title: Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers.
abstract: We used aromatic phosphonates with substituted phenyl rings with different molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface in order to engineer the energy-level alignment at hybrid inorganic/organic semiconductor interfaces, with an oligophenylene as organic component. The work function of ZnO was tuned over a wide range of more than 1.7 eV by different SAMs. The difference in the morphology and polarity of the SAM-modified ZnO surfaces led to different oligophenylene orientation, which resulted in an orientation-dependent ionization energy that varied by 0.7 eV. The interplay of SAM-induced work function modification and oligophenylene orientation changes allowed tuning of the offsets between the molecular frontier energy levels and the semiconductor band edges over a wide range. Our results demonstrate the versatile use of appropriate SAMs to tune the energy levels of ZnO-based hybrid semiconductor heterojunctions, which is important to optimize its function, e.g., targeting either interfacial energy- or charge-transfer.
authors.forename: Melanie
authors.initials: M
authors.lastname: Timpel
authors.forename: Marco V
authors.initials: MV
authors.lastname: Nardi
authors.forename: Giovanni
authors.initials: G
authors.lastname: Ligorio
authors.forename: Berthold
authors.initials: B
authors.lastname: Wegner
authors.affiliation: ∥Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Pätzel
authors.affiliation: ∥Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Kobin
authors.affiliation: ∥Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Hecht
authors.affiliation: ‡Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Strasse 16, 12489 Berlin, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Koch
keywords: ZnO; energy-level tuning; layered hybrid systems; phosphonic acid; photoelectron spectroscopy; self-assembled monolayer