Graphene phonons are measured as a function of electron doping via theaddition of potassium adatoms. In the low doping regime, the in-plane carbonG-peak hardens and narrows with increasing doping, analogous to the trend seenin graphene doped via the field-effect. At high dopings, beyond thoseaccessible by the field-effect, the G-peak strongly softens and broadens. Thisis interpreted as a dynamic, non-adiabatic renormalization of the phononself-energy. At dopings between the light and heavily doped regimes, we find arobust inhomogeneous phase where the potassium coverage is segregated intoregions of high and low density. The phonon energies, linewidths and tunabilityare remarkably similar for 1-4 layer graphene, but significantly different todoped bulk graphite.
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