STM Study of PTCDA on Pb/Si(111) 1×1

Abstract: The interaction and orbital energy levels of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules on a Pb/Si(111) 1x1 substrate have been investigated. A Si(111) sample was annealed to form the 7x7 configuration. 1.5 monolayer of Pb was evaporated onto the surface, which was then annealed. 0.5 monolayer of PTCDA was applied to the substrate through molecular beam epitaxy (MBE). The surface configuration of the substrate was monitored step by step by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Scanning tunneling spectroscopy (STS) was used to pinpoint the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecules. It was found that the PTCDA molecules formed a herringbone pattern on the substrate. The PTCDA electronic energy levels corresponding to the HOMO and the LUMO were obtained. From these values the energy gap between these orbitals, the molecular bandgap of PTCDA on Pb/Si(111) 1x1, was determined.