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

Jülich/Graz/Trieste, 25 August 2017. The interaction between organic molecules and metallic substrates plays a key role in electronic devices based on organic compounds. Additionally, the charge transfer at molecule-metal interfaces strongly affects the overall physical and magnetic properties of the system, and ultimately the device performance. Scientists from Forschungszentrum Jülich, the Universities of Graz, Austria, and Trieste, Italy, and the synchrotron facility Elettra in Trieste, using state-of-the art theoretical and experimental techniques, succeeded in developing a consistent picture of the adsorption behaviour of nickel tetraphenylporphyrin (Ni-TPP) on copper (Cu(100)) and clarifying the electronic structure of this organic/metal interface. Combining multiple surface techniques, such as scanning tunnelling microscopy (STM) and photoemission tomography, with ab-initio density functional theory (DFT) calculations, they described the interface in detail and observed an unexpected charge transfer involving the organic molecule and the metal substrate. (Nature Communications, DOI: 10.1038/s41467-017-00402-0).

Multi-orbital charge transferProposed adsorption model for Ni-TPP/Cu(100)
Copyright: University of Graz

Molecule-substrate and molecule-molecule interactions often result in charge transfer between the substrate and the lowest unoccupied and highest occupied molecular orbitals. Additionally, in the case of magnetic substrates, the introduction of spin degrees of freedom via the formation of spin polarized hybrid interface states has to be taken into account. “From this perspective, detailed information about changes in the electronic structure of molecules upon adsorption at the interface is crucial to the design and prototype of new devices based on organic compounds”, explains Prof. Claus Schneider from Jülich’s Peter Grünberg Institute (PGI-6). “The main obstacle in this respect is based on the difficulty in predicting, modelling and measuring the electronic and structural properties of the interfaces”, adds Prof. Giovanni Comelli from the University of Trieste.

Porphyrins are extremely versatile molecules, which permit the tailoring of a variety of electronic, magnetic and conformational properties. The porphyrin/metal system is used in several promising organic-based technological applications such as colorimetric gas sensors, organic spin-valves, field-effect transistors, light emitting diodes, optical switches, non-volatile data storage systems and solar cells. In particular, supramolecular multi-porphyrin arrays are being considered as functional components in nanodevices.

Intense experimental and theoretical efforts have targeted the understanding of the behaviour of porphyrin assemblies on metal surfaces. ‘’However, the particular charge transfer behaviour in porphyrin/metal systems is far from being understood” says Dr. Daniel Lüftner from the University of Graz. “DFT calculations performed in our group in Graz show that the exceptional charge transfer surprisingly leads to the filling of the higher unoccupied orbitals, up to LUMO+3 in a Ni-TPP/Cu(100) system” he adds.

“As a consequence, the four phenyl side-groups of Ni-TPP tilt and twist upwards, leading to the four main protrusions visible in STM images”, explains Zhijing Feng from the University of Trieste. “Looking more closely at the STM image of the self-assembled Ni-TPP, the four lobes of the phenyl rings appear asymmetric, revealing a chiral character”, says Dr. Carlo Dri from the University of Trieste. The chirality arises from the twisting of the phenyls as predicted by DFT and confirms that the phenyls are no longer perpendicular to the surface, as they are in the isolated molecule.

Due to this adsorption configuration, scanning tunnelling microscopy cannot reliably probe the states related to the macrocycle. “The photoemission tomography can instead access the Ni-TPP macrocycle electronic states and determine the reordering and filling of the LUMOs upon adsorption”, explains Prof. Peter Puschnig from the University of Graz. “This method implemented in the PEEM setup at the NanoESCA beamline at Elettra shows extremely high efficiency in the study of the adsorption behaviour of molecules on the metal substrates”, adds Dr. Vitaliy Feyer from Jülich’s Peter Grünberg Institute (PGI-6). “This approach allows us to measure the valence band spectra and, at the same time, record the images of the Fourier transform of molecular states. This cannot be done in such a straightforward way by adopting other spectroscopy techniques”, says his colleague Giovanni Zamborlini, who has just started his PhD work at the institute.

Original publication:

G. Zamborlini, D. Lüftner, Zh. Feng, B. Kollmann, P. Puschnig, C. Dri, M. Panighel G. Di Santo, A. Goldoni, G. Comelli, M. Jugovac, V. Feyer, and C. M. Schneider;
Multi-orbital charge transfer at highly oriented organic/metal interfaces;
Nature Communications (2017), DOI: 10.1038/s41467-017-00402-0.

Further information:

Press release from the University of Graz of 25 August 2017 (in German): “Molekulare Alleskönner”

Notification from Forschungszentrum Jülich of 25 August 2017 (in German): "Porphyrine – elektronische Eigenschaften der molekularen Alleskönner entschlüsselt”

Contacts:

Dr. Vitaliy Feyer
Forschungszentrum Jülich, Peter Grünberg Institute (PGI-6)
Phone: +39 040375-8302/8738
Email: v.feyer@fz-juelich.de

Assoz.-Prof. Dr. Peter Puschnig
University of Graz, Department of Physics, Austria
Phone: +43 316 380 -5230
Email: peter.puschnig@uni-graz.at

Prof. Dr. Claus M. Schneider
Forschungszentrum Jülich, Peter Grünberg Institute (PGI-6)
Phone: +49 2461 61-4428
Email: c.m.schneider@fz-juelich.de

Press contact:

Tobias Schlößer
Forschungszentrum Jülich, Corporate Communications
Phone: +49 2461 61 -4771
Email: t.schloesser@fz-juelich.de



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