Paper SS1-WeM2
Effect of Poling Direction on the Reactivity of Ferroelectric Oxide Surfaces

Wednesday, October 17, 2007, 8:20 am, Room 608

Ferroelectric polarization creates high energy surfaces that are expected to reduce their surface energy by reconstructing or strongly adsorbing polar molecules. Because opposite charges must be screened on opposite surfaces, different reactivities are expected on positively and negatively poled surfaces. We have studied the surface atomic composition, structure and reactivity of LiNbO₃(0001) surfaces. The surfaces appeared nearly indistinguishable in spectroscopic and diffraction measurements: both surfaces were (1x1) and appeared almost fully oxygen terminated in low energy ion scattering spectra. Despite the structural similarities, differences in reactivity between positively and negatively poled surfaces were observed using TPD. Polar molecules including 2-propanol and acetic acid adsorbed much more strongly on the positive surfaces as evidenced by desorption peak temperatures over 100 K higher. Further, the TPD curves were found to depend unusually strongly on the heating rate. This efect could be attributed to the change in polarization with temperature - the pyroelectric effect- creating a temperature-dependent heat of adsorption. Including this effect, an 11 kJ/mole higher 2-propanol heat of adsorption was estimated for the positive surface. These results will be compared with adsorption of non-polar molecules where the polarization changing with temperature is not expected to affect the strength of the adsorbate-surface interaction. The results for LiNbO₃ will also be compared with PbZr_xTi_1-xO₃ thin films to determine the generality of the observed phenomena.