Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation

Abstract

Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition.

Application of Doppler broadened gamma spectroscopy to study the surface of graphene

“…The sample consisted of multiple layers of graphene (~ 6 -8 layers) grown on a polycrystalline Cu foil (MLG) purchased from ACS materials. The presence of MLG on Cu was confirmed using Raman spectroscopy and PAES [8]. Figure 2 shows a comparison between the Doppler broadened annihilation gamma spectra obtained at positron energy of 20 keV and at positron energy of 2eV from MLG on Cu.…”

“…Based on previous studies, [9] of the diffusion of positrons in well-annealed Cu after implantation using a variable energy beam, we believe that the data obtained at 20keV represent the Doppler broadened spectra from the bulk of the sample (in this case Cu). Our previous investigations of the intensities of the Auger peak in the positron annihilation induced Auger electron spectra (PAES) obtained with low energy positrons ( ~ 2 eV) [8,10] indicate that the data obtained at 2 eV represent the Doppler broadened annihilation spectra solely from the surface (in this case MLG). It may be seen from Fig.…”

Coincident measurement of the energy spectra of Doppler-shifted annihilation gamma quanta and positron-induced secondary electrons

“…In addition, by analyzing the Doppler-broadened gamma spectrum in coincidence with a range of electrons in the Auger spectrum that may correspond to a multi-Auger process, it may be possible to determine the particular annihilation event that results in multi-Auger emission [8]. Finally, we plan further experiments regarding the VVV Auger process in which the VVV region in the Auger electron spectrum will be measured in coincidence with the region in the gamma spectrum corresponding to valence annihilations [9,10].…”

Ultrafast Hyperspectral Transient Absorption Spectroscopy: Application to Single Layer Graphene

“…Recent observations suggest that, during the early stages of the dynamics of photogenerated carriers, which cannot be accessed with our technique, Auger processes, such as Auger recombination and carrier multiplication, play a significant role [63,[66][67][68].…”