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The photoluminescence of polycrystalline sintered ZnO codoped with Li + and trivalent rate-earth (RE 3+ ) ions (Dy 3+ , Er 3+ , Eu 3+ , Ho 3+ , Nd 3+ , Sm 3+ and Tm 3+ ) has been studied. The luminescence spectra depend upon the nature of the rare earth. The UV excitation of the Eu 3+ -, Dy 3+ -, Sm 3+ -and Tm 3+ -doped samples induces the luminescence of the RE 3+ ions: in addition to the broad ZnO pattern, the spectra show the characteristic lines of the 4f transitions of the RE 3+ ions. The UV excitation of the Er 3+ -, Ho 3+ -and Nd 3+ -doped samples induces only the luminescence of ZnO with a partial reabsorption of the light by the RE 3+ ions, no emission from the RE 3+ ions being observed. Furthermore, the presence of Li + ions increases the absorbance of the RE 3+ ions and enables the observation of their luminescence under direct 4f-4f photoexcitation. Additional experiments, X-ray analysis, scanning electron micrography and energy dispersive spectrometry indicate that the RE 3+ ions are mainly located outside the ZnO microcrystals in the grain boundaries of the polycrystalline matrix where they are closely associated with Li + ions. An energy transfer between ZnO and the RE 3+ ions is evidenced only in the case of Tm 3+ . It is proposed that this energy transfer is a consequence of an electron/hole pair recombination at the grain boundaries involving the Tm 2+ semireduced form. |
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