# Direct Fabrication of Ultrahydrophobic Laser-Induced Graphene for Strain Sensors
## 3 Results and Discussion  
### Surface Material Characterization  
Laser-induced graphene (LIG) from polymeric precursors is usually fabricated using a CO\({}_{2}\) or ultraviolet laser [33; 34; 35; 36]. The electron microscope images in Figure 2a,b reveal certain interesting surface modification features. Interestingly, beside the laser-modified track, a swollen region is observed. The swell is a result of material expansion occurring at laser fluence below the ablation threshold. Multiple factors could contribute to the swell formation, such as non-equilibrium cooling of the polymer melt and polymer surface expansion due to the pressure effect as a result of gaseous and volatile substance release during the ablation process [37; 38]. Furthermore, Figure 2a,b shows random bubbles in the laser modified region. Such a bubble formation can be attributed to a first order phase transition generally observed with laser interaction of relatively transparent polymers. The bulk boiling and the eventual rapid cooling results in the formation of such bubble-like features. Figure 2c,d shows the electron microscope images of the reduced graphene oxide's porous and fibrous network structure. This agrees with the LIG generation mentioned in previous studies which have confirmed a heptagon-pentagon and hexagon hybrid lattice due to the laser-induced localized temperature and pressure at a short timescale [39; 40]. The formation mechanism of the reduced graphene oxide can be attributed to a photothermal pyrolysis process [41; 42]. In comparison to the UV and CO\({}_{2}\) laser wavelengths used to generally perform LIG fabrication, the polyimide has a relatively lower absorption cross-section at 1064 nm. Therefore, a multi-pulse incubation effect initiates the thermal degradation of the polyimide to an amorphous carbon form with a better absorption cross-section, eventually leading to ablation of the material and formation of LIG [43; 44]. The Raman spectra shown in Figure 3 suggest the formation of reduced graphene oxide. Two higher intensity peaks representing G and D bands, typical to carbonaceous materials, are observed. The G band at 1580 cm\({}^{-1}\) corresponds to the graphitic lattice stretching of sp\({}^{2}\) carbon atoms. The disorder band (D-band) at 1335 cm\({}^{-1}\) is attributed to  
Figure 2: Field Emission Scanning electron microscope images of the fabricated LIG, (**a,b**) Side and top view of the ablated and swollen laser-modified region; (**c,d**) magnified images of the porous and fibrous LIG network in the central ablated region. the breathing vibrations of the sp\({}^{2}\) hybridized carbon [45]. The band mainly arises due to the restriction of carbon vibrations due to oxygen-containing functional groups acting as defects to the material structure. Indeed, an Id/Ig ratio of D and G band at 0.99 confirms that the laser-generated graphene structure has multiple defects. Furthermore, a broadened 2D band is observed in the region from 2700 to 3000 cm\({}^{-1}\). This is due to the second order process of double resonance Raman scattering resulting in two phonon emissions near the K point of the Brillouin zone and is strongly dependent on the phonon structure of graphene. Consequently, unlike the presence of a monolayer graphene, which has a single Raman shift at 2700 cm\({}^{-1}\), the broadening in this study indicates the presence of a multi-layered structure. Additionally, the oxygen-based functional groups prevent efficient stacking of the graphene layers, causing a slight shift in the peak appearance [46]. Another interesting observation regards the Raman shift peak at \(\sim\)2930 cm\({}^{-1}\). Few studies have attributed the peak due to the presence of C-H bonds. Also, the peak present at 3165 cm\({}^{-1}\) is correlated to the presence of hydroxyl and carboxylic functional groups [47; 48]. Herein, it is important to emphasize that the term laser-induced graphene (LIG) refers to the formation of a reduced graphene oxide (rGO)-based structure and not a monolayer 2D Graphene.