Effects of Ca2+ doping on upconversion luminescence intensity and crystal field asymmetry of β-NaYF4:Yb3+/Er3+ microcrystals
Introduction
Upconversion (UC) micro- or nano-crystal, generally made up of activator, sensitizer and matrix, can emit high-energy photon through absorption of two or more low-energy photons [1]. Lanthanide ion with rich spectroscopically active levels is suitable to be the activator [2], Yb3+ with large absorption cross-section is an excellent sensitizer, and β-NaYF4 is usually selected as the matrix for its low phonon energy [3]. Taking Er3+ as the activator, β-NaYF4:Yb3+/Er3+ crystal can convert 980 nm light into visible light, which has promising application prospects in the area of biological labeling [4], photocatalyst [5] and solar cells [6]. Nonetheless, the application of β-NaYF4:Yb3+/Er3+ crystal is greatly limited for its low upconversion luminescence (UCL) intensity.
Impurity ion doping can effectively improve the UCL intensity of UC crystal. For example, Lei [7] doped Ca2+ into β-NaGdF4:Yb3+/Er3+ crystal and intensified the UCL intensity by about 200 times, Du [8] introduced Cu2+ into NaYF4:Yb3+/Er3+ crystal and enhanced the UCL intensity by 25–37 times, Yi [9] improved the UCL intensity of β-NaYF4:Yb3+/Er3+ crystal to 22–34 times by Ni2+ doping. The mechanism of UCL intensity enhancement induced by impurity ion doping is generally explained by the following factors: (1) the interactions among the activators/sensitizers that highly depend on their separating distance and can be reflected from the unit cell volume [10]. (2) the surface quenching effect caused by the crystals surface nature and limited by the crystals surface to volume ratio [11,12]. (3) the hypersensitive transition, or in other words, the forbidden 4f-4f electrons transition probability of activators that may be promoted by increasing the asymmetry of local crystal field around activators [13,14]. The first two factors have been directly investigated by XRD patterns, SEM images, FT-IR spectra and other characterization methods. However, the effect of local crystal field asymmetry variation on the UCL intensity enhancement of UC crystal is usually considered in various indirect ways. Such as taking Eu3+ as the structural probe [[15], [16], [17]], inferring the relationship among local crystal field asymmetry, crystal lattice distortion, lattice vacancy and ions migration caused by the incorporation of impurity ion [[7], [8], [9],13,14,[18], [19], [20], [21], [22], [23], [24], [25]]. On the other hand, Judd-Ofelt analysis provides a way to measure the degree of asymmetry of local crystal field [26,27]. Three Judd-Ofelt parameters Ω2,4,6 may be computed according to the absorption spectrum, or the similar excitation spectrum [15], or the diffuse-reflection spectrum after transforming by Kubelka–Munk function [28,29], and Ω2 is always used to study the local crystal field asymmetry [15,28,30,31]. However, Judd-Ofelt analysis is mainly conducted for transparent glass samples [[31], [32], [33], [34]] and not often performed for impurity ion doped UC crystals (powder sample) at present, probably due to the complicated analysis procedure for powder sample and the absorption bands overlap among many luminescent impurity ions (such as Cu2+, Ni2+, Mn2+ …) and activators in visible region [9,35,36].
Considering the above points, we selected Ca2+ rather than the popular luminescent ion in recent years [8,9,[37], [38], [39]] to dope NaYF4:Yb3+/Er3+ crystals. By taking advantage of the excellent performance of Ca2+ in improving the UCL intensity of Yb3+-sensitized/Er3+-activated UC crystals [7,16,17] and the nonexistent absorption bands overlap between Ca2+ and Er3+ in 300–800 nm spectral region [40], we enhanced the UCL intensity of NaYF4:Yb3+/Er3+ crystals greatly and studied the corresponding mechanism in depth.
Section snippets
UC crystals preparation
The raw materials were all from Shanghai Aladdin Industrial Co., Ltd. The Ca2+ doped NaYF4:Yb3+/Er3+ crystals were synthesized using a hydrothermal method. The compositions for synthesizing different samples are listed in Table 1, and the molar ratios of Yb3+ and Er3+ are fixed at 20 mol% and 2 mol% to make this study comparable to the similar studies [7,9,16,17]. Typically, the Y(NO3)3·6H2O, Yb(NO3)3·5H2O, Er(NO3)3·5H2O and CaCl2 were dissolved in 20 mL of ultrapure water. After adding 30 mL
XRD analysis
The XRD patterns of the synthesized crystals are presented in Fig. 1(A). The diffraction peaks are in line with standard β-NaYF4 phase as Ca2+ doping concentration rises from 0 to 35 mol%. With Ca2+ doping concentration further rising, however, the diffraction peaks indexed to α-NaYF4 is observed, which is different from the cases of Ca2+-doped NaYF4:Nd3+ [18], Ca2+-doped NaLuF4:Yb3+/Er3+ [17] and Ca2+-doped NaGdF4:Yb3+/Er3+ [16]. Meanwhile, the diffraction peaks of samples (a)~(e) are all
Conclusions
To sum up, Ca2+ with different concentration were doped into NaYF4:Yb3+/Er3+ microcrystals for UCL intensity enhancement. The phase change from β-NaYF4 ( space group) to α-NaYF4 did not occur until Ca2+ doping concentration exceeded 35 mol%. The UC emissions mechanism of hexagonal UC microcrystals was not affected by Ca2+ doping. But it was found that the incorporation of Ca2+ into hexagonal microcrystals enhanced the unit cell volume, changed the microcrystals morphology especially surface
CRediT authorship contribution statement
Hao Tang: Conceptualization, Methodology, Validation, Writing - original draft, Writing - review & editing. Hua Zhou: Resources, Formal analysis, Investigation, Data curation. Xianhua Cheng: Supervision, Project administration, Funding acquisition.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (No. 51575341), and the Opening Fund of State Key Laboratory of Solid Lubrication in Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (LSL-1703).
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