Abstract
Negative Poisson’s ratio (NPR) in auxetic materials is of great interest due to the typically enhanced mechanical properties, which enables plenty of novel applications. In this paper, by employing first-principles calculations, we report the emergence of NPR in a class of two-dimensional honeycomb structures (graphene, silicene, h-BN, h-GaN, h-SiC, and h-BAs), which are distinct from all other known auxetic materials. They share the same mechanism for the emerged NPR despite the different chemical composition, which lies in the increased bond angle (θ). However, the increase of θ is quite intriguing and anomalous, which cannot be explained in the traditional point of view of the geometry structure and mechanical response, for example, in the framework of classical molecular dynamics simulations based on empirical potential. We attribute the counterintuitive increase of θ and the emerged NPR fundamentally to the strain-modulated electronic orbital coupling and hybridization. It is proposed that the NPR phenomenon can also emerge in other nanostructures or nanomaterials with similar honeycomb structure. The physical origin as revealed in our study 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.
Highlights
For common materials, the lattice along one direction expands or shrinks as the other orthogonal directions are compressed or stretched, respectively
With tensile strain applied along the armchair direction, the six 2D materials share the same mechanism for the emerged negative Poisson’s ratio (NPR) despite the different components, which lies in the increased bond angle θ
By analyzing the evolution of key geometry parameters (bond length (b1) and bond angle (θ)) with strain increasing, it is found that the six 2D materials share the same mechanism for the emerged NPR despite the different components, which lies in the increased bond angle θ
Summary
The lattice along one direction expands or shrinks as the other orthogonal directions are compressed or stretched, respectively. The negative Poisson’s ratio (NPR) is of great interest because the type of materials with NPR typically possess enhanced toughness, shear resistance, and efficient sound/vibration absorption, which enables plenty of novel applications, such as aerospace and defense[2]. Researches of the NPR are mostly conducted on the bulk auxetic structures, which are further extended to nanomaterials and more interesting phenomena are found. The NPR has been reported recently in 2D materials without any external modifications to the structure, shape, or composition, such as the in-plane NPR (for instance, borophene[10], penta-graphene[11], α-silica[12], Be5C213), the out-of-plane NPR (phosphorene[14,15], GeS16, SnSe17, arsenic[18,19], TiN20), and both in-plane and out-of-plane NPR possessed in bidirectional 2D auxetic material Ag2S21
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