# Low Kinetic Energy Oxygen Ion Irradiation of Vertically Aligned Carbon Nanotubes
## 1 Introduction  
Carbon nanotubes (CNTs) are nanostructured material with high technological importance owing to their unique chemical and electronic properties. They have been reported as good candidates to replace silicon in different electronic devices due to their higher carrier velocity, nanoscale structure and relative low-cost large scale production [1]. Nevertheless, their inert surface limits their use in several applications, such as biological and gas sensor devices [2; 3; 4]. To tackle this issue, the CNTs surface properties can be altered through changes of their surface chemistry by functionalization. It has been shown that CNTs functionalization with oxygen groups can enhance their surface chemical reactivity [5; 6] and change their hydrophobicity improving their dispersion in aqueous media [7; 8], property that is of vital importance for their integration in biological systems. Moreover, oxygen groups can act as active sites for further functionalization increasing their potential applications in a vast variety of fields, such as drug delivery, bio imaging, water purification and catalysis, among others [9]. [PERSON] et al. reported in 2010 [10] that amino functionalization of CNTs can be easily implemented by chemical modification of carboxyl groups in oxidized CNTs, engineering CNTs capable of removing Cd\({}^{2+}\) from water solutions. Regarding electronic properties, oxygen groups grafted at the CNTs surface can convert metallic CNTs into semiconducting [11]. [PERSON] et al. [11] showed that even a very small oxygen amount could alter the electronic structure of CNTs without altering their morphology. Functionalization of CNTs has been achieved via different techniques such as wet chemistry, surface plasma treatment and simultaneous functionalization during synthesis or ion irradiation [12; 13; 14; 15]. In order to obtain an optimal tailoring of the CNTs chemical and electronic properties, the functionalization technique used must be fully controlled. In this context, low kinetic ion irradiation consists of irradiating a target with ions of low kinetic energy, these ions interact with the sample in two different ways, they can bond directly to the atoms in the target, mainly in the intrinsic defects, or crosswise the sample creating new defects that can be used by other ions to create new functional groups. Ion irradiation can alter the structure and the chemical, electronic and magnetic properties of carbon materials [16]. Additionally, junctions among carbon nanotubes can be created, which may have application in the reinforcement of materials for construction among others [17]. The ion kinetic energy during ion irradiation is an important parameter for optimal functionalization. It is considered low kinetic energy ion irradiation if the ions have an energy range from 100 eV to a few keV, and high kinetic energy ion irradiation if the range is of several keV to MeV [17; 18]. The degree of interaction of the ions with the sample also depends on the displacement threshold energy \(T_{d}\) of the sample, that is, the minimum energy required by an atom through the impact of an energetic particle to be ejected from its original position [16]. Only if the kinetic energy imparted by the ions is higher than the T\({}_{d}\) of the irradiated material the atoms will be moved and defects created. Unlike others functionalization techniques, low energy ion irradiation is a clean, efficient and site-selective post-growth technique that does not generate liquid waste and allows functionalizing with a precise ion dose control, adjusting the quantity and energy of ions arriving to the sample surface [19]. In addition, this technique offers the possibility of scaling up to produce large quantities of functionalized CNTs for commercial use. Furthermore, different parameters for ion irradiation can be varied reducing the creation of defects on the structure of nanomaterials.