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

The atomic scale surface structures of ZnO ( 0 1 1 ¯ 0 ) non-polar as well as ( 0 1 ¯ 1 1 ) and ±(0 0 0 1) 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 ±(0 0 0 1) polar surfaces. For the ( 0 1 1 ¯ 0 ) 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 ( 0 1 ¯ 1 1 ) polar surface, the oxygen ions at the outmost top layer were directly imaged. a × 2 reconstruction has also been observed at the ( 0 1 ¯ 1 1 ) surface, and its atomic structure has been proposed based on image simulation. Oxygen-terminated ( 0 0 0 1 ¯ ) polar surface is flat and shows no detectable reconstruction. For the Zn-terminated (0 0 0 1) 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 (0 0 0 1) polar surfaces.