π-Plasmon Dispersion in Free-Standing Monolayer Graphene Investigated by Momentum-Resolved Electron Energy-Loss Spectroscopy

Graphene is a single layer of carbon atoms densely packed in a honeycomb lattice with each carbon atom accommodating four valence electrons: one π electron and three σ electrons [1]. The lone unpaired π electron in the half-filled pz orbital would form a π bond with the neighboring carbon atoms leading to delocalized π bands. Three σ electrons, sp 2 -hybridized with neighboring three carbon atoms, form stronger σ bonds within the graphene layer. The π electrons play a predominant role in the electronic excitations below 10 eV while the σ electrons have important contributions at energies above 10 eV [1]. Furthermore, graphene is characterized by the signature linear dispersion of the π electrons near K, K’ points of the Brillouin zone (Dirac cone) [1].