Scanning tunneling and photoemission spectroscopies at the PTCDA/Au(111) interface

We have studied the electronic properties at the interface of a PTCDA molecular film on Au(111) by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and angle resolved ultraviolet photoelectron spectroscopy (ARUPS). Measurements were performed for the clean Au(111) surface as well as for 1 ML and multilayer coverage of PTCDA. STS curves recorded for 1 ML PTCDA film show a feature at the density of states inside the PTCDA gap and a metallic behavior, in contrast to higher coverage, where a semiconductor behavior is observed. By relating this information to the band dispersion of the electronic states close to the Fermi edge, we can assign this electronic feature to the Au(111) Shockley surface state. By comparison of intramolecularly resolved STM images with first-principles calculations of the free molecule we show that the electronic molecular states at the interface are not altered by the gold surface. All these facts indicate a weak interaction of the PTCDA molecules with the An surface and we conclude that the observed metallicity of the interface is provided by the gold substrate rather than by the organic layer. (C) 2006 Elsevier B.V. All rights reserved.