Liu Zhongfan's research group, Peng Hailin's research group and collaborators in the School of Chemistry reported a non-destructive clean transfer method for large-area graphene

The excellent electrical and optical properties of graphene make it have broad application prospects
in high-end electronics, optoelectronic devices and other fields.
The graphene thin film materials prepared by chemical vapor deposition (CVD) method have the advantages
of high quality, large area and controllable number of layers.
However, graphene films usually need to be prepared on a metal substrate, in order to achieve their subsequent functional applications, it is usually necessary to transfer the graphene film prepared on the metal substrate to the target functional substrate, and the large-area graphene transfer process usually produces graphene breakage, folds and pollution, which seriously affects the properties of
graphene after transfer.
Therefore, how to achieve a large area of graphene lossless, clean transfer is still a research technology bottleneck in the field of graphene applications, and it needs to be solved
urgently.

In view of this problem, recently, the research group of Professor Liu Zhongfan and Professor Peng Hailin of the School of Chemistry and Molecular Engineering of Peking University, the research group of Lin Li Distinguished Researcher of the School of Materials Science and Engineering, and the research group of Wei Yujie, researcher of the Institute of Mechanics, Chinese Academy of Sciences, have added hydroxyl-prone volatile small molecules to the traditional transfer medium polymethyl methacrylate (PMMA) by designing the molecular structure of the transfer medium.
The conformal contact between the graphene and the target functional substrate is ensured, and then the separation of the transfer medium and the graphene is realized by the mechanical peeling method, and a clean and complete graphene surface is obtained, and the lossless and clean transfer
of a large area graphene film is realized 。 The research results are titled "Large-area transfer of two-dimensional materials free of cracks, contamination and wrinkles via controllable conformal contact" and recently published in Nature Communications 2022, 13.
4409）
。

The paper points out that the key to the successful transfer of graphene films is to achieve conformal contact
between them and the target substrate through the structural design of the transfer medium.
Due to the good fit of graphene and the target substrate, it is possible to effectively avoid the damage and fold problems
caused by the stress of graphene in the transfer.
For the silicon-based substrate, the research team designed a volatile small molecule containing hydroxyl group embedded in the polymethyl methacrylate (PMMA) chain as a transfer medium, and by heating, the volatilization of the small molecules led to the re-stacking of the PMMA chain, which promoted the conformal bonding of graphene and the target substrate, and successfully achieved complete transfer
of 4-inch wafer graphene 。 For polyethylene glycol terephthalate (PET) substrate with high roughness, by embedding propylene carbonate with low glass transition temperature into the PMMA chain, the polymer glass transition after heating enables graphene to spread the infiltration on the PET surface and realize the complete transfer of A4 size graphene film, with a completeness of up to 99%.

In the traditional graphene transfer method, PMMA is used as a transfer medium, and acetone is used to dissolve and remove PMMA
.
However, PMMA is often difficult to remove completely, resulting in residual contaminants
on the graphene surface.
In this regard, the research team proposes that graphene and the target substrate have a strong force through conformal contact, and then the transfer medium can be directly stripped and removed, which replaces the traditional method of acetone degumming, which greatly saves costs and can achieve batch transfer
.
With the help of atomic force microscopy, it can be clearly shown that the graphene surface is clean and pollution-free
.
In order to evaluate the electrical properties of graphene after transfer, the research team constructed a boron nitride packaged graphene device, which can be tested to have a room temperature mobility of 120,000 cm 2/Vs and a low temperature (4K) mobility of up to 1420,000 cm²/Vs
.
This transfer method is also suitable for the transfer
of two-dimensional materials such as MoS₂.

Non-destructive clean transfer method and property evaluation of large-area graphene films

The paper's co-corresponding authors are Liu Zhongfan, Lin Li, Peng Hailin and Wei Yujie
.
The first co-authors are Zhao Yixuan, a doctoral candidate at the School of Chemistry of Peking University, and Song Yuqing, a postdoctoral fellow, Dr.
Hu Zhaoning of the Beijing Graphene Research Institute, and Wang Wendong
, a postdoctoral fellow at the University of Manchester.
The research work has been funded
by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Beijing Municipal Science and Technology Commission, and the Beijing National Research Center for Molecular Sciences.