Journal of Alloys and Compounds 475 (2009) 452–455 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Controlled synthesis and luminescence properties from cubic to hexagonal NaYF 4 :Ln 3+ (Ln = Eu and Yb/Tm) microcrystals Guofeng Wang, Weiping Qin ∗ , Jisen Zhang, Lili Wang, Guodong Wei, Peifen Zhu, Ryongjin Kim State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China article info Article history: Received 10 June 2008 Received in revised form 9 July 2008 Accepted 10 July 2008 Available online 28 August 2008 Keywords: NaYF4 Rare earth Up/down-conversion luminescence abstract Cubic and hexagonal NaYF 4 :Ln 3+ (Ln = Eu and Yb/Tm) microcrystals were separately synthesized by an EDTA-assisted solvothermal method. Under 393-nm excitation, the emission spectra of NaYF 4 :Eu 3+ micro- crystals show the characteristic Eu 3+ emissions. In the excitation spectra of the 610-nm emission, the 7 F 0 → 5 D 3 is dominant for the cubic microcrystals, while the 7 F 0 → 5 L 6 is dominant for the hexagonal sample. In addition, unusually strong ultraviolet emissions ( 1 I 6 → 3 H 6 , 1 I 6 → 3 F 4 , and 1 D 2 → 3 H 6 ) were observed in the hexagonal NaYF 4 :Yb 3+ /Tm 3+ microcrystals under 980-nm excitation. In comparison with a cubic sample having the same chemical compositions, the hexagonal microcrystals had a markedly enhanced ability of emitting ultraviolet up-conversion luminescence. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Fluorides are efficient hosts for down-conversion (DC) and up- conversion (UC) luminescence of rare earth (RE) ions due to their low phonon energies and optical transparency over a wide wave- length range [1]. Many applications of RE-doped fluorides have been demonstrated, such as lasers [2], optical communications [3], display devices [4], and so on [5]. Among the fluorides reported, NaYF 4 , one of the most efficient DC and UC host lattices, has attracted increasing attention [6–12]. It is well known that NaYF 4 may be prepared in two polymor- phic forms, namely, cubic and hexagonal phases [13,14]. The crystal structure of the hexagonal NaYF 4 was known to have three cation sites, one for Y 3+ ions, one for both Y 3+ and Na + ions, and the third for Na + ions [6]. The cubic NaYF 4 is a fluorite structure, in which Y 3+ and Na + ions are randomly distributed in the cation sites. Previous investigations have shown that the hexagonal NaYF 4 is a much better host lattice for the luminescence of RE ions than the cubic NaYF 4 , which set off a new upsurge of synthesizing hexago- nal NaYF 4 [6]. However, the luminescence properties of NaYF 4 :Eu 3+ have received little attention. Especially, the enhancement of the ultraviolet (UV) UC luminescence in hexagonal NaYF 4 :Yb 3+ /Tm 3+ has never been reported. ∗ Corresponding author. Tel.: +86 431 85168240/8325; fax: +86 431 85168240/8325. E-mail address: wpqin@jlu.edu.cn (W. Qin). In the present study, cubic and hexagonal NaYF 4 :Ln 3+ (Ln = Eu and Tm/Yb) microcrystals were synthesized by a facile solvother- mal method. The luminescence properties were studied in detail. Under 393-nm excitation, the characteristic emissions of Eu 3+ ions were observed in NaYF 4 :Eu 3+ microcrystals. In the excitation spec- tra of the 610-nm emission, the 7 F 0 → 5 D 3 is dominant for the cubic microcrystals, while the 7 F 0 → 5 L 6 is dominant for the hexagonal sample. Under 980-nm excitation, unusually strong UV emissions were observed in the hexagonal NaYF 4 :Yb 3+ /Tm 3+ microcrystals. In comparison with a cubic sample having the same chemical com- positions, the hexagonal microcrystals can emit enhanced UV UC fluorescence. 2. Experimental 2.1. Synthesis Y2O3, Eu2O3, Yb2O3, and Tm2O3 (purity ≥ 99.999%) were supplied by Shang- hai Chemical Reagent Company. Ethylenediamine tetra-acetic acid (EDTA), NaF, and HNO3 were supplied by Beijing Chemical Reagent Company, and were of analytical grade. All the reagents were used as received without further purification. Deionized water was used to prepare solutions. Ln2O3 (Ln = Y, Eu, Yb, and Tm) were dissolved in dilute HNO3 by heating to prepare the stock solution of Ln(NO3)3. In a typical synthesis, 1 mL of 0.5 M Ln(NO3)3 aqueous solution and 0.5 mmol of EDTA were dispensed into 15 mL of ethanol and magnetically stirred for 1 h, forming a chelated Ln–EDTA complex. An aqueous solution of NaF was added into the complex and vigorously stirred for 1h. The mixture was then transferred into a 50mL autoclave and heated at 180 ◦ C for 24 h. After cooling to room tempera- ture, the suspension was centrifuged at 8000 rpm for 10 min. The product was then washed thoroughly and dried in vacuum at 80 ◦ C. To improve the crystallinity of the nanocrystalline powders, all of the resultant products were annealed at 400 ◦ C for 2 h. 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.050