Fingerprints of sp1-carbon hybridization in the core-level spectra of surface-grown materials

The advent of on-surface chemistry and molecular self-assembly opened the way to the realization of organic materials whose lack of stability in vacuo would otherwise forbid their synthesis. A famous example is the formation of graphene nanoribbons with tunable width by polimerization of properly designed molecular precursors. Recently the on-surface coupling of the precursors have also been used to synthesize adsorbed organic systems with chemically vulnerable linear carbon chains with sp1 hybridization. [1] Another advantage of on-surface synthesis is that the full realm of the experimental and theoretical surface science toolbox is available for the investigation. While the method of choice for surface-supported architectures is the scanning tunneling microscope and ultimate resolution is achieved by low-T non-contact atomic force microscopy, electron core-level spectroscopy can provide an even more local and sometimes insightful probe of the electronic properties of the material, as we show here. We present ab initio investigations of the core-level spectral properties of a paradigmatic mixed sp1/sp2 C polymer, recently synthesized on Au(111) in linear/2D forms, and constituted by alternated biphenyl and linear C4 units, to discuss the appearance of the sp1 hybridization typical of linear C chains in the spectra. X-ray photoemission (XPS) and polarized near-edge X-ray absorption fine structure (NEXAFS) spectra are evaluated for the polymer in vacuo and on the metal surface, in the framework of density functional theory simulations with core-excited C potentials that have successfully reproduced spectral dichroisms [2]. We find that, at variance with the XPS spectra that do not easily show the signature of sp1 hybridization, NEXAFS spectra can distinctly fingerprint the sp1 part of the polymer through polarized spectra. Such capability is further facilitated by the different degree of hybridization of the molecular orbitals at the interface for the sp1 and sp2 states. [1] Q. Sun, R. Zhang, J. Qiu, R. Liu, and W. Xu, Adv. Mater. 2018, 30, 1705630 [2] G. Fratesi, V. Lanzilotto, L. Floreano, and G. P. Brivio, J. Phys. Chem. C 2013, 117, 6632