# Graphene-Based Grid Patterns Fabricated via Direct Ink Writing for Flexible Transparent Electrodes
## 3 Results and Discussions  
### Characterization of Graphene Film Fabricated by DIW  
A typical graphene film was fabricated by the DIW technique with a single trace and one layer. The appearance of the film is band-like, as shown in Figure 2. As can be seen in the optical image, the graphene film was continuous and the width was uniform, indicating good rheological behavior of the graphene ink [27]. More importantly, the \"coffee ring\" effect was absent in the as-prepared graphene film using the modified graphene ink, which will be further discussed in the following section. The surface and cross-sectional morphologies of **G-0** and **G-20** graphene films after depositing 15 layers were analyzed by SEM, as presented in Figure 3. Figure 3a shows the flat and continuous surface of the **G-20** film, indicating that the graphene flakes were uniformly distributed in the film. This can be attributed to the shear force induced by the DIW process and hot-pressing [28; 29]. As can be seen in the cross-sectional morphology of the **G-20** film shown in Figure 3b, graphene flakes were densely stacked without folds, voids, and fractures. In contrast, when pure water was used as the dispersion solvent, the graphene flakes were stacked messily, and folds and voids can be clearly observed in Figure 3c. Due to the difference in graphene flake stacking, the thicknesses of the graphene films were different. The thicknesses of **G-0**, **G-10**, **G-20**, and **G-30** were measured by a step meter and are listed in Figure 3d and Table 1. As can be seen, the thickness of graphene film increased with increasing printed layers, which can be attributed to the accumulation of graphene flakes in the film. **G-0** exhibited the largest thickness, while **G-10**, **G-20**, and **G-30** had comparable thicknesses. A two-fold increase in thickness for **G-0** was witnessed after 15 layers were printed on the substrate. In addition, the variation in thickness for the graphene films prepared with different solvent recipes was considered. As can be seen in Figure 3e,f, **G-0** exhibited a large variation compared with **G-20**, indicating the more uniform thickness of graphene films prepared using a water/ethanol mixture solvent. Considering that all the graphene films, using different dispersion solvents, experienced the exact same process during fabrication, i.e., DIW, solvent evaporation, and hot-pressing, it is speculated that the addition of SDS and ethanol to the dispersion solvent contributed to the formation of densely packed and uniformly distributed graphene thin films. Figure 2: Picture of one graphene conductive film prepared by DIW. Subsequently, the sheet resistances of **G-0**, **G-10**, **G-20**, and **G-30** were measured by a resistance meter to evaluate their performance to be used as electrodes, and the results are listed in Table 2. Initially, the sheet resistance decreases rapidly with an increase in the number of printed graphene layers. However, after 15 layers are printed, the resistance reduction plateaus. It is also worth noting that the lowest sheet resistance was achieved by the **G-0** and **G-20** films, while **G-0** was thicker than **G-20**.