Improved conductivity of carbonized polyimide by CO2 laser graphitization

Abstract

Direct laser writing (DLW) is a fast and cost-effective technique for printing conductive structures on flexible substrates such as polyimide (PI) by the conversion of insulative PI to conductive carbon. However, the conductivity (∼10³ S m⁻¹) obtained by this method needs to be improved to compete with ink-jet printing of carbon-based materials. The reason behind the low conductivity achieved by the DLW process is due to the crystallinity and hybridization of bonding in carbonaceous structures. In this work, the DLW process has been implemented in two steps: the first step called carbonization was performed by writing pulsed CO₂ laser on PI to form tracks which consist of amorphous tetrahedral carbon (a mixture of sp² and sp³ hybridized carbon) having intrinsically low conductivity. The second step called graphitization is overwriting of the laser on the pre-carbonized tracks to convert sp³ hybridized bonds to sp² hybridized bonds by the process called laser graphitization. The conductivity of tracks carbonized at (0.21 ± 0.02) W and fluence (3.31 ± 0.32) × 10³ mJ cm⁻² at a repetition rate of 0.1 kHz was 56.1 ± 3.1 S m⁻¹ which increased to 146.7 ± 5.1 S m⁻¹ upon overwriting with the laser at (0.50 ± 0.03) W and fluence (7.88 ± 0.47) × 10³ mJ cm⁻² at the same repetition rate. The photothermal process of carbonization and graphitization is modeled for the DLW process and the threshold power of both the processes is calculated and validated by Raman spectroscopy. Improved conductivity achieved by detailed understanding of the laser and material parameters involved in this transformation enables process optimization leading to future applications in scalable manufacturing of flexible bio-sensors and electrochemical energy storage devices.