  We directly correlate the local (20-nm scale) and global electronic
properties of a device containing mono-, bi- and tri-layer epitaxial graphene
(EG) domains on 6H-SiC(0001) by simultaneously performing local surface
potential measurements using Kelvin probe force microscopy and global transport
measurements. Using well-controlled environmental conditions, where the
starting state of the surface can be reproducibly defined, we investigate the
doping effects of N2, O2, water vapour and NO2 at concentrations representative
of the ambient air. We show that presence of O2, water vapour and NO2 leads to
p-doping of all EG domains. However, the thicker layers of EG are significantly
less affected by the atmospheric dopants. Furthermore, we demonstrate that the
general consensus of O2 and water vapour present in ambient air providing
majority of the p-doping to graphene is a common misconception. We
experimentally show that even the combined effect of O2, water vapour, and NO2
at concentrations higher than typically present in the atmosphere does not
fully replicate the state of the EG surface in ambient air. All doping effects
can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it
is essential to consider naturally occurring environmental effects and properly
separate them from those coming from targeted species.
