Subangstrom profile imaging of relaxed ZnO(101̄0) surfaces

Relaxation is a most basic structural behavior of free surfaces, however, direct observation of surface relaxation remains challenging in atomic-scale. Herein, single-crystalline nanoislands formed in situ on ZnO nanowires and nanobelts are characterized using aberration-corrected transmission electron microscopy combined with ab initio calculations. For the first time, displacements of both Zn and O atoms in the fresh (101̄0) facets are quantified to accuracies of several picometers and the under-surface distributions of contractions and rotations of Zn-O bonds are directly measured, which unambiguously verify the theoretically predicted relaxation of ZnO (101̄0) free surfaces. Finally, the surface relaxation is directly correlated with the size effects of electromechanical properties (e.g., elastic modulus and spontaneous polarization) in ZnO nanowires.