# Low Kinetic Energy Oxygen Ion Irradiation of Vertically Aligned Carbon Nanotubes
## 3 Results and Discussion  
The macroscopic morphology and alignment of v-CNTs were evaluated with SEM microscopy before and after the oxygen ion irradiation in order to estimate if disorder was induced by ions impacting onto the sample surface. We observed that neither the density nor the alignment of CNTs were affected by the ion irradiation treatment (Figure 1). The understanding of the dependency of the oxygen bonding configurations in v-CNTs on the different parameters used during ion irradiation is a key step for optimal controlling functionalization and, consequently tailoring CNTs properties for applications such as in sensing. X-ray photoelectron spectroscopy (XPS) is a surface technique that, besides providing information on the relative atomic concentration of the elements present on a sample, is also suitable to characterize changes in the bonding configuration of the atoms via the chemical shift, i.e., via the analysis of the shift in the binding energy of the core electrons of the different atoms composing a sample. We first recorded XPS survey spectra to evaluate if the oxygen ion irradiation had grafted undesired elements other than oxygen at the CNT surface. Figure 2 shows typical survey spectra recorded on v-CNTs before and after 5 min of irradiation with oxygen ions with 1 keV of kinetic energy. We could observe in the spectrum recorded before the ion irradiation a high intensity peak centered at 284.4 eV, generated by photoelectrons emitted from the C1s electronic level, and a very low intensity peak at 532.6 eV associated to oxygen contamination during the v-CNTs synthesis, evaluated to be 1.7 at. %. After ion irradiation the relative intensity of the peak generated by C1s photoelectrons decreased and the intensity of the peak at 532.6 eV corresponding to O1s photoelectrons increased remarkably. No other elements were found in the samples. Figure 1: Cross section SEM images of v-CNTs before (**a**) and after 5 min of irradiation with oxygen ions with 1 keV of kinetic energy (**b**). Figure 3 summarizes the change in the relative oxygen concentration when v-CNTs were irradiated with oxygen ions with different kinetic energies and irradiation times. Increasing the irradiation time (1, 2.5, 5 and 10 min) the relative concentration of oxygen increased from 1.7 to 20.0 at. % (Figure 3a). For a fixed irradiation time (5 min) we could divide the resulting amount of oxygen ion grafting in two regimes (Figure 3b): from 0.5 to 2 keV, the relative oxygen concentration remained almost constant (15.0 at. %) independent of the ion energy. As the number of oxygen ions arriving at the sample surface was fixed throughout the experiment, we could conclude that the ion kinetic energy in this range did not affect the rate of ions grafted at the CNT surface. The amount of oxygen ions implanted thus depended only on the treatment time. The second regime was observed for oxygen ion irradiation from 0 up to 0.5 keV (Figure 3b), it is clear that when CNTs were irradiated with ions with 0 keV kinetic energy, i.e., the grid supply at the extractor was set to 0 keV, the amount of oxygen grafted was lower. Under these conditions no specific ion kinetic energy was selected, and all types of species generated inside the plasma chamber were able to reach the sample surface, including neutrals species. It is important to mention that the distance between the exit of the plasma cup in the Tectra ion source and the surface of the sample was about 12 cm, thus it was a remote plasma treatment in which the species with short-life time recombine before reaching the sample surface [28]. Under these conditions instead of ion irradiation, functionalization occurred via a plasma treatment. The quantity of oxygen grafted during this remote oxygen plasma treatment was 8.0 at. % about half that produced by ion energy irradiation, with ion kinetic energies from 0.5 to 2 keV (Figure 3b).