Frontier electronic energy levels of tetrahedral (Td) fullerenes

Fluorine 1s and Cu 2p core level binding energies of (Tl0.5Pb0.5)Sr1.6Ba0.4Ca2Cu3Oy/Fx for nominal x=0.0, 1.0, 2.2 and 3.0 were measured by X-ray photoelectron spectroscopy (XPS) in order to identify the F site in the overdoped superconductors. Detailed data analysis indicate that about two thirds of the incorporated F replace O in Cu-O planes of the Tl-1223 structure. Most of the remaining one third of F replaces O in the Ba/Sr-O planes and a small amount of F substitutes for O in the Tl/Pb-O planes of Tl-1223 structure. Cu 2p3/2 binding energy was found to be decreasing with increasing x. This indicates that the valency of Cu in Tl-1223/Fx decreases as x increases. The reduction of charge carriers in the overdoped high temperature superconductors is discussed in terms of anion substitution.