Analysis of the line shape of a low energy Auger emission (VVV) initiated by a valence hole to investigate the surface electronic states of graphene, highly oriented pyrolytic graphite (HOPG) and Si (100).

Recently, our group reported the direct observation of an Auger electron emission process from single layer graphene (SLG) that is initiated by a hole created by the annihilation of a valence electron with a surface-bound positron (Chirayath et al., Nature Comm. 8, 16116, 2017). The annihilation induced valence hole decays through an intra-band Auger relaxation process and is termed as VVV. The ability to model all the features of the VVV Auger peak using valence and conduction band density of states of a freestanding SLG shows the sensitivity of the process to the surface electronic states. Energy conservation requires that the valence bandwidth be greater than the work function of the sample for electron emission into the vacuum through a VVV Auger process and hence, the VVV peak should be detectable in materials where this condition is satisfied. Here we report on positron annihilation induced Auger electron spectroscopy (PAES) measurements, which shows the presence of VVV Auger peak in Si (100) and HOPG. The VVV Auger peaks from Si (100) and HOPG are compared to the VVV Auger peak from SLG and to the theoretically generated VVV Auger spectrum to understand the ability of VVV-PAES to investigate unoccupied surface states in wideband materials.