title: Interplay between Energy-Level Position and Charging Effect of Manganese Phthalocyanines on an Atomically Thin Insulator.
abstract: Understanding the energy-level alignment and charge transfer of organic molecules at large bandgap semiconductors is of crucial importance to optimize device performance in organic electronics. We have studied submonolayer coverage of manganese phthalocyanine (MnPc) on hexagonal boron nitride (h-BN) on Rh(111) as a model system by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The adsorbed molecules show three distinctly different bias-dependent topographic signatures, which depend on their adsorption positions on the h-BN. Among these three types of MnPc, one shows pronounced charging because of the proximity of the highest occupied molecular orbital (HOMO) to the Fermi level on the decoupling h-BN substrate. The charging of the MnPc from its neutral to the MnPc(+) state leads to a down shift of the Mn 3d-related orbital by 840 meV as determined from the difference in energy position between high- and low-bias charging. We find that the charging field is linearly related to the HOMO position with respect to the Fermi level, with a clear correlation to the adsorption orientations of the MnPc. Our results show how critically energy level alignment and field-induced charge transfer process can depend on adsorption configurations, even on an apparently low-interacting substrate like metal supported monolayer h-BN.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: Liwei
authors.initials: L
authors.lastname: Liu
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Dienel
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: Roland
authors.initials: R
authors.lastname: Widmer
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: Oliver
authors.initials: O
authors.lastname: Gröning
keywords: charging effect; hexagonal boron nitride; molecular adsorption; molecular orbital; scanning tunneling microscopy; scanning tunneling spectroscopy