# Processing of Single-Walled Carbon Nanotubes with Femtosecond Laser Pulses
## 3 Results and Discussion  
The as received SWCNTs were characterized by the transmission and scanning electron microscopy before femtosecond laser irradiation. The representative high-resolution transmission electron microscope (TEM) image of SWCNT ropes and the typical scanning electron microscope (SEM) image of SWCNT networks are shown in Figure 2a,b, respectively. The TEM image is showing several SWCNTs in the form of ropes. The zoomed image of SWCNT ropes is shown in the inset of Figure 2a. The average diameter of a single wall carbon nanotube is ~1.67 nm, as estimated from the TEM. It is a well-established fact that a metal catalyst is indispensable for the synthesis of SWCNTs through the chemical vapor deposition process. Therefore, particles inside the SWCNT ropes are metal catalyst particles. The most widely used transition metal catalysts for SWCNTs growth are Fe, Co, Ni, etc. and are reported in several studies [34; 35]. It is obvious from the representative image of the SEM that the ropes of the SWCNTs have a complex network pattern after dispersing. A large number of samples of SWCNTs were irradiated by the femtosecond laser to investigate the effect of laser parameters such as pulse wavelength, fluences, and laser power etc. to join the SWCNT ropes into bundles. Initially, the first set of experiments were performed with the first harmonic (infrared wavelength) for various power settings, and the results varied according to the laser power and plane of irradiation. The experiment was performed in a way by varying the laser scan rate so that the networks of SWCNTs scribed by femtosecond laser for a fraction of a second. Since the femtosecond laser gush energy over a temporal scale was shorter than the phonon-electron relaxation time [36], it is unlikely that they become oxidized substantially during this shorter period of exposure time. However, the slight oxidization, if occurred, is favorable in the sticking of SWCNTs through oxidation debris in the form of bundles in accordance with the literature [37]. The majority of experiments showed that this infrared wavelength ablates the SWCNT bundles at the line of irradiation and largely burned the nanotube bundle network. Typical images for the 1 st harmonic experiments with the effect of reduction in fluence are shown in Figure 3. Figure 3a,c show the full and partial destruction of SWCNTs at the laser irradiated area, respectively. The burned nanotubes are clearly shown in Figure 3b in the zoomed picture. Keepingfirst harmonics, if we reduce the power further, then less damaging effects are observed as having partial ablation as shown in Figure 3c,d. The quantity of burned nanotubes is substantially lower, with a significant reduction in power with resulting fluences of 169.77 J/cm\({}^{2}\) to 21.22 J/cm\({}^{2}\). However, few joining could be suggested at the interface of intact and damaged nanotubes as indicated by the arrows in Figure 3a,c. There is an interesting feature that most of the suggested joining are found at the two sides of the plane of irradiation, as shown by the arrows, and this pattern is observed for most of the experiments. Several experiments that were performed with changing the plane of irradiation and power variations for the 1 st harmonics showed similar outcomes. Overall, low power settings for the 1 st harmonic experiments offer somehow better results, with less damaging of the SWCNTs predominantly at the edges of the irradiation line. Figure 3: SEM images of SWCNT samples after IR laser irradiation with fluence reduction effects. (**a**) 169.77 J/cm\({}^{2}\) and (**c**) 21.22 J/cm\({}^{2}\) with 15 mm lens; (**a,c**) macroscopic view of area of SWCNTs irradiation, and (**b,d**) microscopic view of side regions showing connectivity. Figure 2: Images of single-walled carbon nanotubes before irradiations. (**a**) High resolution TEM image and (**b**) SEM image of SWCNTs. To improve the results, the second set of experiments was performed by adopting the green wavelength (GR) 2 nd harmonic from the harmonic generator.