Integer Charge Transfer Model-PTCDA on MgO(001)/Ag(001) Probing the Transition from Single to Double Integer Charge Transfer.

For weakly interacting adsorbate/substrate systems, the integer charge transfer (ICT) model describes how charge transfer across interfaces depends on the substrate work function. In particular, work function regimes where no charge transfer occurs (vacuum level alignment) can be distinguished from regions where integer charge transfer by electron tunneling from substrate to adsorbate or vice versa takes place (Fermi level pinning). While the formation of singly integer charged molecular anions and cations of organic semiconductors on various substrates has been well described by this model, the double integer charging regime has so far remained unexplored and experimentally elusive. Here, we extend the integer charge transfer model to the transition from single to double integer charging. This was made possible by combining a molecular adsorbate with high electron affinity (Perylenetetracarboxylic-dianhydride (PTCDA)) with a substrate with tunable work function (ultrathin MgO(001) films on Ag(001)). Our results, obtained with scanning tunneling microscopy (STM), photoemission spectroscopy (PES), work function measurements and density function theory (DFT) calculations, show that after completing the single negative charging of all molecules in a PTCDA monolayer in the first Fermi level pinning regime, the system transitions to a vacuum level alignment regime for singly charged molecules when the substrate work function is reduced, and finally enters the second Fermi level pinning regime at very low substrate work function, in which the molecules become doubly negatively charged.