Randomly organized and self-assembled Na0.5Bi0.5TiO3 nanodots elaborated by sol-gel and pulsed laser deposition routes

This work reports for the first time on the elaboration, by both chemical (sol-gel) and physical (pulsed laser deposition) routes, of lead-free ferroelectric Na0.5Bi0.5TiO3 nanodots deposited on bare c-sapphire single crystal substrates presenting a 5° miscut angle along the [110] direction. Prior to any deposition, the sapphire substrates were treated at 1350 °C, during 24 h in air, in order to increase the height of the surface steps, reaching by this way ~8 nm. The experimental parameters adjusted for the growth of Na0.5Bi0.5TiO3 dots were the concentration of sols and the number of laser pulses (50 and 100) for the sol-gel and pulsed laser deposition routes, respectively. Whereas the sol-gel route leads to randomly organized Na0.5Bi0.5TiO3 nanodots in respect to the surface steps, the pulsed laser deposition route provokes the self-assembly for some important proportion of these dots along the same surface steps. Despite the lack of organization for the sol-gel dots, the latter present a much more regular distribution in size (~100 and ~10-20 nm as an average lateral dimensions and height, respectively) compared to dots deposited by laser ablation, where three different populations of grains can be observed. In each case, the dots do not seem to be epitaxially grown.