Insights into the growth mechanism of REF3 (RE = La-Lu, Y) nanocrystals: hexagonal and/or orthorhombic

Effect of alkali metal ions introduction on the fluorescence properties of Er-Tm-Yb synergistically sensitized phosphors

Upconversion fluoride phosphors Na1-xMxY1-a-b-cF4:Er3+a, Tm3+b, Yb3+c (M = Li+/K+) have been synthesized by low-temperature combustion method. The optimal doping ratios of ions in the matrix lattice were determined by orthogonal experiments with the control variable method. It was found that when a certain amount of Tm3+ ions were doped into the lattice of Er3+ ions, the upconversion fluorescence intensity and red-to-green ratio of the samples were significantly enhanced. When a small amount of Yb3+ ions was introduced into the Er3+-Tm3 + ions co-doped samples, the upconversion fluorescence intensity of the samples was continued to be enhanced, but the red-to-green ratio was slightly decreased. The mechanism of the influence of the upconversion fluorescence intensity and the red-to-green ratio of the multidoped samples with lanthanide ions was also systematically investigated. Based on the results of orthogonal experiments, the optimal component formulations were determined and alkali metal ions were further introduced. The upconversion fluorescence enhancement mechanism of the samples after the introduction of alkali metal ions was systematically investigated. In this work, the upconversion fluorescence intensity of the prepared samples was significantly enhanced by synergistic sensitization between the ions. In addition, by adjusting the red-to-green ratio of the fluorescence of the samples, a new idea is provided for the preparation of upconversion phosphors with high color purity.

Co-doping of Ho-Yb ion pairs modulating the up-conversion luminescence properties of fluoride phosphors under 1550 nm excitation

In this study, up-conversion fluoride phosphors NaY1-x-y-m-nMxF4:Er3+y,Ho3+m,Yb3+n (M = Lu3+/Gd3+) were synthesized by a low-temperature combustion method. The optimal ionic ratios in the matrix lattice were also determined by a controlled variable method. It was confirmed that doping a small amount of Ho3+ ions and Yb3+ ions in the Er-doped sample matrix lattice can form a mutual sensitizer and a transient energy capture center to enhance the sample's up-conversion luminescence under excitation at the 1550 nm band, respectively. It was also found that the lanthanide ion introduced can modulate the red-to-green ratio of the up-conversion luminescence of the sample. The phase composition and morphology of phosphors were investigated using X-ray diffraction and scanning electron microscopy. The up-conversion luminescence mechanism of Er-Ho-Yb tri-doped samples excited at the 1550 nm band was also investigated. This work presents a novel approach for improving up-conversion luminescence with high color-purity phosphors for display lighting applications when excited at 1550 nm.

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Orthorhombic-to-Hexagonal Phase Transition of REF3 (RE = Sm to Lu and Y) under High Pressure

For the famous functional REF₃ family, there exist two typical structures, that is, orthorhombic phase and hexagonal phase. In the present work, high pressure behaviors of the orthorhombic phase REF₃ (RE = Sm to Lu and Y) were investigated by experimental methods and first-principles calculations. The pressure-induced phase transitions of GdF₃, TbF₃, YbF_3, and LuF₃ were studied by using in situ photoluminescence measurements in the diamond anvil cell. At room temperature, all these four compounds follow the phase transition route from orthorhombic to hexagonal phase at 5.5-20.6 GPa. The pressure ranges of phase transition are 5.5-9.3, 8.4-11.9, 13.5-20.3, and 14.8-20.6 GPa for GdF₃, TbF₃, YbF_3, and LuF₃, respectively. In combination with first-principles calculations, we infer that all orthorhombic REF₃ members from Sm-Lu and Y obey the same orthorhombic-to-hexagonal phase transition rules under high pressures. For lanthanide trifluorides, the transition pressures increase as zero pressure volumes of REF₃ in the orthorhombic phase become smaller. As the calculation results show, this is because the difference in value of energy from the two structures is larger. This work not only provides precise structural change but also benefits the understanding of two typical structures for rare-earth trifluorides, which may play a significant role in the applications of REF₃.

Systematic Crystallization of NH4Ln(MoO4)2 as a Family of Layered Compounds (Ln = La-Lu and Y), Derivation of Ln2Mo4O15, Crystal Structure, and Photoluminescence

NH₄Ln(MoO₄)₂ (Ln = La-Lu lanthanide, Y) was crystallized via hydrothermal reaction as a new family of layered materials, from which phase-pure Ln₂Mo₄O₁₅ was successfully derived via subsequent annealing at 700 °C for the series of Ln elements excluding Ce and Lu. Detailed structure analysis revealed that the ionic size of Ln³⁺ decisively determined the crystal structure and Mo/Ln coordination for the two families of compounds. NH₄Ln(MoO₄)₂ was analyzed to be orthorhombic (Pbcn space group, no. 60) and monoclinic (P2/c, no. 13) for the larger and smaller Ln³⁺ of Ln = La-Gd and Ln = Tb-Lu (including Y), respectively, where both the crystal structures have a layered topology featured by the alternative stacking of a [Ln(MoO₄)₂]^- three-tier infinite anionic layer and interlayer NH₄⁺. Four types of crystal structures were found for the Ln₂Mo₄O₁₅ series, which are monoclinic (P2₁/a, no. 14) for Ln = La, triclinic (P1̅, no. 2) for Ln = Pr-Sm, triclinic (P1̅, no. 2) for Ln = Eu and Gd, and monoclinic (P2₁/c, no. 14) for Ln = Tb-Yb (including Y). The photoluminescence of NH₄Ln(MoO₄)₂ (Ln = Eu, Tb) and Ln₂Mo₄O₁₅:Eu³⁺ (Ln = La, Gd, Y) was thoroughly investigated in terms of spectral features, quantum efficiency, fluorescence decay, and CIE chromaticity. The thermal stability of luminescence was also studied for Ln₂Mo₄O₁₅:Eu³⁺, and the observed charge-transfer excitation components were successfully correlated with the features of the Mo-O polyhedron/unit.

Selective growth and upconversion photoluminescence of Y-based fluorides: from NaYF4: Yb/Er to YF3: Yb/Er crystals

Y-based fluorides have been recognized as most efficient host materials for upconversion photoluminescence (UC-PL). Herein, we have produced a series of Yb/Er doped Y-based fluorides with specific crystal structures, shapes and sizes. The selective growth process is governed by our pre-designed surfactant 4, 4'-((2,5-bi's (2-(diethylamino) ethoxy) -1,4-phenylene) bis (ethyne-2,1-diyl)) dibenzoic acid (DBA) and selective solvents. It is shown that highly pure hexagonal microprisms and cubic microspheres of NaYF₄: Yb/Er could be selectively grown in water at low and high content of DBA, respectively, while only orthorhombic nanowires and microflowers of YF₃: Yb/Er could be obtained in ethanol. Finally, all these materials obtained exhibit strong UC-PL signal while the UC emission intensity of the NaYF₄: Yb/Er hexagonal microprisms is much higher than those of the cubic microspheres and orthorhombic YF₃ nanowires and microflowers. This work provides a novel method for selective crystal growth of Y-based fluorides with specific shape, size, crystal phase and highly UC-PL efficiency by breaking the intrinsic limitation of crystal growth habit, which could be possibly extended to the controlled synthesis of other related materials.

Temperature sensing performance based on up-conversion luminescence in hydrothermally synthesized Yb3+/Er3+ co-doped NaScF4 phosphors

NaScF₄:Yb³⁺/Er³⁺ crystals were successfully hydrothermally synthesized using distilled water as a single solvent.

Tailoring the Synthesis of LnF3 (Ln = La-Lu and Y) Nanocrystals via Mechanistic Study of the Coprecipitation Method

Here, 15 LnF₃ nanocrystals are synthesized using coprecipitation method with citrate stabilization to allow the fast, easy, and reproducible synthesis of several nanoscaled structures in water. General trends related to the behavior of LnF₃ nanocrystals are highlighted due to their broad range of application in several fields (e.g., medical applications). The same nature for all Ln³⁺ cations is expected due to the internal role of f orbitals. However, we found that the use of different lanthanide elements is crucial in the final size, shape, assembly, and crystalline structure. In addition, the decrease of the cation size of the lanthanide series changes the behavior of these compounds, resulting in hexagonal, orthorhombic, and cubic crystalline structures. In addition, we are able to tune the cubic crystalline phase to pure orthorhombic by modifying the pH of the system using HBF₄ instead of tetramethylammonium citrate. Via ¹¹B NMR, we demonstrated the mechanism of HBF₄ as fluorinating agent if an additional source of F^- is not added during the synthesis. ¹H NMR and IR techniques were performed to unravel the picture of the surface chemistry of the two representative metal cations (Y and La). Finally, HRTEM and SAED were performed to uncover the shape of the obtained nanocrystals and the preferential orientation of the assembled particles, giving crucial information on the involved mechanisms. This study reveals not only the dependence of the crystalline structure on the used metal and pH but also ability to achieve LnF₃ assembled particles depending on the final shape and temperature.