# Low Kinetic Energy Oxygen Ion Irradiation of Vertically Aligned Carbon Nanotubes
## 3 Results and Discussion
The increase in the grafting of oxygen for increasing ion kinetic energy could be associated to an increase in the probability of defect creation as discussed by [PERSON] et al. in 2010 [29]. These authors, using atomistic computer simulations based on analytical potential and density-functional theory models, showed that for carbon nanostructures the probability of defect creation during low-mass ion irradiation, had a sharp increase up to 0.5 keV kinetic energy followed by a gradual decrease. This shows that the functionalization efficiency using low kinetic energy ion irradiation was higher than in the remote plasma treatment, since the impacting oxygen species were more energetic increasing the probability of creating defect sites into the CNT lattice [30]. Additionally, the oxygen species shorter lifetimes increased the possibility for recombination into stable gas-phase species unlikely to react with the tubes. Figure 2: XPS survey spectra comparison between v-CNTs before (black line) and after 5 min of irradiation with oxygen ions with 1 keV of kinetic energy (blue line). A detailed analysis of the C1s XPS spectrum gives information on the chemical configuration of the oxygen grafted in the graphitic network. Figure 4 shows a comparison between the C1s spectra recorded on the pristine v-CNTs and after 10 min of oxygen ion irradiation at 1 keV of ion kinetic energy and the result of the curve fitting reproducing each spectrum. Figure 4: XPS analysis of v-CNTs pristine (**a**) and v-CNTs after 10 min of irradiation with oxygen ions with ion kinetic energy of 1 keV (**b**). C1 is assigned to \(sp^{2}\)-C (284.4 eV), C2 to \(\pi\)-plasmon excitations (290.9 eV), C3 to \(sp^{3}\)-C (285.0 eV), C4 to peroxide groups C–O–C (286.6 eV), C5 to carbonyl groups C=O (287.5 eV) and C6 to carboxyl groups –COOH (288.8 eV). Components peaks result from a least-squares fitting procedure. Figure 3: Carbon and oxygen content in v-CNTs before and after oxygen functionalization in function of ion irradiation time (**a**) and ion kinetic energy (for a fixed time: 5 min) (**b**). When the ion irradiation time is zero it corresponds to v-CNTs pristine. For ion energy equal to 0 keV the oxidation occurs through a remote plasma treatment rather than to an ion-implantation process. The pristine C1s spectrum is asymmetric, due to a re-adjustment of the Fermi level caused by electron-hole interaction after the electron scattering. In order to reproduce this asymmetry, the fitting of C1s peak was made with the Doniach-Sunjic function [31]. The peak had a binding energy of 284.4 eV and was reported to be generated by photoelectrons emitted from \(sp^{2}\) carbon atoms [11]. We also observed a low intensity broad peak centered at 290.9 eV, corresponding to photoelectrons that lose kinetic energy to \(\pi\)-plasmon excitations. To reproduce this peak a Gaussian-Lorentzian was used (Figure 4a). The C1s XPS spectra recorded after v-CNTs irradiation with oxygen ions show a drastic change, widening in the range 285-290 eV, due to the bonding of oxygen to carbon atoms (Figure 4b, supplementary). To reproduce the spectrum for the sample irradiated 10 min it was necessary to use six components, the two used in the fitting of pristine v-CNTs and four other symmetric peaks (Gaussian-Lorentzian). These components were centered at binding energies of 285.0 eV, 286.6 eV, 287.5 eV and 288.8 eV. The component at 285.0 eV was assigned to \(sp^{3}\) carbon bonding occurring at defects introduced in the carbon lattice by ion irradiation. This could include interlayer and intertube cross-linking defects, amorphization and carbon bonding to CO\({}_{\mathrm{x}}\) species. Induced in a controllable way, these defects can cause beneficial changes in chemical and electronic properties of the CNTs, promoting their application in different fields [16]. The components at 286.6 eV, 287.5 eV and 288.8 eV were associated to carbon bonded to oxygen.