The integration of graphene and other 2D materials into large-scale applications, such as silicon-based technologies, necessitates reliable transfer methods that maintain material quality. Current methods often result in cracks, wrinkles, and contamination, significantly impacting performance. The challenge is amplified with increasing film size. Graphene's flexibility makes it particularly vulnerable to damage during transfer. While supporting films have been used, achieving conformal contact between the graphene and the target substrate remains a critical hurdle. Non-conformal adherence leads to free-standing regions in the graphene, susceptible to tearing during transfer medium removal. Furthermore, conventional polymer-based transfer methods often require aggressive chemical treatments, leaving behind contaminants that degrade graphene quality. This study addresses these limitations by introducing a novel approach to achieve controllable conformal contact, ensuring crack-free, contamination-free, and wrinkle-free transfer of large-area 2D materials.

Existing literature highlights the importance of conformal contact for crack-free transfer of graphene. However, previous methods, despite mentioning the role of conformal contact, often resulted in surface contamination, cracks, and wrinkles, limiting carrier mobility. Various supporting foils have been proposed, but the challenges of achieving uniform conformal contact over large areas persist. Polymer-free techniques, while avoiding contamination, often lack scalability for large-area transfer. The need for a universal, contamination-free transfer method applicable to large-area 2D materials remains a key research focus.

The researchers incorporated oxhydryl groups-containing volatile molecules (OVMs) or low-glass-transition-temperature (Tg) polymers (polypropylene carbonate, PPC) into poly(methyl methacrylate) (PMMA) supporting films. These additives enable controllable conformal contact by inducing deformation of the supporting film upon heating. For transfer onto SiO2/Si substrates, OVMs were embedded in PMMA via hydrogen bonding. Upon heating, OVM evaporation causes PMMA chain restacking, leading to conformal contact. For transfer onto PET substrates, layer-by-layer blade coating of PPC and PMMA created a film where blending upon heating and subsequent TRT release facilitated conformal contact. A bubbling-based delamination technique was used to detach the graphene from the growth substrate (Cu), allowing for substrate recycling. The clean graphene transfer was confirmed using atomic force microscopy (AFM) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), showing significantly reduced contamination compared to conventional methods. The conformal contact was assessed using AFM, revealing minimal height difference between the graphene and the substrate. Molecular dynamics simulations supported the observed enhanced adhesion due to reduced separation distance. Device fabrication involved hBN encapsulation to minimize substrate interference, and electrical measurements evaluated carrier mobility. The method's versatility was demonstrated by transferring graphene onto Nafion foils and gold electrodes onto various substrates.

The addition of OVMs or PPC to PMMA supporting films enabled controllable conformal contact during the transfer of large-area graphene films, eliminating cracks, wrinkles, and contamination. The resulting conformal contact improved graphene-substrate adhesion, facilitating direct delamination without chemical treatments. The 4-inch graphene single-crystal wafers transferred onto Cu wafers and A4-sized graphene films transferred onto Cu foils exhibited macro-intactness and micro-intactness values of approximately 99%. ToF-SIMS analysis showed a significant reduction (approximately four orders of magnitude) in PMMA residue concentration. The high carrier mobilities measured after transfer (70,000–120,000 cm²V⁻¹s⁻¹ at room temperature and 800,000–1,420,000 cm²V⁻¹s⁻¹ at 4 K after hBN encapsulation) are comparable to exfoliated graphene, highlighting the high quality of the transferred material. The method's versatility was demonstrated through successful transfer onto different substrates including PET, Nafion foils, and the creation of a graphene/MoS2 heterostructure. The transfer of gold electrodes also showcased the method's general applicability to other materials.

The findings demonstrate a significant advancement in the transfer of large-area 2D materials. The controllable conformal contact approach effectively addresses the long-standing challenges associated with cracks, wrinkles, and contamination, leading to improved electronic properties and device performance. The avoidance of chemical treatments and the possibility of Cu substrate recycling contribute to the method's environmental friendliness and potential for scalability. The high carrier mobility values obtained suggest the potential for this method to enable the fabrication of high-performance devices based on 2D materials. The demonstrated versatility of the method for different substrate materials and diverse 2D materials expands its potential impact.

This study presents a novel, scalable, and environmentally friendly method for transferring large-area 2D materials with exceptional quality. The achievement of controllable conformal contact through modified supporting films eliminates the need for aggressive chemical treatments, resulting in significantly improved material quality and device performance. This approach opens new avenues for high-performance electronics and other large-area applications of 2D materials. Future research could focus on further optimization of the supporting film composition and exploring the method's applicability to other 2D materials and heterostructures.

While the method shows significant improvements, there might be limitations related to the specific OVMs or PPC used, which could influence the final quality and transfer efficiency. The optimal conditions for heat treatment might need to be adjusted depending on the size and type of 2D material being transferred. Further research is required to fully understand and address the potential influence of substrate roughness on the final conformity.