Electronic Excitations in Graphene in the 1–50 eV Range: The π and π + σ Peaks Are Not Plasmons

The field of plasmonics relies on light coupling strongly to plasmons as collective excitations. The energy loss function of graphene is dominated by two peaks at ∼5 and ∼15 eV, known as π and π + σ plasmons, respectively. We use electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope and density functional theory to show that between 1 to 50 eV, these prominent π and π + σ peaks are not plasmons, but single-particle interband excitations.