Phase Separation and Charge Transfer in a K-doped C60 Monolayer on Ag(001)

We have performed a scanning tunneling microscopy and spectroscopy study of potassium-doped C60 monolayers (KxC60) on Ag(001) in the regime of x ~ 1. Low temperature annealing (640 K) leads to the formation of two well-ordered KxC60 phases that exhibit differing levels of electron charge transfer. Further annealing (710 K) distills out the higher electron-doped phase from the lower electron-doped phase, leaving behind a third C60 phase completely devoid of K. Spectroscopic measurements indicate that the electron-doping level of the higher electron-doped KC60 phase is anomalously large.