Kelvin probe force microscopy (KPFM), also known as surface potential microscopy, is one member of a suite of electrical characterization methods available in atomic force microscopes. It maps the contact potential difference (CPD) between a surface and the cantilever, containing information about the surface potential and work function. The work-function is defined in solid-state physics as the energy needed to remove an electron from the Fermi level of a solid to the vacuum and is thus, a surface property and not related to the bulk.
KPFM is a surface-sensitive method that probes at and near the surface only. It is often used as a qualitative technique to obtain contrast based on the surface potential. Quantitative scanning Kelvin probe force microscope measurements of the local work function are generally performed in vacuum and require a model to describe the electrostatic interactions between the tip and sample as well as knowing the work function of the tip. In ambient conditions, the contact potential difference is influenced by surface contaminations of tip and sample as well as surface water films, which additionally hampers the calculation of the work function of a sample surface from the CPD.