Electron cascades and secondary electron emission in graphene under energetic ion irradiation

TL;DR

This study investigates electron cascades and secondary electron emission in graphene caused by energetic ion impacts, using advanced simulations to understand energy loss and electron behavior in this two-dimensional material.

Contribution

It introduces combined Monte Carlo and TDDFT simulation approaches to analyze electron dynamics and emission in graphene under ion irradiation, highlighting the importance of electron emission in energy deposition.

Findings

Electron emission depends linearly on deposited energy in MC simulations.

Electrostatic interactions make emission behavior sublinear, aligning with TDDFT results.

Electron capture probability decreases with higher ion velocity, while emission dominates at high velocities.

Abstract

Highly energetic ions traversing a two-dimensional material such as graphene produce strong electronic excitations. Electrons excited to energy states above the work function can give rise to secondary electron emission, reducing the amount of energy that remains the graphene after the ion impact. Electrons can either be emitted (kinetic energy transfer) or captured by the passing ion (potential energy transfer). To elucidate this behavior that is absent in three-dimensional materials, we simulate the electron dynamics in graphene during the first femtoseconds after ion impact. We employ two conceptually different computational methods: a Monte Carlo (MC) based one, where electrons are treated as classical particles, and time-dependent density functional theory (TDDFT), where electrons are described quantum-mechanically. We observe that the linear dependence of electron emission on deposited energy, emerging from MC simulations, becomes sublinear and closer to the TDDFT values when the electrostatic interactions of emitted electrons with graphene are taken into account via complementary particle-in-cell simulations. Our TDDFT simulations show that the probability for electron capture decreases rapidly with increasing ion velocity, whereas secondary electron emission dominates in the high velocity regime. We estimate that these processes reduce the amount of energy deposited in the graphene layer by 15\,\% to 65\,\%, depending on the ion and its velocity. This finding clearly shows that electron emission must be taken into consideration when modelling damage production in two-dimensional materials under ion irradiation.