石墨烯等二维（2D）材料有望应用于多个领域

对于普通材料，沿一个方向的晶格分别随着其他正交方向的压缩或拉伸而扩展或收缩。泊松比（ν）是用于量化物理属性的参数，在大多数情况下为正值。但对于某些特殊材料（所谓“膨胀材料”），ν可以为负。负泊松比（NPR）引起人们极大的兴趣，因为具有NPR的材料类型通常具有更强的韧性、抗剪切性和有效的降噪能力，从而可以实现许多新颖的应用，如航空航天和国防等。从历史上看，对NPR的研究大多是在体膨胀结构上进行的，进一步扩展到纳米材料后，发现了更多有趣的现象。如，近来在碳纳米管和金属纳米板中发现了NPR。另外，石墨烯等二维（2D）材料有望应用于多个领域，而其也已通过特殊工程实现了NPR，可切成纳米带、引入空位缺陷、在极高温度下产生周期性多孔和波纹弯曲等。但2D材料的NPR机制如何尚不清楚。

来自湖南大学的秦光照和郑州大学的秦真真基于第一性原理方法，合作研究了石墨烯、硅烯、h-BN、h-GaN、h-SiC和h-BAs等新型二维蜂窝状材料在单轴应变下的机械响应，预测上述材料在沿扶手椅方向时均存在负泊松比现象。尽管它们元素成分不同，但其NPR均由键角的反常增加导致。这种键角的反常增加无法通过传统层面的几何结构和力学响应的观点来解释（如基于经验势函数的经典分子动力学模拟研究）。该工作通过对应力调控下的关键几何参数及轨道杂化作用的变化趋势进行分析，从电子结构层面阐明了键角反常增加及NPR的底层物理机制。该机制同样可适用于其他具有NPR现象的纳米结构。

Negative Poisson’s ratio in two-dimensional honeycomb structures

Guangzhao Qin and Zhenzhen Qin

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.

责任编辑:pj