Abnormal conductivity in low angle twisted bilayer graphene

Materials scientists can control the interlayer twist angle of materials to offer a powerful method to tune electronic properties of two-dimensional (2-D) van der Waals materials. In such materials, the electrical conductivity will increase monotonically (constantly) with the decreasing twist angle due to enhanced coupling between adjacent layers. In a new report, Shuai Zhang and a team of scientists in functional materials, engineering, nanosystems and tribology, in China, described a setup for non-monotonic angle-dependent vertical conductivity across the interface of bilayer graphene containing low twist angles. The vertical conductivity enhanced gradually with the decreasing twist angle, however, after further decrease in the twist angle, the conductivity of the material notably dropped. The scientists revealed the abnormal behavior using density functional theory (DFT) calculations and scanning tunneling microscopy (STM) and credited the outcome to the unusual reduction in average carrier density originating from local atomic reconstructions. Atomic reconstruction can occur due to the interplay between the van der Waals interaction energy and the elastic energy at the interface, leading to intriguing structures. The impact of atomic reconstruction was significant on vertical conductivity for low-angle, twisted 2-D van der Waals materials; providing a new strategy to design and optimize their electronic performance.
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