Introduction With the development of society, two-dimensional displays can no longer meet people’s needs. Three-dimensional displays can completely reproduce the scene of world, showing the depth of the object, the sense of hierarchy, and reality because the real world is also three-dimensional. At present, there are a lot of techniques to realize three-dimensional displays, including two-dimensional analog three-dimensional displays, binocular parallax stereoscopic displays, and three-dimensional stereoscopic displays. Nevertheless, two-dimensional simulation three-dimensional display does not produce physical depth of field; Binocular parallax stereoscopic display is limited because of the disadvantages of small angle zoom, causing visual fatigue, and needing to assistant equipment. In contrast, three-dimensional stereoscopic display technology has attracted attention due to its realistic display effect, real-time interaction, and multi-angle observation. Among the related technologies, the static volume display technology based on transparent upconversion luminescent materials shows the most significant advantages.
Methods Appropriate composition design is a necessary condition for the nucleation and growth of fluoride nanocrystals in glass. Inthis study, rare earth doped pure red upconversion luminescent oxyfluoride transparent glass ceramics were obtained via the conventional melt-quenching-thermal annealing method. The glass compositions (in mole) are 60SiO2-6Al2O3-7Na2CO3-19NaF- xErF3-yTmF3-(8–x–y)YF3 (x=0–1.6%，y =0–0.4%). The raw materials are SiO2, Al2O3, Na2CO3(Sinopharm Chemical Reagents Co., Ltd.), and high-purity NaF, ErF3, TmF3, YF3(Shanghai Macklin Biochemical Technology Co., Ltd), which were weighed, mixed and ground in an agate mortar. The obtained mixtures were then placed in a sealed alumina crucible and melted at 1 550 ℃ for 45 min under ambient atmosphere. Subsequently, the glass melt was poured into a 200 ℃ preheated copper mould and annealed at 400 ℃ for 5 h to relinquish the internal stress. Finally, the obtained bulk glass was heat-treated at 650 ℃ for 2 h to induce in-situ fluoride crystallization inside the glass to obtain oxyfluoride glass ceramics containing NaYF4 nanocrystals (NaYF4@glass).
Results and discussion For Er3+/Tm3+ co-doped NaYF4@glass, the influence of Er3+ doped content was explored and an optimal Er3+concentration of 15% was confirmed. Meanwhile, with the increase of co-doped Tm3+ content, the red emission of Er3+ gradually enhanced due to the energy transfer process between Er3+ and Tm3+. Correspondingly, the color purity of red emission increased to realize the pure red emission upon the 980 nm near-infrared laser excitation. In addition, the temperature dependent spectra (from 80 K to 470 K) of 15%Er–2%Tm: NaYF4@glass were tested. Owing to the electrons at the 2H11/2, 4S3/2 and 4F9/2 levels of Er3+ were populated independently, there was no direct temperatures-dependent electron correlation between the green (2H11/2, 4S3/2) and red (4F9/2) levels of Er3+ ions. With the increase of temperature, the shape of the emission peak remained basically unvaried, indicating excellent chromatic stability. Ultimately, the Er3+/Tm3+ co-doped NaYF4@glass-based three-dimensional prototype display device showed three-dimensional static/dynamic designed red patterns with high color purity and high stability, providing its potential use in new-generation three-dimensional stereoscopic display technology.
Conclusions In summary, Er3+/Tm3+ co-doped NaYF4@glass can be successfully prepared via conventional melt-quenching- thermal annealing method. The optimized 15%Er–2%Tm co-doped NaYF4@glass can achieve pure red upconversion luminescence due to the energy transfer process between Er3+ and Tm3+. Owing to that the electrons in the 2H11/2, 4S3/2 and 4F9/2 levels of Er3+ are independently populated, there is not direct temperature-dependent electron correlation between the green energy level (2H11/2, 4S3/2) and the red energy level (4F9/2) of Er3+. Moreover, the Er3+/Tm3+ co-doped NaYF4@glass-based three-dimensional prototype display device is constructured. This study can provide a new idea for the development of pure color transparent upconversion materials for three-dimensional stereoscopic display.