**5. Transfer procedures**

Once graphene has been synthesized on Cu foils, its transfer to more suitable substrates for its study or to be used in different applications is necessary. There are several procedures that have been developed for this transfer step, having as purposes: a) to separate the graphene film from the original metal substrate where it was deposited and b) to protect the graphene after the transference [54]. In general, these procedures can be classified in two groups, the wet chemical methods and the dry chemical methods.

#### **5.1 Wet chemical methods**

These methods are based generally on the coating of a protective polymeric layer over the graphene film (e.g., PDMS or PMMA), followed by the immerse in an etching solvent (e.g., FeCl3, HCl, HNO3, Fe(NO3)3, CuCl2) to remove the metal substrate, so the graphene can be positioned then in another substrate (e.g., SiO2) [55]. **Figure 2** depicts a flowchart of the transfer process using PMMA. Despite the good results that could be obtained with this general technique, in recent years, new variants have been developed to overcome its disadvantages (i.e., the presence of impurities and the formation

**Figure 2.**

*Schematic representation of the wet transfer process using spin-coated PMMA.*

of cracks in the material). Such is the case of An and collaborators, who developed a reverse transfer technique by locating the graphene/PMMA bilayer into a flexible substrate (e.g., PET), reducing the density of cracks and the presence of impurities [56]. Lin and coworkers found a way to avoid the use of the polymeric layer, achieving better quality of transferred graphene (free of organic residues). They used isopropyl alcohol (IPA) and ammonium persulfate solution ((NH4)2S2O8) as the etchant agent, which was later substituted with DI water and IPA to reduce the surface tension and ease the transference to the desired substrate [57]. Furthermore, the need of the etching step has been also avoided in some techniques, such as Chandrashekar *et al.* reported a method to separate the Cu foil by oxidizing it in hot water, modifying its nature from hydrophobic to hydrophilic, easing its delamination from graphene [58].
