Electronic and structural coupling of pentacene on NiO(001)

Transition-metal oxides (TMOs) are pivotal in modern applications, with recent years seeing intensified research into their interplay with molecular layers as well as the potential of antiferromagnetic TMOs in spintronic applications. In this work we combine both approaches and investigate the adsorption of pentacene on the (001) surface of NiO. By employing a variety of methods such as scanning tunneling microscopy, low energy electron diffraction, angle-resolved photoelectron spectroscopy and density functional theory, we extract the geometrical arrangement of the molecules and their energy level-alignment. Induced by the substrate-molecule interaction, pentacene forms a self-assembled monolayer in a superstructure commensurate with the NiO substrate. Through photoemission orbital tomography, we identify the first three highest occupied molecular orbitals (HOMO, HOMO-1 and HOMO-2) in the photoemission spectra of the NiO/pentacene interface. The absence of the lowest unoccupied molecular orbital (LUMO) suggests negligible charge transfer at the interface, a finding supported by calculations. Nevertheless, we can observe an induced degeneracy of the HOMO-1 and HOMO-2 orbitals as well as an accumulation of molecular electron density toward the substrate. This preservation of the molecules free electron character of frontier orbitals points to potential applications in the optical control of THz spin dynamics in antiferromagnetic NiO, opening a promising pathway for engineering molecule-based functionalization of antiferromagnetic surfaces.