Tuning of $C60$ energy levels using orientation-controlled phthalocyanine films

The interface electronic structure of $C60/CuPc$ and $C60$ heterojunctions on $SiO2$ and highly oriented pyrolytic graphite has been studied using ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, and synchrotron based photoelectron spectroscopy. Fermi level pinned to the negative integer charge transfer state of $C60$ molecules on the standing CuPc film has been observed, while nearly vacuum-level alignment is observed for $C60$ on the lying CuPc film. We also found small vacuum-level shifts for $C60$ on both standing and lying $F16CuPc$ films, which can be attributed to the rearrangement of underlying $F16CuPc$ molecules. With the use of orientation-controlled CuPc and $F16CuPc$ thin films, $C60$ highest occupied molecular orbital energy levels relative to the substrate Fermi level can be tuned from 1.9 eV for $C60$ on the standing CuPc film to 1.0 eV on the standing $F16CuPc$ film.