Polarity in Oxide Nano-objects

Polar surfaces of compound materials have been the subject of intense activity in the past. Electrostatic arguments, based on a model of rigid charges, predict that they should have an infinite surface energy, because of the presence of a macroscopic dipole, and thus should never be observed. Moreover, peculiar electronic surface states may lie in the gap of the oxide and induce enhanced basic or acid character at surface atoms, with important implications on reactivity. Finally, surface reconstructions that are sometimes consequences of the polar instability can be used as nanostructured substrates to grow artificial structures with predetermined conformations. Polar nano-objects raise a number of very new questions related to the relevance of electrostatic interactions and the role of dimensionality (2D or 3D) in driving the polar instability. As compared to semi-infinite surfaces, additional mechanisms of polarity compensation exist at the nanoscale, involving complete changes of structures, strong lattice relaxations, inhomogeneous charge redistributions, among others.