Atomic structure and crystallographic shear planes in epitaxial TiO2 anatase thin films

We report on the high resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) study of TiO2 anatase thin films grown by pulsed laser deposition on LaAlO3 substrates. The analysis provides evidence of a peculiar growth mode of anatase on LaAlO3 that is characterized by the formation of an epitaxial layer at the film/substrate interface. In particular, the film is split into two adjacent slabs of about 20 nm each, both displaying the same Bravais lattice compatible with the anatase tetragonal cell. The formation of two different families of crystallographic shear (CS) superstructures is observed within the film, namely (103)- and (101)-oriented CS plane structures, occurring in the outer film region and in proximity of the film/substrate interface, respectively. HAADF analysis and Energy Dispersive Spectroscopy highlight the occurrence of Al interdiffusion from the substrate into the film region. By combining HRTEM results, image simulation techniques and DFT calculations we determine the atomic structure of the CS planes, and show that they are cubic-TiO-based structures analogous to the TinO2n-1 Magnéli phases derived from rutile.