Structure and bonding of large aromatic molecules on noble metal surfaces: The example of PTCDA

Recent efforts to understand the interaction of large aromatic molecules with metal surfaces are discussed. We focus exclusively on work involving the model molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and the noble metal surfaces of Cu, Ag, and Au. Using this material system as an example, salient features of the (chemical) bond between an extended pi-conjugated electron system and a metallic substrate are illustrated. Interface structures are a valuable indicator of the metal-molecule interaction strength. Consistent with the trend observed for small molecule adsorption, they indicate that the interaction strength of PTCDA with the metal substrate decreases in the order Cu-Ag-Au. The interfaces of PTCDA with the Au(111) and Ag(111) surfaces have been studied in particular detail. The interaction of Au(111) with PTCDA is weak, leading to point-on-line coincidence between the lattices of the substrate and the molecular overlayer. Experimental results on this surface are generally consistent with a predominantly physisorptive bonding of PTCDA. The situation is different on Ag surfaces, and in particular on Ag(111), where clear signs of PTCDA chemisorption are observed in many ensemble averaging and single molecule spectroscopies. Issues of electronic and geometric structure as well as electron-vibron interaction, and their relation to the chemical molecule-substrate interaction, are discussed in detail. (c) 2007 Elsevier Ltd. All rights reserved.