Positron Impact Induced Secondary Electron Spectra of Graphene and Graphite

We present the positron impact-induced secondary electron (PIISE) energy spectra and yield from single-layer graphene (SLG) and multi-layer graphene (MLG) grown on a polycrystalline Cu substrate. The energy spectrum and yield of secondary electrons resulting from low energy positrons incident on a graphene surface will be compared to results obtained from highly oriented pyrolytic graphite (HOPG) and clean polycrystalline Cu. Since the primary positron is distinguishable from the secondary electrons, the PIISE signal does not include redistributed primaries thus making it possible to obtain the true “secondary” spectra [1]. However, the PIISE spectra measured using a Time of Flight (ToF) spectrometer, where the ToF is obtained from the time difference between the emission of positron annihilation gamma and the detection of the emitted secondary electrons, is modified by the delayed gamma emitted following the annihilation of long-lived ortho positronium (o-Ps). We have obtained the true ToF of the secondary electrons by iteratively deconvoluting the measured spectra using an instrument response function obtained which includes the effect of delayed annihilation of triplet Ps. The response function was obtained using a combination of a Monte Carlo simulation of o-Ps transport [2] and GEANT4 simulations of the detection of the 2 γ singlet and the 3 γ triplet annihilation of o-Ps. Apart from the true PIISE energy spectra from SLG and MLG, we also obtain the Ps fraction which is compared to the Ps fraction obtained using the GEANT4 simulations of the detected gamma energy spectra. We will also discuss how an analysis of the PIISE energy spectra obtained using low positron energies (20 eV – 900 eV) can provide information regarding the unoccupied density of states of graphene.

1. 1. Weiss et al., Appl. Surf. Sci., 116, (1997) 311-317.
   
   2. Lotfimarangloo et al., NIM B, 542, (2023) 99-109.