Nevertheless, the out-of-plane (OOP) symmetry is well preserved in TMD monolayers, hindering their applications in certain areas. The IP asymmetry, together with strong spin–orbit coupling (SOC) generates Zeeman-type spin splitting as well as valley polarization in TMD monolayers. For instance, the IP asymmetry generates finite Berry curvatures and orbital magnetic moments with opposite signs in two valleys, which lead to novel valley-dependent optical characteristics.
As a representative class of 2D materials, transition metal dichalcogenide (TMD) monolayers possess intrinsic in-plane (IP) asymmetry, and distinctive electronic and optical characteristics. In addition, some other Janus 2D monolayers have also been extensively investigated, such as Janus graphene oxide nanosheets, Janus silicene, and asymmetric double-sided bilayer WSe 2. The electronic structures could be modulated by choosing n-type or p-type functional groups. For instance, by surface covalent functionalization, Janus graphene was synthesized in experiments. Therefore, a large variety of strategies have been proposed to break the symmetry in 2D materials and engineer the electronic/magnetic/optical properties. Twisted bilayer graphene with inversion asymmetry gives rise to tunable second harmonic generation.
For instance, inversion symmetry breaking engineered by a vertical electric field (EF) can control the electronic gap of graphene bilayer. Structural symmetry breaking provides a remarkable platform for inducing a wealth of fascinating properties in 2D materials.
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