Plasmon-induced charge spatial distribution of Au dimer/TiO₂ nanostructure. (a) Schematic of Kelvin Probe Force Microscopy measurement of the Au nanoparticle dimers (NDs) on the TiO₂ substrate. Plasmonic holes confined at Au/TiO₂ interface which locates nanogap region between Au NDs. (b) Measured (Mea.) and restored (Res.) SPV profiles along the center horizontal line of the Au NDs under 620 nm light illumination. Inset: three-dimensional (3D) atomic force microscopy (AFM) image of Au NDs/TiO₂. (c) SPV measured at the spot of Au NDs/TiO₂ interface as a function of incident light polarization angle. The black bidirectional arrows indicate the direction of the light polarization. The angle 0° corresponds to the polarization perpendicular to the Au NDs axis. Error bars indicate the statistical uncertainty in the SPV. (d) Sectional view of the simulated electromagnetic (EM) field intensity distribution when the polarization angles are 0° and 90°. (e) Schematic illustration of plasmon-induced charge spatial separation of the Au NDs/TiO₂ system. SCR stands for surface charge region. The downward purple arrow means the direction of plasmonic electron transfer from Au NPs to TiO₂.