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 asdeposited (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. (C) 2018 Elsevier B.V. All rights reserved.