Source identification and method for drastic reduction of Fe contamination on wet transferred graphene

Graphene is a 2D material with extraordinary electronic and mechanical properties and has a wide range of applications. Among existing synthesis methods, chemical vapor deposition (CVD) onto Cu foil is the most prominent because it allows large area high quality graphene to be prepared at relatively low costs and then subsequently wet transferred to arbitrary substrates. Typically, PMMA and FeCl₃ are used as a polymeric support layer and a metal etchant respectively during wet transfer. Unfortunately, this method leaves graphene susceptible to contamination, degrading transferred graphene's extraordinary properties. In this paper, contamination in the form of black particulates is identified to be Fe wrapped in graphene and a model of how it originates was developed. Then, a procedure to reduce this contamination during wet transfer was developed and its effectiveness was systematically studied. Transfers were characterized using Raman spectroscopy and optical microscopy with image segmentation. Experimental data obtained indicates that this simple and economic method is very effective in reducing Fe contamination on transferred CVD graphene, offering a way to improve the wet transfer process. - Highlights: • Metal etchant was found to cause iron contamination in wet transferred graphene. • Its reduction using an O₂ plasma method was presented and systematically studied. • This was found to be more effective than wet transferring with extended washing.