Controlled growth and nucleation of ferroelectric and dielectric single-crystal nano-rods inside nano-porous aluminum oxide

Single-crystal rods of the ferroelectric materials potassium nitrate (PN) and Rochelle salt (RS) were grown inside a matrix of aluminum oxide nano-pores, having a nanometer-size diameter, with a uniform crystallographic orientation towards the longitudinal axis of the pores. The dense porous array is prepared by electro-chemical anodization of an Al substrate. A liquid solution of RS or PN is inserted into the pores using strong capillary forces. Finally, the liquid solution temperature is slowly reduced, to promote preferred nucleation of the solute at the pore bottom. This preference of nucleation at the pore bottom is explained thermodynamically, based on the geometrical parameters of the pore and the nucleus, such as surface curvature and diameter. The initial nucleus is grown to a single-crystal completely filling the pore volume. The crystallographic orientation of these single-crystal nano-rods can be readily controlled by altering the saturation temperature of the solution, and the cooling rate during nucleation and growth. Moreover, such a composite thin film exhibits pronounced polarization values and improved thermal stability of the ferroelectric phase in comparison to the bulk-size material, for the case of RS.