We have investigated the evolution of work function in epitaxial correlated perovskite SmNiO3 (SNO) thin film spanning the metal-insulator transition (MIT) by Kelvin probe force microscopy (KPFM). Combining contact-mode atomic force microscopy, KPFM and electrostatic force microscopy (EFM), we present charge writing processes associated with point defect engineering in the SNO thin films. Surface potential tuning in two-terminal devices is demonstrated and compared to thermal control by proximity to the phase transition boundary. The charge distribution, retention, and diffusion on SNO were systematically examined. Local compositional changes by AFM-tip induced electric fields is shown to be a viable approach to spatially engineer electronic properties of correlated oxides towards eventual applications in electronics.