Spectroscopic properties of Er3+ in sol-gel derived ZrO2 films

Films of (x/2)Er2O3-100ZrO2 (x=0, 0.1, 1 and 5) were prepared by the sol-gel and dip-coating methods, and the influences of the type of ZrO2 phase on Er3+ spectroscopic properties were investigated. X-ray diffraction patterns show the existence of metastable tetragonal phase at low temperatures, which are probably caused by both the excess surface energy due to the small crystallite-size effect (J. Phys. Chem. 4 (1965) 1238) and the stabilization of ZrO2 with the addition of Er2O3 (J. Phys. Chem. B 106 (2002) 1909; J. Ceram. Soc. Jpn. 107 (1999) 1111). Under excitation at 488 nm, the green (525+545 nm) downconversion fluorescence due to the (2H11/2+4S3/2)→4I15/2 transition and the red (670 nm) downconversion fluorescence due to the 4F9/2→4I15/2 transition were observed, and their intensities increased with increasing the heat-treatment temperature regardless of the type of the ZrO2 phase. On the other hand, the intensity of red upconversion fluorescence also increased monotonically with the heat-treatment temperature under excitation at 780 nm, while the green upconversion fluorescence was hardly observed in the tetragonal phase (for the film with x=1, below 800 deg. C; x=5, below 700 deg. C), but was observed in the cubic (x=1, at 800 deg. C or above; x=5, at 700 deg. C or above) and monoclinic (x=0.1, at 600 deg. C or above) phases under excitations at 647 and 780 nm. The reason is probably that the f-f transition probability concerning the 4I13/2, from which the excited state absorption occurs, varied sensitively with the local structure of Er3+ ions in the different phase of ZrO2