Influence of the Anatase and Rutile phases on the luminescent properties of rare-earth-doped TiO2 films

For a long time, the development of new and more efficient light-emitting materials has been the driving force to investigate the presence of trivalent rare-earth (RE³⁺) ions into solid hosts. It happens because RE³⁺ ions provide efficient and well-defined optical transitions whose intensities are determined by phenomena regarding excitation−recombination mechanisms or, ultimately, by the RE³⁺ atomic surroundings. Motivated by these aspects, this paper presents a comprehensive study of the influence exerted by TiO₂ (under the amorphous and crystalline forms) onto the light emission due to Sm³⁺ and Tm³⁺ ions. To this end, Sm-, Tm-, and SmTm-doped TiO₂ films were prepared and their structural−electronic properties were investigated by optical spectroscopic techniques − following appropriate thermal annealing treatments. As a result of the deposition method and conditions, the as-deposited (stoichiometric) TiO₂ films present an amorphous structure and single RE concentrations around 0.5 at.%. Also, whereas thermal annealing the films at increasing temperatures resulted in almost no changes in their elemental composition, extensive variations were observed in their atomic structure and RE³⁺-related light emission characteristics. Based on the experimental results, it became clear the role played by the amorphous or crystalline (Anatase or Rutile) structures of the TiO₂ films onto the RE³⁺-related luminescence features, and the likely mechanisms behind the phenomenon are presented and discussed in detail.