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

Abstract

This work is dedicated to Prof. Peter S. Pershan on his 85th birthday. His many pioneering contributions to physics research have provided an inspiring model.We present initial results that demonstrate the ability of Doppler broadening spectroscopy (DBS) to extract elemental information from the topmost atomic layer of a sample surface. Doppler broadening spectra from graphene grown on a polycrystalline Cu substrate was compared to the Doppler broadened spectra obtained after the partial removal of the bilayer graphene from the Cu surface. The Doppler broadening of the annihilation photopeak showed a clear difference between the as-inserted and the partially sputtered sample. We posit that this difference was due to the contribution of the gamma photons originating from the annihilation of positrons with the substrate (Cu) electrons to the total Doppler broadened spectrum. The Doppler broadened spectrum of the partially sputtered sample was well fit with a line shape produced by combining the Doppler broadened gamma spectrum of the as-inserted sample with the theoretically calculated annihilation gamma photo peak from bulk Cu. The fraction of the calculated Doppler broadened spectrum from bulk Cu in the fit spectrum is consistent with the increase in intensity of the Cu M2,3VV Auger peak observed in the positron annihilation induced Auger electron spectra (PAES) of the partially sputtered sample. This suggests that analysis of the high momentum region of the gamma photopeak due to photons emitted following the annihilation of surface trapped positrons can provide a quantitative estimate of the elemental composition of the top surface. Our results provide evidence for the feasibility of using DBS as a new spectroscopic technique for the elemental characterization of the topmost atomic layer of surfaces.