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

The photoluminescence of polycrystalline sintered ZnO codoped with Li+ and trivalent rate-earth (RE3+) ions (Dy3+, Er3+, Eu3+, Ho3+, Nd3+, Sm3+ and Tm3+) has been studied. The luminescence spectra depend upon the nature of the rare earth. The UV excitation of the Eu3+-, Dy3+-, Sm3+- and Tm3+- doped samples induces the luminescence of the RE3+ ions: in addition to the broad ZnO pattern, the spectra show the characteristic lines of the 4f transitions of the RE3+ ions. The UV excitation of the Er3+ -, Ho3+ - and Nd3+ - doped samples induces only the luminescence of ZnO with a partial reabsorption of the light by the RE3+ ions, no emission from the RE3+ ions being observed. Furthermore, the presence of Li+ ions increases the absorbance of the RE3+ 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 RE3+ 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 RE3+ ions is evidenced only in the case of Tm3+. It is proposed that this energy transfer is a consequence of an electron/hole pair recombination at the grain boundaries involving the Tm2+ semi-reduced form.