# Processing of Single-Walled Carbon Nanotubes with Femtosecond Laser Pulses
## 3 Results and Discussion
In this series of experiments, different power reduction levels as low as 541.14 J/cm\({}^{2}\) were used for GR. The energy per pulse remains in the range of nanojoules. Typical SEM images of irradiated SWCNTs at different power levels are shown in Figure 4. In general, it is observed that less ablating effects are witnessed as compared to IR with GR irradiation on nanotubes. Initially, 848.85J/cm\({}^{2}\) fluence was opted with a 15 mm lens. The results shown in Figure 4a,b are almost similar with the first harmonic irradiation but substantially less burning is observed. The joining is believed to occur, as shown by the circles in Figure 4b. The laser parameters were further tuned at green wavelength by changing the power settings as low as fluence 541.14 J/cm\({}^{2}\) and replacing the lens to 10X. The results are shown in Figure 4c,d. At this lower fluence and lens setting, the ablation has been reduced considerably and joining is suspected at the end (white phase) where the laser interacts with the bundles. The arrows/circles in Figure 4c,d show suspected welded nanotubes represented by the white phase contrast. The best results achieved so far are shown in Figure 4e,f. The selected area in Figure 4e has been zoomed in Figure 4f. The image is showing clearly suspected welded nanotubes with the second harmonic wavelength. [PERSON] et al. applied a continuous fiber laser on multiwall carbon nanotubes to join them. The authors proposed that the accumulation of absorbed energy as the irradiation time increases leads to the breaking of C-C bonds, and the formation of a robust graphene layer junction that leads to the bonding among MWCNTs. In the current study, with the exposure of the femtosecond laser at very low power and second harmonics, leads to better joining of nanotubes in agreement with the literature [26]. As SWCNTs have a single wall, therefore, they need substantially less energy and time to join and therefore, a more damaging effect was observed during the experimentation on SWCNTs. It is well established that most of the SWCNTs are semiconducting in nature while a small fraction consists of a metallic nature [38]. Furthermore, the metallic nanotubes have a higher thermal conductivity as compared to semiconducting. Moreover, the glass substrate on which nanotubes are dispersed has lower thermal conductivity values. Therefore, it is anticipated that due to the accumulation of absorbed energy that a localized heating has occurred, and results in the form of joining at the end where the ropes/bundles are exposed to the laser irradiation. The SWCNTs are present in the form of a network of ropes/bundles in thin film, while some of the nanotubes are comprised of semiconducting and others are metallic in electrical characteristics. Since each category responds differently with the irradiation of the femtosecond laser, it is difficult to quantify the results of joining due to the complex nature of carbon structures. However, the SEM images suggest that the ropes of SWCNTs within a bundle are joined at the end point where the laser has burned them. An interesting observation is that both the absorbance and fluence have a substantial impact on the femtosecond laser processing of SWCNTs. For instance, a comparison between the IR and GR irradiation effect is conspicuous by analysing Figure 3a,b and Figure 4a,b, where the IR irridation, even with less fluence (169.77 J/cm\({}^{2}\)), ablated SWCNTs completely on the line of action as compared to GR (848.85 J/cm\({}^{2}\)), respectively. One of the possible reasons that GR has less destruction of SWCNTs than IR could be the lesser penetration depth of GR as compared to IR in the nanostructured materials [39; 40]. Furthermore, optical properties for 15 mm and 10X Zeiss lenses, as well as different fluence values, affect the size of the irradiated zone and welding patterns.