Dual-mode blue emission, enhanced up-conversion luminescence and paramagnetic properties of ytterbium and thulium-doped Ba 2 GdF 7 multifunctional nanophosphors

Synthesis, photo- and thermo-luminescent properties of Mg2P2O7:Tm3+ phosphors

In this work, both undoped and Thulium (Tm3+) doped (0.3-10 mol%) magnesium pyrophosphate (Mg2P2O7) powders were synthesized by the solvent evaporation method to study their photo-and thermoluminescent properties. Two crystalline phases were observed in the powders by X-Ray diffraction (XRD), the main phase being Mg2P2O7 and the second one thulium phosphate (TmPO4). The superficial morphology was analyzed by scanning electron microscopy (SEM), which revealed that the powders are agglomerates with an undefined form and grains with non-uniform size distribution. The photoluminescence (PL) emission spectra of Tm3+ doped powders show the 1D2 → 3F4 transition, associated with Tm3+ ions, at 452 and 458 nm. The thermoluminescence (TL) properties were analyzed in the undoped and Tm3+ doped powders exposed to 90Sr beta source. The TL glow curve of Tm3+ doped powders exhibits three maxima at about ⁓64-66 °C, ⁓198-202 °C, and ⁓301 °C. The TL dose-response is sub-linear from 0.11 to 0.54 Gy, linear between 0.79 and 24.95 Gy, and supra-linear from 34.99 to 599.95 Gy. Acceptable repeatability with a coefficient of variation of ∼1% was obtained after ten cycles of irradiation and readout. At 63 d of storage, the powders show fading of 30%, and at 1.6 years (585 d), the integrated TL intensity decays by 47%. The kinetic parameters of activation energy and frequency factor were evaluated using the Initial Rise, Booth, Bohun, and Porfianovitch (BBP) and Hoogenstraaten methods and Glow Curve Deconvolution with a general order kinetic model.

Energy Manipulation in Lanthanide-Doped Core-Shell Nanoparticles for Tunable Dual-Mode Luminescence toward Advanced Anti-Counterfeiting

Developing advanced luminescent materials and techniques is of significant importance for anti-counterfeiting applications, and remains a huge challenge. In this work, a new and efficient approach for achieving efficient dual-mode luminescence with tunable color outputs via Gd³⁺ -mediated interfacial energy transfer, Ce³⁺ -assisted cross-relaxation, and core-shell nanostructuring strategy is reported. The introduction of Ce³⁺ into the inner core not only serves the regulation of upconversion emission, but also facilitates the ultraviolet photon harvesting and subsequent energy transfer to downshifting (DS) activators in the outer shell layer. Furthermore, the construction of the core@shell nanoarchitecture enables the spatial separation of upconverting activators and DS centers, which greatly suppresses their adverse cross-relaxation processes. Consequently, efficient and multicolor-tunable dual-mode emissions can be simultaneously observed in the pre-designed NaGdF₄ :Yb/Ho/Ce@NaYF₄ :X (X = Eu, Tb, Sm, Dy) core-shell nanostructures under 254 nm ultraviolet light and 980 nm laser excitation. The proof-of-concept experiment demonstrates that 2D-encoded patterns based on dual-mode emitting nanomaterials are very promising for anti-counterfeiting applications. It is believed that this preliminary study will advance the development of the fluorescent materials for potential applications in anti-counterfeiting and optical multiplexing.

Design of novel lanthanide-doped core-shell nanocrystals with dual up-conversion and down-conversion luminescence for anti-counterfeiting printing

Development of advanced luminescent nanomaterials and technologies is of great significance for anti-counterfeiting applications in global economy, security, and human health, but has proved to be a great challenge. In this work, we design, synthesize, and characterize mono-disperse, dumbbell-shaped lanthanide-doped NaYF₄@NaGdF₄ core-shell nanoparticles (CSNPs) with dual-mode fluorescence by coating the NaGdF₄:Ln'³⁺ shell onto NaYF₄:Ln³⁺ core nanospheres via a two-step oleic acid mediated thermal decomposition process. Different from the conventional synthesis method to produce spherical nanoparticles, the epitaxial growth of the NaGdF₄:Ln'³⁺ shell onto the nanosphere cores and the lattice mismatch between β-NaGdF₄ and β-NaYF₄ nanocrystals enable the formation of dumbbell-shaped CSNPs, as evidenced by the morphological evolution of CSNPs and as explained by the Ostwald ripening growth mechanism. By tailoring different doped lanthanide ions in the core and the shell, the resultant CSNPs exhibit tunable but different up-/down-conversion luminescence under the irradiation of a 980 nm laser and 254 nm UV light, respectively. Finally, these hydrophilic CSNPs are further fabricated into environmentally benign luminescent inks for inkjet printing to create a variety of dual-mode fluorescent patterns (peacock, temple, and a logo of "Hunan University of Technology") on different paper-based substrates (A4 paper, envelope, and postcard). Our dual-mode light-responsive CSNPs, along with an easy fabrication method, provide a simple and promising material and technique for anti-counterfeiting applications.

Heavy doping of ceria by wet impregnation: a viable alternative to bulk doping approaches

To avoid the deleterious effects of dopant segregation, synthesis methods that facilitate a homogenous dopant distribution in the ceria lattice were employed. Though doping ceria by wet impregnation was also credited to induce a homogeneous solid solution even in the heavy regime (concentration ≥20%, A. Corma, P. Atienzar, H. Garcia and J. Chane-Ching, Nat. Mater., 2004, 3, 394-397), no follow up investigation has been reported. Herein, we investigated ceria nanoparticles (1%Tm-CeO2 and 1%Eu-CeO2) wet-impregnated with trivalent rare-earth (Yb, 20%), bivalent (Ca, 20%) and isovalent (Zr, 30%) metals, followed by annealing in air. Homogeneity of the solid solutions of Yb-impregnated ceria was confirmed by a two-feature characterization toolbox that included X-ray diffraction, Raman spectroscopy, transmission electron microscopy, as well as up-conversion emission as a probe tool. Since the up-conversion emission of Tm was not detectable in the absence of Yb while its efficiency depends on the average distance between Yb and Tm ions, the Yb incorporation and its migration from the surface to the lattice bulk sites in wet-impregnated ceria can be "visualized" and compared with that of the Yb bulk-doped counterpart. The use of Eu luminescence as a local probe confirmed the homogeneity of solid solutions of Ca and Zr-impregnated ceria and also sustained the opposite roles of Ca and Zr as the repeller and the scavenger of oxygen vacancies, respectively. All these results suggested that heavy doping of ceria by wet impregnation with metals with +2, +3 and +4 valencies represent a facile alternative to conventional doping approaches. Therefore, the effects of the amount and the type of metal dopant on the structural properties of CeO2 could be investigated in a more systematic and probably a more reproducible manner, which would significantly increase the potential of ceria in catalysis and other applications.