C1s core-level binding energy shift dependence from carbon atoms position in graphenenanoflakes C96 and polycyclic aromatic hydrocarbon C96H24: a dft study

The hexagon-shape graphene nanoflakes (GNFs) limited by zigzag edges only (with doubly and triply coordinated atoms) have unique increased reactivity. Despite the high systems symmetry (D6h) the Carbon atoms in GNFs occupy non-equivalent positions. Can such physical and chemical characteristics of GNFs, which depend of the atom position in the cluster, definition? This characteristic together with the simplicity of its calculation makes it possible to predict the properties of nanoflakes obtained from GNFs by introducing single and multiatomic vacancies into them or by replacing Carbon atoms with electron withdrawing and electron donating atoms. This characteristic includes the C1s core-level binding energy shifts, the maxima of which characterize the C atoms of a certain type. The proposed work is devoted to quantum chemical calculations of the electronic density of states (DOS) of pristine hexagon-shape GNF C96 (multiplicity, M=5), their saturated counterpart –polycyclic aromatic hydrocarbon(PAH) C96H24 (M=1) and their derivatives with one and two single vacancies in the ground electronic state (GES). All calculations were performed using the density functional theory (DFT) method with the involvement of the valence-split basis set 6-31G (d,p). Systems with open shells were considered using the UB3LYP exchange-correlation functional. The obtained spectra were fitted using Gaussian curve fitting program to determine the binding energy for each peak. The Gaussian function distribution of the theoretically calculated C1s core-level binding energy shifts of GNFs testified the presence of six peaks, each of which refers to a certain type of Carbon atoms. The C1s peak with the highest binding energy (-285.57 eV) is caused by contributions from the doubly coordinated edge cyclic chain (ECC) Carbon atoms. The C1s orbitals of the central hexagon (CHex) atoms and the first cyclic chain (FCC) atoms form delocalized molecular orbitals (MOs) in different parts of the cluster. The analogous spectrum of PAH C96H24 is slightly shifted to the region of lower binding energies and contains only two well-defined peaks. The peak with a higher binding energy (-284.36 eV) is generated by the 1s states of the CHex atoms and the atoms of the FCC, which are bounded to the CHex atoms. The electronic DOS difference in C1s core-level spectra of GNF C96 (M=5) and their saturated counterpart PAH C96H24 is established due to the presence of two weakly bounded π-systems in GNF and common conjugated system in PAH. The electronic DOS of defect-containing cluster C96-1(1) (M=3) (one CHex atom has been removed from the C96nanoflake) is generated by the C1s core-level atoms of the second cyclic chain (SCC), which are located at the different distances from the center of the nanoflake. The peak of the lowest intensity (-284.63 eV) appears in the spectrum as a reflection of the appearance of doubly coordinated Carbon atoms surrounding the single vacancy in the C96-1(1) nanoflake. The analysis of the electronic DOS of the C1s core-level spectrum of the C96-2(1) nanoflakeis shown, that doubly coordinated Carbon atoms, concentrated around two single vacancies, are essentially non-equivalent. If the MO with the lowest binding energy is localized on two of them – the MO with the highest binding energy is localized on the third atoms (one around each single vacancy). The electronic C1s core-level DOS spectrum of defect-containing molecular systems with one C96-1(1)H24 and two C96‑2(1)H24 single vacancies are similar to the analogous spectrum of PAH C96H24. In the first of them – one additional maximum appears due to C1s atoms surrounding the single vacancy. In the second – there are two additional maxima, each of which is generated by C1s core-level atoms adjacent to individual vacancies.