Oriented cube-on-cube nanocrystal assembly of SrTiO3 tubules

We report on an unusual crystallization phenomenon that results in the self-assembly of sub-micron tubules of crystalline SrTiO3. The deposition of the tubular structures was done in the pores of anodized aluminum oxide templates by the electrophoretic deposition of SrTiO3 sols and subsequent annealing. Homogeneous nucleation inside the pores produces a critical number of crystallites leading to their self-organization when the nanocrystals reach sizes that equal the mean free distances between the nuclei. Due to steric constraints the crystals start to organize in order to most efficiently fill the available surface of the pore walls. This process leads to the formation of domains containing a large number of idiomorphic SrTiO3 nano-cubes that are self-aligned into almost perfect cube-on-cube and cube-to-wall registry, which makes up the walls of the tubules. The described mechanism shows the ability of nanocrystals with well defined morphologies to adapt spatial constraints and self-organize into desired geometries.