Abstract
In this work, the series of Tb3+/Eu3+ co-doped xerogels and derivative glass-ceramics containing CaF2 nanocrystals were prepared and characterized. The in situ formation of fluoride crystals was verified by an X-ray diffraction technique (XRD) and transmission electron microscopy (TEM). The studies of the Tb3+/Eu3+ energy transfer (ET) process were performed based on excitation and emission spectra along with luminescence decay analysis. According to emission spectra recorded under near-ultraviolet (NUV) excitation (351 nm, 7F6 → 5L9 transition of Tb3+), the mutual coexistence of the 5D4 → 7FJ (J = 6–3) (Tb3+) and the 5D0 → 7FJ (J = 0–4) (Eu3+) luminescence bands was clearly observed. The co-doping also resulted in gradual shortening of a lifetime from the 5D4 state of Tb3+ ions, and the ET efficiencies were varied from ηET = 11.9% (Tb3+:Eu3+ = 1:0.5) to ηET = 22.9% (Tb3+:Eu3+ = 1:2) for xerogels, and from ηET = 25.7% (Tb3+:Eu3+ = 1:0.5) up to ηET = 67.4% (Tb3+:Eu3+ = 1:2) for glass-ceramics. Performed decay analysis from the 5D0 (Eu3+) and the 5D4 (Tb3+) state revealed a correlation with the change in Tb3+–Eu3+ and Eu3+–Eu3+ interionic distances resulting from both the variable Tb3+:Eu3+ molar ratio and their partial segregation in CaF2 nanophase.
Highlights
Accepted: 2 February 2021In recent years, the materials doped with rare earths (RE3+ ) are considered to be indispensable in the development of optoelectronics, offering promising applications in LEDs [1], displays [2], lasers [3], or optical thermometry [4]
The Tb3+ /Eu3+ energy transfer has been extensively explored and described in various types of phosphors, e.g., LaBWO6 [13], Tb2 MoO3 O12 [14], ScPO4 [15], or KAlP2 O7 [16]; such studies have not been as common for glass-ceramic materials so far, where Tb3+ and Eu3+ ions could be distributed between the amorphous glassy host and crystal phase characterized by different decay rates
In the case of Tb3+ ions, we excluded an involvement of the cross-relaxation process on luminescence lifetimes of the 5 D4 state based on our previous results for SiO2 -PbF2 :Tb3+ sol-gel glass-ceramics, for which we reported the non-radiative relaxation mechanism when the molar ratio of Tb3+
Summary
The materials doped with rare earths (RE3+ ) are considered to be indispensable in the development of optoelectronics, offering promising applications in LEDs [1], displays [2], lasers [3], or optical thermometry [4]. The proper adjustment of emission (i.e., generation of appropriate color purity and maintaining suitable luminescence lifetimes) usually requires the involvement of several RE3+ ions incorporated into the same host material [5,6,7,8,9,10,11,12]. Among numerous combinations of RE3+ in doubly- or triply-doped materials, the optical system consisting of Tb3+ and Eu3+ is a very promising strategy for the generation of multicolor luminescence, which plays a key role in the development of red-green-blue (RGB) optical materials.
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