Theoretical study of vacuum level shift at the C6H6/Al(111) interface

We have investigated the interface dipole induced at C6H6/Al(111)) by using density functional theory (DFT) within the generalized gradient approximation (GGA). We calculated the vacuum level shift from a simple induced density of interface state (IDIS) model and compared it with that estimated by self-consistent DFT-GGA calculations. Both results agree reasonably well with each other in weakly interacting cases, where the charge donation from the highest occupied molecular orbital level dominates, while they deviate from each other in strongly interacting cases, where the lowest unoccupied molecular orbital level starts to be partially filled owing to the back donation from the substrate. Even in weakly interacting cases, the actual charge density distribution is more complex than that assumed in the IDIS model. We discuss the validity and the limitations of the IDIS model to describe the vacuum level shift at organic/metal interfaces. Copyright (C) 2008 John Wiley & Sons, Ltd.