Graphene transfer from growth substrates to target surfaces is a critical step in realizing its applications in electronics, optoelectronics, and flexible devices. The most common growth substrates are copper (Cu) and nickel (Ni), typically via chemical vapor deposition (CVD). However, transferring graphene without introducing defects, cracks, or residues remains a challenge. Several transfer techniques have been developed, each with advantages and limitations. The most widely used methods include polymer-assisted wet transfer, electrochemical delamination, and roll-to-roll processes. Each method must address key issues such as residue removal, crack prevention, and yield optimization to ensure high-quality graphene films.

Polymer-assisted wet transfer is the most established method for graphene transfer. The process involves coating the graphene-grown metal substrate with a polymer support layer, typically polymethyl methacrylate (PMMA), followed by etching the metal substrate using an acidic solution such as iron chloride (FeCl₃) for Cu or ammonium persulfate ((NH₄)₂S₂O₈) for Ni. The polymer-graphene stack is then rinsed in deionized water to remove residual etchant and transferred onto the target substrate. Finally, the polymer is dissolved in acetone or other organic solvents. Despite its widespread use, this method often leaves polymer residues on the graphene surface, which can degrade electrical and optical properties. Residual PMMA can be minimized by optimizing polymer concentration, using alternative polymers like polycarbonate (PC), or employing post-transfer annealing in hydrogen or argon atmospheres. Another drawback is the potential for cracks due to mechanical stress during handling. To mitigate this, researchers have developed modified techniques such as using a second support layer or adjusting the etching rate to reduce strain.

Electrochemical delamination, also known as the bubbling transfer method, offers an alternative to wet etching. In this process, the graphene-coated metal substrate acts as the cathode in an electrochemical cell filled with a dilute alkaline solution, such as sodium hydroxide (NaOH). Applying a voltage induces hydrogen bubble formation at the graphene-metal interface, gently delaminating the graphene film. The detached graphene, often still attached to a polymer support, is then transferred to the desired substrate. This method reduces exposure to harsh chemicals and minimizes metal etching residues. However, the process requires precise control of voltage and electrolyte concentration to prevent damage to the graphene. Over-application of voltage can lead to excessive bubbling, causing tears or wrinkles. Recent improvements include the use of lower voltages and buffered electrolytes to enhance transfer uniformity. Electrochemical delamination is particularly advantageous for large-area transfers, as it avoids the mechanical stress associated with direct etching.

Roll-to-roll (R2R) transfer processes are essential for industrial-scale graphene production. This method involves continuous transfer of graphene from a metal foil to a flexible target substrate, such as polyethylene terephthalate (PET) or polyimide (PI). A typical R2R process begins with laminating a polymer support layer onto the graphene-metal foil. The metal is then etched in a bath while the graphene-polymer film is wound onto a take-up roll. Alternatively, electrochemical delamination can be integrated into R2R systems for a milder transfer. The key advantage of R2R is its scalability, enabling meter-long graphene films to be produced in a single run. However, challenges include maintaining uniformity across large areas and minimizing defects introduced during lamination and etching. Innovations such as optimized roller pressure and in-situ monitoring systems have improved yield and quality. Despite progress, R2R processes still face hurdles in achieving defect-free graphene at commercial scales, particularly for high-performance electronic applications.

Residue removal is a persistent challenge across all transfer methods. Polymer residues, metal impurities, and etchant byproducts can adversely affect graphene's properties. Post-transfer cleaning techniques include thermal annealing, solvent rinsing, and chemical treatments. Annealing at temperatures between 200°C and 400°C in inert or reducing atmospheres effectively removes most organic residues. Solvent rinsing with mixtures of acetone, isopropanol, and chloroform can further reduce contaminants. For metal residues, chelating agents or mild acids are sometimes employed. However, excessive cleaning can introduce new defects, necessitating a balance between purity and structural integrity.

Crack prevention is another critical consideration. Graphene's atomic thickness makes it susceptible to tearing during transfer. Strategies to improve mechanical stability include using thicker polymer supports, optimizing etching rates, and employing sacrificial layers. For example, adding a temporary cellulose acetate layer beneath the graphene can provide additional support during transfer. Another approach is to use hydrophobic target substrates, which promote better adhesion and reduce strain during drying. Pre-stretching flexible substrates before transfer has also been shown to minimize post-transfer wrinkling.

Yield improvements are essential for industrial adoption. Variations in transfer conditions, such as temperature, humidity, and solution composition, can significantly impact success rates. Automated systems with controlled environments help standardize the process. Statistical process control methods are increasingly used to identify and mitigate sources of variability. Additionally, machine learning algorithms are being explored to predict optimal transfer parameters based on initial film characteristics.

Industry adoption of graphene transfer technologies faces several challenges. The high cost of CVD-grown graphene and the complexity of transfer processes limit widespread use in cost-sensitive applications. Consistency in large-area transfers remains problematic, particularly for electronics requiring defect-free monolayers. Integration with existing semiconductor manufacturing processes also poses compatibility issues, such as thermal budget constraints and chemical resistance of target substrates. Furthermore, environmental and safety concerns related to chemical etchants and solvents must be addressed to meet regulatory standards.

In summary, transferring graphene from growth substrates to target surfaces involves trade-offs between scalability, quality, and practicality. Polymer-assisted wet transfer remains the most accessible method but suffers from residue and crack issues. Electrochemical delamination offers a gentler alternative but requires precise control. Roll-to-roll processes enable large-scale production but must overcome uniformity and defect challenges. Advances in residue removal, crack prevention, and process optimization are critical for improving yield and facilitating industrial adoption. As research progresses, hybrid techniques combining the strengths of multiple methods may emerge as the most viable path forward for high-quality graphene integration.