Kinetic effects and ablation thresholds in laser-induced graphene

Abstract

Laser-induced graphitization of polymers is promising for flexible electronics applications. While the effects of laser power on the structure and properties of the laser-induced graphene (LIG) have been elucidated, the role of kinetic effects is still largely unexplored. We use a unique approach of lasing individual electrodes on a tilted polyimide sheet, which allows scanning a continuum of laser fluence on a single lased line as the degree of beam defocus varies spatially. This approach is ideal for revealing discrete morphological transitions and mapping structural and chemical changes. To reveal how LIG formation is energetically or kinetically controlled, three morphological transitions are revealed: T1, from porous to cellular networks; T2, from cellular networks to wooly fibers; and T3, ablation threshold that occurs only at low speeds (<300 mm/s). This newly identified transition is characterized by a cratered top and hierarchical porosity with improved graphene quality. We demonstrate that wooly fiber formation is highly speed-dependent and that fibers only form at high speeds (>300 mm/s). Moreover, by integrating experimental measurements with thermal modeling, we find that both the cellular-network formation and the ablation transitions are rate-independent, as they occur at the temperatures of polyimide graphitization and ablation of graphitic material, respectively. In contrast, our findings show that the formation of wooly fibers is highly rate dependent, as it occurs at temperature ramp rate of ≈10⁷ °C/s. Hence, our results provide new insights into the kinetics of the physicochemical LIG process, toward tailoring porous graphene for different applications.