Charge Transfer at Organic/Metal Interfaces

Organic-based device performances have been rapidly improving in the last years, making them suitable for large-scale industrial applications, involving photo-voltaic cells, light emission systems and building of larger flexible electronics. In parallel, basic research has intensively focused on the chemical and physical properties of semiconducting π-conjugated organic molecules, as they appear to be promising for organic-based device construction. In particular, in controlling the charge injection on such devices, a predominant role is played by the molecule-substrate interaction. Charge transfer at the molecule-metal interface strongly affects the overall physical and magnetic properties of the system, and ultimately, the device performance.

On the perspective of possible technological applications, such as colorimetric gas sensors, organic spin-valves, field-effect transistors, etc., porphyrin represent a class of extremely versatile molecules, allowing for tailoring a variety of electronic, magnetic and conformational properties. In particular, supramolecular multi-porphyrin arrays are considered as functional components in nanodevices. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules (Ni-TPP) adsorbed on Cu(100).

NiTPP molecules tend to form well-ordered islands on the Cu(100) surface already at low coverages, in particular STM measurements reveal the presence of two rotational domains mirrored with respect to the [001] direction of Cu(100). These two domains are also commensurate with the substrate (see Fig. 1a). The molecule-substrate interaction strongly affects the molecular adsorption geometry and electronic structure. Indeed, hybrid functional density functional theoretical (DFT) calculations suggest a significant charge transfer from Cu to the molecule, resulting in the occupation of the gas-phase LUMO/LUMO + 1 and LUMO + 3 molecular orbitals, accompanied by a back donation of charge from the molecule to the substrate. As a consequence of this strong interaction with the substrate, the porphyrin's macrocycle approaches the surface very closely (∽2 Å), forcing the phenyl ligands to bend upwards (Fig. 1b). Therefore, the STM contrast arises mainly from the electronic states of the phenyl rings preventing the STM tip to reliably probe the states related to the macrocycle.

This limitation can be overcome by photoemission tomography (PT) which combines momentum microscopy with DFT calculations. This approach gives a direct access to the molecular orbitals by looking at their signature in the angular distribution of the photoemitted electrons from the molecular film.

PT provides a relatively simple interpretation of momentum maps, since the photoemission intensity becomes proportional to the modulus squared of the Fourier transform calculated from the real space molecular orbital. On this respect, the PhotoEmission Electron Microscope (PEEM) installed at the NanoESCA beamline at Elettra synchrotron is an ideal set-up to measure momentum maps within a single image acquisition.

Using PT we have shown that the remarkable charge transfer takes place at NiTPP/Cu(100) interface and it leads to filling of the higher unoccupied orbitals up to LUMO+3, thereby confirming the DFT predictions. Our results emphasize the importance of complementary STM and PT measurements for characterizing complex organic systems. A multi-technique approach, including electronic structure calculations, permits to develop a consistent picture of the adsorption behavior and electronic properties of interfaces between non-planar molecules and metallic surfaces.

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, C.M. Schneider, “Multi-orbital charge transfer at highly oriented organic/metal interfaces”, Nature Communications 8 (2017) 335, https://doi.org/10.1038/s41467-017-00402-0