Transmission Electron Microscopy Studies of Surface Phenomena in Polar Oxides
Arun Subramanian | Download entire thesis

Atomic scale processes at surfaces dictate the macroscopic properties of solid surfaces and have been the subject of numerous theoretical and experimental studies. Polar surfaces, due to their non-vanishing dipole moment along the surface normal, are unstable from classic electrostatics arguments. The ionic model often used to describe this class of materials falls short in explaining the experimental observations of stable polar surfaces. A change in the electronic structure of the surface has been proposed as a stabilization mechanism, but until this study has not been experimentally verified. The research approach described in this manuscript involves a combination of computational (Direct Methods) and experimental (TEM) methods for investigation of polar oxide surface structures. The results from the studies of the surfaces of three different polar oxides, namely MgO~(111), NiO~(111) and SrTiO3~(110), are presented in this dissertation. The atomic structure and charge density of the MgO~(111)-rt3 x rt3 R30 reconstruction has been experimentally measured and verified by \textit{ab initio} calculations. The structure and densities of two other reconstructions on the NiO~(111) surface, namely the (rt3 x rt3 R30) and (2x2), are also presented. The phase transition between these two surface phases of NiO is accompanied by a novel step ordering pheneomenon is documented using TEM and SEM. Finally, the results from the studies of the atomic struture of the SrTiO3~(110)-3x4 surface reveal similarities with another surface structure (SrTiO$_3$~(100)-c(6x2)).