## 3 Results and Discussions
### Femtosecond Laser Cutting CNT Film
#### 3.3.1 Raman Analysis of Femtosecond Laser Cutting CNT Film
The Raman spectra and SEM topographies are shown in Figure 7. When the pulse energy was low, the Raman spectrum showed that the intensity of the D band did not change much, but the intensity of the G band increased significantly when the laser was processed with a wavelength of 515 nm. The intensity of the D band and the G band increased simultaneously at the wavelength of 1030 nm. The relative area ratio \(I_{\rm D}/I_{\rm G}\) was 0.42 and 0.48, respectively. Figure 6: Raman spectra of two unprocessed areas of CNT film. Through the irradiation of the femtosecond laser, the composition of the material underwent specific changes. The binder in the CNT film is preferentially ablated by the laser, exposing the CNTs wrapped therein, and the film cutting region was detected as a plurality of the CNT layer with a certain inclination angle. Thus, the intensity of the G band increased. The microscopic topography of the SEM showed a clear cutting groove and no damage on the surface of the film. Therefore, a wavelength of 515 nm could cut the CNT film without damaging the substrate material, and the degree of defect of the display material of the Raman spectrum did not change much. When the wavelength was increased to 1030 nm, the energy of a photon was lower than that of a photon at short wavelength. Under the same parameters, unbroken CNTs existed in the groove after laser cutting, and the edge of the slit was partially detached from the substrate due to the pulsed laser-induced thermal elastic force [35]. The CNT film could not be completely cut, and the cutting edge quality was poor with these processing parameters. Films in this case may cause short circuits if used as microelectrodes. When the laser pulse energy was increased to \(E=500\) nJ, the intensity and half-width of the D band and G band increased significantly. The D band shifted to higher wavenumbers after Lorentz fitting, and the G band shifted to lower wavenumbers, as shown in Figure 8. The relative area ratios were 0.73 and 0.68, respectively. Figure 7: Raman detection and SEM image of the groove entrance of CNT film by laser cutting at different wavelengths under a low pulse energy. (**a**,**b**) \(\lambda=515\) nm; (**c**,**d**) \(\lambda=1030\) nm. The original structures of CNT films was dramatically changed by irradiation with a high laser energy. The increase of D band intensity and band width at half height indicates that the quality of the CNTs was decreased and amorphous carbon existed [36]. The combination of laser induced material vaporization and air breakdown to form a plasma generated a driving force that resulting in tensile deformation of the CNTs--the Raman D band approached 1366 cm\({}^{-1}\). There were many CNTs with different degrees of tension in the Raman collection area, which proves that the bond of SP\({}^{2}\) hybridization underwent considerable distortion. The G band shifted slightly to lower wavenumbers caused by the tensile deformation of CNTs [37]. A higher femtosecond laser energy caused damage to the substrate material and coagulated on the CNT film at the same time, which changed the lifetime of scattered phonons during G band generation, resulting in an increase in band width at half height and an increase in the acromion strength of the G band. A large number of recrystallized particles were on the surface of CNT film and in the groove through the micro-morphology, as shown in Figure 8. The processing quality dropped sharply, so laser parameters were no longer suitable for cutting CNT film materials.