Structural modification and band gap engineering of carbon nano-onions via sulphur doping: Theoretical DFT study

Density functional theory (DFT) calculations were performed to investigate the structures and electronic properties of S-doped carbon nano-onions (S-CNOs). We report a detailed study of the stoichiometric and defective (monovacancy (MV) and divacancy (DV)) CNOs with different doping contents and distribution patterns. The results indicate the natural tendency of sulphur to occur in groups around the edges of the local deformations and at the defects that are in very good qualitative agreement with the experimental data. [22] More importantly, the five-membered rings containing disulphide bridges are the most stable binding motif in the structure of CNOs. However, the S-S motif can also be located in the six-membered ring either with both two-coordinated S atoms or with one-and three-fold coordinated S atoms. For defective systems, we also studied the energetic stability of the MV-and DV-containing S-CNOs. The results reveal that the most stable position of sulphur is the two-fold coordinated carbon site in the first case, whereas in the second case, it is the one linking octagonal and pentagonal rings. Furthermore, the electronic analysis indicates that the position and concentration of dopant and the type of defect unambiguously influence the band gap energy in both stoichiometric and defective CNOs.