  We have studied the adsorption of gas molecules (CO, NO, NO2, O2, N2, CO2,
and NH3) on graphene nanoribbons (GNRs) using first principles methods. The
adsorption geometries, adsorption energies, charge transfer, and electronic
band structures are obtained. We find that the electronic and transport
properties of the GNR with armchair-shaped edges are sensitive to the
adsorption of NH3 and the system exhibits n type semiconducting behavior after
NH3 adsorption. Other gas molecules have little effect on modifying the
conductance of GNRs. Quantum transport calculations further indicate that NH3
molecules can be detected out of these gas molecules by GNR based sensor.
