Plasma Engineering on Flexible Polyimide Substrates for 2D Material Exfoliation

Abstract

2D transition-metal dichalcogenides (TMD) with piezoelectric properties (e.g., Tungsten Diselenide (WSe2)) have potential applications in flexible energy-harvesting devices. To characterize the piezoelectric response under strain through substrate bending, the flake must be transferred onto a flexible substrate. However, direct exfoliation onto a flexible substrate (e.g., Kapton polyimide) remains challenging because 2D flakes generally fail to adhere to non-polar, hydrophobic surfaces. In this work, surface modification of the standard flexible, polymeric substrate was investigated to improve the success rate of the 2D flake transfer process. The polyimide surface was treated with oxygen plasma at a pressure of 5 Pascal and a flow of 50 sccm, with varying power levels of 50 W to 200 W for a time duration of 30 seconds to 1 minute. The 2D WSe2 flakes were exfoliated on the oxygen plasma-treated polyimide substrate and analyzed in the optical microscope by placing the SiO2 (silicon dioxide)-coated silicon wafer under the film for enhanced optical contrast. This study showed that exfoliation onto the plasma-treated polyimide substrate contained 2D flakes of over 30 µm. These results demonstrated a simple, inexpensive imaging strategy for locating WSe₂ flakes on flexible, transparent substrates and identified oxygen plasma treatment as a promising approach to improving the success rate of the large-area exfoliation. Together, these modifications offer practical improvements for preparing WSe₂ samples for future piezoelectric material characterization.