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Upconversion luminescence (UPL) characteristics and effects of Li+ ion on the UPL of ZnWO4:Yb,Er polycrystalline phosphors were investigated. It was shown that introduction of Li+ ions could reduce the calcination temperature by about 200 °C and increase the crystallinity of ZnWO4:Yb,Er by a liquid-phase sintering process via formation of Li2WO4 and other intermediates. UPL efficiency is remarkably promoted by Li+ ions. Moreover, the UPL spectrum of Li+-doped ZnWO4:Yb,Er presents a red shift, and the strongest peak position shifts from 553 to 559 nm. These can be attributed to a shift in the 4f level barycenter to lower energy, which results from lowering of the symmetry of the crystal field around Er3+.

I. INTRODUCTION

Zinc tungstate exhibits interesting photoluminescence properties1; in addition, it is nonhygroscopic and nontoxic. It may be applied as a possible material for x and ␥ ray scintillators,2 in a laser host, in acoustics, in optical fibers, and as a sensor material for detection of NO2 gas. Zinc tungstate has low crystal symmetrical monoclinic wolframite structure with C2h point-group symmetry, P2/c space group.3,4 Moreover, it presents low phonon energies (ប␻ ⳱ 199.5 cm−1).5 These make us believe it to be a promising upconversion host material for fabrication of UPL biological sensors. However, investigations into UPL of ZnWO4 have rarely been reported before.6 It is therefore of great interest to examine the UPL properties of ZnWO4 material co-doped with rare-earth elements. It is well known that Li+ ions dopants frequently play important roles in morphology as well as in luminescent properties of phosphors. Watterich et al.7,8 demonstrated that after x irradiation at 77 K in a ZnWO4:Li single crystal, a defect was accompanied by a charge compensation; the local C2 symmetry was then reduced to C1. Hatayama et al.9 observed a surprisingly 1000-fold enhancement of the blue emission in Li+ co-doped ZnS:Tm. Park et al.10 demonstrated that cathodoluminescence efficiency at low voltages of Gd2O3:Eu3+ was remarkably improved by Li+ doping and that it also benefited the spherical morphology. Lopez et al.11 also reported that,

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0275 2078

http://journals.cambridge.org

J. Mater. Res., Vol. 23, No. 8, Aug 2008 Downloaded: 13 Mar 2015

with the addition of 1.0 at.% Li, the blue emission intensity of Y3Al5O12:Tm increases by as much as 87%. Zhou et al.12,13 reported that introduction of impurity atoms (N or Li) into the ZnO:Er specimen led to formation of a lower-symmetry C4v structure around Er3+, while the ZnO host crystal lattice showed a [ZnO4] tetrahedral structure. In this paper, we investigate UPL characteristics and effects of Li + ions on UPL of ZnWO4:Yb,Er phosphors. II. EXPERIMENTAL

In the present study, a solid-state reaction method was adopted to prepare the ZnWO4:Yb,Er and ZnWO4: Yb,Er,Li phosphors. All chemicals used were analyticgrade reagents without further purification. Nominal compositions of t