An Ultra-Smooth rGO Nano-Thin Film from a Homogeneous Thin Liquid Film Confined by a Conical Fiber Array: Toward the Highly Sensitive Pressure Sensor

Reduced graphene oxide (rGO) thin films have demonstrated various advantages in flexible electronics. So far, solution processes are widely used for making rGO thin films for their mild operation conditions and low cost. However, wrinkles are frequently formed due to the capillary contraction during drying, which severely deteriorates the conductivity and the transparency of rGO thin films. Here, an ultra-smooth rGO nano-thin film, featuring wrinkle-free and a rather low surface roughness of 1.38 nm is developed, which is attributable to a steady and homogeneous thin liquid film confined by a conical fiber array. The thin liquid film facilitates both in-plane stacking of nanosheets and reducing the buried solvent under nanosheets. Notably, with the capillary flow replenishing the tri-phase contact line evaporation, a semi-dry film near the tri-phase contact line is produced, that enables the force equilibrium of multi-directional capillary forces on dispersed nanosheets for depositing a wrinkle-free nanofilm. The as-prepared rGO nanofilm gives a low sheet resistance of 8.3 kΩ sq⁻¹ and a high transmittance of 91.9%, showing better performances than other reported rGO nanofilms. On this basis, it is demonstrated that a high-sensitive pressure sensor with a detection limit as low as 0.02 Pa, highlighting the solution-processed innovative ultra-smooth 2D nanomaterial thin films, and devices.