Enhancing the static green up-conversion luminescence of NaY(MoO4)2:Yb/Er microcrystals via an annealing strategy for anti-counterfeiting applications

Structure-Dopant Concentration Relations in Europium-Doped Yttrium Molybdate and Peak-Sharpening for Luminescence Temperature Sensing

A set of Eu3+-doped molybdates, Y2-xEuxMo3O12 (x = 0.04; 0.16; 0.2; 0.4; 0.8; 1; 1.6; 2), was synthesized using a solid-state technique and their properties studied as a function of Eu3+ concentration. X-ray diffraction showed that the replacement of Y3+ with larger Eu3+ resulted in a transformation from orthorhombic (low doping concentrations) through tetragonal (high doping concentrations), reaching monoclinic structure for full replacement in Eu2Mo3O12. The intensity of typical Eu3+ red emission slightly increases in the orthorhombic structure then rises significantly with dopant concentration and has the highest value for the tetragonal Y2Mo3O12:80mol% Eu3+. Further, the complete substitution of Y3+ with Eu3+ in the case of monoclinic Eu2Mo3O12 leads to decreased emission intensity. Lifetime follows a similar trend; it is lower in the orthorhombic structure, reaching slightly higher values for the tetragonal structure and showing a strong decrease for monoclinic Eu2Mo3O12. Temperature-sensing properties of the sample with the highest red Eu3+ emission, Y2Mo3O12:80mol% Eu3+, were analyzed by the luminescence intensity ratio method. For the first time, the peak-sharpening algorithm was employed to separate overlapping peaks in luminescence thermometry, in contrast to the peak deconvolution method. The Sr (relative sensitivity) value of 2.8 % K-1 was obtained at room temperature.

Barium molybdate up-conversion nanoscale particles with IR-LED chip, temperature sensing, and anti-counterfeiting applications

Barium molybdate nanoparticles exhibiting up-conversion luminescence were synthesized via the solvothermal method. Analysis revealed a prominent signal corresponding to the (112) plane in the XRD pattern, indicating the tetragonal structure of the synthesized nanoparticles. Raman spectroscopy detected the symmetric stretching frequencies of MoO4. When excited at 980 nm, the nanoparticles emitted a green spectrum with peaks at 532 and 553 nm. The luminescence intensity varied with the excitation light source, supporting the mechanism involving energy transfer from Yb-doped Er ions via the two-photon effect of the up-conversion phosphor. Moreover, the synthesized nanoparticles exhibited diminished luminous intensity with increasing temperature, suggesting potential for flexible composite sensor fabrication. Integration with a 980 nm LED chip yielded a green emission color. Furthermore, when applied to banknotes, plastic cards, fabrics, and artwork, the opaque solution mixed with polymers remained invisible to the naked eye; however, under 980 nm laser irradiation, the distinct green color became apparent, offering a viable approach for anti-counterfeiting measures.

Low-temperature in situ preparation of Eu3+/Tb3+-doped CaMoO4/SrMoO4 nanoparticle thin films and their application in anti-counterfeiting

Rare earth-doped metal oxide thin films exhibit remarkable potential for application in anti-counterfeiting, owing to their exceptional fluorescent properties. However, the existing fabrication techniques for these rare earth-doped luminescent thin films are predominantly complex and necessitate high-temperature conditions. In light of this issue, we present a low-temperature method for in situ fabrication of luminescent Ca1-xMoO4:Eux3+ and Sr1-xMoO4:Tbx3+ nanocrystal thin films by a solution deposition process. The developed method has the advantages of simple operation, rapid and low-temperature synthesis. The optimal chemical compositions of molybdate-based luminescent films are Ca0.90MoO4:Eu0.103+ and Sr0.90MoO4:Tb0.103+. Moreover, we evaluate the practical feasibility of luminescent nanoparticle films in the field of anti-counterfeiting by combining the unique fluorescent properties of rare earth ions and designing customized fluorescent patterns.

Tuning multicolour emission of Zn2GeO4:Mn phosphors by Li+ doping for information encryption and anti-counterfeiting applications

Traditional fluorescent materials used in the anti-counterfeiting field usually exhibit monochromatic luminescence at a single-wavelength excitation, which is easily forged by sophisticated counterfeiters. In this work, Zn₂GeO₄:Mn,x%Li (x = 0 and 20), Zn₂GeO₄-NaLiGe₄O₉:Mn,x%Li (x = 50 and 70) and NaLiGe₄O₉:Mn micro-phosphors with multi-chromatic and multi-mode luminescence have been successfully synthesized via a hydrothermal approach followed by an annealing treatment. As expected these Li⁺ doped Zn₂GeO₄:Mn and Zn₂GeO₄-NaLiGe₄O₉:Mn phosphors exhibit a double peak emission including a long green afterglow (∼540 nm) and red photoluminescence (∼668 nm). By tuning Li⁺ doping concentrations, a gradual colour output and a tuneable afterglow duration are achieved. In particular, the Zn₂GeO₄:Mn,Li and NaLiGe₄O₉:Mn phosphors exhibit excellent performance as security inks for printing luminescent numbers and anti-counterfeiting patterns, which show an afterglow time-dependent or excitation wavelength-dependent luminescence colour evolution. This work proves the feasibility of the Li⁺ doping strategy in emission tuning, which can stimulate further studies on multi-mode luminescent materials for anti-counterfeiting applications.

NaY(MoO4)2-based nanoparticles: synthesis, luminescence and photocatalytic properties

We report on a novel synthesis method, which produces NaY(MoO₄)₂ nanoparticles having an almost spherical shape and hydrophilic character. The procedure is also suitable for the preparation of NaY(MoO₄)₂-based nanophosphors by doping this host with lanthanide cations (Eu³⁺, Tb³⁺ and Dy³⁺), which, under UV illumination, exhibit intense luminescence whose color is determined by the selected doping cation (red for Eu³⁺, green for Tb³⁺ and yellow for Dy³⁺). The effects of the cations doping level on the luminescent properties are analyzed in terms of emission intensities and luminescent lifetime, to find the optimum phosphors. Finally, the performance of these nanophosphors and that of the undoped system for the photocatalytic degradation of rhodamine B, used as a model compound, is also analyzed.