  Graphene has demonstrated great promise for technological use, yet control
over material growth and understanding of how material imperfections affect the
performance of devices are challenges that hamper the development of
applications. In this work we reveal new insight into the connections between
the performance of the graphene devices as environmental sensors and the
microscopic details of the interactions at the sensing surface. Specifically,
we monitor changes in the resistance of the chemical-vapour deposition grown
graphene devices as exposed to different concentrations of ethanol. We perform
thermal surface treatments after the devices are fabricated, use scanning probe
microscopy to visualize their effects on the graphene sensing surface down to
nanometer scale and correlate them with the measured performance of the device
as an ethanol sensor. Our observations are compared to theoretical calculations
of charge transfers between molecules and the graphene surface. We find that,
although often overlooked, the surface cleanliness after device fabrication is
responsible for the device performance and reliability. These results further
our understanding of the mechanisms of sensing in graphene-based environmental
sensors and pave the way to optimizing such devices, especially for their
miniaturization, as with decreasing size of the active zone the potential role
of contaminants will rise.
