Small torsion angle bilayer graphene has been prepared

In recent years, with the discovery of novel quantum phenomena in bilayer graphene with torsion angles in the magic angle range (~ 1.1 °), the study of torsional bilayer graphene has received increasing attention. Therefore, a new field of research has spawned—twistronics. Conventional bilayer graphene is a stable structure formed through AB stacking, while for torsional bilayer graphene, its surface will show molar fringe superlattice, and this superlattice cycle and bilayer graphene band structure will change with the change of torsion angle. At present, laboratory small torsion angle bilayer graphene is mostly prepared by micromechanical stripping method and manual stacking method. How to directly prepare small torsion angle bilayer graphene by growth method is an important issue to be solved at present.

Recently, Wang Haomin, a researcher at the Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, in collaboration with Li Qunyang, a professor at Tsinghua University, reported for the first time the preparation of bilayer graphene with different torsion angles by chemical vapor deposition on the surface of hexagonal boron nitride (h-BN), and found the effect of local atomic remodeling of small-torsion angle bilayer graphene on its vertical conductance. The relevant research results were published online in Science Advances (Science Advances, 6, eabc5555, 2020) under the title "Abnormal conductivity in low-angle twisted bilayer graphene."

The research group of Shanghai Institute of Microsystems proposed a new method for the preparation of bilayer graphene with different torsion angles on h-BN surfaces by non-equilibrium chemical vapor deposition: a monolayer polycrystalline graphene film was first prepared on the h-BN surface, followed by graphene single-crystal domain growth on its surface, and finally bilayer graphene with different torsion angles on the h-BN surface was prepared. The researchers collaborated with Li Qunyang's team to compare the vertical conductance of bilayer graphene with different torsion angles using conductive atomic force microscopy (C-AFM) measurements and found the abnormal angle dependence of vertical conductance in small torsion angle bilayer graphene. Researchers collaborated with Gao Lei's group, an associate professor at Beijing University of Science and Technology, to further reveal that the abnormal conductance behavior at small torsion angles stems from the reduction of the average carrier concentration caused by local atomic reconstruction of bilayer graphene with the help of atomic-level interface contact quality models, density functional theory calculations, and high-resolution characterization by scanning tunneling microscopy. This study reveals for the first time the contribution of atomic-level reconstruction to vertical conductivity in van der Waals materials, provides guidance for understanding the unique physical behavior of van der Waals materials with small torsion angles, and provides new ideas for designing and optimizing the electrical properties of two-dimensional materials.

Zhang Shuai, Song Aisheng, PhD students of Tsinghua University, and Chen Lingxiu, PhD, of Shanghai Institute of Microsystems, are co-first authors of the paper, and Li Qunyang, Wang Haomin, and Gao Lei are corresponding authors of the paper. The research work was funded by the National Key R & D Program, the National Natural Science Foundation of China, the Strategic Pilot Science and Technology Special Project of the Chinese Academy of Sciences (Category B), and the Science and Technology Commission Fund of Shanghai Municipality.

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