PTCDA on Au(1 1 1), Ag(1 1 1) and Cu(1 1 1): Correlation of interface charge transfer to bonding distance

The electronic structure at the interfaces of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and the metal surfaces Au(1 1 1), Ag(1 1 1) and Cu(1 1 1) was investigated using ultraviolet photoelectron spectroscopy (UPS). By combining these results with recent X-ray standing wave data from PTCDA on the same substrates clear correlation between the electronic properties and the interface geometry is found. The charge transfer between the molecule and the metal increases with decreasing average bonding distance along the sequence Au–Ag–Cu. Clear signatures of charge-transfer-induced occupied molecular states were found for PTCDA on Ag(1 1 1) and Cu(1 1 1). As reported previously by Zou et al. [Y. Zou et al., Surf. Sci. 600 (2006) 1240] a new hybrid state was found at the Fermi-level (EF) for PTCDA/Ag(1 1 1), rendering the monolayer metallic. In contrast, the hybrid state for PTCDA/Cu(1 1 1) was observed well below EF, indicating even stronger charge transfer and thus a semiconducting chemisorbed molecular monolayer. The hybridisation of molecular and Au electronic states could not be evidenced by UPS.