# Processing of Single-Walled Carbon Nanotubes with Femtosecond Laser Pulses
## 3 Results and Discussion
This is evident from the SEM images with GR in Figure 4e where the microscope lens (10X Zeiss) showed a more localized effect with less width of the irradiated zone, as compared to the 15 mm lens in Figure 4a with a broadened width of irradiated zone indicated by the red arrows. The SEM images in Figure 4 show that interaction between SWCNTs film and femtosecond laser is predominantly a non-thermal mechanism [41], as described in the two-temperature model [42]. However, at higher fluence it partially ablates the materials accompanied by the joining of nanotubes. It is further highlighted that the SWCNT ropes are tiny clusters of limited atoms as their aspect ratio is high, so there is a possibility of global melting at some points in the film without thermal melting, as reported in the literature and observed in this study [42]. Despite the fact that in nanosecond laser irradiation a similar phenomenon of heating, melting, and evaporation occurred, such a thermal mechanism in the processing of SWCNTs with the femtosecond laser is improbable to exist. This is because the electron's lattice thermal coupling, as well as thermal diffusion to the lattice, takes a substantially longer time than the laser pulse width. Consequently, electrons do not have enough time to transfer energy to lattice [42]. In non-thermal heating, at the  
Figure 4: SEM images of SWCNTs sample after irradiation with (**a,b**) green wavelength (GR) with a fluence of 848.85 J/cm\({}^{2}\) with 15 mm lens; (**c,d**) GR with a fluence of 541.14 J/cm\({}^{2}\) with 10X Zeiss lens, and (**e,f**) GR with a fluence of 2705.71 J/cm\({}^{2}\) with 10X Zeiss lens. fluence above a surface threshold value [41], the electrons are excited, ejected, and weaken the bonds at surface atoms. This results in surface melting which is beneficial for the joining of nanostructures without damaging the whole entity [41; 43]. Figure 5 shows the proposed mechanism for thermal effects of femtosecond and nanosecond laser pulses on SWCNT. For a longer pulse duration, the entire nanotube is heated, as the thermal effects are dominant, and it is equivalent to annealing by the continuous laser wave or conventional thermal sintering [41]. Whereas for femtosecond laser irradiation, only the surface heating will be observed, as laser pulse duration is considerably smaller than the characteristic time for thermal diffusion. Consequently, it is favorable for the joining of nanotubes [44]. As shown in Figure 4d,f, the morphology of the joined nanotubes is almost unchanged and they well kept their original shape. This suggests surface melting, which is somehow attributed to non-thermal effects of the femtosecond laser. The number and quality of nanotube joints are high in femtosecond irradiation of SWCNTs, as clearly shown in Figure 4f, compared to a similar study of continuous fiber laser on multi-walled carbon nanotubes (MWCNTs) [26]. Other authors have also shown similar phenomenon of non-thermal heating in the joining of nanostructures by femtosecond laser irradiation [41; 42]. This preliminary study suggests that the femtosecond joining technique for SWCNTs is simple and cost-effective but needs further refinement of experimental parameters, and special sample preparation for specialized applications. The simplicity of this technique overcomes a number of challenges that come across in the conventional joining technique for applications in microelectronic devices and sensors. Further investigations on the characteristics of welded/joined sections are part of our ongoing studies.