Optical properties of Tb3+ doped KLaF4 in cubic and hexagonal symmetries
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 f–f transitions or f–d 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 f–f 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.