Phonons in potassium doped graphene: the effects of electron-phonon interactions, dimensionality and ad-atom ordering

TL;DR

This study investigates how potassium doping affects graphene phonons, revealing doping-dependent phonon behavior, phase segregation, and differences from bulk graphite, highlighting the complex interplay of electron-phonon interactions and dimensionality.

Contribution

It provides new insights into phonon behavior in potassium-doped graphene, especially regarding non-adiabatic effects and inhomogeneous phases across different doping levels.

Findings

G-peak hardens and narrows at low doping

G-peak softens and broadens at high doping

Inhomogeneous potassium coverage forms between doping regimes

Abstract

Graphene phonons are measured as a function of electron doping via the addition of potassium adatoms. In the low doping regime, the in-plane carbon G-peak hardens and narrows with increasing doping, analogous to the trend seen in graphene doped via the field-effect. At high dopings, beyond those accessible by the field-effect, the G-peak strongly softens and broadens. This is interpreted as a dynamic, non-adiabatic renormalization of the phonon self-energy. At dopings between the light and heavily doped regimes, we find a robust inhomogeneous phase where the potassium coverage is segregated into regions of high and low density. The phonon energies, linewidths and tunability are remarkably similar for 1-4 layer graphene, but significantly different to doped bulk graphite.