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

Abstract

We present Doppler broadened gamma spectra, obtained using the newly developed advanced positron beam at the University of Texas at Arlington, from a sample consisting of 6 to 8 layers of graphene (MLG) on polycrystalline Cu. The kinetic energy of the positron beam was varied form 2 eV to 20 keV allowing for a depth resolved measurement. The ratio curves formed by dividing the measured Doppler broadened gamma spectra obtained at low positron kinetic energies (~2eV) to the gamma spectra obtained at 20 keV were compared to ratio curves found by dividing the calculated spectra of bulk graphite to bulk Cu. The ratio curves obtained from the measured results show qualitative agreement with those obtained from the calculated spectra. In particular, both sets of curves indicate a much reduced intensity at high momentum. The agreement between the measured and calculated curves is consistent with the hypothesis that the 2eV spectra correspond to the Doppler broadened spectra from the thin overlayer of Graphene (which we anticipate should be similar to the spectra obtained from bulk graphite) and that the spectra taken at 20 keV corresponds to bulk Cu due to the fact that most of the positrons implanted at this energy annihilate in the Cu substrate. The results taken at 2 eV provide evidence that it is possible to obtain chemically sensitive information from the top atomic layers of surfaces (both internal and external) from an analysis of the high momentum tail of the Doppler broadened gamma spectra obtained from the annihilation of positrons at the surface.

A multi-stop time-of-flight spectrometer for the measurement of positron annihilation-induced electrons in coincidence with the Doppler-shifted annihilation gamma photon

“…The multifunctional nature of the advanced positron beam allows it to characterize the topmost atomic layer of surfaces as well as the sub-surface region, hidden interfaces, and inner surfaces of porous materials. The ability of Doppler broadening spectroscopy to selectively characterize the chemical nature of the topmost layer of the external surfaces was demonstrated recently using the advanced positron beam [45][46]. These investigations provided the proof-ofprinciple that chemical characterization using Doppler broadening spectroscopy provides the same level of surface selectivity as that obtained with PAES [13].…”

Efficient machine learning approach for optimizing the timing resolution of a high purity germanium detector

“…The experimental decomposition of the Doppler broadened annihilation γ spectra can provide a valuable benchmark for theoretical calculations that aim to simulate the annihilation characteristics of positrons on surfaces [39]. Additionally, it may be the first step in standardizing the new inner surface characterization method entirely based on Doppler broadened γ spectroscopy [40,41].…”

Elemental Characterization of the Topmost Atomic Layer of Surface Using Doppler Broadening Spectroscopy

“…The wavefunction of the surfacetrapped positrons decays rapidly and has appreciable overlap only with the electron wavefunctions spilling into the vacuum from the top surface. Weiss et al [7] proposed that the gamma photon emitted following the annihilation of surface trapped positrons should provide highly surface selective chemical information and had shown that the high energy (equivalently high momentum) region of the annihilation gamma from multiple layers of a graphene sample on a Cu substrate was similar in shape to the high energy region of the theoretically calculated annihilation gamma from the bulk of graphite.…”