Relying on first principles simulations of stoichiometric MgO, ZnO, and NaCl ($1\times 1$) ultrathin (111) films, we demonstrate the existence of a critical thickness below which polarity is uncompensated: the surface charges are bulklike, and the total dipole moment and the formation energy grow linearly with thickness. This study reveals novel facets of the problematics of polarity akin to the nanoscopic size of the objects and opens stimulating perspectives on polar nanostructures with surface properties and reactivity unaffected by charge compensation as in macroscopic samples.

• Received 6 November 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.205701