Effects of Ca2+ doping on upconversion luminescence intensity and crystal field asymmetry of β-NaYF4:Yb3+/Er3+ microcrystals

https://doi.org/10.1016/j.jlumin.2020.117086Get rights and content

Highlights

  • The green and red UC emissions were improved by 102.8 and 43.1 times respectively.

  • Ca2+ doping changed the crystal structure and morphology of NaYF4:Yb3+/Er3+.

  • Judd-Ofelt analysis was performed to study the local site asymmetry of Er3+.

  • Local site asymmetry variation of Er3+ was less dominant in affecting UC emission.

Abstract

The green and red upconversion (UC) emissions of 35 mol% Ca2+ doped β-NaYF4:Yb3+/Er3+ microcrystals were improved to 102.8 and 43.1 times higher than those of Ca2+-free counterparts. X-ray powder diffraction spectra, scanning electron microscopy, Fourier transform infrared spectroscopy, upconversion luminescence (UCL) spectroscopy and Judd-Ofelt analysis were performed to investigate the mechanism of UCL intensity enhancement caused by Ca2+ doping. The hexagonal-to-cubic phase transition was observed when Ca2+ doping concentration exceeded 35 mol%. The incorporation of Ca2+ into hexagonal microcrystals lattice induced unit cell volume enhancement and microcrystals surface to volume ratio variation, which affected the UCL intensity by hindering the non-radiative energy loss process via Yb3+/Er3+ ions and changing the amount of surface quenchers adsorbed on the microcrystals surface. The hypersensitive transition related local crystal field asymmetry around Er3+ in the grown hexagonal microcrystals was also investigated by Judd-Ofelt analysis but unexpectedly found to be the less dominant factor for the UCL intensity variation. This study helps to realize the mechanism of impurity ion doping resulted UCL intensity enhancement, which is instructive for high-performance UC microcrystals synthesis.

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 (P6 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).

References (62)

  • A. Awang et al.

    Enhanced spectroscopic properties and Judd–Ofelt parameters of Er-doped tellurite glass: effect of gold nanoparticles

    Curr. Appl. Phys.

    (2013)
  • Y. Qian et al.

    Optical spectroscopy and laser parameters of Zn2+/Er3+/Yb3+-tridoped LiNbO3 crystal

    J. Lumin.

    (2012)
  • Q. Nie et al.

    Luminescence resonance energy transfer probes based on NaYF4:Yb,Er-Ag nanocompounds for sensitive detection of Mn2+ ions

    J. Alloys Compd.

    (2017)
  • Y. Zhang et al.

    Improved LRET-based detection characters of Cu2+ using sandwich structured NaYF4@NaYF4:Er3+/Yb3+@NaYF4 nanoparticles as energy donor

    Sensor. Actuator. B Chem.

    (2018)
  • X. Wu et al.

    Enhance the luminescence properties of Ca14Al10Zn6O35:Ti4+ phosphor via cation vacancies engineering of Ca2+ and Zn2+

    Ceram. Int.

    (2019)
  • W. Mnasri et al.

    The first one-pot synthesis of undoped and Eu doped β-NaYF4 nanocrystals and their evaluation as efficient dyes for nanomedicine

    Mat. Sci. Eng. C-Mater.

    (2019)
  • R.S. Yadav et al.

    Surface analysis and enhanced photoluminescence via Bi3+ doping in a Tb3+ doped Y2O3 nano-phosphor under UV excitation

    J. Alloys Compd.

    (2017)
  • P. Villanueva-Delgado et al.

    Judd–Ofelt analysis of β-NaGdF4: Yb3+, Tm3+ and β-NaGdF4:Er3+ single crystals

    J. Lumin.

    (2017)
  • M.P. Hehlen et al.

    50th anniversary of the Judd–Ofelt theory: an experimentalist's view of the formalism and its application

    J. Lumin.

    (2013)
  • G. Yao et al.

    Calculation of Judd-Ofelt parameters for Er3+ in β-NaYF4: Yb3+, Er3+ from emission intensity ratios and diffuse reflectance spectra

    J. Lumin.

    (2015)
  • E. Cantelar et al.

    Judd–Ofelt analysis of powder samples: LiNbO3:Er3+ submicron-sized particles as a model case

    Opt. Mater.

    (2015)
  • L. Yao et al.

    Simultaneous enhancement of upconversion luminescence and thermometric property of upconversion nanoparticles by tuning crystal field

    J. Lumin.

    (2019)
  • H. Chen et al.

    Optical properties of Dy3+-doped CaYAlO4 crystal

    J. Lumin.

    (2018)
  • G. Bilir et al.

    Judd–Ofelt analysis and near infrared emission properties of the Er3+ ions in tellurite glasses containing WO3 and CdO

    Optic Commun.

    (2011)
  • X. Cheng et al.

    Er3+ sensitized photon upconversion nanocrystals

    Adv. Funct. Mater.

    (2018)
  • A. Nadort et al.

    Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties

    Nanoscale

    (2016)
  • B.D. Ravetz et al.

    Photoredox catalysis using infrared light via triplet fusion upconversion

    Nature

    (2019)
  • L. Lei et al.

    Highly intensified upconversion luminescence of Ca2+‐doped Yb/Er:NaGdF4 nanocrystals prepared by a solvothermal route

    Chem. Asian J.

    (2014)
  • K. Du et al.

    Enhanced upconversion luminescence and controllable phase/shape of NaYF4:Yb/Er crystals through Cu2+ ion doping

    Crystengcomm

    (2018)
  • M. Yi et al.

    Upconversion effective enhancement of NaYF4:Yb3+/Er3+ nanoparticles by Ni2+ doping

    J. Mater. Sci.

    (2018)
  • S. Wen et al.

    Advances in highly doped upconversion nanoparticles

    Nat. Commun.

    (2018)
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