Energetic and Kinetic Factors Governing the Direct Fabrication of Laser Induced Graphene Microelectrodes on Flexible Substrates

Direct laser writing on flexible polymers like polyimide has been shown to generate highly functional graphene-based patterns. These conductive high surface area graphene electrodes are promising for electrochemical biosensing and supercapacitors, among other emerging flexible device applications. Hence, 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 of lasing individual graphene microelectrodes on a tilted polyimide film, which allows scanning a continuum of laser fluence values that are spatially distributed on the sample, with tilt angle as the key parameter describing this spatially controlled laser fluence. Combined with Gaussian beam modeling, this approach uniquely enables revealing discrete morphological transitions of morphology and their corresponding fluence value. Moreover, we can map both these morphological and chemical changes as a function of energetics and kinetics. In particular, we use laser fluence as a measure of the average energy delivered during lasing, while we use scanning speed-dependence to elucidate kinetically controlled morphology transitions. Based on Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction, the concomitant changes in atomic structure and heteroatom content are also revealed. 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. We also correlate these morphological changes to electrical conductivity measurements and identify the most promising ones for target applications. We complement the experimental results with analytical beam modeling and numerical finite element thermal modeling to advance our process understanding and control. Hence, our findings provide insight into the fundamental mechanisms underlying the bottom-up fabrication of functional graphene microelectrodes with tailored morphology/chemistry and desired properties.