Anchor Group versus Conjugation: Toward the Gap-State Engineering of Functionalized ZnO(10(1)over-bar0) Surface for Optoelectronic Applications

Molecular sensitization of the single-crystal ZnO (10 (1) over bar0) surface through absorption of the catechol chromophore is investigated by means of density functional approaches. The resulting type II staggered interface is recovered in agreement with experiments, and its origin is traced back to the presence of molecular-related states in the gap of metal oxide electronic structure. A systematic analysis carried out for further catecholate adsorbates allows us to identify the basic mechanisms that dictate the energy position of the gap states. The peculiar level alignment is demonstrated to be originated from the simultaneous interplay among the specific anchoring group, the backbone conjugation, and the lateral functional groups. The picture derived from our results provides efficient strategies for tuning the lineup between molecular and oxide states in hybrid interfaces with potential impact for ZnO-based optoelectronic applications.