Figure 1: Time of flight positron annihilation-induced Auger electron spectrum.
TOF spectrum of electrons emitted from a single layer of graphene (magenta) on a polycrystalline Cu substrate following the creation of holes via positron annihilation. Also shown is the spectrum of electrons emitted from the polycrystalline Cu substrate (black) after removal of the graphene layer. The top axis shows the energy of the electrons calculated from their TOFs. Analysis in this paper shows that the strong peak at ∼4 eV (∼1.65 μs) in the spectrum from graphene corresponds to emission of electrons as a result of an Auger transition in which the energy is provided by the filling of a deep hole in the 20 eV wide valence band of the graphene layer created by matter–antimatter annihilation (Fig. 2). This peak is notably absent in the spectrum taken from the clean Cu substrate, consistent with the fact that the Auger electrons resulting from the filling of holes in the relatively shallow valence band of Cu do not have enough energy to leave the surface. The TOF-PAES spectrum of the graphene also shows peaks corresponding to the Auger relaxation of core holes in C (KVV), adsorbed O (KVV) and in Cu (MVV) and the spectrum from clean Cu substrate show peaks corresponding to the M2,3VV and M1VV Auger peaks. The use of an incident 1.25 eV positron beam to create the holes eliminates the normally overwhelming beam induced secondary electron background that prevents other photon or electron based techniques from making a direct measurement of the valence Auger process.
