  Using first-principles calculations we show that the work function of noble
metals can be decreased or increased by up to 2 eV upon the adsorption of
self-assembled monolayers of organic molecules. We identify the contributions
to these changes for several (fluorinated) thiolate molecules adsorbed on
Ag(111), Au(111) and Pt(111) surfaces. The work function of the clean metal
surfaces increases in this order, but adsorption of the monolayers reverses the
order completely. Bonds between the thiolate molecules and the metal surfaces
generate an interface dipole, whose size is a function of the metal, but it is
relatively independent of the molecules. The molecular and bond dipoles can
then be added to determine the overall work function.
