指前因子.本文还研究了TLZ 发光玻璃的力学性能,发现其维氏硬度大小为7.50 GPa,断裂韧性大于1.20 MPa·m^1/2. 此外,还对TLZ玻璃在808 nm激光激发下的上转换发光性能进行了研究,实验结果显示光谱中有三个强发射谱峰. 优异的上转换发光性能以及良好的热稳定性和机械性能表明,这类新材料在上转换器件的实际应用中具有很大的潜力.     

Investigation on the upconversion luminescence of Sr3AlO4F:Yb3+, Er3+, Ho3+ phosphors

To develop new emission-tunable upconversion (UC) phosphors, the Sr₃AlO₄F:5%Yb³⁺, xEr³⁺, yHo³⁺ (0 ≤ x ≤ 1%, 0 ≤ y ≤ 1%) samples were prepared by conversional solid-state reaction method, and their luminescence properties upon 980 nm excitation were studied. Upon 980 nm excitation, Yb³⁺-Er³⁺ codoped Sr₃AlO₄F shows a predominant emission peak between 645 and 700 nm which is attributed to the ⁴F_9/2-⁴I_15/2 transition of Er³⁺, and the Er³⁺ green emissions have been almost quenched. In this case, the yellowish green emitting light is obtained. The possible reason was interpreted by the energy level diagram and the proposed UC mechanism. For Yb³⁺-Ho³⁺ codoped Sr₃AlO₄F, three emissions are observed obviously which are all derived from the Ho³⁺ ion. The corresponding chromaticity coordinates indicate a red emission has been gained. To realize the tunable emission, the typical Sr₃AlO₄F:5%Yb³⁺, 0.2%Er³⁺, 1%Ho³⁺ phosphor was developed, and its emission spectrum includes the emission peaks of both Er³⁺ and Ho³⁺. Correspondingly, the sample gives a yellow emission.     

Frenkel Defect-modulated Anti-thermal Quenching Luminescence in Lanthanide-doped Sc2(WO4)3

Although large amount of effort has been invested in combating thermal quenching that severely degrades the performance of luminescent materials particularly at high temperatures, not much affirmative progress has been realized. Herein, we demonstrate that the Frenkel defect formed via controlled annealing of Sc₂(WO₄)₃:Ln (Ln=Yb, Er, Eu, Tb, Sm), can work as energy reservoir and back-transfer the stored excitation energy to Ln³⁺ upon heating. Therefore, except routine anti-thermal quenching, thermally enhanced 415-fold downshifting and 405-fold upconversion luminescence are even obtained in Sc₂(WO₄)₃:Yb/Er, which has set a record of both the Yb³⁺-Er³⁺ energy transfer efficiency (>85 %) and the working temperature at 500 and 1073 K, respectively. Moreover, this design strategy is extendable to other hosts possessing Frenkel defect, and modulation of which directly determines whether enhanced or decreased luminescence can be obtained. This discovery has paved new avenues to reliable generation of high-temperature luminescence.     

Synthesis and surface modification of ultrasmall monodisperse NaYF4:Yb3+/Tm3+ upconversion nanoparticles

The emerging upconversion nanoparticles (UCNP) have gained substantial consideration in the field of bioanalytical as well as diagnostic applications. Therefore, great progress has been made in the synthesis and surface modification of luminescent UCNPs over the last two decades. In this paper, we have reported monodispersed and high luminescent upconversion nanoparticles NaYF₄: 20%Yb³⁺, 2%Tm³⁺ have been synthesized using a solvothermal method, followed by a coating of the NaYF₄ shell with a thin layer of SiO₂ on the surface to afford the core-shell NaYF₄:Yb³⁺, Tm³⁺@SiO₂ nanoparticles (NP@SiO₂). The prepared nanoparticles were of strong upconversion fluorescent emission intensity, hexagonal phase, and with an average size of about 8 ± 1 nm, which have been characterized by luminescence spectroscopy, powder X-ray diffraction (P-XRD), Dynamic light scattering (DLS), and Transmission electron microscopy (TEM). The results indicate that the NP@SiO₂ can be used for the conjugation of fluorescent probes for various biomolecules and can find applications in cancer cell imaging and disease diagnosis.     

Upconversion Luminescence Properties of a LiNb0.3Ta0.7O3:Er3+,Yb3+ Ceramic

A co-doped LiNb_0.3Ta_0.7O₃:Er³⁺,Yb³⁺ ceramic was prepared by a high temperature solid state procedure. Under the excitation of 980 nm laser radiation, intense 660 nm red light and 550 nm green light emissions corresponding to the ⁴F_9/2→⁴I_15/2 and ²H_11/2/⁴S_3/2→⁴I_15/2 transitions of Er³⁺ were observed. The change of Yb³⁺ concentration has a more significant influence on luminous intensity than the Er³⁺ concentration. The emission of red and green lights is attributed to a two-photon process. The upconversion luminescence mechanisms were analyzed in detail.     

Confining Excitation Energy in Er3+-Sensitized Upconversion Nanocrystals through Tm3+-Mediated Transient Energy Trapping

A new class of lanthanide-doped upconversion nanoparticles are presented that are without Yb³⁺ or Nd³⁺ sensitizers in the host lattice. In erbium-enriched core–shell NaErF₄:Tm (0.5 mol %)@NaYF₄ nanoparticles, a high degree of energy migration between Er³⁺ ions occurs to suppress the effect of concentration quenching upon surface coating. Unlike the conventional Yb³⁺-Er³⁺ system, the Er³⁺ ion can serve as both the sensitizer and activator to enable an effective upconversion process. Importantly, an appropriate doping of Tm³⁺ has been demonstrated to further enhance upconversion luminescence through energy trapping. This endows the resultant nanoparticles with bright red (about 700-fold enhancement) and near-infrared luminescence that is achievable under multiple excitation wavelengths. This is a fundamental new pathway to mitigate the concentration quenching effect, thus offering a convenient method for red-emitting upconversion nanoprobes for biological applications.     

Vacuum ultraviolet and near-infrared excited luminescence properties of Ca3(PO4)2:RE, Na (RE=Tb, Yb, Er, Tm, and Ho)

Tb³⁺, Yb³⁺, Tm³⁺, Er³⁺, and Ho³⁺ doped Ca₃(PO₄)₂ were synthesized by solid-state reaction, and their luminescence properties were studied by spectra techniques. Tb³⁺-doped samples can exhibit intense green emission under VUV excitation, and the brightness for the optimal Tb³⁺ content is comparable with that of the commercial Zn₂SiO₄:Mn²⁺ green phosphor. Under near-infrared laser excitation, the upconversion luminescence spectra of Yb³⁺, Tm³⁺, Er³⁺, and Ho³⁺ doped samples demonstrate that the red, green, and blue tricolored fluorescence could be obtained by codoping Yb³⁺-Ho³⁺, Yb³⁺-Er³⁺, and Yb³⁺-Tm³⁺ in Ca₃(PO₄)₂, respectively. Good white upconversion emission with CIE chromaticity coordinates (0.358, 0.362) is achieved by quadri-doping Yb³⁺-Tm³⁺-Er³⁺-Ho³⁺ in Ca₃(PO₄)₂, in which the cross-relaxation process between Er³⁺ and Tm³⁺, producing the ¹D₂-³F₄ transition of Tm³⁺, is found. The upconversion mechanisms are elucidated through the laser power dependence of the upconverted emissions and the energy level diagrams.     

Synthesis,characterization, and in vitro toxicity evaluation of upconversion luminescence NaLuF4:Yb3+/Tm3+ nanoparticles suitable for medical applications

Synthesis, characterization, and in vitro toxicity evaluation of upconversion luminescence NaLuF₄:Yb³⁺/Tm³⁺ nanoparticles (UCLNPs) are reported in the current study. Initially, the synthesized lanthanide trifluoroacetate (Ln(OOCCF₃)₃) precursor was used to fabricate NaLuF₄ nanoparticles doped with Yb³⁺ and Tm³⁺ metal ions. The nanoparticles were coated with calcium carbonate (CaCO₃) after removing the hydrophobic species on them to enhance their biocompatibility. The in vitro methylthiazolyldiphenyl-tetrazoliumbromide (MTT) test was used to evaluate the toxicity of synthesized NaLuF₄:Yb³⁺/Tm³⁺ nanoparticles (NLF-5) on L929 mouse fibroblast cell lines. The transmission electron microscopy image showed that the particle size of NaLuF₄:Yb³⁺/Tm³⁺ was 32 nm. The synthesized NLF-5 nanoparticles have both α-cubic and β-hexagonal crystalline structures that provided a superb near-infrared-to-near-infrared upconversion luminescence signal when excited at 980 nm. MTT test results show that the death of L929 fibroblast cells was observed only at concentrations above 250 μg/mL of NaLuF₄:Yb³⁺/Tm³⁺ nanoparticles. In addition, with an increase in patrol time of 24, 48, and 72 hr, cell toxicity increased significantly, while the coated nanoparticles did not have any toxic effects. The synthesized nanoparticles could be used as a suitable material for medical applications due to their small particle size, high photoluminescence emission intensity, and low toxicity.     

Mesoporous hybrids containing Eu complexes covalently bonded to SBA-15 functionalized: Assembly, characterization and photoluminescence

A novel series of luminescent mesoporous organic-inorganic hybrid materials has been prepared by linking Eu³⁺ complexes to the functionalized ordered mesoporous SBA-15 which was synthesis by a co-condensation process of 1,3-diphenyl-1,3-propanepione (DBM) modified by the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC), tetraethoxysilane (TEOS), Pluronic P123 surfactant as a template. It was demonstrated that the efficient intramolecular energy transfer in the mesoporous material Eu(DBMSi-SBA-15)₃phen mainly occurred between the modified DBM (named as DBM-Si) and the central Eu³⁺ ion. So the Eu(DBMSi-SBA-15)₃phen showed characteristic emission of Eu³⁺ ion under UV irradiation with higher luminescence quantum efficiency. Moreover, the mesoporous hybrid materials exhibited excellent thermal stability as the lanthanide complex was covalently bonded to the mesoporous matrix.     

Confining Excitation Energy in Er3+‐Sensitized Upconversion Nanocrystals through Tm3+‐Mediated Transient Energy Trapping

A new class of lanthanide‐doped upconversion nanoparticles are presented that are without Yb³⁺ or Nd³⁺ sensitizers in the host lattice. In erbium‐enriched core–shell NaErF₄:Tm (0.5 mol %)@NaYF₄ nanoparticles, a high degree of energy migration between Er³⁺ ions occurs to suppress the effect of concentration quenching upon surface coating. Unlike the conventional Yb³⁺‐Er³⁺ system, the Er³⁺ ion can serve as both the sensitizer and activator to enable an effective upconversion process. Importantly, an appropriate doping of Tm³⁺ has been demonstrated to further enhance upconversion luminescence through energy trapping. This endows the resultant nanoparticles with bright red (about 700‐fold enhancement) and near‐infrared luminescence that is achievable under multiple excitation wavelengths. This is a fundamental new pathway to mitigate the concentration quenching effect, thus offering a convenient method for red‐emitting upconversion nanoprobes for biological applications.     

Novel rare earth ions-doped oxyfluoride nano-composite with efficient upconversion white-light emission

Transparent SiO₂-Al₂O₃-NaF-YF₃ bulk nano-composites triply doped with Ho³⁺, Tm³⁺ and Yb³⁺ were fabricated by melt-quenching and subsequent heating. X-ray diffraction and transmission electron microscopy measurements demonstrated the homogeneous precipitation of the β-YF₃ crystals with mean size of 20 nm among the glass matrix, and rare earth ions were found to partition into these nano-crystals. Under single 976 nm laser excitation, intense red, green and blue upconversion emissions were simultaneously observed owing to the successive energy transfer from Yb³⁺ to Ho³⁺ or Tm³⁺. Various colors of luminescence, including bright perfect white light, can be easily tuned by adjusting the concentrations of the rare earth ions in the material. The overall energy efficiency of the white-light upconversion was estimated to be about 0.2%.     

Structure-dependent luminescence properties of Eu3+-doped CsBPO4 (B = Mg,Zn)

Eu³⁺-doped monophosphate CsBPO₄ (B = Mg, Zn) was synthesized by conventional solid state reaction. The crystal phase formations and morphology of the phosphates were analyzed by X-ray powder diffraction (XRD) and scanning electron microscope (SEM) measurements, respectively. The luminescence properties, including the photoluminescence excitation and emission spectra, decay curve (lifetime), the color coordinates and the internal quantum efficiency were investigated. Two Eu³⁺-doped monophosphates display very distinct luminescence properties. CsMgPO₄:Eu³⁺ presents the dominant reddish-orange emission from magnetic dipole transition ⁵D₀ → ⁷F₁ and a long luminescence lifetime. In contrast, CsZnPO₄:Eu³⁺ has a pure red color with the dominant induced electric-dipole transition ⁵D₀ → ⁷F₂ and a fast decay. Different results on the luminescence features of CsBPO₄ (B = Mg, Zn):Eu³⁺ were discussed on the base of crystal structure. Eu³⁺ has structure-dependent transitions due to the special microstructure occupied in a given host.     

Synthesis and spectral properties of Eu-doped YF3 nanobundles

YF₃:Eu³⁺ nanobundles were synthesized by a facile microemulsion method. Analysis of X-ray diffraction, scanning electron microscope, and transmission electron microscopy reveals that each nanobundle consists of numerous nanowhiskers with a mean length of ∼500 nm and a mean diameter of ∼2 nm. Under 393-nm excitation, the luminescence was dominated by ⁵D₀ → ⁷F₁ transition, indicating the inversion symmetry of Eu³⁺ site. The luminescence intensity increased with increasing Eu³⁺ concentration, up to about 30 mol%, and then decreased abruptly. The peak positions and spectral shapes of emissions were independent of Eu³⁺ concentration. Finally, the critical distance of energy transfer was calculated.     

双光子敏化Eu3+高效发光活细胞成像纳米生物探针

将Eu(tta)₃dpbt (dpbt: 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine; tta:thenoyltrifluoroacetonato)包埋在甲基丙烯酸甲酯-苯乙烯共聚物、正辛基三甲氧基硅及其水解缩合产物组成的杂化基质中, 制备了Eu(tta)₃dpbt 质量分数为40%的荧光纳米粒子, 其平均粒径为45 nm. 所制备的发光纳米粒子在水中分散稳定性高、光稳定性好、细胞毒性低、长波敏化Eu³⁺发光性能优良, 适宜作为生物分析的发光标记物. 所制备的发光纳米粒子的可见区激发峰位于415 nm, 激发峰尾部延展至475 nm, 其发光量子产率为0.31(λ_ex=415 nm, T=23 ℃), 最大双光子激发作用截面为5.0×10⁵ GM (λ_ex=830 nm, 1 GM=10^-50 cm⁴·s·photo^-1×particle^-1). 以转铁蛋白修饰上述发光纳米粒子表面制备的纳米生物探针被成功应用于活的HeLa肿瘤细胞的特异性标记和双光子激发Eu³⁺发光成像.     

An efficient upconversion luminescence energy transfer system for determination of trace amounts of nitrite based on NaYF4:Yb3+, Er3+ as donor

Based on NaYF₄:Yb³⁺, Er³⁺ upconversion nanocrystals as donor and 4-((4-(2-aminoethylamino)naphthalen-1-yl)diazenyl) benzenesulfonic acid dihydrochloride (ANDBS) as acceptor, an efficient luminescence energy transfer (LET) system was developed for selective and sensitive determination of trace amounts of nitrite. Based on Griess Reaction, ANDBS was generated by the quantitative reaction of nitrite, sulfanilamide and N-(1-naphtyl)-ethylenediamine dihydrochloride (N1NED). The degree of the overlaps between the emission spectrum of NaYF₄:Yb³⁺, Er³⁺ and the absorption spectrum of ANDBS were effective for luminescence energy transfer. Under the optimal condition, the upconversion luminescence quenching of NaYF₄:Yb³⁺, Er³⁺ was in proportion to the trace amounts of nitrite. The detection limit for nitrite achieved is 0.0046 μg mL^?1 and the system shows high sensitivity towards nitrite at 0.008000–0.2500 μg mL^?1 range.     

Synthesis and upconversion luminescence properties of Yb/Tm-codoped BaSiF6 nanorods

One-dimensional BaSiF₆:Yb³⁺(20%)/Tm³⁺(1.2%) nanorods were synthesized by a facile microemulsion method for the first time. X-ray topographic analysis found that the nanorods have a pure rhombohedral structure. Under 980 nm excitation, bright-blue upconversion luminescence was presented in the nanorods, indicating that BaSiF₆ is a new host material for producing desirable upconversion luminescence.