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Understanding the competition between epitaxial strain and thermodynamics in TiO2: Structural, morphological, and property evolution

  • Author(s): Apgar, BA
  • Martin, LW
  • et al.

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We explore the evolution of epitaxial TiO2 films on a wide range of (001)-oriented perovskite substrates. We observe epitaxial stabilization of anatase to >150 °C above the bulk anatase-to-rutile transition temperature for films grown on substrates with -2.0% to 6.0% lattice mismatch. Continuum elastic models are used to calculate the strain energy density and to construct a model for the preferred epitaxial orientation of anatase. This model is consistent with experimental observations that the strain energy density dominates and leads to the stabilization of 00l- and h00-oriented anatase below and above lattice mismatch values of 5%, respectively. Additionally, TiO 2 nanocrystallite size is found to decrease with lattice mismatch and is discussed in terms of energy competition and possible changes in the nucleation and growth process. To further probe the competition between bulk free, surface, interface, and strain energies in metastable (kinetically limited) as-grown TiO2 films, ex post facto annealing was completed to assess the equilibrium state of the films. These studies confirm the continuum elastic model and highlight the relative importance of the different energies. We then implement our understanding of energy competition to deterministically increase surface area and enhance light absorption via in situ growth processes and ex post facto annealing. © 2014 American Chemical Society.

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