Single crystal rods, having nanometer-size diameter, were grown with uniform crystallographic orientation inside a matrix of alumina nanopores. The fabrication process of this nanocomposite structure consists of several stages. First, a highly dense array of alumina pores (about 1011cm−2 and an average diameter of about 35nm) is prepared by an electrochemical anodization process of pure Al substrate. Then, the pores are filled with a liquid solution aided by the pores’ capillary forces. Finally, the temperature of the liquid solution is slightly decreased to a supersaturated state where precipitation starts only at the pore bottom. The nucleation preference at the pore bottom is explained thermodynamically based on the contact angle, geometrical parameters of the nucleus, surface curvature, and pore diameter. In each pore the nucleus is grown to a single crystal that completely fills its volume. The crystallographic orientation of the single crystals inside the pores can be controlled by temperature and composition during growth. The nucleation and growth processes in the alumina nanopores are demonstrated with Rochelle salt (NaKC4H4O6∙4H2O) and potassium nitrate (KNO3).
Single-crystal rods of the α-phase of potassium nitrate were grown with [010] uniform crystallographic orientation inside a matrix of aluminum oxide nanopores. The pores were prepared by anodization of polished aluminum substrates. The potassium nitrate single crystals were grown inside the pores from a supersaturated aqueous solution at various temperatures. Dielectric polarization measurements of the porous film filled with the potassium nitrate crystals show an electric field induced reversible transition from the nonferroelectric α-phase to the ferroelectric γ-phase at about 200kV∕cm. The ferroelectric γ-phase has a coercive field of about 169kV∕cm and remnant polarization of about 0.216μC∕cm2.
Growing ferroelectric nano single-crystals with uniform polar direction in-vertical to the substrate plane is of high scientific and technological interest for understanding ferroelectric behavior in small dimensions and developing future devices. Single-crystals of Rochelle Salt (RS) and potassium nitrate (PN), with preferred orientation, were grown inside a highly-dense array of aluminum oxide nano-pores, oriented in-vertical to the substrate plane. Preferred orientation was also obtained for potassium niobate crystals. Under certain conditions, nucleation occurred only at the pore bottom due to a tight control over temperature, liquid composition, and pore size. Nucleation at the bottom is necessary for the formation of identically-aligned singlecrystals within the pores. Non-linear dielectric behavior was observed in the case of RS and PN crystals inside the pore. The pores stabilize the ferroelectric phase of RS up to 30°C above the upper Curie temperature of bulk (24°C). In the case of PN-filled pores, the crystals grow in the nonferroelectric orthorhombic phase. Upon applying an electric field of 200 kV/cm, a transition to the ferroelectric phase occurs, and a hysteresis loop appears. This phenomenon was not observed in bulk PN and is attributed to the effect of the hydrostatic pressure applied by the pore walls combined with the applied electric field.
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