Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic-Metal Interfaces.

Thomas Georg Boné,Florian Lackner,Marie S Sättele,Georg Koller,Holger F Bettinger,Thomas Jauk,Thomas Chassé,Katharina Greulich,Peter Puschnig,Michael G Ramsey,Larissa Egger,Heiko Peisert,Martin Sterrer,Andreas Windischbacher

doi:10.1021/acs.jpcc.1c01306

Abstract

Charge-transfer processes at molecule–metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal’s work function and the adsorbate’s electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate’s orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic–metal interfaces through the adjustable molecular alignment.

Highlights

When organic molecules adsorb on a metal surface, a number of processes such as molecular conformational changes,[1,2] substrate reconstructions,[3,4] or chemical reaction pathways[5] are related to the realignment of the electronic levels at the interface
We demonstrate that monolayers of heptacene on Cu(110) behave markedly different compared to all previously studied oligoacene monolayers on coinage metal surfaces, and, to the best of our knowledge, are dissimilar to any other polycyclic aromatic hydrocarbons adsorbed on metal substrates studied so far.[16,26,46,47]
These findings are fully confirmed by our density functional theory (DFT) calculations and demonstrate that the charge transfer and level alignment at organic−metal interfaces depend on intrinsic properties of the adsorbate molecule and substrate but that the adsorption geometry, which could be tuned by suitable growth conditions, may play a crucial role

Summary

■ INTRODUCTION

When organic molecules adsorb on a metal surface, a number of processes such as molecular conformational changes,[1,2] substrate reconstructions,[3,4] or chemical reaction pathways[5] are related to the realignment of the electronic levels at the interface. For heptacene still face-on but rotated by 90°, significantly less charge is transferred to the molecule, resulting in only the LUMO being filled and the molecular energy levels being shifted significantly toward the Fermi edge These findings are fully confirmed by our DFT calculations and demonstrate that the charge transfer and level alignment at organic−metal interfaces depend on intrinsic properties of the adsorbate molecule and substrate but that the adsorption geometry, which could be tuned by suitable growth conditions, may play a crucial role. Photoemission[57] approximating the final state as a plane wave,[43] modified by an exponential damping factor introduced between the substrate and the organic molecule to mimic the mean free path of the photoemitted electrons.[56]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES