# Low Kinetic Energy Oxygen Ion Irradiation of Vertically Aligned Carbon Nanotubes
## 3 Results and Discussion
This therefore confirmed that the remote plasma was only adding very low energy isolated oxygen atoms to the nanotube surface (probably primarily neutrals) with minimal damage and disruption of the carbon layers. Epoxide oxygen could also be removed through relatively gentle reducing processes. In contrast, the ion-implantation results show high concentrations of carbonyl and carboxyl groups, demonstrating that the lattice was being disrupted and carbon dangling bonds created, which were being oxygenated. This was consistent with a picture where incoming charged oxygen ions were arriving with kinetic energies far above the knock-on threshold energies of the lattice carbon, resulting in carbon displacements and increased local damage. This was also visible in the production of \(sp^{3}\)-C carbon atoms as collateral damage. Thus the two processes were complementary, allowing choice of oxygen chemical functional group addition depending on end-application requirements. Figure 5: Variation in relative area of the components of carbon \(sp^{2}\)-C (C1) and carbon \(sp^{3}\)-C (C3) used to reproduce the C1s XPS spectrum of v-CNTs as a function of the treatment time (ion kinetic energy 1 keV; (a)) and the ion kinetic energy (fixed treatment time: 5 min; (b)). We could further understand the oxidation processes occurring during ion irradiation by exploring the change in relative concentration of different C-O species over time (Figure 7a). Just as for the plasma treatment, epoxide species start forming on the basal plane from the beginning of the treatment, and gradually increase in concentration over the time. In parallel there was an extremely initial rapid formation of carbonyl groups. This presumably involved functionalization of tube tips, which are much more chemically reactive than the tube sidewalls and represent preferential addition sites [34]. During  
Figure 6: XPS analysis of v-CNTs functionalized by plasma treatment (0 keV of ion kinetic energy) (**a**) and by ion irradiation with 2 keV of ion kinetic energy (**b**). The treatment time was 5 min for the two samples. 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 epoxide 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 7: Variation in relative area of the components of carbon bonded to oxygen (C4 (C–O–C), C5 (C=O) and C6 (–COOH)) used to reproduce the C1s XPS spectrum of v-CNTs as a function of the treatment time (ion energy 1 keV; (**a**)) and the ion kinetic energy (treatment time 5 min; (**b**)). this period there was a gradual increase in _sp\({}^{3}\)_-C indicating the ion irradiation process is beginning to damage the nanotube carbon network (Figure 5a). After 5 min the network was sufficiently damaged that holes and internal edges are formed. These were C=O and -COOH edge-terminated, giving rise to a corresponding increase in the C=O and -COOH C1s peaks in the XPS. The carbonyl groups were intermediate states and could oxygen saturate, transforming to carboxyl groups, helping to explain the relative stability in carbonyl concentration and large increase in -COOH in the C1s peak from 5 to 10 min. This process would be kinetically dependent on the rate of damage site and hole formation (i.e., new dangling bond creation rate), and was also reflected in the ion energy study (Figure 7b). As the ion kinetic energy increased there was a steady decrease in epoxide concentration, primarily because at higher energies there is an increased chance of damaging the lattice and so inserting oxygen as other (carbonyl or carboxyl) forms. The slight decrease in the _sp\({}^{3}\)_-C component observed when the sample was irradiated with 0.1 keV ions could be associated to a preferential removal of physically adsorbed species (amorphous carbon and oxygen species including water) occurring during the synthesis and due to the exposure to ambient air.