Charging and interaction of individual impurities in a monolayer organic crystal.

We use a scanning tunneling microscope to study the charging of Ag-doping centers in a monolayer of C(60) molecules supported on a thin Al(2)O(3) film grown on the NiAl(110) surface. Differential conductance spectroscopy shows that charging affects the conduction through C(60) molecules located around the doping centers. This effect is used to observe the electrostatic interaction of a pair of centers. Charging of one doping center affects the energy levels of the other, an analogue of the field-effect action.