Phonons in potassium-doped graphene: The effects of electron-phonon interactions, dimensionality, and adatom ordering

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, nonadiabatic 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 notably very similar for one- to four-layer potassium-doped graphene, but significantly different to bulk potassium-doped graphite.

Figure 1

The Raman spectra of graphene in order of increasing potassium doping. The approximate positions of the peaks are marked by arrows. Representative spectra are offset from one another and normalized to the $G$ peak for clarity. The different doping regimes are denoted to the right of the plot: (a) undoped, (b) lightly doped, (c) inhomogeneous, (d) intermediate, and (e) heavily doped. Also shown are the Raman spectra for the GICs (f) KC${}_8$ and (g) KC${}_{24}$.

Figure 2

The correlation between the $G$-peak energy and width for the different doping regimes: undoped (black $\blacksquare$), lightly doped (purple $•$), inhomogeneous (red $▴$), intermediate (yellow $⧫$), and heavily doped (green $★$). These are compared with results on gated graphene from Refs.  3 and 24 (magenta/thick line). The arrows are guides to the eye depicting the trend with increasing doping.

Figure 3

(a) The evolution of the $G$ peak of inhomogeneously doped graphene with doping. (b) The $G$ peak of one- to four-layer graphene for three dopings including saturation, and the $G$ peak of a saturation doped $\sim$100-layer graphite flake. (c) The effect of doping the $G$-peak integrated intensity (normalized to the 520 cm${}^{-1}$ Si peak); labels are as in Fig. 2.