Universal 2D material film transfer using a novel low molecular weight polyvinyl acetate
Graphical abstract
Introduction
Various two-dimensional (2D) materials including graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides and black phosphorus have recently attracted a great deal of attention due to their exceptional physical and chemical properties [1], [2]. Numerous researchers have utilized 2D materials for different applications such as electronic and optoelectronic devices, solar cells, and water splitting catalysts [3], [4], [5], [6]. Until now, extensive effort has been put into obtaining high-quality large-area 2D materials via various synthetic routes [7], [8]. Of numerous synthetic methods reported so far, chemical vapor deposition (CVD) route has been considered the most promising approach to obtain high-quality uniform films over a large area [9]. In general, when using the CVD method to synthesize 2D materials, several synthetic parameters should be taken into consideration, including precursor, substrate, gas flow rate, growth temperature, growth time, as well as the distance between precursor-substrate [10], [11]. Hence, through careful control of the above parameters during the synthesis process, the high-quality large-area 2D materials can be fabricated by CVD. However, when using CVD method for the growth of 2D materials, the as-synthesized 2D materials need to be transferred to other appropriate substrates as they are usually grown on metal substrates. Furthermore, obtaining clean and large-area 2D material films with desirable electrical properties after the transfer is still far from satisfaction due to many outstanding limitations incurred during the transfer process, such as unwanted impurities, structural defects, and poor surface flatness [12], [13]. To overcome these outstanding issues, extensive effort has been directed towards developing novel transfer methods employing various rigid polymer supports to facilitate the transfer of 2D materials including polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), polyvinyl alcohol (PVA), as well as rosin [14], [15], [16], [17], [18], [19], [20], [21] on to arbitrary substrates. Unfortunately, these transfer methods suffer from several critical drawbacks, such as complicated transfer procedure, rough surface, and structural damages. For instance, in general, PMMA has been employed as a supporting layer for transferring 2D materials. However, the long-chain structure (the structure formula of PMMA is shown in Fig. S1a in Supporting Information), large molecular weight (~120,000) and the low solubility in chemical solvent (such as acetone) result in large amount of PMMA residues on the surface of 2D materials [21], [22]. Removing PMMA via heating (generally at 50–300 °C) process introduces defects or damage to the surface of 2D materials [22]. Thus, there has been great demand on developing a low-cost universal transfer technique that produces high-quality clean and large-area 2D materials surmounting these outstanding issues. Compared with PMMA, polyvinyl acetate (PVAc, as depicted in Fig. S1b in Supporting Information) has smaller molecular weight (~100,000), which can be easily removed from the surface of the 2D materials during the dissolving process. The PVAc provides excellent room-temperature solubility in chemical solvents (including acetone, methanol, and ethanol), robust support, as well as low-cost. These advantages indicate that PVAc will be an excellent supporting layer for 2D materials transfer. However, the systematic investigation of a universal transfer technique for obtaining high-quality 2D materials by using PVAc as a supporting layer has been yet reported.
In this work, PVAc is used as a rigid support layer during transfer process of large area CVD-grown 2D materials onto desirable target substrates without incorporating structural and chemical damages. We demonstrate the advantages of our transfer method by applying to the large-area graphene film grown on Cu foil and h-BN film grown on Pt foil by taking unique advantages of PVAc as a rigid support. The PVAc assisted transfer technique in this work enables graphene on h-BN to achieve a high carrier mobility of up to ~ 10,000 cm2 V−1 s−1 at room temperature, indicating that the quality of the as-transferred 2D materials is not only mainly determined by the quality of the as-synthesized 2D materials, but also by defects incurred during the transfer process. The results demonstrate our transfer approach is a promising method for large scale and flexible electronics based on 2D materials.
Section snippets
Preparation of PVAc solution
PVAc solution was prepared by dissolving 0.6 g of PVAc powder (Sigma-Aldrich, average molecular weight: ~ 100,000, (A.R.)) in 15 ml of toluene and stirring at 80 °C for 2 h.
PVAc assisted transfer of CVD-grown graphene on Cu foil
Monolayer graphene was grown on Cu foil using low pressure chemical vapor deposition (LPCVD) method following our previously developed synthetic method reported elsewhere [23], which is given in Supporting Information. A schematic depicting the transfer procedure of graphene using PVAc as a robust support is shown in Fig. 1.
Results and discussion
Schematic illustrating the PVAc assisted transfer of CVD-grown graphene on Cu foil is provided in Fig. 1. Firstly, PVAc was coated on graphene/Cu foil; secondly, the Cu foil was etched using 0.1 M of ammonium persulfate; graphene film was then transferred onto the target substrate. To more effectively demonstrate that our transfer method can be used for an effective transfer of large area 2D materials, we transferred the graphene film with a size of 8 cm × 10 cm by using PVAc, as shown in Fig.
Conclusion
We developed an effective transfer method that enables large-area 2D materials such as graphene and h-BN to be reliably transferred onto target substrates via PVAc as a rigid support layer. The PVAc provides many advantages, including an excellent room-temperature solubility in chemical solvents such as acetone, methanol, or ethanol, a robust support, clean surface of 2D materials after PVAc layer is removed, as well as low-cost. Compared with PMMA and PVA assisted transfer methods, the PVAc
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Research Foundation of Korea grant funded by the Korean government (2017R1D1A1B03034847). Qian Y. wishes to thank the financial support through China Scholarship Council (CSC, 20180820016).
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