π-Orbital mediated charge transfer channels in a monolayer Gr-NiPc heterointerface unveiled by soft X-ray electron spectroscopies and DFT calculations

• Published in: Nanoscale 2022-09, Vol.14 (36), p.13166-13177
• Main Authors: Casotto, Andrea, Drera, Giovanni, Perilli, Daniele, Freddi, Sonia, Pagliara, Stefania, Zanotti, Michele, Schio, Luca, Verdini, Alberto, Floreano, Luca, Di Valentin, Cristiana, Sangaletti, Luigi
• Format: Article
• Language: English
• Subjects: Channels; Density functional theory; Fine structure; Graphene; Mathematical analysis; Molecular orbitals; Monolayers; Optoelectronics; Photoelectric emission; Photoelectrons; Silicon substrates; Soft x rays; Spectrum analysis; Valence band; X ray absorption; X ray photoelectron spectroscopy
• ISSN: 2040-3364; 2040-3372
• DOI: 10.1039/d2nr02647c

With the aim to identify charge transfer channels underlying device development and operation, X-Ray Photoelectron Spectroscopy (XPS), Near-Edge X-Ray Absorption Fine Structure (NEXAFS), and Resonant Photoelectron Spectroscopy (ResPES) have been employed to characterize a novel heterointerface obtained by the controlled evaporation of a Nickel Phthalocyanine (NiPc) monolayer on a single layer of Graphene (Gr) on SiC substrate. Indeed, the Gr-NiPc interface could be a promising candidate for different applications in the field of photonics, optoelectronics, and sensing, provided that clear information on the charge transfer mechanisms at the Gr-NiPc interface can be obtained. The analysis of the spectroscopic data has shown the effective functionalization and the horizontally-flat disposition of the NiPc complexes over the Gr layer. With this geometry, the main intermolecular interaction experienced by the NiPc species is the coupling with the Gr substrate, through π-symmetry orbitals, as revealed by the different behaviour of the valence band photoemission at resonance with the N K-edge and Ni L 3 -edge. These results have been supported by the analysis of density functional theory (DFT) calculations, that allowed for a rationalization of the experimental data, showing that charge transfer at the interface occurs from the doubly degenerate e g LUMO orbital, involving mainly N and C (pyrrole ring) p z states, to the holes in the p-doped graphene layer. The charge transfer process at a graphene-Ni phthalocyanine interface is investigated by soft X-ray electron spectroscopies and DFT calculations, thus providing fundamental information to develop electronic devices based on this heterojunction.