  Integrated, highly sensitive and reversible sensor devices for toxic and
hazardous gases in environmental pollution monitoring can be realized with
graphene-based materials. Here we show that, single layer graphene grown on SiC
can be utilized to implement sensor devices being extremely sensitive towards
NO2 showing an n-type response. A second type of sensor with an added NiO layer
on top of the single layer graphene changed its response to p-type but did not
reduce its sensitivity. We show that the conduction switch from n-type to
p-type was not a consequence of an alteration of the graphene layer but is
found to be an effect of the NiO layer. We find that the NiO leads to lowering
of the Fermi level to a point that a crossing of the Dirac Point in the
graphene switched the conduction type. These sensors were tested in the 100 ppb
NO2 regime, showing good response and a detection limit extrapolated to be
below 1 ppb. This new NiO/graphene/SiC configuration can be an attractive
p-type sub-ppb sensor platform for NO2 and related gases.
