Antiresonances in the Mid-Infrared Vibrational Spectrum of Functionalized Graphene

We report anomalous antiresonances in the infrared spectra of doped and disordered single layer graphene. Measurements in both reflection microscopy and transmission configurations of samples grafted with halogenophenyl moieties are presented. Asymmetric transparency windows at energies corresponding to phonon modes near the Gamma and K points are observed, in contrast to the featureless spectrum of pristine graphene. These asymmetric antiresonances are demonstrated to vary as a function of the chemical potential and defect density. We propose a model that involves coherent intraband scattering with defects and phonons, thus relaxing the optical selection rule forbidding access to q not equal Gamma phonons. This interpretation of the new phenomenon is supported by our numerical simulations that reproduce the experimental features.