## 3 Results and Discussions
### Femtosecond Laser Cutting CNT Film
#### 3.3.2 Influence of Laser Parameters on Processing Quality  
It can be seen from the above analysis that relatively a good quality can be obtained at the wavelength of 515 nm when the other parameters are the same. As such, the wavelength of 515 nm was selected as the further experimental parameter.  
Laser energy was the most critical factor affecting the material cutting, so pulse energies of 45, 100, 200 and 500 nJ were selected to cut the CNT film. The other parameters were a pulse duration of 300 fs, a repetition frequency of 40 kHz, and a scanning speed of 0.1 mm/s. The SEM results are shown in Figure 9.  
Figure 8: Raman detection and SEM image of the groove entrance of CNT film by laser cutting at different wavelengths under high pulse energy. (**a**,**b**) \(\lambda\) = 515 nm; (**c**,**d**) \(\lambda\) = 1030 nm.  
The groove width increased with the increasing pulse energy, as shown in Figure 9. The cutting edges of CNT film were clean at the pulse energy of 45 nJ, but some CNT filaments were not entirely broken. When the pulse energy is 100 nJ, the cutting groove was smooth and the CNT film was wholly cut off with no apparent CNT filaments. When the pulse energy increased to 200 nJ, there was a large amount of condensate in the groove, the width of the groove bottom became smaller, and re-condensed particles could be observed on the cutting edge, which reduced the cutting quality. At the pulse energy of 500 nJ, the quality of the cut edge deteriorated and a large amount of recompensed matter was simultaneously generated on the surface of the film. Thus far, this parameter was not suitable for cutting.  
The laser scanning speed is also an essential factor affecting the absorption of energy per unit area of the film. High-quality cutting could be obtained when the pulse energy was 100 nJ and the scanning speed was 0.1 mm/s. The cutting quality decreased, the cutting groove became uneven, and the straightness decreased as the laser scanning speed increased to 1.0 mm/s. The reason for this was the reduction in the number of pulses acting on a unit area and the overlap rate of adjacent pulses, which affected the cutting of the CNT film, resulting in an irregular groove, as shown in Figure 10. The method of increasing the repetition frequency and the scanning speed in equal proportions can be used to improve processing efficiency without influencing processing quality. When the laser scanning parameters of laser frequency was in the range from 40 kHz to 1 MHz and the laser scan speed was from 0.1 mm/s to 1.3 mm/s, the uniformity and straightness of the machined grooves did not change much. Due to the vibration of the moving platform, the straightness decreased after the laser speed exceeded 1.3 mm/s, resulting in a low processing quality.  
In summary, the processing quality in the femtosecond laser cutting of CNT film first increased and then decreased with the increasing laser pulse energy. The pulse energy of 100 nJ and laser  
Figure 10: Different laser scanning speeds of the cutting morphology of the CNT film (**a**) \(v=0.1\) mm/s; (**b**) \(v=0.5\) mm/s; and (**c**) \(v=1.0\) mm/s.  
Figure 9: SEM images of CNT film ablation grooves with different single pulse energies (**A**) \(E=45\) nJ; (**B**) \(E=100\) nJ; (**C**) \(E=200\) nJ; (**D**) \(E=500\) nJ.  
scanning speed of 0.1 mm/s were selected to pattern microstructures on the CNT film. The pattern morphologies were investigated in detail by SEM, as shown in Figure 11. The results exhibited a fine line curve and even area in the patterned region, both of which demonstrate a method of fabricating a film structure to provide a feasible means for the fabrication of integrated circuits [38] and CNT film transistors [39].