  We describe scanning tunneling spectroscopy (STS) measurements performed on
individual cobalt (Co) atoms deposited onto backgated graphene devices. We find
that Co adatoms on graphene can be ionized by either the application of a
global backgate voltage or by the application of a local electric field from a
scanning tunneling microscope (STM) tip. Large screening clouds are observed to
form around Co adatoms ionized in this way, and we observe that some intrinsic
graphene defects display a similar behavior. Our results provide new insight
into charged impurity scattering in graphene, as well as the possibility of
using graphene devices as chemical sensors.
