## 3 Results and Discussion
Spectra were recorded before and after grafting on the Cu surface to identify the functional groups attached to the metal surface. Furthermore, the spectra after sonication of the samples to determine the bonding strength between grafted organic groups and the metal surface were collected (Figure 3). The FT-IR spectrum of the aminophenyl-grafted Cu surface exhibited a weak broad peak at \(\sim\)3400 cm\({}^{-1}\), which was attributed to N-H stretching [77]. A similar broad peak has been observed in the FT-IR spectra of polymerized diamines, whereas crystalline diamines exhibit sharp peaks in this region [77; 78]. Clear peaks were observed in the 1350-1610 cm\({}^{-1}\) region, specifically at 1496, 1511, and 1608 cm\({}^{-1}\), which were attributed to C=C bond stretching in the aromatic rings [60; 77; 78]. The band observed at 1259 cm\({}^{-1}\) was attributed to C-NH\({}_{2}\) stretching, whereas that at 1178 cm\({}^{-1}\) was attributed to CH bending. The peak at 831 cm\({}^{-1}\) corresponded to CH out-of-plane bending, and that at 630 cm\({}^{-1}\) could correspond to ring deformation [78]. The observation of peaks around 2300 cm\({}^{-1}\) indicated the presence of trace CO\({}_{2}\) in the atmosphere. No peak corresponding to the N\(\equiv\)N stretching mode of diazonium at 2280 cm\({}^{-1}\) was observed, which supports the proposed metal-organic reaction [60]. The peak intensities decreased as the reaction temperature decreased from 65 to 25 \({}^{\circ}\)C on the FTIR peaks in Figure 3, which suggests that this grafting reaction is dependent on the temperature. Alternatively, this behavior could be due to multilayer formation, owing to the reactions between in situ generated diazonium molecules. For comparison purposes, the FTIR of pure 4-phenylenediamine was recorded (Figure S4), where the main features discussed in Figure 3 are also visible. The covalent nature of Cu-C bonds has been reported by [PERSON] et al., as has multilayer formation on Cu substrates via diazonium salt-Cu metal reactions [61]. However, in our work, strong peak intensities were observed after sonication, which suggests the formation of strong bonds between the metal surface and the organic groups. Furthermore, an EDAX analysis (not shown) indicated that the atomic percentage of C increased upon grafting of the organic groups to the metal surface. As shown in Figure 4A, vertically standing CNTs bonded to Cu metal were obtained from promoting the bond formation between the aminophenyl groups grafted on the metal  
Figure 3: FTIR spectra of organic grafted Cu surface. Normalized FTIR spectra of (**a**) polished Cu surface, (**b**) aminophenyl group attached to Cu surface at 25 \({}^{\circ}\)C, (**c**) aminophenyl group attached to Cu surface at 25 \({}^{\circ}\)C after sonication, (**d**) aminophenyl group attached to Cu surface at 65 \({}^{\circ}\)C, and (**e**) aminophenyl group attached to Cu surface at 65 \({}^{\circ}\)C after sonication. surface and the oxidized CNT open ends. Afterward, the embedding polymer around the HD-CNTs was physically removed using forceps while the CNTs remained anchored on the metal surface. The forceful removal of the polymer appeared to tear the CNTs at the outer surface of the HD-CNTs, but the number of removed CNTs was minimal considering the large number of them on the cylindrical structure that remained standing (Figure 4A). The three-dimensional nature of the assembly was clearly observed, and the interface between the CNTs and the Cu metal surface is visible in Figure 4B. Similar attempts with physisorbed embedded CNTs typically removed both the polymer and the CNTs at once, suggesting that the chemical bonding to the metal substrate in the HD-CNT samples was extremely strong for the cross-linked HD-CNTs compared with simple adhesion. The interface between the CNTs and the Cu substrate is shown at a higher magnification in Figure 4B, and the connections joining the CNT bundles to the Cu surface are clearly visible.