 The work function is not a characteristic of a bulk material, but rather a property of the surface of the material (depending on crystal face and contamination).The work function for a given surface is defined by the difference
where is the charge of an electron, is the electrostatic potential in the vacuum nearby the surface, and is the Fermi level (electrochemical potential of electrons) inside the material.The term is the energy of an electron at rest in the vacuum nearby the surface.