Direct observation of the vanishing electron energy loss spectroscopy cross section in graphene

In transmission electron energy loss (EEL) spectroscopy, the cross section in two dimensions (2D) is quenched by kinematic effects once the momentum transfer becomes smaller than a critical value set by qz, the momentum loss parallel to the beam. Our highly momentum (Δq=0.02Å-1) and energy (ΔE=45 meV) resolved setup is instrumental in delivering the experimental verification of quenched 2D EEL spectra on freestanding graphene at momentum transfers q below 0.06Å-1. We retrieve the intrinsic uniform dielectric response of graphene from measured spectra by quantifying the kinematic suppression.