Influence of edge defects on Raman spectra of graphene
The effect of the vacancy edge defect (bonds breaking between carbon atoms) of a graphene sheet on the Raman spectrum was studied. The spectra were calculated using the density functional theory (DFT) using the Gaussian-09 program. Modeling of a graphene sheet fragment with a break of one bond between carbon atoms at the edge showed that changes in the Raman spectrum were common for the presence of nitrogen and oxygen atoms. In this case, the strongest D lines appear in functionalized graphene with oxygen atoms, and the Dʹ lines - in nitrogen-doped graphene. This is confirmed by the experimental Raman spectra as well as calculated ones. The strongest 2D line is characteristic of the graphene model without impurities; the presence of other oxygen or nitrogen atoms leads to a decrease in its intensity. Modeling of a graphene sheet fragment with a break of one bond between carbon atoms at the edge showed that changes in the spectrum are common for both cases of graphene nitrogen-doping and functionalization with oxygen atoms. A comparison of the experimental data with the simulation results showed good agreement between the data. Moreover, the vibrations of hydrogen atoms linked by covalent bonds with neighboring carbon atoms moved towards each other in disturbed hexagonal structures located on the edge of the graphene sheet. Such a steric effect led to a decrease in the frequency of the stretching vibration q(CH), which appeared in the region of ~2700 cm-1 and corresponded to the 2D vibration. The results of the vacancy edge defect modeling for the graphene sheet fragment being nitrogen-doped and functionalized by oxygen atoms were compared with experimental data.