Observation of phonon anomaly at the armchair edge of single-layer graphene in air.

Confocal Raman spectroscopy is used to study the phonon modes of mechanically exfoliated single-layer graphene sheets in ambient air. We observe that ambient gas induces obvious shifts in the G band frequency as well as the change in intensity ratio of 2D and G bands, I(2D)/I(G), owing to the Fermi energy change by ambient gas doping. The change in I(2D)/I(G) for the armchair edge is significantly larger than those for the graphene center or zigzag edge in our graphene samples. Also, the G band phonon anomaly, the G band frequency softens and peak width broadens at the charge neutral (Dirac) point, is clearly identified at the armchair edge but not for the zigzag edge or graphene center. We conclude that Fermi level of the armchair edge is close to the Dirac point, making the phonon anomaly visible. However, the charge carrier concentration at the graphene center was too high (Fermi level away from the Dirac point), which completely smears out the phonon softening phenomenon. This study proves that the phonon anomaly can occur at the armchair edge as predicted by Sasaki et al. (J. Phys. Soc. Jpn. 2010, 79, 044603). Our results also demonstrate that the phonon property of an edge or center site in single-layer graphene is very sensitive to its local carrier concentration.