Synthesis, structure, and upconversion emission of Er3+ and Yb3+ co-doped YBO3
Graphical abstract
Introduction
Upconversion (UC) is the anti-Stokes photoluminescence phenomenon emitting photons which have higher energy than the individual absorbed photons by absorbing more than two photons sequentially [[1], [2], [3]]. Phosphors with doped rare-earth (RE) ions have been widely studied for efficient UC emission because of the well-defined 4f energy levels in the RE ions [4,5]. Many researchers have been interested in UC phosphors because of their unique optical characteristic and potential application [[6], [7], [8]]. Among the RE ions, we chose the Er3+ ion as an activator because it has fascinating green and red emissions. Importantly, the metastable energy levels of Er3+ ions make it possible to obtain transitions over a large range of wavelengths from infrared (IR) to ultraviolet (UV) [9,10]. Interestingly, many UC phosphors have been synthesized with co-doping Yb3+ ions because Yb3+ ions have a large absorption cross section of around 980 nm [11,12]. Research on Er3+ and Yb3+ co-doped UC phosphors has been widely performed [[13], [14], [15], [16]].
Rare earth orthoborates (RBO3, R: rare earth element) have attracted much attention as host materials. Among the various RBO3 compounds, YBO3 has been widely studied for applications in optoelectric devices. In fact, the phonon energies in YBO3 (1300 cm−1) are higher than those in other materials such as fluorides (500–600 cm−1) and oxides (500 cm−1) materials [17]. In spite of this disadvantage, the RE-doped YBO3 have long fluorescence time and good chemical stability [[18], [19], [20]]. And also, YBO3:Er,Yb phosphors have quite dominant red UC emission relative to green UC emission due to its high phonon energies while Er3+ and Yb3+ doped fluorides and oxides materials have relatively stronger green UC emission than red. Several works have studied RE-ion doped YBO3 phosphors under UV excitation. Bi3+ and Dy3+ co-doped YBO3 samples have showed large energy transfer from Bi3+ to Dy3+ and bright blue emission under 270 nm excitation [21]. Dubey et al. observed intense 594, 612, and 628 nm emissions from Eu3+ in YBO3:Eu3+ under 395 nm excitation [22]. Sohn et al. reported that Tb3+ doped YBO3 can be practical materials for the green component in plasma displays because of their strong absorption in the vacuum UV range [23]. Although studies on YBO3 phosphors under UV excitation have been performed by many researchers, there have been only a few studies on the UC emission properties of YBO3 compounds. Grzyb et al. investigated the energy transfer from Yb3+ to Tb3+ and observed strong green and red UC emissions from Tb3+ in YBO3:Yb3+,Tb3+ under near-IR excitation [24]. Yang et al. investigated the effect of the matrix with an inverse opal structure on the UC emission in YBO3:Yb3+,Er3+ [25].
Motivated by these considerations, we synthesized Er3+ and Yb3+ co-doped YBO3 phosphors and investigated their structural and UC emission properties systematically. To analyze the structural properties in detail, we measured X-ray diffraction and performed the Rietveld refinement. In the UC emission spectra, we observed bright red UC emission from the Er3+ and Yb3+ co-doped YBO3 under 980 nm near-IR excitation. The UC emission intensity was found to depend strongly on the concentration of the Er3+ and Yb3+ ions. The power-dependent UC emission spectra were also measured to investigate the number of absorbed photons in the UC process.
Section snippets
Sample preparation
The Er3+ and Yb3+ co-doped YBO3 (YBO3:Er,Yb) polycrystals were synthesized using the solid-state reaction method. We chose Y2O3, H3BO3, Er2O3, and Yb2O3 as the raw materials and mixed Y2O3, H3BO3, Er2O3, and Yb2O3, in the ratios of 0.95-x: 1: x: 0.05 and 0.95-y: 1 : 0.05: y where x = 0.03, 0.05, and 0.1, and y = 0.02, 0.05, and 0.1 for YBO3:xEryYb. The mixed powders were calcinated at 700 °C for 7 h. The calcinated powder pressed into pellets, and then sintered at 1200 °C for 30 h.
Analysis methods
The X-ray
Structural analysis of YBO3:Er,Yb
Fig. 1 shows the XRD results of the YBO3:Er,Yb samples in this study. We observed that the YBO3:Er,Yb samples have identical patterns to pure YBO3 (JCPDS 16-0277). The main peaks of our samples are significantly sharp and strong, indicating that the samples have sufficient crystallinity. To investigate the degree of the size mismatch between the dopants and the substituted ions, the difference in the ionic radii, DR (%), was quantitatively estimated aswhere R(Y3+
Conclusion
We investigated the UC emission properties of Er3+ and Yb3+-ion doped YBO3:Er,Yb synthesized by the solid state reaction method. The XRD patterns indicate significant crystallinity of the YBO3:Er,Yb samples without any secondary phase. The PL spectra of the samples showed some UV emissions originated from the f-f transitions in the Er3+ ions. Under near-IR excitation, we identified weak green and strong red emissions at 547 and 660 nm respectively, which indicate that the UC process occurred
Credit author statement
Hyeontae Lim: Investigation, Writing- Original draft preparation, Writing - Review & Editing, Jongcheol Won: Investigation, Sangwon Wi: Investigation, Writing - Review & Editing, Soyeong Jang: investigation, J.-S. Chung: Supervision, and Y. S. Lee: Conceptualization, Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (NRF-2018R1D1A1A02086130).
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