Mapping Unoccupied Electronic States of Freestanding Graphene by   Angle-Resolved Low-Energy Electron Transmission
  We report angle-resolved electron transmission measurements through freestanding graphene sheets in the energy range of 18 to 30 eV above the Fermi level. The measurements are carried out in a low-energy electron point source microscope, which allows simultaneously probing the transmission for a large angular range. The characteristics of low-energy electron transmission through graphene depend on its electronic structure above the vacuum level. The experimental technique described here allows mapping the unoccupied band structure of freestanding two-dimensional materials as a function of energy and probing angle, respectively in-plane momentum. Our experimental findings are consistent with theoretical predictions of a resonance in the band structure of graphene above the vacuum level [V. U. Nazarov, E. E. Krasovskii, and V. M. Silkin, Physical Review B 87, 041405 (2013)]. 
Flavio Wicki, Jean-Nicolas Longchamp, Tatiana Latychevskaia, Conrad   Escher and Hans-Werner Fink
cond-mat.mes-hall cond-mat.mtrl-sci