Impact of High Ytterbium(III) Concentration in the Shell on Upconversion Luminescence of Core–Shell Nanocrystals

After coating 20 Yb/2 Er:NaGdF₄ core nanocrystals with a NaYbF₄ shell, upconversion emission of the rare earth ions weakens. So far, the exact reason for this phenomenon is still unclear due to lack of the direct evidence. In this report, a core@shell@shell sandwich‐like structure is designed and fabricated to investigate this phenomenon. We find that high Yb³⁺ concentration in the shell has mainly two adverse impacts: it promotes not only the deleterious back energy transfer from Er³⁺ in the core to Yb³⁺ in the shell but also the energy transfer from Yb³⁺ in the core to Yb³⁺ in the shell. To obtain nanocrystals with high upconversion efficency, appropriate Yb³⁺ concentration should be introduced into the shell or the transition layer.     

Spectroscopic Properties and Upconversion Studies in Ho3+/Yb3+ Co‐doped Calcium Scandate with Spectrally Pure Green emission

The optical properties of a Ho³⁺/Yb³⁺ co‐doped CaSc₂O₄ oxide material are investigated in detail. The spectral properties are described as a function of doping concentrations. The efficient Yb³⁺→Ho³⁺ energy transfer is observed. The transfer efficiency approaches 50 % before concentration quenching. The concentration‐optimized sample exhibits a strong green emission accompanied with a weak red emission, showing perfect green monochromaticity. The results of the spectral distribution, power dependence, and lifetime measurements are presented. The green, red, and near‐infrared (NIR) emissions around 545, 660, and 759 nm are assigned to the ⁵F₄+⁵S₂→⁵I₈, ⁵F₅→⁵I₈, and ⁵F₄+⁵S₂→⁵I₇ transitions of Ho³⁺, respectively. The detailed study reveals the upconversion luminescence mechanism involved in a novel Ho³⁺/Yb³⁺ co‐doped CaSc₂O₄ oxide material.     

Upconversion Luminescence from Ho3+ and Yb3+ Codoped α-NaYF4 Single Crystals

The Ho³⁺/Yb³⁺ co-doped α-NaYF₄ single crystal was grown successfully for the first time by a modified Bridgman method in which KF was used as assisting flux and a large temperature gradient (70-90 oC/cm) of solid-liquid interface was adopted. Upconversion emissions at green ～544 nm, red ～657 and ～751 nm were obtained under 980 nm laser diode excitation. The intensity at ～544 nm was much stronger than those of ～657 and ～751 nm. The mechanisms of the upconversion emissions were investigated by studying the relationship between the upconversion intensity and pump power. The optimized Yb³⁺ concentration was about 8.08 mol% when Ho³⁺ concentration was hold at about 1.0 mol%. The results showed that Ho³⁺/Yb³⁺+ doped α-NaYF₄ single crystal was a possible candidate upconversion material for the green solid-state laser.     

Hydrothermal Synthesis and Up‐conversion Luminescence of Ho3+/Yb3+ Co‐doped PbTiO3

Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals with different content of dopant were successfully prepared via a facile hydrothermal method. The purity, morphology, element distribution, chemical state and up‐conversion (UC) photoluminescence (PL) of PbTiO₃ nanocrystals affected by Ho³⁺ dopant are investigated systematically. X‐ray diffraction (XRD) results illustrate that PbTiO₃ samples with the doping Ho³⁺ concentration ranging from 0 to 5 mol‐% are perovskite structure. The doping Ho³⁺ ions have no change on the crystal structure of perovskite PbTiO₃. Owing to the non‐equivalent substitution of Ho³⁺ to Ti⁴⁺ in PbTiO₃, the particle size of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples is decreased as well as the particle agglomeration is detected. Moreover, Ho and Yb ions have uniform distributions in the PbTiO₃ nanoparticles as the presence of Ho³⁺ and Yb³⁺ cations. The up‐conversion spectra demonstrate that Ho/Yb co‐doped PbTiO₃ samples have up‐conversion emissions centered at 550 nm, 660 nm and 755 nm, corresponding to the transitions of ⁵F₄(⁵S₂)→⁵I₈, ⁵F₅→⁵I₈ and ⁵S₂(⁵F₄)→⁵I₇ of Ho³⁺ ions. Additionally, the effect of temperature on the UC PL property of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ system is further investigated. The sensitivity and the trend of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples in temperature from 298 k to 493K are calculated on the basis of fluorescence intensity ratio (FIR) method. Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals are verified the high potential in the optical temperature sensing.     

Controllable Multicolor Upconversion Luminescence by Tuning the NaF Dosage

Multicolor upconversion (UC) luminescence of NaYF₄:Yb³⁺/Er³⁺ nanoparticles (NPs) was successfully tuned by simply controlling the NaF dosage. Unlike UC nanocrystals previously reported in the literature with multicolor emission obtained by varying the rare‐earth dopants, the current work developed a new approach to tune the UC emission color by controlling the NaF concentration without changing the ratio and dosage of rare‐earth ions. TEM and powder XRD were used to characterize the shape, size, and composition of the UC luminescent nanocrystals. The luminescence images, emission spectra, and multicolor emission mechanism of the NPs have also been demonstrated. As a result of the excellent ability of this new method to manipulate color emission, this will open up new avenues in the areas of bioprobes, light‐emitting devices, color displays, lasers, and so forth. To demonstrate their biological applications, the water‐stable, biocompatible, and bioconjugatable NaYF₄:Yb³⁺/Er³⁺@poly(acrylic acid) NPs were synthesized by this developed strategy and applied in targeted‐cell UC luminescence imaging.     

Designing Upconversion Nanocrystals Capable of 745 nm Sensitization and 803 nm Emission for Deep‐Tissue Imaging

A crystal design strategy is described that generates hexagonal‐phased NaYF₄:Nd/Yb@NaYF₄:Yb/Tm luminescent nanocrystals with the ability to emit light at 803 nm when illuminated at 745 nm. This is accomplished by taking advantage of the large absorption cross‐section of Nd³⁺ between 720 and 760 nm plus efficient spatial energy transfer and migration through Nd³⁺→Yb³⁺→Yb³⁺→Tm³⁺. Mechanistic investigations suggest that a cascaded two‐photon energy transfer upconversion process underlies the emission mechanism. This protocol enables deep‐tissue imaging to be achieved while mitigating the attenuation effect associated with the visible emission and the overheating constraint imposed by conventional 980 nm excitation.     

BiF<Subscript>3</Subscript>:Ho<Superscript>3+</Superscript> system for upconversion of 2-μm laser radiation into visible emission

Ho³⁺-doped bismuth(III) fluoride was suggested for upconversion of 2-μm laser radiation (1.9–2.1 µm) into visible emission. The process is possible owing to direct excitation of the ⁵I₇ level, followed by the excitation of the ⁵F₅ level with the emission threshold of 1.4 W.     

Intense Upconversion Luminescence of CaSc2O4:Ho3+/Yb3+ from Large Absorption Cross Section and Energy‐Transfer Rate of Yb3+

Concentration‐optimized CaSc₂O₄:0.2 % Ho³⁺/10 % Yb³⁺ shows stronger upconversion luminescence (UCL) than a typical concentration‐optimized upconverting phosphor Y₂O₃:0.2 % Ho³⁺/10 % Yb³⁺ upon excitation with a 980 nm laser diode pump. The ⁵F₄+⁵S₂→⁵I₈ green UCL around 545 nm and ⁵F₅→⁵I₈ red UCL around 660 nm of Ho³⁺ are enhanced by factors of 2.6 and 1.6, respectively. On analyzing the emission spectra and decay curves of Yb³⁺: ²F_5/2→²F_7/2 and Ho³⁺: ⁵I₆→⁵I₈, respectively, in the two hosts, we reveal that Yb³⁺ in CaSc₂O₄ exhibits a larger absorption cross section at 980 nm and subsequent larger Yb³⁺: ²F_5/2→Ho³⁺: ⁵I₆ energy‐transfer coefficient (8.55×10^?17 cm³ s^?1) compared to that (4.63×10^?17 cm³ s^?1) in Y₂O₃, indicating that CaSc₂O₄:Ho³⁺/Yb³⁺ is an excellent oxide upconverting material for achieving intense UCL.     

Strong Single‐Band Blue Emission from Colloidal Ce3+/Tm3+‐Doped NaYF4 Nanocrystals for Light‐Emitting Applications

An intense single‐band blue emission at λ=450 nm is observed from Tm³⁺ ions through Ce³⁺ sensitization, for the first time, in colloidal Ce³⁺/Tm³⁺‐doped NaYF₄ nanocrystals. The intense Tm³⁺ emission through broad‐band excitation is advantageous for developing luminescent nanocomposites because they can be easily incorporated into polymers. The composites can easily be coated over UV light‐emitting diodes (LEDs) to develop phosphor‐based blue LEDs.     

Preparations,Structures and Luminescence Properties of Penta‐rare‐earth Incorporated Tetravacant Dawson Selenotungstates and Their Ho3+/Tm3+ Co‐doped Derivatives

A family of penta‐rare‐earth incorporated tetravacant Dawson selenotungstates [H₂N(CH₃)₂]₁₀H₃[SeO₄RE₅(H₂O)₇(Se₂W₁₄O₅₂)₂] ? 40H₂O [RE=Ho³⁺ ( 1 ), Er³⁺ ( 2 ), Tm³⁺ ( 3 ), Tb³⁺ ( 4 )] were synthesized. It should be noted that a penta‐RE [SeO₄RE₅(H₂O)₇]¹¹⁺ central core connecting two tetra‐vacant Dawson‐type [Se₂W₁₄O₅₂]^12? subunits generates a dimeric assembly of [SeO₄RE₅ (H₂O)₇(Se₂W₁₄O₅₂)₂]^13? in the structures of 1 – 4 . Meanwhile, a class of Ho³⁺/Tm³⁺ co‐doped derivatives based on 1 with a Ho³⁺/Tm³⁺ molar ratio of 0.75:0.25–0.25:0.75 were also prepared and characterized by energy‐dispersive spectroscopy (EDS) analyses. Moreover, their luminescence properties were systematically investigated, which indicate that Tm³⁺ ions can sensitize the emission of Ho³⁺ ions in the visible region and prolong the fluorescence lifetime of Ho³⁺ ions to some extent. Energy transfer from Tm³⁺ ions to Ho³⁺ ions was probed by time‐resolved emission spectroscopy (TRES), and the CIE 1931 diagram has been applied to evaluate all possible luminescence colors.     

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.     

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%.     

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.     

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.     

Green upconversion emission dependence on size and surface residual contaminants in nanocrystalline ZrO2:Er3+

The luminescence lifetime of the green upconversion emission Er³⁺ ions in ZrO₂ nanocrystals was found to be sensitive to the particle size and crystalline phase, as well as to the residual surface contaminants. Erbium doped (0.2 mol% Er₂O₃) ZrO₂ nanocrystals ranging from 54 to 120 nm in size were prepared by a sol–gel process with the presence of nonionic PLURONIC P127 surfactant, and the upconversion emission was characterized. PLURONIC P127 at a molar ratio of 0.0082 promoted both an enhancement in the green upconversion emission as well as a strong reduction of surface contaminants such as CO, CH₂, and OH. A fluorescence decay analysis via a simple microscopic rate equation model suggests that crystallite size and nonradiative relaxation mechanisms to different surface contaminants have to be taken into account to explain the observed green luminescence quenching. XRD, FTIR and luminescence lifetime measurements allow the quantification of the nonradiative processes that lead to the green luminescence quenching; and prove the relevance of using nonionic surfactants in the synthesis to reduce residual surface contaminants.     

Synthesis,structure and photoluminescence properties of Ho3+ doped TTB–BaTa2O6 phosphors

Ho³⁺ doped TTB–BaTa₂O₆ phosphors were produced by the solid state reaction method. XRD analysis confirmed the formation of BaTa₂O₆ single phase which was accomplished by heat treatment at 1425 °C for 20 h. The crystal structure of TTB–BaTa₂O₆ allowed doping concentration of Ho³⁺ ions up to 10 mol%, maintaining a single phase composition. A second phase of HoTaO₄ begins to appear at 15 mol%. The lattice structure and the crystallite sizes changed with the concentration of Ho³⁺. In SEM analysis, it was also shown that BaTa₂O₆ grain sizes changed with the concentration of Ho³⁺. EDS analysis revealed that the Ta/Ba ratio increased on the grains depending on Ho³⁺ concentration. Charge balance of the structure was formulated through the EDS results. In fluorometer analysis, a strong green emission (λ_em = 546.9 nm) was observed in the visible spectral region. The emission increased with the doping concentration of up to 2.5 mol%, and above this level decreased due to concentration quenching.     

Lanthanide-doped LiYF4 nanoparticles: Synthesis and multicolor upconversion tuning

We report the synthesis of tetragonal-phase LiYF₄ nanoparticles doped with upconverting lanthanide ions. The nanoparticles have been characterized by XRD, TEM, and luminescence decay studies. The size of the as-synthesized LiYF₄ nanoparticles can be tuned by varying the precursor ratio of F to lanthanide ions. Passivated by oleic acid ligands, the LiYF₄ nanoparticles can be readily dispersed in a wide range of nonpolar solvents including hexane, cyclohexane, dichloromethane, and toluene. The lanthanide-doped (Yb³⁺, Er³⁺, Tm³⁺, Ho³⁺) LiYF₄ nanoparticles show intense upconversion emissions upon near infrared excitation at 980 nm. By varying composition and concentration of the dopant ions, the color output can be precisely modulated under single wavelength excitation with a diode laser.     

Effect of SnO2 Nanocrystals on the Emission of Eu3+ Ions in Silica Matrix

Silica xerogels containing Eu³⁺ ions and SnO₂ nanocrystals were prepared in the sol‐gel process, and characterized by x‐ray diffraction (XRD) and photoluminescence spectra. Under the excitation at 393 nm, characteristic emission of Eu³⁺ ions at 614 nm was enhanced with increasing amount of SnO₂ nanocrystals. Moreover, when the Eu³⁺/SnO₂ co‐doped samples were excited at 345 nm, corresponding to the sideband of SnO₂ nanocrystals, the emission of Eu³⁺ ions at 614 nm was clearly observed, while no emission of Eu³⁺ ions for the Eu³⁺‐doped sample. It may be ascribed to the energy transfer from SnO₂ conduction band to Eu³⁺ conduction band. Further experimental results suggest that the energy transfer may be achieved through surface transition state.     

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.     

Up/downconversion luminescence rare-earth ion-doped Y2O3 1D nanocrystals

Multicolor luminescent rare-earth ion-doped Y2O3 nanocrystals(NCs) were prepared by a solvethermal method.The as-synthesized NCs yielded nanosheets,nanowires(NWs) and nanorods(NRs) with the increase of alkali(NaOH) in oleic acid system.Moreover,Y2O3 nanowires with controllable size have also been obtained.After sintering,the PL intensity of Y2O3:Ln 3+ nanocrystals increased with the changed morphology of the precursor,that is,Y(OH) 3 nanocrystals.Both downconversion(red emission for Y2O3:Eu 3+ and green emission for Y2O3:Tb 3+) and upconversion(red emission for Y2O3:Yb/Er 3+) luminescence of the as-prepared nanocrystals have been demonstrated in this work.We also found that the PL intensity of Y2O3:Ln 3+ NCs dispersed in polar solvent was stronger than that in nonpolar solvent.Their up/downconversion fluorescence and controllable morphology might promise further fundamental research and biochemistry such as nanoscale optoelectronics,nanolasers,and ultrasensitive multicolor biolables.