Surface modification of ZnO(0001)-Zn with phosphonate-based self-assembled monolayers: Binding modes, orientation, and work function

We used partially fluorinated alkyl and aromatic phosphonates as model systems with similar molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface. The introduced surface dipole moment allows tailoring the ZnO work function to tune the energy levels at the inorganic-organic interface to organic semiconductors, which should improve the efficiency of charge injection/extraction or exciton dissociation in hybrid electronic devices. By employing a wide range of surface characterization techniques supported by theoretical calculations, we present a detailed picture of the phosphonates' binding to ZnO, the molecular orientation in the SAM, their packing density, as well as the concomitant work function changes. We show that for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding.