Microstructure selection and control of flexible laser-induced graphene (LIG) based on strain response

Laser-induced graphene (LIG) is a fabrication technique for graphene-based flexible devices with significant commercial potential. However, different flexible devices may exhibit entirely opposite requirements for the strain response of their functional layers. To investigate and regulate the strain response of LIG for meeting diverse strain requirements of different flexible devices, this paper takes the microstructure of LIG with polyimide as the carbon precursor as the starting point. On the one hand, from the perspective of deformation response, we investigated the adaptability of LIG with different microstructures in flexible devices through finite element analysis (FEA) and scanning electron microscope (SEM) observations; On the other hand, from the perspectives of power density and irradiation time, the evolution process of microstructures was studied using time-resolved reflectance (TRR) analysis, high-speed photography and other methods. A method for inducing different microstructures by adjusting laser parameters was proposed. Finally, the gauge factor (GF), response/recovery time and cyclic response of porous crater structure and surface nanofibers structure were tested. The results reveal that the two microstructures are composite microstructures, which are composed of four basic microstructures: porous PI (3.0 kW/cm²), pores (3.0 kW/cm²), cellular network (7.6 kW/cm²) and nanofibers (7.6 kW/cm²). During the LIG process, there can be a liquid phase up to 100 ms, and when the irradiation time is more than 3 ms, the surface nanofibers structure will transform into a porous craters structure. The gauge factor of the surface nanofibers structure ranges from 0.38 to 1.45. Porous crater structure that of the porous crater structure spans 0.77 to 18.95, which has a faster response speed, but the resistance of the porous crater structure cannot be restored. Therefore, the interaction between gas phase and liquid phase during laser irradiation is the main reason for the formation of various microstructures of LIG. By changing the laser power density and irradiation time, the microstructure of LIG can be freely adjusted and the strain response of LIG can be further controlled. Porous crater structure is more suitable for one-time measurement of small strain, such as crack detection of brittle materials. Surface nanofiber structure is suitable for flexible gas sensors, supercapacitors and other flexible devices requiring low strain response.