Designed growth of large bilayer graphene with arbitrary twist angles

A “prestacked substrates-angle replication production” strategy is developed to grow centimetre-scale twisted bilayer graphene with arbitrary twist angles (accuracy, <1°).
Designed growth of large bilayer graphene with arbitrary twist angles
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Twisted bilayer graphene (TBG) has emerged as a fantastic material in both scientific research and engineering applications, wherein the twist angle provides a new degree of freedom to tune the physical properties ranging from metallic, semiconducting, and superconducting to even topological insulating state. At present, the fabrication of TBG mostly relies on the post-stacking/folding techniques or the random growth during some specific growth processes. However, issues such as low throughput, small size, interlayer contamination, or poor angle control are unavoidable in these prevailing methods. Therefore, to fully explore the potential of TBG in electronics and twistronics, large-scale fabrication of TBG with precise arbitrary twist angles is highly desirable.

Our group has been working on the growth of large-scale single-crystal graphene [1] and the production of single-crystal Cu foils with different kinds of facets [2]. Previous studies demonstrate that single-crystal monolayer graphene can be directionally modulated by the periodic potential of the substrate. On this basis, if two substrates are prestacked with a designed rotation angle, TBG can be obtained after the monolayer epitaxial growth and the rotation angle can be replicated as the twist angle between the substrates.

Here our group, including researchers from Peking University, ShanghaiTech University, Renmin University of China, and other collaborators, developed a unique “prestacked substrates-angle replication production” strategy for the designed growth of centimetre-scale TBG with a clean interface, intrinsic van der Waals couple, and arbitrary twist angles (size of ~2 cm × 1 cm, and angle accuracy of < 1°) on Cu(111) substrates. In our experiments, graphene monolayers were first grown between two Cu foils at the growth temperature of 1050 °C. Then at an elevated temperature of 1074 °C, the Cu foils become soft and their surfaces are tuned to be atomically flat, which permits the two layers of graphene to attach well and generate strong interlayer coupling. In addition, the twist angle of bilayer graphene is locked by the two Cu foils due to the stronger interaction between graphene and Cu compared with that between the two graphene monolayers. Therefore, TBG with arbitrary twist angles (even a small angle like 1°) could be achieved by this designed macroscale manipulation method.

Figure 1. a Schematics for the growth design of twisted bilayer graphene. b Photograph of two stacked Cu foils with a rotation angle of α. The ruler number indicates millimetres. c-e Characterizations of twisted bilayer graphene with the designed twist angles α of 14°. f-i Universal preparation of twisted bilayer graphene with different twist angles.

We then need to expose the fabricated TBG for the subsequent characterization. Here, we carried out a custom-developed equipotential surface etching method to remove the copper foil on one side. Here two parallel Pt electrodes were immersed into an (NH4)2S2O8 solution to establish a uniform electric field. Cu2+ ions can be equably dissolved from the surface of the Cu foil to ensure homogeneous etching. Meanwhile, an amperometric method was applied to monitor the accurate time of graphene exposure. The well homogeneity of the obtained TBG was further verified by the large-area Raman mapping, photocurrent measurement, and selected area electron diffraction (SAED) statistical results. Samples with different twist angles (0°, 5°, 12°, 14°, 30°) can be successfully obtained, demonstrating the flexibility in twist angle design of our method. We believe that this strategy can be extended to the preparation of other twisted bilayer materials and thus provides an alternative way for the future applications of twistronics at the integration level.

 

Reference

[1]. Wu, M., Zhang, Z., Xu, X. et al. Seeded growth of large single-crystal copper foils with high-index facets. Nature 581, 406–410 (2020).

[2]. Xu, X., Zhang, Z., Dong, J. et al. Ultrafast epitaxial growth of metre-sized single-crystal graphene on industrial Cu foil. Sci. bull. 62, 1074-1080 (2017).

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