Negative and near-zero Poisson's ratios in 2D graphene/MoS₂ and graphene/h-BN heterostructures

Negative and near-zero Poisson's ratio materials are mechanical metamaterials that host an improved strain energy absorption property with respect to conventional materials. By using first-principles calculations, we report the negative and near-zero Poisson's ratios in van der Waals (vdW) heterostructures, such as graphene/hexagonal molybdenum disulfide (G/MoS₂) and graphene/hexagonal boron nitride (G/h-BN), sharing distinct crystal structures from all other known negative and near-zero Poisson's ratio materials. However, the Poisson's ratios of the interfacial layer in bilayer MoS₂ and h-BN are positive and near-zero along the out-of-plane direction, respectively, which are quite different from the Poisson's ratios of G/MoS₂ and G/h-BN heterostructures. Therefore, these behaviors are unique to vdW heterostructures. We attribute these negative Poisson's ratio behaviors to the special interaction of p_z orbitals between the interfacial layers. Furthermore, we observed among them high-order elastic constants in the out-of-plane direction, which exhibit nonlinear stress-strain responses. These results are significant for the preparation of G/h-BN and G/MoS₂ heterostructures. The combinations of tunable negative/near-zero Poisson's ratios with the excellent electrical/optical properties of G/MoS₂ and G/h-BN heterostructures will pave the way to a wide range of applications in novel auxetic molecular sieves and electrodes.