Profile imaging of reconstructed polar and non-polar surfaces of ZnO

The atomic scale surface structures of ZnO (0110) non-polar as well as (0111) and (0001) polar surfaces have been directly imaged by high-resolution transmission electron microscopy (HRTEM). The observations were made on clean surfaces created by irradiating a single ZnO nanobelt using 400 keV electron beam in TEM, under which ZnO dots were grown epitaxially and in situ on the surface of the nanobelt. A technique is demonstrated for directly distinguishing the surface polarity of the +/-(0001) polar surfaces. For the (0110) non-polar surface, HRTEM images and simulation results indicate that the Zn ions in the first and second layer suffer from inward and outward relaxation, respectively; the oxygen ions in the first and second layer prefer shifting to vicinal Zn ions to shorten the bonding distance. For the oxygen-terminated (0111) polar surface, the oxygen ions at the outmost top layer were directly imaged. a x 2 reconstruction has also been observed at the (0111) surface, and its atomic structure has been proposed based on image simulation. Oxygen-terminated (0001) polar surface is flat and shows no detectable reconstruction. For the Zn-terminated (0001) polar surface, HRTEM may indicate the existence of Zn vacancies and a possibly c-axis, random outward displacement of the top Zn ions. Our data tend to support the mechanism of removal of surface atoms for maintaining the stability of (0001) polar surfaces. (c) 2006 Elsevier B.V. All rights reserved.