Elsevier

Optical Materials

Volume 36, Issue 2, December 2013, Pages 396-401
Optical Materials

Optical properties of Tb3+ doped KLaF4 in cubic and hexagonal symmetries

https://doi.org/10.1016/j.optmat.2013.09.031Get rights and content

Highlights

  • Tb3+-doped cubic and hexagonal KLaF4 nanocrystals were obtained by solution method.

  • Both cubic and hexagonal KLaF4:Tb3+ showed strong green emission under UV light.

  • The average lifetime of the (5D4  7F5) emission was 1–3 ms.

  • Tb3+/Yb3+ co-doped cubic KLaF4 showed intense green upconversion.

  • Cubic and hexagonal KLaF4:Tb3+ exhibited paramagnetic character at RT.

Abstract

Tb3+ doped cubic and hexagonally ordered KLaF4 nano-sized crystals have been synthesized by solution based method with the simultaneous evaluation of their optical and magnetic properties. While higher amounts of Tb3+ (10%) can be doped in cubic KLaF4 lattice, only 3% of Tb3+ can be doped in hexagonal KLaF4 by this method. Cubic KLaF4:Tb3+ samples show a very strong green emission centered at 545 nm (5D4  7F5) on excitation with 337 nm laser, the intensity increases monotonically with Tb3+ content. The relative intensity of the excited state 5D3 violet emission is weaker than that of 5D4 emissions even at the low Tb3+ amounts. It is proposed that the 5D3  5D4 cross-relaxation may be responsible for the decrease in the 5D3 decay rate with increasing Tb3+ concentrations. The average emission decay lifetime of the green emission (545 nm; 5D4  7F5) of Tb3+ doped cubic KLaF4 samples are in the order of 1–3 ms. The absence of the characteristic emissions from 5D3 level in hexagonal KLaF4:Tb3+ phosphor suggests the pronounced non-radiative cross relaxation between the 5D3 to 5D4 energy levels. Further, these cubic KLaF4:Tb3+ samples show intense green upconversion emission when co-doped with moderate concentrations of sensitizer Yb3+ ions. Both cubic and hexagonal KLaF4:Tb3+ samples show paramagnetic behavior at room temperature with χg value ranging from 1.627 × 10−6 to 1.356 × 10−5 emu/g.

Introduction

The study of excitation mechanisms of lanthanide ions in a host matrix is a crucial point in the development of the technologies of synthesis in the light-emitting materials, and generation of light using blue and UV incoherent excitation sources. Lanthanide ions can be excited resonantly through ff transitions or fd transitions or non-directly in charge transfer process (CT) or dipole–dipole energy transfer (ET) [1]. The energy position of excitation band related to this process depends strongly on the ions local chemical environment in contrary to well defined excitation bands related to ff transitions. Among the inorganic phosphors, fluoride host matrices are considered to be good hosts as they possess high refractive index and low phonon energy which cause longer lifetimes of their excited states and high luminescence yields [2], [3], [4], [5]. Also, the mixed metal fluorides have optical transparency over a wide wavelength range and crystallize in such a symmetry that allows site-selective doping capability. Therefore, their use in wide range of applications, such as in solid state light emitting applications, scintillators and thermally stimulated luminescence (TSL), bioseparation, Magnetic Resonance Imaging (MRI) and vivo imaging of tissues and cells are being probed [6], [7], [8], [9], [10].

The versatile nature of cubic KLaF4 system has been exemplified by the demonstration of efficient green upconversion (UC) on Er3+ doping and the strong red emission on Eu3+ doping [11], [12]. Recently, our first time synthesis of hexagonally ordered KLaF4, promoted by the transformation nanocrystals by controlled aging at 65 °C, showed that this host lattice possessed phonon energy (262 cm−1) lower than the widely investigated NaYF4 (360 cm−1) system [13]. Also, it showed a strong green UC upon Er3+ doping. Other than these examples, study on the normal green emission by the emitter Tb3+ having high magnetic moment in these two host matrices will be of importance as such process will add both optical and magnetic functions to the lattice. Such combinations are being used in MRI systems [8].

In this paper, the successful synthesis of Tb3+-doped cubic and hexagonal KLaF4 nanosized crystals along with their optical and magnetic properties are described. Though, efficient green emission is observed from both the cubic and hexagonal polymorphs of KLaF4, the emission intensity is higher from hexagonal sample than the cubic KLaF4 samples. The decay lifetimes of the emission from the Tb3+ ions in this lattice has been explained by the cross relaxation mechanism. Additionally, intense green upconverted emission has been observed from cubic KLaF4 sample doped with Yb3+ (20%) and Tb3+ (2%). Tb3+-doped samples showed paramagnetic behavior at room temperature suggesting its potential use in applications requiring both effective emission and paramagnetic character.

Section snippets

Experimental

For the experiments, La(acac)3·xH2O [Sigma Aldrich, 0.4362 g, 1 mol] and KF [Merck, GR 0.2324 g, 4 mol] were used. The reactants were dissolved in 20 mL of methanol separately. The solution containing the lanthanum ion was added drop-wise under constant stirring to KF solution. The resulting white colored suspension was subjected to two different reaction conditions. In one set, the reaction was continued at room temperature under constant stirring for ∼12 h. In the other, the suspension was aged at

Results and discussion

PXRD patterns of Tb3+ (1, 3, 5 and 10 mol%) doped KLaF4 (Fig. 1) indicate the formation of monophasic cubic polymorph of KLaF4 with the positions and the intensities of the observed reflections matching well with the JCPDS file no.75-2020. Le-Bail fitting of the PXRD pattern of 3% Tb3+ doped KLaF4 sample yields the lattice dimension of a = 5.9340 (3) Å (inset of Fig. 1) [14]. The PXRD pattern of Tb3+ (3%) doped hexagonal form of KLaF4 matched well with JCPDS File No. 75-1927. Rietveld refinement of

Conclusion

Versatility of KLaF4 as a host lattice both in cubic and hexagonal symmetries as an efficient green emitter by doping with various concentrations of Tb3+ ions has been demonstrated. Additionally, efficient color tuning from bluish-green to green with varying Tb3+ ion concentration has also been observed. The 5D3  5D4 cross-relaxation process has been observed which decreased the 5D3 decay rate for all the Tb3+ concentrations in the host lattice. Furthermore, relatively higher lifetimes in the

Acknowledgements

The authors sincerely thank and acknowledge DU-DST PURSE grant and Department of Information Technology (DIT), Govt. of India (under Photonics Development Program Ref: 12(1)/2008-PDD) for their financial support to carry out this work. One of the authors, Shahzad Ahmad, wishes to record his sincere thanks to CSIR, New Delhi, India for a SRF fellowship. This work is part of the High-Impact Research Scheme of IIT Delhi. The technical help from project staff, Mr. A.A. Reddy and Mr. Packiya Raj is

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    Current address: Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC.

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