We report the observation of Fano-type phonon resonance using infrared Fourier transform spectroscopy at room temperature in few-layer $\left(\sim 3\right)$ graphene synthesized by chemical-vapor deposition on copper substrate. We subsequently demonstrate a continuous tuning of Fano line shape from antiresonance to phonon dominated by ammonia chemical doping. The few-layer Fano resonance exhibits a strong asymmetric characteristic between $n$-doped and $p$-doped graphene, different from that observed in double-layer graphene gated by external electrodes. This asymmetry probably arises from nonuniform charge-distribution among the layers of graphene due to the substrate effect on the bottom layer and adsorbed ammonia molecules on the top layer. Simultaneous measurements of Fano line shape and infrared signature of adsorbed ammonia molecules have also revealed the orientation of ammonia molecules on graphene and confirmed the charge-transfer mechanism of chemical doping.

• Received 5 August 2010

DOI:https://doi.org/10.1103/PhysRevB.82.155435