X-ray Free Electron Laser Studies of Electron and Phonon Dynamics of Graphene Adsorbed on Copper

TL;DR

This study uses femtosecond X-ray spectroscopy and ab-initio theory to investigate electron and phonon dynamics in graphene on copper, revealing indirect excitation mechanisms and decay pathways involving hot electron-hole pairs and optical phonons.

Contribution

First direct femtosecond X-ray absorption study of graphene on copper, elucidating excitation and decay mechanisms with theoretical analysis.

Findings

Graphene excitation is dominated by indirect excitation from copper's hot electron-hole pairs.

Decay involves rapid excitation of strongly coupled optical phonons.

Copper's hot electron-hole lifetime influences graphene's electronic excitation lifetime.

Abstract

We report optical pumping and X-ray absorption spectroscopy experiments at the PAL free electron laser that directly probe the electron dynamics of a graphene monolayer adsorbed on copper in the femtosecond regime. By analyzing the results with ab-initio theory we infer that the excitation of graphene is dominated by indirect excitation from hot electron-hole pairs created in the copper by the optical laser pulse. However, once the excitation is created in graphene, its decay follows a similar path as in many previous studies of graphene adsorbed on semiconductors, i e. rapid excitation of SCOPS (Strongly Coupled Optical Phonons) and eventual thermalization. It is likely that the lifetime of the hot electron-hole pairs in copper governs the lifetime of the electronic excitation of the graphene.