Negative Poisson's ratio (NPR) in auxetic materials is of great interest due to the typically enhanced toughness, shear resistance, sound and vibration absorption, which enables plenty of novel applications such as aerospace and defense. Insight into the mechanisms underlying the NPR is significant to the design of auxetic nanomaterials and nanostructures. Currently, the understanding of the NPR phenomena is dominated by the geometry analysis in literature. The auxetic effect is generally thought to be independent of chemical composition and electronic structure, which originates from the special reentrant structures or the rigid building blocks linked by flexible hinges.
In this study, by employing first-principles calculations, we report intrinsic NPR in a class of two-dimensional honeycomb structures (graphene, silicene, h-BN, and h-GaN), which are distinct from all other known auxetic materials. Their honeycomb structures possess no reentrant or hinge-like building block. The electronic effect rather than the mechanical factor is found responsible for the intrinsic NPR. The four 2D materials share the same mechanism for the emerged NPR despite the different components, which lies in the increased bond angle. The increase of bond angle is quite intriguing and anomalous, which cannot be explained in the traditional point of view of the geometry structure and mechanical response, such as in the framework of classical molecular dynamics (MD) simulations based on empirical potential. We attribute the counter-intuitive increase of bond angle and the emerged NPR foundamentally to the strain modulated electronic orbital coupling and hybridization. We further propose that the NPR phenomenon can also emerge in other nanostructures or nanomaterials with similar honeycomb structure. Our study not only make a comprehensive investigation of the intrinsic NPR in the four 2D materials with honeycomb structure, but also reveals the physical origins, which deepens the understanding on the NPR and would shed light on future design of modern nanoscale electromechanical devices with special functions based on auxetic nanomaterials and nanostructures.