Optical and electronic interaction at metal-organic and organic-organic interfaces of ultra-thin layers of PTCDA and SnPc on noble metal surfaces

We investigate the interaction mechanisms at metal-organic and organic-organic interfaces in highly-ordered ultra-thin layers of the dye molecules 3,4,9,10-perylene-tetracarboxylic- dianhydride (PTCDA) and tin(II)-phthalocyanine (SnPc) on single crystalline noble metals. The ultra-thin films are characterized by means of in situ differential reflectance spectroscopy (DRS), followed by an extraction of the optical functions by application of a numerical algorithm. For the first time, DRS data of PTCDA and SnPc films on Ag(111) are presented. We found that for the contact layers of PTCDA and SnPc the well-known covalent interaction between adsorbate and substrate is manifested in broad and structureless absorption spectra. Surprisingly, the optical spectra of the respective first monolayers on Ag(111) are almost identical despite of the rather different electronic structure of the free molecules. The special character of the optical spectra is emphasized by a comparison with PTCDA and SnPc monolayers on Au(111) where the electronic interaction at the metal-organic interface is much weaker. Quite differently from the contact layer, the second layer of the same molecule on Ag(111) clearly shows monomeric behavior which can only be observed if the electronic and optical coupling with the surrounding molecules and the substrate is faint. However, a very weak out-of-plane electronic interaction remains as concluded from the comparison with the spectra obtained on inert mica substrates. We also present structural data acquired with low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) of SnPc on Au(111). (C) 2013 Elsevier B.V. All rights reserved.