Identifying Electronic Doorway States in the Secondary Electron Emission from Layered Materials

Abstract

Secondary electron emission after primary low- energy (<200 eV) electron impact is one of the most fundamental processes of particle-solid in- teraction. However, its underlying mechanisms are yet to be fully understood. Secondary electron spectra, especially the low-energy part (<20 eV) are often considered featureless, how- ever, coincidence measurements of primary scat- tered and emitted secondary electrons can re- veal distinct structures. For example, by de- tecting correlated pairs of two electrons, the so-called X-peak in highly-oriented pyrolytic graphite (HOPG) [1] could be explained recently based on a hybridisation of interlayer states [2]. Here we further explore the emission mechanism of secondary electrons by studying low-energy electron emission from quasi-freestanding single- and bilayer graphene besides HOPG, spanning the transition from two-dimensional to bulk carbon systems. Our measurements reveal pronounced layer-dependent resonance signatures above the vacuum threshold, with some states opening only for multilayer systems, see Figure 1. While the peak at 3.3 eV is only prominent for HOPG, there is a peak at 7.7 eV for bilayer graphene and only a washed-out distribution for the monolayer. These obvious differences in the secondary elec- tron spectra are surprising as the density of states and band structure of all three materials are sim- ilar. Hand-in-hand with density functional the- ory, we could link the observed features to Fes- hbach resonances, where quasi-bound states of 2.0 1.5 1.0 2.0 1.5 1.0 2.0 1.5 1.0 single-layer graphene 3.3 eV 7.7 eV bilayer graphene HOPG 0 5 10 15 E2 - Evac (eV) 5 (MAIN), Germany Institute of Photonics, TU Wien, Vienna, Austria *corresponding author: anna@iap.tuwien.ac.at Figure 1: Secondary electron spectrum of single- layer graphene, bilayer graphene, and HOPG from coincidence measurements, where the sec- ond electron had a kinetic energy between [0,140 eV]. Taken from [3]. the material above the vacuum level couple to the continuum and act as doorway states for electron emission. These results provide new insights into low-energy electron emission in graphene-based systems and open up new perspectives for under- standing emission processes in layered materials. References [1] H. Yamane et al. Phys. Rev. B 64 113407 (2001) [2] W. S. M. Werner et al. Phys. Rev. Lett. 125 196603 (2020) [3] A. Niggas et al. under review (2025)