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

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.

Hexacene on Cu(110) and Ag(110): Influence of the Substrate on Molecular Orientation and Interfacial Charge Transfer

“…POT has already been applied successfully to explain several acene/coinage metal systems. 3 , 13 , 24 , 59 …”

“…With UPS, we also obtain the work functions of both interfaces in focus. 13 , 60 Molecules close to a metal interface tend to have a reduced HOMO–LUMO gap by the polarization of the metal. 61 − 64 Consequently the LUMO level moves closer to the Fermi level E F .…”

“… 10 Despite this, the formation of comparably stable, ordered film structures was recently reported for hexacene ( 6A ) and heptacene ( 7A ). 3 , 8 , 11 − 13 Additionally, in particular on-surface synthesis opened an entrance to larger acenes up to dodecacene. 14 − 20 …”

“… 21 − 24 Furthermore, for heptacene ( 7A ) on Cu(110), it was demonstrated that even the adsorption geometry affects the interfacial electronic structure. 13 …”

“…Recently, it was demonstrated for heptacene monolayers on Cu(110) that the orientation of the adsorbate may be even crucial for the charge transfer processes. 13 …”