Shielding copper atoms by distortion of phthalocyanine ring on Si(1 1 1)

The adsorption behavior of CuPc on Si(1 1 1) and the substrate effect on the valence state of the Cu atoms in CuPc is investigated by synchrotron-based photoemission and X-ray absorption spectroscopy. The monolayer of CuPc on Si(1 1 1) is disordered, in contrast to the ordered multilayer CuPc. The formation of Si–C and Si–N bonds at the interface is clearly observed in the Si 2p and C 1s core level spectra. For the CuPc monolayer, the valence state for a portion of Cu atoms is reduced from 2+ to 1+. Further post-annealing significantly enhances the formation of Si–C and Si–N bonds between the molecules and the substrate, but the Cu 2p core level spectra unambiguously reveal that the Cu valence state remains unchanged. This suggests that the Cu atoms are well shielded from the surrounding environment by the distortion of the CuPc molecules.