Tuning of C60 energy levels using orientation-controlled phthalocyanine films

The interface electronic structure of C-60/CuPc and C-60 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 C-60 molecules on the standing CuPc film has been observed, while nearly vacuum-level alignment is observed for C-60 on the lying CuPc film. We also found small vacuum-level shifts for C-60 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, C-60 highest occupied molecular orbital energy levels relative to the substrate Fermi level can be tuned from 1.9 eV for C-60 on the standing CuPc film to 1.0 eV on the standing F16CuPc film. (C) 2010 American Institute of Physics. [doi:10.1063/1.3475716]