  The polar covalent bond between a single Au atom terminating the apex of an
atomic force microscope tip and a C atom of graphene on SiC(0001) is exposed to
an external electric field. For one field orientation the Au-C bond is strong
enough to sustain the mechanical load of partially detached graphene, whilst
for the opposite orientation the bond breaks easily. Calculations based on
density functional theory and nonequilibrium Green's function methods support
the experimental observations by unveiling bond forces that reflect the polar
character of the bond. Field-induced charge transfer between the atomic
orbitals modifies the polarity of the different electronegative reaction
partners and the Au-C bond strength.
