M. Abdulhafez, S. Ghosh, M. Sahaluddin, G. Tomaraei, M. Bedewy
University of Pittsburgh,
Keywords: heater, flexible device, graphene, laser processing
Summary:Direct-laser write of graphenic patterns on flexible polymers like polyimide is promising for scalable fabrication of conductive high-surface-area nanocarbons with tunable resistivity. Hence, these laser-induced graphene (LIG) material is attractive for many electrode applications, such as for electrochemical biosensing and supercapacitors, among other emerging flexible device applications. Importantly, a fundamental understanding of the energetic and kinetic factors governing the polymer-to-graphene transformation during lasing is needed for building comprehensive process-structure-property relationships. Here, we develop a unique approach for controlling the spatiotemporal evolution of laser fluence based creating topographic features at a length-scale that is commensurate with relevant range of beam defocus. Moreover, we leverage multiple lasing passes at different powers and speeds in order to boost the graphitization process and further reduce resistivity. Combined with Gaussian beam modeling, this approach uniquely enables revealing discrete transitions of morphology, chemistry, and conductivity. Since, the temperature achieved in resistive heaters directly depends on the value of local value of resistivity, creating LIG patterns with spatially varying morphology and resistivity leads to spatial maps of temperature. Our results show that there are four distinct morphologies: (1) isotropic pores; (2) anisotropic cellular networks; (3) aligned wooly fibers, and (4) ablated hierarchically porous structures, each of which has different resistivity. Accordingly, we present different methods to create LIG-based heaters with designed spatial thermal gradients. Combining dynamic beam defocusing and relasing, we demonstrate flexible heaters with periodic bands of high temperature and low temperature using IR imaging and thermocouple measurements. Hence, our technique is an enabling technology for such applications as thermotherapy and deicing.