Observation and analysis of ordered and disordered structures on the ZnO(0001) polar surface

Using low temperature scanning tunneling microscopy (STM), we discovered locally ordered patches of O adatoms and single Zn vacancies on the Znterminated ZnO(0001) polar surface. Such patches are determined to be metastable ordered structures on the surface. Density functional theory (DFT) calculations show that Zn atoms bonded to an O adatom encounter a larger reaction barrier for leaving lattice sites, explaining the observed general disordered nature of the Zn-terminated surface that is populated by cavities of different shapes and sizes and disordered distribution of adatoms. The interplay among different driving mechanisms provides valuable insight as to how a polar surface of an ionic crystal achieves its lowest energy reconstructed surface structure. Comparisons between the charge on surface vs bulk layers for a relaxed (1 × 1) slab and a slab bounded on two ends by reconstructed surfaces with stoichiometric changes reveal that in a neutral environment, the polar surface of an ionic crystal tends to go through reconstructions that allow cations to donate electrons to anions in amounts very similar to that in the bulk.