High-resolution TEM investigation of structure and composition of polar Pd/ZnO interfaces

In several metal/oxide interfaces, it is well known that polar facet of the oxide influences the formation of a stable metal/oxide interface. However, the formation mechanism of such polar interface is still unclear. In this work, Pd/(0001)ZnO and Pd/(0001̄)ZnO interfaces were used as model systems to investigate the polar metal/oxide interfaces. High-resolution transmission electron microscopic (HRTEM) observation was performed on atomic scale. Quantitative analysis of the HRTEM image revealed specific local atomic structures near the polar interfaces. The Pd/(0001)ZnO interface is found to be terminated with Zn and only these Zn atoms were relaxed to compensate for the interfacial lattice mismatch as large as 18%. Moreover, the Zn atoms occupy lattice sites of the Pd crystal. On the other hand, the Pd/(0001̄)ZnO interface is terminated with O. In this case, the Pd crystal was relaxed to terminate dangling bonds of all terminating oxygen atoms. These results were complemented by semiquantitative first principles calculations, where the charge transfer across each interface could be analyzed. The different terminations and charge transfers across the interfaces preserved the total charge balance of the ZnO crystal. The relaxations of lattice mismatch at the interfaces formed chemically interactive geometries depending on the polarity of ZnO.