Two step charge transfer process for photoluminescence of Eu3+/Gd3+ codoped Bi2Ti2O7 pyrochlore dielectric thin films

The photoluminescent and dielectric properties of Eu3+/Gd3+ co-doped Bi2Ti2O7 ((Bi1.5-xGdxEu0.5)Ti2O7, BGET) pyrochlore thin films, with Gd content varying in the full range of 0 <= x <= 1.5, have been investigated. The thin films were synthesized by a conventional chemical solution deposition method. The typical intra-4f transitions of Eu3+ ions can be excited by UV radiation. The close correlation between the excitation spectra and the optical band-gap energy of the BGET thin films indicates the essential role of host sensitization in the excitation of Eu3+ luminescence. The host sensitization mechanism can be explained by a resonant energy transfer between host fundamental emission and Eu-O bond absorption. A two-step charge transfer (CT) process is proposed based on the energy band structure determined by XPS technique and spectral analysis. The maximum PLE efficiency can be obtained when the greatest overlap between host emission and Eu -> O CT absorption spectra is achieved. The essential role of the host energy-band structure is confirmed by these results.