## 2 Materials and Methods  
### Sample Preparation  
Initial undoped carbon nanotubes were sourced from the \"MWCNT-1\" brand (manufactured by the G.K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences (SB RAS)), obtained via chemical vapor deposition (CVD) [50].  
The sample preparation procedure is schematically depicted in Figure 2. In the initial step, a suspension was prepared by mixing MWCNT powder with ethanol (0.005 g of MWCNT powder per 15 mL of ethanol), followed by ultrasonic treatment (Jeken Ultrasonic Cleaner model PS-06A, Dongguan City, Guangdong Province, China) for 5 h. Subsequently, the prepared suspension was aerosol-sprayed onto a preheated titanium substrate using an airbrush, with the substrate temperature maintained at 65-70 \({}^{\circ}\)C for around 5-7 min (Figure 3a). This process yielded a layer of MWCNTs approximately 650-700 nm in thickness (Figure 3b). Ultimately, the formed MWCNT layer was irradiated by a continuous ion beam using an accelerator located at the Omsk Scientific Center, SB RAS (Figure 3c). A schematic representation of the ion source in this accelerator is shown in Figure 3d. The principle of operation of the source is based on the Penning discharge and is as follows: after creating a preliminary vacuum of about 10\({}^{-4}\) Torr in the accelerator chamber, the working gas enters the ion source for ionization in crossed magnetic and electric fields.  
The ions of the obtained low-temperature plasma, when an accelerating voltage is applied, bombard the surface of the samples placed on the sample table. It should be noted that such a design of the ion source makes it possible to change the beam cross-section in accordance with the dimensions of the substrate; thus, it is possible to irradiate the surfaces of materials with an area of up to 8000 cm\({}^{2}\). In our case, the pressure of residual gases in the accelerator chamber after pumping out was 3 \(\times\) 10\({}^{-5}\) Torr. After the working gas (dry N\({}_{2}\)) was admitted, the pressure was 3 \(\times\) 10\({}^{-4}\) Torr. This irradiation involved N\({}^{+}\) ions with an average energy of 20 keV and the beam current was about 100 mA. The choice of average ion energy considered the range of nitrogen ions within a layer of specific thickness, as determined using the SRIM-2013 software package (refer to Figure 3e) [51]. Experimentally, the beam fluence at this stage was selected based on visual inspection to minimize surface damage. An optimal beam fluence of 1 \(\times\) 10\({}^{17}\) ion/cm\({}^{2}\) (which corresponds to a current density of -50 \(\upmu\)A/cm\({}^{2}\)) was determined to yield satisfactory results; thus, this modification mode was adopted for the initial experiments.  
Figure 3: (**a**) Photograph of an MWCNT layer deposited via aerosol spraying on a Ti substrate; (**b**) SEM image of cross-section of an MWCNT layer on a Ti substrate; (**c**) general view of the ion implanter; (**d**) ion source scheme: 1—magnets, 2—cathode, 3—anode, 4—gas inlet, 5—sample table, 6—sample; (**e**) calculation of the projective range of N\({}^{+}\) ions in a layer of nanotubes 650 nm thick on a Ti substrate.  
Figure 2: Scheme of preparing N-MWCNTs using nitrogen ion irradiation. The arrows show the individual stages from the beginning to the end of obtaining modified nanotubes.