Photoelectron spectroscopic studies of ultra-thin CuPc and PTCDA layers on Cu(100)

The initial interaction and interface formation between Cu(100) and the organic semiconductors 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and copper phtalocyanine (CuPc) has been studied by means of angle-resolved UV photoelectron spectroscopy (ARUPS). Both CuPc and PTCDA are known to adsorb strongly on the Cu(100) surface. The bonding interaction is revealed via interface-related structures in photoemission spectra. However, the spectra develop rather differently in the low coverage regime: while CuPc molecular states are observed below monolayer coverage, the first PTCDA layer is found to be reacted such that the adsorbate-induced emission is strongly modified relative that of intact PTCDA molecules. We find a number of structures that are neither PTCDA- nor Cu-derived, and that the oxygen-related component of the PTCDA spectrum is completely missing in spectra from the monolayer. Importantly for device applications, we find evidence of interfacial electronic states in the form of new peaks located in the former HOMO-LUMO gap for both molecules. In the case of PTCDA these support a chemisorptive bonding model, whereas in the case of CuPc we interpret the state as a monolayer-specific resonance. (C) 2007 Elsevier B.V. All rights reserved.