Monovalent impurities on graphene could be divided into ionically and covalently bond impurities. The covalent impurities with one chemically active electron cause universal mid-gap states as the carbon atom next to the impurity was effectively decoupled from the graphene π bands. The electronic structure of graphene suppresses migration of these impurities and makes the universal mid-gap very stable. This effect was strongest for neutral covalently bond impurities. The ionically bond impurities have migration barriers of typically less than 0.1eV, which was about an order of magnitude less than their typical binding energies. An asymmetry between anions and cations regarding their adsorption sites and topology of their potential-energy landscape was predicted. In addition, the migration barrier for oxygen adatoms on graphene and their electronic structure was discussed. The barrier was found to be similar to that of monovalent covalently bond impurities.

Impurities on Graphene: Midgap States and Migration Barriers. Wehling, T.O., Katsnelson, M.I., Lichtenstein, A.I.: Physical Review B, 2009, 80[8], 085428