# Non-universal Transverse Electron Mean Free Path through Few-layer Graphene

## TL;DR

This study measures the transverse electron mean free path in few-layer graphene, revealing it is non-universal and varies with layer number, challenging previous assumptions of a universal MFP curve.

## Contribution

It provides the first direct measurements of transverse electron MFP in graphene and shows it varies with layer number, supported by theoretical calculations.

## Key findings

- Total MFP is just a few graphene layers at low energies.
- Elastic MFP oscillates with layer number.
- Experimental results agree with theoretical calculations.

## Abstract

In contrast to the in-plane transport electron mean-free path in graphene, the transverse mean-free path has received little attention and is often assumed to follow the 'universal' mean-free path (MFP) curve broadly adopted in surface and interface science. Here we directly measure transverse electron scattering through graphene from 0 to 25 eV above the vacuum level both in reflection using Low Energy Electron Microscopy and in transmission using electron-Volt Transmission Electron Microscopy. From this data, we obtain quantitative MFPs for both elastic and inelastic scattering. Even at the lowest energies, the total MFP is just a few graphene layers and the elastic MFP oscillates with graphene layer number, both refuting the 'universal' curve. A full theoretical calculation taking the graphene band structure into consideration agrees well with experiment, while the key experimental results are reproduced even by a simple optical toy model.