Application of the Taguchi Method to Optimize Graphene Coatings on Copper Nanoparticles Formed Using a Solid Carbon Source

Abstract

Graphene has attracted much recent interest as an electronic material due to its large electron mobility. Large-area graphene has been synthesized using chemical vapor deposition (CVD). However, it is difficult to apply this process to grow graphene on nanoparticles (NPs) because of their small radius of curvature, which results in a large defect density. In this work, we used the Taguchi method to optimize the deposition of graphene on nanoparticles. We used polyvinylpyrrolidone (PVP) to coat copper NPs via CVD and optimized the process conditions using a minimal number of experiments. The PVP served as the solid carbon source, forming graphene when heated to 875℃. To improve the quality of the graphene coatings on the Cu NPs, the following process parameters were varied: gas conditions (ratio of Ar to H₂), process time and temperature, the amount of PVP solution, and the molecular weight of PVP. We identified optimal process conditions using only eight experiments. Raman spectroscopy was used to analyze the quality of the graphene coatings by comparing two-dimensional (2D) spectra and I_D/I_G ratios of the different coatings. A decrease in I_D/I_G, in combination with sharper Raman bands, is indicative of the thickness and crystal quality of the graphene layer. The quality of the graphene layer was also evaluated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).The optimal conditions for the formation of graphene-coated Cu NPs were: a temperature of 875℃, a deposition time of 2 minutes, an Ar-to-H₂ ratio of 1:1, PVP with a molecular weight of M_W = 3,500 (K-12) during the polyol process, and a 50-wt.% PVP solution with M_W = 45,000 (K-30). Using the Taguchi method, we identified trends relating defect density versus process conditions and successfully obtained a graphene coating with a minimal defect density.