Graphene, a two-dimensional material, holds immense potential due to its exceptional electrical and mechanical properties, leading to its exploration in various applications, such as membranes and pressure sensors. While exfoliation methods were initially used, their limitations in reproducibility and scale have shifted the focus towards chemical vapor deposition (CVD) on copper substrates. However, transferring the graphene from copper to other substrates remains a significant hurdle. Most existing techniques focus on optimizing the transfer process itself or post-transfer cleaning steps. This research introduces a novel pre-treatment method for the graphene/copper material before transfer to improve the process quality and reproducibility. The method is inspired by work on reduced graphene oxide (rGO) membranes and aims to decorate the graphene with stabilizing functional carbon groups, eliminating the need for a polymer support layer during the wet-chemical transfer process, a significant advancement over traditional methods.

Existing graphene transfer methods often rely on polymer support layers like PMMA, PVA, or paraffin to prevent graphene from tearing during transfer. However, removing these polymers post-transfer is challenging and often leads to polymer residue contamination on the graphene surface, which degrades the material’s properties. Previous attempts to avoid or remove these residues involve additional chemicals, extended heating, or are substrate-specific. The proposed pre-treatment method offers a significant departure from these existing approaches by entirely eliminating the need for polymer support.

The pre-treatment involves flattening the graphene/copper sample using a commercially available vacuum bag sealer with an ethylene-vinyl acetate copolymer (EVAC) foil. This step attaches functional carbon groups to the graphene surface. The copper is then etched away using an ammonium persulfate (APS) solution, and the graphene is transferred to a target substrate using a pure water environment—no supporting layer is necessary. The effectiveness of the method was evaluated through various techniques: optical microscopy to observe the graphene's behavior during and after transfer; Raman spectroscopy to assess defect density and charge carrier concentration; atomic force microscopy (AFM) to analyze surface cleanliness; scanning electron microscopy (SEM) to examine the copper substrate's surface; X-ray photoelectron spectroscopy (XPS) to analyze surface composition changes; and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to identify surface adsorbates. Contact angle measurements were also conducted to quantify surface hydrophobicity and surface energy. The Owens-Wendt equation was used to analyze the polar and dispersive interactions between the graphene surface and liquids.

The pre-treatment significantly improved graphene transfer success rate. Optical microscopy showed that pre-treated graphene maintained its shape during transfer, unlike untreated graphene which crumbled. Raman mapping confirmed a much lower defect density (average AD/AG intensity ratio of ~0.04) in pre-treated graphene compared to conventionally transferred graphene with PMMA (~0.2). The pre-treated graphene also exhibited higher charge carrier density (1.5 × 10¹³ cm⁻²) than PMMA-transferred graphene (5 × 10¹² cm⁻²), likely due to charge transfer from the acetate groups in the EVAC. AFM revealed a significantly cleaner graphene surface after the pre-treatment, with minimal polymer residue unlike PMMA-transferred graphene. SEM images showed no significant difference in copper substrate smoothness between treated and untreated samples, suggesting that the improvement originates from graphene surface modification. Analysis of transfer success rate and contact angle as a function of flattening time revealed a clear correlation between increased hydrophobicity and successful transfer. XPS data indicated an increase in sp³-hybridized carbon and C-O bonds after pre-treatment, attributed to adsorbates from the EVAC. ToF-SIMS confirmed the presence of acetate groups from the EVAC on the graphene surface. Contact angle measurements showed a decrease in surface energy after pre-treatment, but importantly a significant reduction in the polar component of the surface energy, suggesting that the improved mechanical stability is caused by the suppression of polar interactions with water during the transfer process.

The results demonstrate that the improved mechanical stability during wet-chemical transfer is not due to changes in overall surface energy, contrary to common assumptions. Instead, the key factor is the significant reduction in polar interactions between the graphene and water, preventing the graphene from tearing. The pre-treatment technique effectively hydrophobizes the graphene, increasing its mechanical stability and charge carrier density without leaving significant polymer residues. The high charge carrier density and low defect density are beneficial for improved charge carrier mobility, making this technique promising for various graphene applications. The finding challenges the existing understanding of graphene transfer and offers a new, effective strategy for high-quality graphene transfer.

This study presents a simple, scalable, and cost-effective method to enhance graphene's mechanical stability and properties during wet-chemical transfer. The pre-treatment using an EVAC foil effectively hydrophobizes the graphene, suppressing polar interactions with water and preventing tearing. The resulting graphene exhibits high crystallinity, low defect density, and increased charge carrier density. Future research could explore the optimization of this technique for different graphene growth methods and substrates, and investigate the long-term stability of the functionalized graphene.

While the current study demonstrates significant improvements in graphene transfer, the long-term stability of the functional groups attached to the graphene surface needs further investigation. The influence of different copolymer materials on the effectiveness of the pre-treatment also warrants further study. The analysis focused on single-layer graphene; future work should evaluate the effectiveness of this technique on multi-layer graphene.