Metal oxides exhibit intriguing physical phenomena, including superconductivity, ferroelectricity, magnetism, metal-to-insulator transitions, resistive switching, catalytic activity, and sensing. While the band theory well explains the metals and semiconductors, intensive research efforts are still necessary to establish theoretical backgrounds for the electronic structures and transport properties of metal oxides. In particular, the surface and interface of the metal oxide, where periodic structures are truncated, can show peculiar electrical, magnetic, and optical properties quite distinct from those of the bulk counterparts. The underlying mechanism has been attributed to the strong interaction among charge, spin, orbital, and lattice at the surface and interface of the metal oxide. 

In this talk, I will introduce some of the recent research activities in my group – simultaneous measurements of surface work function (WS) and conductivity of metal oxide heterostructures. A Kelvin probe force microscopy (KPFM) system, equipped with a glove box and a variable temperature stage, enables us to measure WS of the sample. Heterointerface of LaAlO3 and SrTiO3, where two dimensional electron gas (2DEG) phenomena occur, shows close relationship between surface potential and interfacial conductance, under local electrical stress, ambient gas change, and light illumination, clearly revealing the strong surface-and-interface coupling. Nearly grain-boundary-free VO2/TiO2 thin films allow us to observe metallic and insulating domains with distinct WS values while varying the sample temperature. The percolation model well explains the evolution of the domain distribution and resistance during the metal-insulator transition of the VO2 thin films. ZnO/Ag nanogratings show significantly enhanced photoluminescence with the help of surface plasmon polaritons (SPPs). KPFM measurements reveal how SPP excitation influences surface photovoltage of the nanograting. All these results suggest that surface work function measurements using KPFM are very useful to unveiling origins of fascinating physical properties of metal oxide heterostructures.