Defect-induced Fermi level pinning and suppression of ambipolar behaviour in graphene

We report on systematic study of electronic transport behaviour of low-biased, disordered graphene nanowires. We reveal the emergence of unipolar transport as the defect concentration increases beyond 0.3% where an almost insulating behaviour is observed for n-type conduction whilst a metallic behaviour is observed for p-type conduction. The conductance shows a plateau that extends through the entire side above the Dirac point (n-branch) where the conductivity coincides with the minimum conductivity at the Dirac point. Raman spectroscopy and X-ray photoemission spectroscopy were used to probe the nature of the defects created by helium ion irradiation and revealed the presence of oxygen-carbon bonds as well as sp3 configurations on the irradiated samples from C KLL Auger spectrum. The observed behaviour is attributed to oxygen groups created after the sputtering of carbon atoms by incident helium ions. These groups act as charge traps, pinning the Fermi level to the Dirac point.