## 3 Results and Discussion
### Raman Characterization  
Raman characterization was performed on CFs in order to elucidate the induced structural changes upon laser irradiation and assess the uniformity of the modified CF surfaces. All spectra (Figure 7) contained prominent vibrational modes near 1343 cm\({}^{-1}\), 1582 cm\({}^{-1}\), and 2685 cm\({}^{-1}\), which correspond to the D, G, and the 2D bands, respectively [17; 43; 44]. The D band is associated with the presence of defects such as vacancies and strain in graphitic networks. The G band is representative of in plane  
Figure 6: Scanning electron microscopy (SEM) micrographs of the irradiated samples. (**a**,**b**) bTCF, (**c**,**d**) TCF\({}_{3.75\,W}\), (**e**,**f**) TCF\({}_{4.25\,W}\), (**g**,**h**) TCF\({}_{4.75\,W}\), (**i**,**j**) TCF\({}_{7.5\,W}\), (**k**,**l**) TCF\({}_{10\,W}\), (**m**,**n**) bPCF, and (**o**,**p**) PCF\({}_{4.25\,W}\). (More detailed SEM micrographs can be found in the supporting information, Section S1, Figures S8–S16).  
sp\({}^{2}\) bonding and originates from first-order Raman scattering, whereas the 2D band results from a double resonance Raman process. The shoulder peak in the G band (D\({}^{\prime}\)), is a band indicative of finite graphite crystals and graphene edges. Figure 8 shows the correlation between the fluence (J/cm\({}^{2}\)) and the I\({}_{\text{D}}\)/I\({}_{\text{G}}\) intensity ratio of the irradiated samples. It is apparent that as the fluence increased to higher levels, the I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratio increased as well, indicating the existence of more defective structures, which correlated well with the increased \"roughening\" at higher fluences observed from the SEM analysis. More specifically, at a fluence of 37 J/cm\({}^{2}\) (3.75 W) the ratio was calculated as \(\sim\) 0.94, at a fluence of 42 J/cm\({}^{2}\) (4.25 W) as \(\sim\)1.02, and at a fluence of 47 J/cm\({}^{2}\) (4.75 W) as \(\sim\)1.10 for TCF fibers. However, PCF fiber (Unidirectional cloth) at 42 J/cm\({}^{2}\) exhibited a larger intensity ratio (\(\sim\)1.12), when compared to TCF exposed at the same fluence and was comparable to TCF exposed to a fluence of 47 J/cm\({}^{2}\). More Raman spectra of the PCF irradiated samples as a function of different laser powers can be found in the supporting information, Section S2. Raman characterization of the PCF UD cloths are presented in Figure S17. The intensity of the 2D band gradually increased with increasing fluence, showing a very well-defined sharp band at the higher fluence samples (TCF\({}_{4.75\text{\,W}}\) and PCF\({}_{4.25\text{\,W}}\)) implying a high degree of graphitization. It is insinuated, that the TCFs seem to require relatively larger fluence (by 5 J/cm\({}^{2}\)) to achieve a high degree of surface graphitization, when compared to the PCFs. This difference can be attributed to either different graphitization responses of the fibers upon irradiation or to the thick sizing agent of TCF (T700 SC: 1.1% of sizing agents) [45], which is acting as a shield to the laser irradiation, requiring higher fluence values to achieve the desirable \"roughening\" of the CFs' surface. Another notable observation on the Raman spectra of irradiated PCF samples (Figure S17) was, at fluencies equal to or higher than 44 J/cm\({}^{2}\) (4.5 W), the D and G bands broadened resembling those of non-irradiated bPCF fibers, whereas the 2D band was still present. The reason for the broadening of D and G bands can be attributed to the increased structural disorder in the graphite crystalline structure induced by the higher laser power irradiation [36; 46].  
Figure 7: Raman analysis of the irradiated samples. TCF denotes Toray carbon fibers (T700 SC), while PCF denotes the Pyrofil fibers (TR50S). bTCF and bPCF represent control bare non-exposed carbon fiber (CFs).