Comparative investigation on structure and luminescence properties of fluoride phosphors codoped with Er3+/Yb3+

doi:10.1016/j.saa.2009.06.040

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 74, Issue 2, 1 October 2009, Pages 441-445

https://doi.org/10.1016/j.saa.2009.06.040 Get rights and content

Abstract

This paper reports on comparative investigation of structure and luminescence properties of tetragonal LiYF₄ and BaYF₅, and hexagonal NaYF₄ phosphors codoped with Er³⁺/Yb³⁺ by a facile hydrothermal synthesis. The products were characterized by X-ray diffractometer, scanning electron microscope, and photoluminescence spectroscopy. Intense visible emissions centered at around 525, 550 and 650 nm, originated from the transitions of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺, respectively, have been observed in all the samples upon excitation with a 980 nm laser diode, and the involved mechanisms have been explained. Based on the green up-conversion emission performance, the Yb³⁺ concentrations of Er³⁺/Yb³⁺-codoped LiYF₄, BaYF₅, and NaYF₄ phosphors have been optimized to be 10, 20, and 20 mol.%, respectively. The quadratic dependence of fluorescence on excitation laser power has confirmed that two-photon contribute to up-conversion of the green–red emissions.

Introduction

Over the past several years, considerable interest has been centered on the conversion of infrared radiation to shorter wavelengths by rare-earth (RE)-doped materials for their potential applications in lighting and display, infrared inspection and biological labeling, etc. [1], [2], [3]. Especially, complex fluoride crystals based infrared-to-visible up-conversion (UC) phosphors such as RE-doped NaYF₄ have regarded as promising materials, due to its low phonon energy, high refractive index, optical transparency and excellent luminescence properties [4], [5], [6], [7]. Numerous efforts have been devoted to explore appropriate approaches to synthesize well-defined complex fluoride phosphors. Recently, many reports are focus on modification of nano- to micro-sized fluoride phosphors, which involve selected solvent treatment, ligands and surfactants to control the size and morphology of the product [8], [9]. However, the toxicity of the solvent must be taken account in and the organic ligands with high energy C–H and C–C vibrational oscillators attached on the product surface are severe luminescence quencher of nearby RE ions [10], [11]. Hydrothermal treatment is considered to be a promising approach to prepare various inorganic micromaterial with less pollution, high homogeneity and high purity. On the other hand, the ladder-like energy levels of Er³⁺ enable it an ideally activator for the infrared–visible conversion and Yb³⁺ can enhanced the corresponding UC efficiency significantly for the energy match between Er³⁺ and Yb³⁺ [12].

Herein, we present a facile hydrothermal approach to synthesis the Er³⁺/Yb³⁺-codoped LiYF₄, NaYF₄ and BaYF₅ phosphors. The main objective of this work is to carry out a comparative investigation on structure and luminescence properties of fluoride phosphors: LiYF₄, NaYF₄ and BaYF₅, to elucidate its mechanisms responsible for the UC emission process, and to examine its suitability as potential UC phosphors. Effects of Yb³⁺ concentration on UC emission have also been investigated.

Section snippets

Experimental

All chemicals were of analytical grade and used without further purification. Y₂O₃, Er₂O₃ and Yb₂O₃ were dissolved in nitric acid to form corresponding nitrates Ln(NO₃)₃ (Ln = Y, Er, Yb) solution. For Er³⁺/Yb³⁺-codoped LiYF₄, reactants with an initial mole ratio of 1.5 LiF:(0.98 − x) Y(NO₃)₃:8 NH₄HF₂: 0.02 Er(NO₃)₃: x Yb(NO₃)₃ (x = 0.05, 0.1, 0.2, 0.3, 0.5), were mixed to form suspension with a pH of ca. 2–3 under magnetic stirring, then transferred into a Teflon-lined stain-steel autoclave of 50 ml

Results and discussion

Fig. 1 represents the XRD patterns of as-synthesized products. All the diffraction peaks can be readily indexed to a pure scheelite-type tetragonal LiYF₄ (body-centered lattice, space group I41/a [88], a = 5.16 Å, c = 10.74 Å) [13], gagarinite-type hexagonal NaYF₄ (primitive lattice, space group p63/m [176], a = 5.96 Å, c = 3.53 Å) [14], and distorted fluorite-type tetragonal BaYF₅ (primitive lattice, space group $P \bar{4} 21 m$ [113], a = 12.46 Å, c = 6.78 Å), respectively. The fairly narrow full width at half maximum

Conclusion

In summary, we report on the structural and UC properties of Er³⁺/Yb³⁺-codoped LiYF₄, NaYF₄ and BaYF₅ phosphors by a facile hydrothermal synthesis. Intense UC emissions centered at 525, 550 and 650 nm have clearly been observed and the involved mechanisms are explained. The green emissions at 525 and 550 nm are due to the ²H_11/2 → ⁴I_15/2, and ⁴S_3/2 → ⁴I_15/2 transitions, respectively. The red up-conversion emission at 650 nm has been associated with the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions. The optimal

Acknowledgements

This work is jointly supported by the NSFC (50872036), and DSTG (2006J1-C0491).

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

Reference (20)

J. Alloys Compd.

J. Alloys Compd.

Mater. Lett.

J. Lumin.

J. Lumin.

Appl. Phys. Lett.

Appl. Phys. Lett.

Opt. Spectrosc.

Adv. Funct. Mater.

Appl. Phys. Lett.

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A new type of zero thermal quenching red emitting phosphor β-NaYF<inf>4</inf>:Eu<sup>3+</sup> for NUV LEDs

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Tunable upconversion emission of Ho<sup>3+</sup>/Yb<sup>3+</sup>-doped single β-NaYF<inf>4</inf> microrod