M. Abdulhafez, M. Bedewy
University of Pittsburgh,
Keywords: porous carbon, carbon nanofibers, laser processing, flexible device manufacturing, electrical conductivity
Summary:Direct Laser-based carbonization of commercial polymers, such as polyimide, is a promising alternative to printing conductive carbon electrodes on flexible substrates. These laser-induced nanocarbons (LINCs) are formed along surface patterns based on the irradiation of high intensity beams of CO2 lasers in a direct-write fashion. Considering the variety of morphologies that can be created, this technique is attractive for its flexibility, versatility, and scalability. For example, recent efforts have demonstrated the fabrication of various functional devices like micro-supercapacitors, sensors and microfluidic devices directly on polyimide films. While LINCs have been observed to have a turbostratic carbon structure with different hierarchical porous and fibrous morphologies, the fundamental mechanisms underlying the formation of LINCs is still largely missing. Here, we elucidate the process-structure-property relationships that are needed in order to correlate the laser processing parameters to the resulting micro-scale and nanoscale morphology, as well as to the achieved electrode properties. We study the formation of individual lines by scanning the laser beam over polyimide films at different fluence levels and Gaussian beam intensity profiles using two unique approaches. In the first approach, we control fluence by leveraging the degree of focusing/defocusing in order to lase lines at different spot sizes (vertical position) with the same integrated power. The second approach involves lasing lines on tilted samples, which enables sweeping different fluence values as a spatial map along the points of the laser path. Scanning electron microscopy images show that porous, cellular and fibrous morphologies are observed with increasing fluence. Importantly, two specific threshold values of fluence were identified that correspond to the transition from pores to networked cells, as well as for the transition from networked cells to a forest of fibers. Moreover, Raman spectra enabled quantification of the atomic quality of the nanocarbons formed as a function of the fluence level. In particular, our results show that 2D/G ratio increases (up to 0.55) with higher fluence. However, when the morphology switches to fibers, the 2D peak disappears. We then correlate these findings to the electrical conductivity measured across LINC lines lased at different fluences. We show that higher 2D/G ratio and more cellular morphology at a fluence value of 9 J/cm2 gives the best value of resistance of around 0.4 K/mm (≈5S/cm). Finally, we propose a model to explain the evolution of morphology and atomic structure during LINC formation based on our experimental results. Our model and results offer new insight into elucidating this physicochemical process and enables more precise control of the LINC formation process for tunability of their nanoscale morphology and properties.