Effect of crystalline ordering on the luminescent properties of Eu3+-doped aluminum oxide nanophosphors.

Al 2 O 3 :Eu3+ nanophosphors were synthesized by the microwave assisted solvothermal technique. The structural and photoluminescence characteristics of alumina powders doped with 6.4 ​at.%, 7.5 ​at.%, 9.5 ​at.% and 13 ​at.% Eu, calcined at 900 ​°C, 1000 ​°C, 1100 ​°C and 1200 ​°C for 3 ​h, were investigated by X-ray diffraction, and reflectance, photoluminescent and X-ray fluorescence spectroscopies. The effects of the calcination temperature on the structure and, in turn of it on the photoluminescence properties are investigated. Luminescent decays curves were measured for the emission ascribed to the 5D 0 -7F 2 transition in all the samples and a non-exponential behavior is observed. An increase in the average lifetime correlates well with the formation of the EuAlO 3 perovskite phase as the calcination temperature increases. Judd-Ofelt analyses were performed and some additional quantities were derived from them such as the radiative and nonradiative transition rates and the emission quantum efficiency. The decrease in the Ω 2 parameter value as the calcination temperature increases is compatible with a higher symmetry around the Eu3+ site as well as with a reduction in the Eu3+ covalence bonding within this ion into EuAlO 3 perovskite. Short caption X-ray diffraction patterns and excitation spectra from 6.4 at.% and 13 at.% Eu-doped alumina nanophosphors annealed at 900 °C and 1200 °C, respectively. Long caption X-ray diffraction (XRD) patterns and excitation spectra of 6.4 at.% and 13 at.% Eu-doped alumina nanophosphors annealed at 900 °C and 1200 °C, respectively. The XRD pattern of 6.4 at.% Eu-doped alumina shows the amorphous and γ-Al 2 O 3 phases resulting in an excitation spectrum where the broad peak centered at 270 nm is ascribed to an Eu3+–O2- charge transfer (CT) state in Al 2 O 3 , unlike in the alumina sample doped with 13 at.% Eu annealed at 1200 °C this band is significantly reduced and a new band being associated with an O 2 --Eu3+- CT state in EuAlO 3 appears at around 300 nm. Image 1 • Al 2 O 3 :Eu3+ nanophosphors were synthesized by a microwave assisted solvothermal technique. • Annealing temperature and Eu concentration govern amorphous-Al 2 O 3 → γ-Al 2 O 3 → EuAlO 3 phase transformations. • Luminescent features of the nanophosphors strongly depend on crystal structure. • A new excitation band at 303 nm is plausibly associated with the O2- → Eu3+ charge transfer state of EuAlO 3. [ABSTRACT FROM AUTHOR]