Luminescence properties and charge transfer mechanism of host sensitized Ba2CaWO6:Eu3+ phosphor
Graphical abstract
Introduction
White light emitting diodes are considered as the new generation solid-state lighting sources which would replace the conventional incandescent and fluorescent lamps, due to their high luminous efficiency, long lifetime, fast response, energy saving and environmental friendliness [1], [2], [3]. Most of the commercially available white LEDs are based on phosphor-converted emission method. The present strategy to produce white light is to make use of blue InGaN LED chip in combination with Cerium(III) doped Yttrium Aluminium Garnet (YAG:Ce3+) yellow phosphor. The blue light emitting InGaN chip excites the yellow emitting YAG:Ce3+ phosphor and then the remaining blue light is mixed with this yellow light to produce the desired white light [4]. But a YAG:Ce3 + phosphor based white LED exhibits poor color rendering index (CRI ~ 10–80) and high correlated color temperature (CCT ~ 7750 K) due to the lack of a red component [5], [6], [7]. There are some recent studies which incorporate rare earth ions as activators in phosphor converted white light emitting diode (pc-WLED) to compensate the lack of red components in the emission thereby enhancing the CRI value. With the incorporation of rare earth ions like Tb3+, high CRI could be obtained without any significant decrease in the luminescent efficiency [8]. In order to improve the white light quality, a combination of UV LED chip with tricolor (red, green and blue) phosphors, which offers a higher color rendering index and superior color uniformity can also be used. However, the commercially available red-emitting phosphor for UV LED chip, Y2O2S:Eu3+ is chemically unstable and has a low absorption efficiency in the UV-region [7]. The instability of color temperature due to degradation of different colors and variations in driving current also affects the performance of UV-LEDs [8]. Therefore, it is important to explore efficient red phosphors for pc-WLEDs.
Materials like tungstates are considered as a wise choice for white LEDs because they have a unique structure, excellent luminescent properties, good chemical and thermal stability and wide potential applications [9]. Tungstate based host matrices have wide excitation and emission bands due to the ligand to metal charge transfer (CT) transitions of (W04)2 − and (W06)6 − oxyanion complex [10]. It also shows self-activation and emits blue-green light under ultraviolet excitation and offers more practical applications in the field of quantum electronics as laser host material, scintillators in medical devices, and phosphor materials for fluorescent lamps [11], [12], [13], [14]. Rare earth ions have interesting optical properties because of their partially filled 4f shells, which makes them suitable activators for various host matrices [15]. Most rare earth ions show sharp excitation peaks, but their higher concentration in host lattice leads to concentration quenching. Rare earth activated tungstates have a strong covalent interaction, high stability and intense visible luminescence. They have been widely investigated, due to their broad and intense charge transfer excitation band in the UV region. Tungstate, with double perovskite structure, possesses excellent physical properties such as ferroelectrics, dielectrics, photocatalytic, magnetoresistance, etc. [16] which has made it an interesting luminescent material since the 1950s. A high refractive index and a high X-ray absorption coefficient are some of the other properties of alkaline earth metal tungstates [17]. Eu3+ activated double perovskite tungstate phosphors exhibits a high quantum efficiency in the red or reddish-orange region due to its distinct 4f-4f transitions. So Eu3+ doped tungstate materials have been extensively investigated for the last few years owing to their potentiality in the field of pc-WLEDs [2], [3], [6].
To the best of our knowledge, the spectroscopic Judd-Ofelt investigation of Eu3+ activated Ba2CaWO6, has not been reported. In the present work, we report the energy transfer from host lattices to Eu3+ ions in Ba2CaWO6:Eu3+ phosphor with different concentrations of Europium ions. The structural, morphological and spectroscopic characteristics of the prepared phosphors were investigated. The radiative properties were obtained by Judd-Ofelt analysis from the emission spectrum and lifetime measurements. The host has a wide charge transfer band centered in the UV region, which provides an opportunity for developing a new efficient luminescent material.
Section snippets
Experimental
Ba2Ca1-xWO6:xEu3+ phosphors (where x = 0.0, 0.1, 0.2, 0.25, 0.3, 0.35 and 0.4 wt%) were synthesized using the conventional solid-state reaction technique. Appropriate amounts of BaCO3 (99.99%), CaCO3 (99.99%), WO3 (99.99%) and Eu2O3 (99.99%), obtained from Sigma-Aldrich, were used as the starting materials. The stoichiometric mixture of raw material was well ground in an agate mortar for an hour to achieve a uniform mixture. The mixture was preheated, at 1000 °C, for 6 h in KSL-1700X-S High
XRD and EDS spectra
Fig. 1 shows the XRD profiles of Ba2CaWO6 and Ba2CaWO6:xEu3+ recorded in the range of 10° ≤ 2θ ≤ 80° with a scan rate of 0.02°/sec. The obtained spectra were compared with ICDD No. 73-0136 and the observed excellent agreement helped us to conclude that Ba2CaWO6:xEu3+ has a cubic double perovskite structure with the space group Fmm [18]. The hexavalent tungstates are stabilized in the ordered double perovskite structure A2BB'O6 where A2 is an alkaline earth ion (Ba2+), B is a divalent metal ion (Ca
Conclusions
Eu3+ doped Ba2CaWO6 phosphors were prepared by solid state reaction method. The XRD data confirmed the double perovskite structure. Crystallite size, evaluated through Scherrer and Williamson-Hall plot (W-H), was comparable and the relatively small strain suggests the negligibly small lattice distortion through the introduction of rare earth ions in to the host lattice. The stoichiometry of the sample was verified by the EDS spectrum. The spherical morphology of the sample was investigated
Acknowledgements
Authors are thankful to UGC (Govt. of India) for financial assistance through SAP-DRS (No.F.530/12/DRS/2009 (SAP-1)) and DST-PURSE (SR/S9/Z-23/2010/22 (C, G)) programs. Authors are also thankful to Department of Chemistry, National Institute of Technology, Calicut, for the Decay analysis. The authors Sreeja E, Subash Gopi, Viji Vidyadharan and Remya Mohan P are grateful to UGC, India for the award of Research Fellowship in Sciences to Meritorious Students (RFSMS) Fellowship.
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