  Density functional calculations are used to explain the charge transfer
doping mechanism by which species physisorptively bonded to graphene can
increase its free hole or electron density, without giving rise to defects, and
thus maintain a high carrier mobility. Typical dopants studied are FeCl3,
AuCl3, SbF5, HNO3, MoO3, Cs2O and O2. These systems do not break the {\pi}
bonding of the basal plane are particularly important as these do not degrade
the carrier mobility. In contrast, more reactive radicals like -OH cause a
puckering of the basal plane and thereby act as defects.
