Phase separation and charge transfer in a K-doped C 60 monolayer on Ag(001)

We have performed a scanning tunneling microscopy and spectroscopy study of potassium-doped ${\text{C}}_{60}$ monolayers $({\text{K}}_{\text{x}}{\text{C}}_{60})$ on Ag(001) in the regime of $x\ensuremath{\approx}1$. Low-temperature annealing (640 K) leads to the formation of two well-ordered ${\text{K}}_{\text{x}}{\text{C}}_{60}$ 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 ${\text{C}}_{60}$ phase completely devoid of K. Spectroscopic measurements indicate that the electron-doping level of the higher electron-doped ${\text{KC}}_{60}$ phase is anomalously large.