Surface work function measurements by non-contact and non-destructive Kelvin probe technique: a review

The solid surfaces have unique surface properties. Measurement of the work function of a surface is an important tool of surface characterization. Surface work function is a fundamental electronic property; it provides an understanding of the relative position of the surface Fermi level. For example, corrosion of metal surfaces (including bio-corrosion) can be studied by non-contact and non-destructive methods of measuring surface work function. Any charge insertion or removal, changes the surface work function, as in the case of electro-chromic materials. The electrical properties of modified surface form the basis of sensors working on surface electrical resistance. Presently, the surface modification and tuning of the work function is a requirement for several organic and inorganic semiconductor devices and metal surfaces. Surface modification can be achieved with chemical and physical methods; each method has its own charge transfer process at the surface dipole layer. The surface of even the degenerate semiconductors like tin doped indium oxide (ITO) thin films can be modified. Among the several techniques to measure the surface work function, Kelvin probe has the advantage of keeping the surface virgin even after the measurement because it is a non- contact and non-destructive technique. Kelvin probe technique gives the work function of a surface relative to a reference electrode. The Kelvin probe technique differs from the Kelvin Probe Force Microscopy (KPFM) though both the measurements give the surface work function. Present talk summarizes the basic principles of Kelvin probe technique and presents the status of surface work function measurements on several metals and semiconductor modified surfaces. (author)