# Low Kinetic Energy Oxygen Ion Irradiation of Vertically Aligned Carbon Nanotubes
## 3 Results and Discussion
The electronegative oxygen atoms induce a positive charge in the carbon atoms thus changing the electron screening of their nucleus. This increased consequently the binding energy of the electrons, explaining the shift to higher binding energies for the components in the C1s spectra. Using spin polarized density functional calculations within the local density approximation (LDA) for the interpretation of the C1s XPS peak of oxygen functionalized carbon nanotubes, [PERSON] et al. [12], assigned the component at 286.6 eV to epoxide groups (C-O-C), the component 287.5 eV to carbonyl groups (C=O) and 288.8 eV to carboxyl groups (-COOH). In order to understand the effect of varying the ion irradiation parameters in the chemistry of the v-CNTs, we analyzed the C1s XPS spectrum of each sample (Table 2, Table S1, Figures S1-S7). To corroborate our results, the analysis of O1s XPS spectra was also performed (Table S2, Figures S8-S17). The same number of C-O components were obtained in C1s and O1s analysis. Figure 5 shows the variation in the relative area of each component used to reproduce the C1s peak as a function of the treatment time and the ion kinetic energy. The relative contribution of component C1 (graphitic carbon (\(sp^{2}\)-C)) decreased from the 96.3% in pristine v-CNTs to 62.7% when the treatment time reached 10 min, indicating that the oxygen functionalization occurred through breakdown of \(\pi\) bonding in the \(sp^{2}\)-C bond. The relative area of component C2 (\(sp^{3}\)-C) increased to 20% after 10 min of treatment (Figure 5a). \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Irradiation** & **Ion Kinetic** & ** \begin{tabular}{c} **\(sp^{2}\)-C** \\ **(\%A) C1** \\ \end{tabular}  & ** \begin{tabular}{c} **\(sp^{3}\)-C** \\ **(\%A) C1** \\ \end{tabular}  & ** \begin{tabular}{c} **C-O-C** \\ **(\%A) C3** \\ \end{tabular}  & ** \begin{tabular}{c} **C-O-C** \\ **(\%A) C4** \\ \end{tabular} ** & **\ \begin{tabular}{c} **C=O (\%A)** \\ **(\%A) C6** \\ \end{tabular}  & ** \begin{tabular}{c} **Plasmon** \\ **(\%A) C2** \\ \end{tabular}  \\
5 & 0.1 & 92.9 & 5.5 & 1.3 & 0 & 0.3 & 0 \\
5 & 0.25 & 84.2 & 9.8 & 2.6 & 0 & 1.9 & 1.5 \\
5 & 0.5 & 71.2 & 15.7 & 3.3 & 2 & 5.7 & 2.1 \\
5 & 1.5 & 69.7 & 19.0 & 4.2 & 2.4 & 3.7 & 1.1 \\
5 & 2.2 & 72.0 & 15.5 & 3.9 & 4.2 & 2.7 & 1.7 \\ \hline \When the treatment time was fixed to 5 min and the ion kinetic energy was the variant parameter, we observed that the ion energy did not significantly affect the relative area of the graphitic carbon component (C1), remaining in a range from 69.7% to 72.0% (for the range of energies from 0.5 to 2 keV). Similarly, the area of the component C2 (\(sp^{3}\)-C) remained in a range from 15.7% to 19.0%. These results suggest that for the samples irradiated with ion kinetic energy varying from 0.5 to 2 keV, approximately the same number of ions interact with the carbon nanotubes. At 0 keV kinetic energy (a remote plasma treatment [32]), only 8.8% of the \(sp^{2}\) carbon (C1) was transformed into \(sp^{3}\) (C3) and 5.5% into oxygen-bound carbon species (C4), confirming that the remote plasma treatment generated less defects at the CNT surface than ion irradiation (Figure 5b), with much lower oxygen grafting at the CNT surface (Figure 3b). It is interesting to compare the grafting of oxygen atoms for 0 keV (remote plasma functionalization) with the ion irradiation results (Figures 6 and 7b). In the case of remote plasma functionalization, the only C-O bonding observed comes from epoxide groups. These consist of single oxygen atoms bonded covalently to a \(sp^{2}\)-C surface, forming covalent bonds with two carbon neighbors with the oxygen atom sitting above the C-C bond centre. This is the least disruptive oxygen functional group (indeed epoxide groups can be added to graphene simple trough ozone exposure with UV light [33]), since they do not require carbon dangling bonds, and hence associated hole or edge formation.