Effect of Cu- and Y-Codoping on Structural and Luminescent Properties of Zirconia Based Nanopowders

Zirconia (ZrO₂) has attracted considerable attention because of its mechanical, electric, thermal and luminescent properties offering diverse applications such as catalysts, high temperature and corrosion resistant coatings, sensors, radiation detectors, laser techniques, biological labeling, etc. Different defect-related emission bands can be observed in visible spectral range from pure and/or Y-stabilized ZrO₂materials allowed an application of zirconia for white light emitting devices.

ZrO₂ nanopowders doped with Y and/or Cu were synthesized by a co-precipitation technique using Zr, Y and/or Cu nitrates and calcined at 500-1000^оС for 1 h. The Y content in the samples was fixed at 3 mol%, whereas Cu concentration was varied from 1 to 8 mol%.

Structural, optical and light emitting properties were studied by Raman scattering, PL and PL excitation methods at room temperature. Raman scattering spectra of Y-doped and (Y,Cu)-codoped ZrO₂ samples annealed at 500-800 °C showed the presence of broad phonon bands peaked at 145, 255, 325, 475 and 630 cm^-1 corresponded to tetragonal ZrO₂ phase. The peak’s broadening is caused by a structural distortion. For (Y,Cu)-codoped samples the increase of annealing temperature up to 900°C results in the shift of E_g modes from 475 cm^-1 and 630 cm^-1 towards 466 cm^-1 and 636 cm^‑1, respectively, due to stress relaxation. Besides, the appearance of several phonons corresponding to monoclinic ZrO₂ phase was observed that was the evidence of the t-m phase transformation. An annealing at 1000°C causes the formation of m-ZrO₂ phase only that was confirmed by the quenching of E_2a phonon mode (259 cm^-1). The comparison of these data with those obtained for Y-doped ZrO₂ powders showed that Cu-codoping results in the formation of m-ZrO₂ phase in (Y,Cu) codoped ZrO₂ samples at 900-1000°C contrary to stable t-ZrO₂ phase in Y-doped only materials. It should be noted that the increase of Cu content can lead to the decrease of the annealing temperature of ZrO₂ t-mphase transition.

The analysis of PL spectra of (Y,Cu) codoped samples showed the presence of two main PL components peaked at about 640 and 550 nm in the most samples. The PL excitation spectra demonstrated that the former PL component is excited by the 290-300nm light preferably, whereas 540-nm PL emission is excited by the light with shorter wavelength (250-260nm). An annealing at 500-900 °C results in the slight variation of total PL intensity, while the PL component peaked at 640 nm increases mainly. At the same time the treatment at higher temperature (1000°C) leads to an enhancement of green emission mainly. Besides an appearance of 300-nm peak in PLE spectra demonstrates that the shorter PL component is excited by the longer-wavelength light. The main reason of the transformation of PL spectra of (Y,Cu) codoped samples is ZrO₂ t-mphase transition. This phenomenon as well as PL and PL excitation mechanisms will be discussed in details.

This work was supported by National Academy of Sciences of Ukraine