Revealing large scale surface charge distributions with Scanning Kelvin Probe Microscopy

Scanning Kelvin Probe Microscopy (SKPM) is a macroscale (100 μm - 10 cm) technique to map the electrostatic potential above a surface by scanning a vibrating conducting probe and measuring induced currents. Typically, SKPM is used to measure localized differences in a material's work function e.g. in metals or semi-conductors. For insulators, such as oxide layers or thin polymers, a measurable potential difference can arise from localized surface charge, hence it is an excellent tool for studying charge patterns caused by adhesion, liquid evaporation, etc. However, there is a problem—back conversion from the measured voltage signal to the surface charge density that causes it is extremely complicated. In this talk, I will show how quantitative charge density can be backed out from SKPM data by deconvolution with the experimentally determined Green's function of the system. This presents an advantage over similar techniques at smaller scales (e.g. Kelvin Probe Force Microscopy), where the Green's function is estimated with simulations. Using this approach, we have begun investigating the electrostatic charging patterns caused by several interesting large-scale surface phenomena including peeling of soft polymers from surfaces and evaporation of liquids/solvents. By backing out charge, we take a step towards a quantitative understanding of the charging mechanisms in these and other electrified soft materials.