Failure of Fermi Level in Referencing Chemical Shift of Molecules on Solid Surfaces

Core level chemical shifts using Fermi level as an orthodox reference level are extensively used to detect the forming/breaking of bonds (i.e., change in oxidation states) during chemical reactions and are broadly used as an experimental proof of charge transfers across organic interfaces by several research communities for many decades. Based on core level and valence photoemission experimental measurements a paradoxical relationship between core-level shift and charge transfer is shown, demonstrating the failure of conventional practice of using Fermi level to measure chemical shifts of molecules on solid surfaces. Thus, great care needs to be taken when interpreting experimentally observed shifts in core level binding energies as well as assigning core level energy peaks associated with certain oxidation states. To resolve this paradoxical problem, it is shown that measurement of interface dipoles is essential in investigating the charge transfers of molecules to/from solid surfaces.