title: Core level binding energies of functionalized and defective graphene.
abstract: X-ray photoelectron spectroscopy (XPS) is a widely used tool for studying the chemical composition of materials and it is a standard technique in surface science and technology. XPS is particularly useful for characterizing nanostructures such as carbon nanomaterials due to their reduced dimensionality. In order to assign the measured binding energies to specific bonding environments, reference energy values need to be known. Experimental measurements of the core level signals of the elements present in novel materials such as graphene have often been compared to values measured for molecules, or calculated for finite clusters. Here we have calculated core level binding energies for variously functionalized or defected graphene by delta Kohn-Sham total energy differences in the real-space grid-based projector-augmented wave density functional theory code (GPAW). To accurately model extended systems, we applied periodic boundary conditions in large unit cells to avoid computational artifacts. In select cases, we compared the results to all-electron calculations using an ab initio molecular simulations (FHI-aims) code. We calculated the carbon and oxygen 1s core level binding energies for oxygen and hydrogen functionalities such as graphane-like hydrogenation, and epoxide, hydroxide and carboxylic functional groups. In all cases, we considered binding energy contributions arising from carbon atoms up to the third nearest neighbor from the functional group, and plotted C 1s line shapes by using experimentally realistic broadenings. Furthermore, we simulated the simplest atomic defects, namely single and double vacancies and the Stone-Thrower-Wales defect. Finally, we studied modifications of a reactive single vacancy with O and H functionalities, and compared the calculated values to data found in the literature.
authors.affiliation: Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland ; University of Vienna, Faculty of Physics, Strudlhofgasse 4, A-1090 Vienna, Austria).
authors.forename: Toma
authors.initials: T
authors.lastname: Susi
authors.affiliation: Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland ; Present address: Helsingin matematiikkalukio, Kuusikkotie 3, 00630 Helsinki, Finland.
authors.forename: Markus
authors.initials: M
authors.lastname: Kaukonen
authors.affiliation: Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland ; Present address: Suomen ympäristökeskus, Mechelininkatu 34a, 00260 Helsinki, Finland.
authors.forename: Paula
authors.initials: P
authors.lastname: Havu
authors.affiliation: LOMA, Université Bordeaux 1, 351 Cours de la Libération, 33405 Talence, France.
authors.forename: Mathias P
authors.initials: MP
authors.lastname: Ljungberg
authors.affiliation: University of Vienna, Faculty of Physics, Strudlhofgasse 4, A-1090 Vienna, Austria).
authors.forename: Paola
authors.initials: P
authors.lastname: Ayala
authors.affiliation: Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland.
authors.forename: Esko I
authors.initials: EI
authors.lastname: Kauppinen
keywords: X-ray photoelectron spectroscopy; core level; defects; density functional theory; graphene