  Interactions between atomic and molecular objects are to a large extent
defined by the nanoscale electrostatic potentials which these objects produce.
We introduce a scanning probe technique that enables three-dimensional imaging
of local electrostatic potential fields with sub-nanometer resolution.
Registering single electron charging events of a molecular quantum dot attached
to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we
quantitatively measure the quadrupole field of a single molecule and the dipole
field of a single metal adatom, both adsorbed on a clean metal surface. Because
of its high sensitivity, the technique can record electrostatic potentials at
large distances from their sources, which above all will help to image complex
samples with increased surface roughness.
