Facile composite synthesis and photoluminescence of NaGd(MoO 4 ) 2 : Ln 3+ (Ln = Eu, Tb) submicrometer phosphors
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NaGd(MoO4)2:Ln3þ (Ln = Eu, Tb) submicrometer phosphors have been synthesized by a composite method including the solid state reaction process at room temperature and the hydrothermal process. It is revealed that temperature and humidity have an influence on the reaction rate and that higher temperature and humidity can speed up the reaction process. Crystalline water is necessary for the solid phase reaction at room temperature. The x-ray diffraction (XRD) patterns indicate that NaGd(MoO4)2:Ln3þ (Ln = Eu, Tb) submicrometer phosphors crystallize well with the scheelite structure. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images illustrate that the average grain size of NaGd(MoO4)2:Ln3þ is about 225 nm without conglomeration. The luminescent lifetime and quantum efficiency for NaGd(MoO4)2: Eu3þ are determined.
I. INTRODUCTION
Recently there has been a growing concern about phosphor-converted light-emitting diodes (LEDs) and tricolor phosphors, which are widely considered as the next generation of solid state illumination materials.1–4 In these devices, the tricolor phosphors are pumped by UV-InGaN chips or blue GaN chips and generate white light. Nowadays one of the challenges for the new generation of lighting based upon GaN comes from the development of novel families of phosphors that are optimized for excitations at longer wavelengths in the near UV (350–400 nm). The current phosphor materials of choice for solid state lighting based upon near UV GaN-LEDs are Y2O2S:Eu3þ for red, ZnS:(Cuþ, Al3þ) for green, and BaMgAl10O17:Eu2þ for blue. However, the efficiency of the Y2O2S:Eu3þ red phosphor is much less than that of the green and blue phosphors except that its lifetime is severely limited by the dose of UV radiation.5,6 Therefore, the primary interest of the present work is to search for novel red phosphors for near UV InGaN chip-based white light-emitting diodes. Double molybdates ARE(MoO4)2 (A = Liþ, Naþ, Kþ, Rbþ, Csþ; RE = trivalent rare earth ions) which share scheelite-like (CaMoO4) structure, have been widely studied because of their laser applications.7–10 In the past few years much research focused on the relationship between the cations’ radii and their structure.11,12 The scheelite-like structure of CaWO4 is a tetragonal crystal system with space group symmetry I41/a, and Ca2þ can be substituted a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0035 32
J. Mater. Res., Vol. 24, No. 1, Jan 2009
http://journals.cambridge.org
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by a combination of monovalent and trivalent cations, leading to the formation of MR(MoO4)2 (M = Li, Na, R = Ln, Y, Bi) compounds with a statistical distribution of Mþ and R3þ. In MRE(MoO4)2 (RE = Ln, Y) compounds, Mo6þ is coordinated by four oxygen atoms in a tetrahedral site, and the rare-earth/sodium site is eight coordinated, with two sets of rare earth–oxygen distances. MLn(MoO4)2 compounds are considered to be ideal luminescent hosts due to their excellent thermal an
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