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Size-dependent reactivity
The atomic polarization in ferroelectric materials such as BaTiO3 and PZT can cause bending of the electronic band structure at the surface of the material or at the interfaces between crystallites if the material is polycrystalline. The electronic bands can either bend up or down, depending on the direction of the polarization vector at the surface. For example, a c+ domain at the surface of a BaTiO3 crystal has a polarization vector pointed out of the surface. This causes downward band bending in the surface region. This effect has been used to decorate ferroelectric domains with metal particles using a photochemical reaction. The ferroelectric sample is immersed in a metal cation solution, and electron-hole pairs are excited by shining super band gap light on the material. The electrons are drawn toward the surface of c+ domains, where they can react with metal cations. In figure 1(a), the domain structure at the surface of a BaTiO3 crystal has been imaged by piezoresponse force microscopy (PFM). The topographic structure after the photochemical deposition of Ag (b) and Pd (c) is also shown. In the first figure, (d-f) are a schematic of the band bending effect at the surface. It is also possible to control the domain orientation at the surface of ferroelectric materials such as thin films of PZT using scanning probe microscopy. The photodeposition of metals can then be directed to form patterned structures, as demonstrated in the second figure.
Piezoresponse force microscopy (PFM) image (a) of a multidomain BaTiO3 crystal. Both c+ (bright) and c- domains (dark) are visible. The topographic structure after the deposition of Ag (b) and after the removal of Ag and deposition of Pd (c) indicates that photoreduction of Ag and Pd occurs selectively at the surface of c+ domains.
Illustration of ferroelectric nanolithography. Surface topography (a) and piezoresponse force microscopy (PFM) images (b) of PZT film. PFM images of stripe (c) and checkerboard (e) domain patterns show that local control of ferroelectric domain orientation is possible. Surface topography (d) and SEM (f) images of these surfaces after photodeposition of Ag.
S. V. Kalinin, D. A. Bonnell, T. Alvarez, X. Lei, Z. Hu, J. H. Ferris, Q. Zhang, S. Dunn, Nano Letters 2 (2002) 589-593. Atomic polarization and local reactivity on ferroelectric surfaces: a new route toward complex structures