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.
