Site-Specific Organic/Metal Interaction Revealed from Shockley-Type Interface State

In order to discuss the site specificity in the organic/metal interfacial interaction, the electronic structure of the well-ordered interface of cobalt phthalocyanine (CoPc) adsorbed on Au(111) is studied by angle resolved photoemission spectroscopy (ARPES) with synchrotron radiation. The core-level and valence-band ARPES spectra show an evidence for the strong CoPc/Au(111) chemical interaction localized at the Co site and the resultant formation of the interface states. The CoPc/Au(111) interface states are formed around the highest occupied molecular orbital (HOMO) and just below the Fermi level (E-F) with the nondispersive and dispersive characteristics, respectively. The nondispersive interface state around the HOMO is attributed to the intramolecular pi(pc)-d(Co) mixed orbital induced by the site-specific interfacial interaction. The dispersive interface state below EF originates from the adsorbate-induced modification of the Au(111) Shockley state, which shows difference between CoPc and metal-free Pc (H2Pc) and dependence on the temperature. The Shockley-type interface state enables us to characterize organic/metal interface energetics quantitatively in terms of the site-specific interfacial interaction.