Multi-orbital charge transfer at highly oriented organic/metal interfaces

Giovanni Zamborlini,Matteo Jugovac,Giovanni Comelli,Mirko Panighel,Bernd Kollmann,Vitaliy Feyer,Zhijing Feng,Andrea Goldoni,Carlo Dri,Claus Michael Schneider,Peter Puschnig,Giovanni Di Santo,Daniel Lüftner

doi:10.1038/s41467-017-00402-0

Abstract

The molecule–substrate interaction plays a key role in charge injection organic-based devices. Charge transfer at molecule–metal interfaces strongly affects the overall physical and magnetic properties of the system, and ultimately the device performance. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules adsorbed on Cu(100). The exceptional charge transfer leads to filling of the higher unoccupied orbitals up to LUMO+3. As a consequence of this strong interaction with the substrate, the porphyrin’s macrocycle sits very close to the surface, forcing the phenyl ligands to bend upwards. Due to this adsorption configuration, scanning tunneling microscopy cannot reliably probe the states related to the macrocycle. We demonstrate that photoemission tomography can instead access the Ni-TPP macrocycle electronic states and determine the reordering and filling of the LUMOs upon adsorption, thereby confirming the remarkable charge transfer predicted by density functional theory calculations.

Highlights

The molecule–substrate interaction plays a key role in charge injection organic-based devices
We unequivocally determine the electronic occupancy of the molecular frontier orbitals, i.e., HOMOs and LUMOs, of the adsorbed nickel tetraphenyl porphyrin (Ni-TPP) on the Cu(100) surface by means of photoemission tomography (PT)
By comparing scanning tunneling microscopy (STM) and density functional theory (DFT)-simulated images, we demonstrate that the topography contrast arises mainly from the electronic states of the porphyrin phenyl rings, which are strongly tilted upwards

Summary

Introduction

The molecule–substrate interaction plays a key role in charge injection organic-based devices. The molecule–substrate and the molecule–molecule interactions often result in charge transfer between the substrate and the lowest unoccupied and highest occupied molecular orbitals (LUMO and HOMO, respectively)[8, 9] and possibly, in case of magnetic substrates, introduce spin degrees of freedom[10, 11] via the formation of spin-polarized hybrid interface states[12, 13] From this perspective, detailed information about the changes in the electronic structure of molecules upon adsorption at the interface is crucial to design and prototype new devices based on organic compounds[14]. This approach, applied to similar systems of π-conjugated molecules, may help to unravel the electronic structure of the molecular/metal interface, even in the presence of strong hybridization, improving the organic-based device-engineering

Methods

Results

Conclusion