Gate-controlled ionization and screening of cobalt adatoms on a graphene surface

Graphene impurities provide both a source of mobility-limiting disorder and a means to desirably alter graphene electronic structure. Adsorbates on graphene can, for example, induce Coulomb scattering(1,2), alter electron-phonon interactions(3), shift the chemical potential(1,2), change the effective dielectric constant(4) and-in cases such as 'graphane'(5)-form whole new two-dimensional materials. While these effects have thus far been primarily studied with spatially averaged techniques, understanding the microscopic physics of such behaviour requires local-probe exploration of the subnanometre-scale electronic and structural properties of impurities on graphene. Here we describe scanning tunnelling microscopy and spectroscopy measurements made on individual Co atoms deposited onto back-gated graphene devices. We find that the electronic structure of Co adatoms can be tuned by application of the device gate voltage, and that the Co atoms can be reversibly ionized. Large screening clouds are observed to form around Co adatoms ionized in this way, and we observe that some intrinsic graphene defects also show charging behaviour. Our results provide new insight into charged-impurity scattering in graphene, as well as the possibility of using graphene devices as chemical sensors.