▎ 摘　　要

We report that the behavior of a low-energy pi plasmon excitation in a single layer graphene (SLG) can be modified by doping external potassium (K) atoms, a feature demanded to realize the graphene plasmonics. Using high-resolution electron-energy-loss spectroscopy, we find that upon K-doping the pi plasmon energy increases by 1.1 eV due to the K-induced electron density up to n=7 x 10(13) cm(-2) in SLG. The four modified dispersions for different K-dopings, however, are found to merge into a single universal curve when plotted in the dimensionless coordinates indicating that the unique plasmonic character of SLG is preserved despite the K-dopings. (C) 2011 American Institute of Physics. [doi:10.1063/1.3630230]