## 3 Results and Discussion  
Freshly microtomed HD-CNTs from the same fiber were functionalized at their open ends. For this purpose, positioning CNTs while maintaining their orientations and protecting their side walls within the assembly was required. We accomplished this by assembling CNTs into macroscopic fibers, where the CNTs had linear orientations and maintain their pristine nature of individual CNTs while their side walls were encapsulated within a polymer [40; 41]. Aside from protecting the side walls, the encapsulating polymer prepared as film facilitated handling of the microscale HD-CNTs without ever making contact with their open ends. The CNTs in Figure 2A,B illustrate the carboxylic functionalized ends that were employed to react with amino functional groups at the Cu and Pt surfaces. In general, covalent bond formation between CNTs and other molecules requires functionalization of the CNT surfaces. A variety of methods for CNT covalent functionalization have been reported, including free radical addition, oxidation, carboxyl-based coupling, fluorination, and addition or substitution reactions. These functionalization reactions have mainly been used to enhance CNT dispersion and compatibility [65; 66]. Typically, functionalization occurs at the edges of five- or seven-membered ring defects in the basal planes of the CNTs or the tip of the CNTs [66; 67]. Unfortunately, most of those approaches for functionalization do not allow the positioning of functional groups to be controlled. With the current method, access to all carbon atoms at the open-ended CNTs was made easier, and their complete functionalization would be ideal. To date, quantification of the degree of functionalization has been limited to theoretical studies, where the optimum number of functional groups at the open ends has been determined [68]. The thin films obtained by microtoming that held the HD-CNTs had two important functions: (1) holding the CNTs of a uniform length packed in a cylindrical structure so that the majority of the continuous CNTs pass through the film and have open ends on both sides of the film and (2) covering the sidewalls of the CNTs so that only the open ends of the CNTs are exposed, which allows for chemical modification of the CNT tips, conserving the pristine CNT structure. It was assumed that the number of open-ended CNTs in the HD-CNT film samples were similar because multiple samples could be prepared from the same CNT fiber using microtoming. This is a unique advantage of this approach, because it allows the number of CNTs assembled within a particular fiber to be controlled by limiting the width of the CNT array employed for fiber assembly, as demonstrated previously [40; 69]. Wet chemistry functionalization is preferential at the open ends of CNTs [70; 71]. We selected carboxylic functionalization at the open ends of the CNTs, because carboxylic acid functionalization is a well-established single-step functionalization reaction for CNTs which gives a reactive end for a second molecule attachment [72]. To achieve a high carboxylic density on the CNT open ends, the functionalization time with HNO\({}_{3}\) (2 M) was increased to 24 h. The Raman spectra were recorded for CNT cross-sections chemically attached to metal surfaces (Figure S3). The peak at ~1585 cm\({}^{-1}\) was attributed to the G band, which originated from the in-plane tangential stretching of the C-C bonds in CNTs, whereas the peak at ~1334 cm\({}^{-1}\) was attributed to the D band. The ratio between these bands (\(I_{\text{C}}/I_{\text{D}}\)) indicated a high degree of functionalization. However,as the assemblies contained multi-walled CNTs with a relatively high D band intensity, quantitative information about functionalization could not be obtained by comparing only the (\(\left(I_{\text{G}}/I_{\text{D}}\right)\) ratio. However, EDAX analysis consistently supported an increase in the oxygen content, although by no more than 2%.