Liu Ning, Hong Jiawang, Zeng Xiaowei, Pidaparti Ramana, Wang Xianqiao
College of Engineering, University of Georgia, Athens, GA 30602, USA.
Phys Chem Chem Phys. 2017 May 24;19(20):13083-13092. doi: 10.1039/c7cp01033h.
The outstanding mechanical performance of nacre has stimulated numerous studies on the design of artificial nacres. Phosphorene, a new two-dimensional (2D) material, has a crystalline in-plane structure and non-bonded interaction between adjacent flakes. Therefore, multi-layer phosphorene assemblies (MLPs), in which phosphorene flakes are piled up in a staggered manner, may exhibit outstanding mechanical performance, especially exceptional toughness. Therefore, molecular dynamics simulations are performed to study the dependence of the mechanical properties on the overlap distance between adjacent phosphorene layers and the number of phosphorene flakes per layer. The results indicate that when the flake number is equal to 1, a transition of fracture patterns is observed by increasing the overlap distance, from a ductile failure controlled by interfacial friction to a brittle failure dominated by the breakage of covalent bonds inside phosphorene flakes. Moreover, the failure pattern can be tuned by changing the number of flakes in each phosphorene layer. The results imply that the ultimate strength follows a power law with the exponent -0.5 in terms of the flake number, which is in good agreement with our analytical model. Furthermore, the flake number in each phosphorene layer is optimized as 2 when the temperature is 1 K in order to potentially achieve both high toughness and strength. Moreover, our results regarding the relations between mechanical performance and overlap distance can be explained well using a shear-lag model. However, it should be pointed out that increasing the temperature of MLPs could cause the transition of fracture patterns from ductile to brittle. Therefore, the optimal flake number depends heavily on temperature to achieve both its outstanding strength and toughness. Overall, our findings unveil the fundamental mechanism at the nanoscale for MLPs as well as provide a method to design phosphorene-based structures with targeted properties via tunable overlap distance and flake number in phosphorene layers.
珍珠层优异的力学性能激发了众多关于人造珍珠层设计的研究。磷烯作为一种新型二维（2D）材料，具有晶体平面内结构以及相邻薄片间的非键相互作用。因此，磷烯薄片以交错方式堆积的多层磷烯组件（MLP）可能表现出优异的力学性能，尤其是卓越的韧性。为此，进行了分子动力学模拟，以研究力学性能对相邻磷烯层重叠距离和每层磷烯薄片数量的依赖性。结果表明，当薄片数量等于1时，通过增加重叠距离可观察到断裂模式的转变，从由界面摩擦控制的韧性失效转变为由磷烯薄片内共价键断裂主导的脆性失效。此外，可通过改变每层磷烯薄片的数量来调整失效模式。结果表明，极限强度与薄片数量遵循指数为 -0.5 的幂律关系，这与我们的分析模型高度吻合。此外，当温度为1 K时，每层磷烯薄片数量优化为2，以便有可能同时实现高韧性和高强度。而且，我们关于力学性能与重叠距离之间关系的结果可以用剪切滞后模型很好地解释。然而，需要指出的是，提高MLP的温度可能会导致断裂模式从韧性转变为脆性。因此，为了同时实现其优异的强度和韧性，最佳薄片数量在很大程度上取决于温度。总体而言，我们的研究结果揭示了MLP在纳米尺度上的基本机制，并提供了一种通过调节磷烯层中的重叠距离和薄片数量来设计具有目标性能的磷烯基结构的方法。
