  A Fano-like phonon resonance is observed in few-layer (~3) graphene at room
temperature using infrared Fourier transform spectroscopy. This Fano resonance
is the manifestation of a strong electron-phonon interaction between the
discrete in-plane lattice vibrational mode and continuum electronic excitations
in graphene. By employing ammonia chemical doping, we have obtained different
Fano line shapes ranging from anti-resonance in hole-doped graphene to
phonon-dominated in n-type graphene. The Fano resonance shows the strongest
interference feature when the Fermi level is located near the Dirac point. The
charged phonon exhibits much-enhanced oscillator strength and experiences a
continuous red shift in frequency as electron density increases. It is
suggested that the phonon couples to different electronic transitions as Fermi
level is tuned by chemical doping.
