(101b) Growth, Dissolution and Stabilization of Polar Oxide Surfaces

Polar crystal surfaces play a key role in catalysis, semiconductors, gas sensing systems, etc. and so their stability has been studied in great detail. These are classified as “Tasker Type 3” surfaces¹ that arise from an alternate stacking of positive and negatively charged lattice planes. A non-zero dipole moment perpendicular to the surface results in very high surface energies and makes these surfaces unstable. However, polar crystal surfaces do exist in nature and are found to be quite stable. The {0001} planes of ZnO wurtzite crystal structure have a non-zero dipole moment perpendicular to them and are polar surfaces. Several mechanisms have been suggested in the literature for the stabilization of these polar ZnO surfaces. Atomic microscopy studies on the (0001) ZnO surface^2,3 have provided the experimental evidence of one of these stabilization mechanisms - the removal of 25% of the surface ions eliminates the perpendicular dipole moment. This proposed mechanism drives the formation of triangular shaped pits and islands that are found on the Zn-terminated polar surface.

The thermodynamics of the formation of triangular pits on the (0001) surface of ZnO has been studied here to understand the stabilization of this polar oxide surface. An atomistic nucleation model has been implemented to calculate the free energy change for the formation of pits of different sizes and shapes. The microscopy experiments that were carried out at ultra-high vacuum (UHV) conditions correspond to an undersaturated vapor. Therefore, the 2-D nucleation theory has been adapted for dissolution on the crystal surface. It was found that the formation of triangular pits with O-terminated edges stabilizes the surface to the greatest extent. The long-range coulombic interactions have a significant effect on the surface energy contribution for these polar surfaces. The formation of triangular pits was found to be an activated process with a critical pit size beyond which the pits form spontaneously. This model can be applied more generally to study the stabilization of polar surfaces of inorganic crystals and can be used to predict the morphology of the nanostructures formed on polar surfaces in a wide variety of applications.

References:
1. Tasker, P. W. J. Phys. C 12, 4977 (1979).
2. Dulub, O., Boatner, L. A., and Diebold, U. Surf. Sci. 519, 201–217 (2002).
3. Ostendorf, F., Torbrugge, S., and Reichling, M. Phys. Rev. B 77, 041405(R) (2008).