The electron-acceptor molecule TCNQ is found in either of two distinct integer
charge states when embedded into a monolayer of a charge transfer complex on a
gold surface. Scanning tunneling spectroscopy measurements identify these
states through the presence or absence of a zero-bias Kondo resonance.
Increasing the (tip-induced) electric field allows us to reversibly induce the
oxidation or reduction of TCNQ species from their anionic or neutral ground
state, respectively. We show that the different ground states arise from
slight variations in the underlying surface potential, pictured here as the
gate of a three-terminal device.