Enhanced Strain Resistance of Fractal Fiber Laser-Induced Graphene for Flexible Electrodes via Annealing and Oxygen Plasma Etching

1. Introduction<br/>Laser-induced graphene (LIG) is a low-cost, three-dimensional (3D) porous carbon material with excellent electrical properties, fabricated via laser direct writing (LDW). Its easy fabrication makes LIG suitable for applications ranging from healthcare wearables to soft electronics that need flexible, stretchable materials with good stress resistance. However, LIG's natural brittleness limits its strain resistance. To address this, we propose a dual treatment strategy of annealing and oxygen plasma etching to enhance the strain resistance of LIG.<br/><br/>2. Method<br/>A 6 mm × 6 mm square LIG pattern was prepared on a polyimide film using a 10.6 μm carbon dioxide (CO₂) infrared laser with a power of 3.6 W and a speed of 90 mm/s. The LIG was then modified through a 25-minute oxygen plasma etching process and a 4-minute low-temperature annealing at 180°C, resulting in a fractal fiber structure.<br/><br/>3. Result and Discussion<br/>LIG was prepared through the LDW technique. We defined energy density E_d (mJ/mm) as follows:<br/>E_d=P_laser/V_laser<br/>where P_laser is the laser power, and V_laser is the laser scanning speed.<br/>Data show that increasing E_d leads to greater LIG thickness, lower sheet resistance, and reduced resistivity. Scanning electron microscope (SEM) images of LIG surface morphologies were analyzed under a constant E_d(4.0 mJ/mm)with varying P_laser. As laser power increased from 3.6 W to 6 W, LIG's entangled fiber structure transitioned into a layered, uniform 3D porous structure. Thus, P_laser = 3.6 W and V_laser = 90 mm/s were chosen for fabrication, yielding a fibrous morphology that maintains good electrical interconnectivity through lateral slippage under tensile or bending strain.<br/>To enhance strain resistance, we optimized LIG's microstructure using low-temperature annealing and oxygen plasma etching. A 4-minute 180°C annealing treatment repaired surface oxidation defects in the graphene and reduced internal stress, enhancing mechanical performance. Oxygen plasma etching then introduced defects and pores, increasing surface roughness. This combined treatment balanced structural integrity with microstructural modifications, enhancing LIG's overall mechanical performance and strain resistance.<br/>SEM images revealed changes in LIG's microstructure after treatment. After 25 minutes of oxygen plasma etching (ET 25), the LIG structure was significantly degraded, with almost no intact fibers. Conversely, 4 minutes of low-temperature annealing at 180°C (AT 4) resulted in a denser fiber structure. Following AT 4 and ET 25, the microstructure became rougher, forming multiple filamentous structures within each fiber while maintaining the overall fiber morphology as fractal microfibers ranging from hundreds of nanometers to a few nanometers. XPS data showed increased C-C bonds (from 68.62% to 78.85%) and decreased C-O, and C=O bonds after dual treatment, while Raman spectroscopy indicated good graphitization with low I_D/I_G values.<br/>We studied the strain resistance of LIG patterns transferred to polydimethylsiloxane films, preparing three samples: untreated LIG, AT 4 treated, and AT 4 followed by ET 25 treated. Stretching tests showed similar gauge factors (GF, calculated as GF=ΔR/(R₀×ε)) of approximately 14 for untreated and AT 4 treated samples. However, LIG treated with AT 4 and ET 25 exhibited a lower GF of 7.43, demonstrating superior strain resistance with minimal changes in electrical performance.<br/><br/>4. Conclusion<br/>Considering both mechanical stability and electrical performance retention, we chose manufacturing parameters of P_laser = 3.6 W and V_laser = 90 mm/s, with LIG treated by 4-minute low-temperature annealing followed by 25-minute oxygen plasma etching. This process produces fractal fiber LIG with high strain resistance, ensuring mechanical reliability and stable electrical performance for LIG electrodes in stretchable devices, meeting the requirements for flexibility and durability in practical use.