# Graphene-Based Grid Patterns Fabricated via Direct Ink Writing for Flexible Transparent Electrodes
## 3 Results and Discussions
### Formation Mechanism of Well-Aligned Graphene Flakes in DIW Films  
By comparing the morphology and electrical conductivity of graphene films prepared with different dispersion solvents, it can be concluded that the addition of SDS and ethanol in the solvent plays an important role in obtaining a well-stacked and uniformly distributed graphene thin film with high conductivity. Therefore, the effects of SDS and ethanol are discussed in this section. The uniform distribution of graphene flakes is crucial in affecting the electrical conductivity of as-deposited graphene conductive films. The presence of voids and other defects in the films would induce excessive resistance between adjacent graphene flakes. One of the main difficulties in forming uniformly distributed graphene flakes in the deposited films is the \"coffee ring\" effect during solvent evaporation [34]. The \"coffee ring effect\" refers to the phenomenon that when a droplet evaporates from a solid surface, the edge line of the droplet is pinned and the surface tension creates capillary flows toward the edge, which carry the suspended material to the edge of the droplet for deposition, resulting in uneven deposition of the material after evaporation [35]. The addition of small ionic surfactants such as sodium dodecyl sulfate (SDS) causes an increase in the surfactant concentration on the edge line so that the surface tension of the droplet is locally reduced. The gradient of surface tension triggers a Marangoni vortex toward the center of the droplet, which eventually leads to the homogeneous deposition of the remaining particles [36; 37; 38]. As can be seen from cross-sectional SEM images (Figure 3b) and step meter results (Figure 3d inset), the thickness of graphene conductive films along the lateral direction was uniform, indicating that the \"coffee ring\" effect was suppressed well with 0.5 wt.% SDS addition. Additionally, shear forces exerted during DIW printing can improve the alignment of graphene flakes, as shown in Figure 5.  
The effect of ethanol is discussed as follows: When the deposited ink was heated and the temperature reached the boiling point of the water/ethanol mixture, ethanol molecules absorbed the thermal energy to form a reversible covalent bond with the graphene sheet in the interlayer. Conversely, water molecules absorbed the thermal energy to overcome intermolecular attraction with the graphene and escape from the graphene interlayer as vapor. The ethanol molecules that are covalently bonded to graphene flakes can improve the formability and flexibility of the deposited graphene film upon deformation [39]. Consequently, when the deposited graphene film was hot-pressed, the defects or voids formed due to solvent evaporation could be eliminated without changing the structure of graphene flakes and the connections between each flake could be improved (see Figure 5). Figure 5: Schematic of graphene flake alignment by DIW and hot-pressing. As a result, a densely packed graphene film with fewer folds and pores, a smoother surface, and lower thickness was obtained. The DIW strategy employed in this study, which involves the addition of SDS and ethanol, offers remarkable advantages over traditional methods of modifying graphene dispersions with surfactants alone when it comes to preparing graphene thin films. While conventional surfactant-modified graphene dispersions can somewhat improve graphene dispersion, they often fail to achieve a highly uniform distribution of graphene during complex deposition processes, thereby producing graphene films with wrinkles and voids between flakes due to the lack of structural optimization during film formation [40,41]. In this study, the synergistic effect of SDS and ethanol not only eliminates the coffee ring effect via Marangoni flow, resulting in a more uniform deposition of graphene on the substrate, but also enhances the alignment of graphene flakes during hot-pressing.