# Low Kinetic Energy Oxygen Ion Irradiation of Vertically Aligned Carbon Nanotubes
## 2 Materials and Methods  
Vertically aligned multi-walled carbon nanotubes (v-CNTs) were synthesized by thermal catalytic chemical vapor deposition technique using C\({}_{2}\)H\({}_{4}\) as a carbon source [27]. A multilayer system composed of Si/Al/Fe was the catalyst. Si wafers were used as substrates. An Al thin film (30 nm) was deposited on Si wafers and then, after its oxidation, a 1.5 nm Fe layer was deposited and heated to 300 \({}^{\circ}\)C for three hours to form nanoparticles, which are responsible for the CNTs growth. The Al and Fe layers were prepared by magnetron sputtering using a Quorum device. For the v-CNTs growth, the reactor was heated to 750 \({}^{\circ}\)C in atmospheric pressure under He flow (500 sccm). The catalyst was placed inside the reactor and H\({}_{2}\) flow (200 sccm) was introduced into the reactor for 20 min. After that, C\({}_{2}\)H\({}_{4}\) flow (60 sccm) was introduced into the reactor for 24 min. After the growth, H\({}_{2}\) and C\({}_{2}\)H\({}_{4}\) flows were switched off and a flow of He was used to clean the reactor (500 sccm). The outer diameter distribution of the v-CNTs obtained was 10-40 nm. The low kinetic energy oxygen ion irradiation in v-CNTs was performed in a vacuum chamber system (PREVAC-541) using as an ion source a Tectra plasma source TPIS, where microwave energy is used to create gas plasma at the plasma cup from which ions are extracted. Microwaves with a frequency of 2.45 GHz were generated by a magnetron coupled to a coaxial feedthrough structure, which guided the microwaves into the plasma cup containing low pressure O\({}_{2}\) gas. The electrons generated in the plasma undergo cyclotron resonance motion increasing their perpendicular kinetic energy due to a magnetic quadrupole arranged around the discharge chamber. Subsequently, when the energized free electrons collide with the gas in the volume, they cause ionization of the gas if their kinetic energy is larger than the ionization energy of the atoms or molecules. The ion extraction optics was composed of two grids located at the open end of the plasma cup, a positive voltage was applied to the first grid to accelerate ions extracting them from the plasma out to the sample, the other grid had a negative voltage and assisted in the ion extraction and the beam current. Oxygen ion irradiation was performed varying separately the irradiation time and the ion kinetic energy. For the samples functionalized with different irradiation time the ion kinetic energy was fixed at 1 keV and the time was varied from 1, 2.5, 5 and 10 min. The samples functionalized varying the ion kinetic energy had irradiation time fixed to 5 min, the different ion kinetic energies were 0.1, 0.25, 0.5, 1, 1.5 and 2 keV. For all samples the current induced by the ion irradiation energy was 10 \(\mu\)A, certifying that the number of ions reaching the samples were the same (Table 1). The vertical alignment and the morphology of v-CNTs were characterized before and after oxygen ion irradiation by scanning electron microscopy (SEM) using a JEOL-JSM-7500F-Field Emission Scanning Electron Microscope operated at 15 kV and transmission electron microscopy (TEM) using a JEM-1011 Jeol Ltd. Tokyo, Japan. The chemical composition of the v-CNTs was studied with X-ray photoelectron spectroscopy (XPS) using a VERSARPROBE PHI 5000 from Physical Electronics, Chanhassen, MN U.S. equipped with a monochromatic Al K\(\alpha\) X-ray source. The XPS analysis chamber is connected to the ion irradiation vacuum chamber system to avoid contaminations. C1s and O1s core level spectra were recorded for each sample with an energy resolution of 0.6 eV. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Irradiation Time (min)** & **Ion Kinetic Energy (keV)** & **Oxygen Content (at. \%)** & **Carbon Content (at.