Photoluminescence of polycrystalline zinc oxide co-activated with trivalent rare earth ions and lithium. Insertion of rare-earth ions into zinc oxide

• Published in: Journal of luminescence 1997-07, Vol.75 (1), p.35-49
• Main Authors: Bachir, S., Azuma, K., Kossanyi, J., Valat, P., Ronfard-Haret, J.C.
• Format: Article
• Language: English
• Subjects: Excitation transfer mechanism; Grain boundaries; Polycrystalline semiconductor; Rare-earth luminescence; Rare-earth oxides; ZnO
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/S0022-2313(97)00093-8

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.